18 cm

Figure 3.16. Chimney #10 from Garden Banks block 382. The ages
shown here are based on the 228Th/228Ra assays.

53



Table 4.4. Annual discharge estimates based on data from barite-bearing seeps in the Garden Banks and Mississippi Canyon produced in this study, Discharges

from the Mississippi River into the Guif of Mexico are shown for comparison purposes.

Source * Fa Coy * Aner * Anara ° Fay 7 Boacra * Brossa ” Fey
(Liyr) (mg/L) (dpmv/L) (dpm/L) (glyr) (dpm/yr) (dpmv/yr) (dpm/yr)
Garden Banks 250 1,173 3,656 2,453 2.3 x 10" 7.3.x 108 4.9 x 10 1.2.x 10
Mississippi Canyon 2,920 91 739 491 21x 10!” 1.7.x 10" Lx 10! 2.8 x 10"4
Mississippi River 507 x 10"? 8 x 107 0.18 0.12 4.4 x 10! 9.1 x 10! 6.1 x 108 1.5 x 10!

' Garden Banks; GB-382 and GB-338; Mississippi Canyon: MC-929.
? Seeps fluid fluxes data from Table 4.3; Mississippi River discharge is the 46 year mean of 507 km’/yr (Dinnel and Wiseman, 1986).
* Ba concentration in seep pore fluids from Table 3.4; Mississippi River data from Buerkert (1997).

“5 Ra activities in seep pore fluids from Table 3.3; Mississippi River data from Krest (1995),

678 F = Raya x C (or A): Annual discharges from seeps are estimated over the Louisiana upper slope area of 15,552 km’ assuming a barite-seep distribution of

0.1% of area and a density of 5 point-sources per square meter (see text).

° Total radium flux @*Ra + ?Ra),

Lit



19

FAUNA

a.

Chemoautotrophs
Gnmussels)

———_*

Shell

226Ra, Ba

TM,

Heterotrophs
(crabs, starfish, etc.)

Body

Ne

226Ra Ba

Figure. 2.6. Flow chart of chemical and isotope assays of fauna.



Coastal Marine Institute

Fate and Effects of Barium and

OCS Study

MMS 2001-004

Radium-Rich Fluid Emissions from
Hydrocarbon Seeps on the Benthic Habitats
of the Gulf of Mexico Offshore Louisiana

Authors

Paul Aharon
Dan Van Gent
Baoshun Fu
L. Max Scott

January 2001

Prepared under MMS Coniract
14-35-0001-30660-19946

by

Coastal Marine Institute
Louisiana State University
Baton Rouge, Louisiana 70801

Published by

U.S. Department of the Interior
Minerals Management Service
Gulf of Mexico OCS Region

Cooperative Agreement
Coastal Marine Institute
Louisiana State University



113

estimate the radium and barium fluxes from a point source. Radium fluxes are
derived first.

Given the radioactivity of fluid seepage and the growth rate of a radioactive
barite chimney, the flux of radium in dpm/yr can be determined according to
the following equation:

F = (VpARajit (4.6)

where F is the average flux of radium in dpm/yt; V is the volume of the
barite deposit in cm3; p is the density of the barite deposit in gm/em3 ; t is the
duration of growth (Ra-rich fluid emission) in years; and Raj is the average
total initial radium activity in dpm/g scavenged by the barite. The Raj is the
summation of the two dominant radium isotopes, 226Ra and 228Ra, During the

initial fluid emission at t = 0, this value can be calculated from the actual
measurements of Ra activities as follows:

ARa; = 226Raj + 228Ra; (4.7)

where 226Raj and 228Rai are the initial activities in dpm/g of the two Ra

isotopes. Due to the long half life of 226Ra (1,620 yrs) and the young ages of
the barite deposits (< 30 yrs), the measured 226Ra values can be used as 226Rai.

228Ra: can be obtained from the following equation:
228Ra; = 226Ra,(228Ra/226Ra); (4.8)

where (228Ra/226Ra); is the initial 228Ra/226Rq activity ratio of the barite
deposits.

Equation (4.6) can be used to calculate the radium fluxes of the barite
deposits. For illustrative purposes, let's assume that a barite deposit forming on
top of a point-source emission site has a truncated conical-shape form (Fig.
4.15), which is typical to the observed morphologies of the chimneys and the
crusts blanketing mud-volcanoes in the Gulf of Mexico. For a truncated
conical-shape barite chimney, the radium flux in dpm/yr units from a point
source can be determined by the following equation:

F = [1/3rh(ry 2419? + ry12)pARa(ijl/t (4.9)

Where rj and rg are the radii of the base and the top, respectively, of a
truncated conical barite feature in cm and h is the height of the chimney in
em (Fig. 4.15).

Because the term h/t in equation (4.9) equals the average growth rate (G)
of the chimney, this equation can be rearranged into:

F = 1/3nG(r2+r9? + r112)pARa(i) (4.10)
where G is the vertical average growth rate in cm/yr.

Using equation (4.10) and the parameters in Table 4.3, fluxes of radium
emission from three barite chimneys are calculated and their values are



62

3.0 T r r 1
2.5 4
E  2.0b ca 4
&
a
vo
a 157 slope=1 1
1.0} i i 4
05 Ff F : \
0.5 1.0 15 2.0 2.5 3.0
Agel (yrs)

Figure 3.20, Relationship between the first and second 228Th/228Ra dating of
pure barites. The second dating was performed two years after the first dating.
The ages are sampling ages. Data converge along the line with a slope of 1:1

indicating that the initial 228Th in the barites is negligible.



73

Table 3.12. Radium-226 content in heterotrophic fauna from seeps. Seep designation “C” denotes carbonate

seep and “B” denotes barite seep.

Sample Biota’ Type | Year |DiveNo> | GOM Seep* | Ra-226°
LD. Block dpm/g
GS-8 GC Skeleton [| 1992 [JSL-3300 | GC-184 Cc [o1+03
GS-8 GC Body 1992 [JSL-3300__[ GC-184 Cc [osto4
97-GLUC1 GC Skeleton_[1997 | JSL-2897__| GB-338 B_ [10204
97-GLI/C1 GC Body 1997 [ JSL-2897 __ | GB-338 B_ | 60226
97T-GLUCZ GC Skeleton [1997 | JSL-2897__| GB-338 B_ [04203
97-GLUC2 GC Body 1997 | JSL-2897___ | GB-338 B | 10.7428
GS-1 SF | Whole body | 1993 _ [3SL-3300 | GC-184 Cc [os+04
OT-SE2 SF__| Whole body [1997 | JSL-2897__ | GB-338 B [17204
97-SF3 SF [ Whole body [1997 | JSL-2897__| GB-338 B_ [20405
97-SF1 SF [ Whole body [1997 | JSL-2897__ | GB-338 B_ [39+06
97-TW1 PY Skeleton [1997 | JSL-2897 | GB-338 B [05403
O7-TW2 PY Skeleton [1997 | JSL-2897 _ [GB-338 B | 10+04

'GC- galatheid crab
SF- starfishes
PY- polychaetes
? Year of collection
3 Submersible dive number; JSL = Johnson-Sea-Link

* Seep type according to predominant geologic products; C = carbonate-dominated seep; B = barite-

dominated seep

* Radium-226 determination with 2-o counting error; BDL = below detection limit



normalized to 87Sr/86sr of NBS-987 = 0.71023 and to 86sr/88sr = 0.1194 with an
estimated analytical error of £1x1075 (20).

226Ra determinations in fluid samples were conducted with a modified
scintillation technique developed by Prichard et al. (1980). A 5 ml aliquot of a
fluid sample was injected under 10 ml of a toluene-based scintillation cocktail
with Liquifluor® (New England Nuclear product) added as the scintillating
agent. The entire two-phase aqueous-organic layer was sealed gas-tight with a
polyseal cap in a 20 ml borosilicate-glass scintillation vial and stored for at
least three weeks prior to analysis. The 222Rn isotope (half-life 3.82 days,
Broecker and Peng, 1982) resulting from alpha decay of 226Ra, and in secular
equilibrium with the parent isotope, was then assayed using a liquid
scintillation counter with a-discrimination capabilities. The total activity of
226Ra in the sample was calculated from the counting data and the detection

efficiency was determined by measuring the 226Ra blanks and standards.
Based on multiple analyses of standards, the average conservative error (lo)

in the 226R activity values is estimated to be 45% or lower.

For 228Ra determination, 5 ml of a fluid sample was acidified to 0.1 N with
dilute hydrochloric acid. The acidified sample was aliquoted onto a mini-

column of Dowex 50W-X12 cation exchange resin. The 228Ra was allowed to
decay to its first daughter product, 228 Ac, over a period of 36 hours.

Subsequently, the 228 Ac was selectively eluted from the column with dilute
lactic acid and collected in 10 ml of a water miscible liquid scintillation
cocktail. The high energy beta particle emissions of 228Ac are measured on
the liquid scintillation counter by virtue of visible microflashes of light
produced by interaction of the beta particles with the liquid scintillation
cocktail. These samples were also compared to a NIST (National Institute of
Standards and Technology) traceable 228Ra standard which was carried
through the identical procedure. Based on multiple analyses of standards, the

average conservative error (1 o) in the 228Ra activity values is estimated to be
+8 % or lower.

2.2. Barite Deposits

The types of imaging, chemical and isotope analyses applied to the barite
chimneys and crusts from barite-bearing seeps are summarized in a flow
chart in Figure 2.2.

2.2.1. Imaging, Chemical and Stable Isotope Assays

The soft and friable chimneys were hardened at room temperature using
Araldite GY-506 water insoluble resin. Polished thin sections were made of
chimneys and crusts. Mineral identifications and textural interpretations were
made using reflected and transmitted light microscopes and a JEOL-840
scanning electron microscope (SEM) equipped with an EDS X-ray and back-
scatter electron-detector units. Quantitative analyses for Ba, Sr, Ca, Na and Fe
in barite were performed using an electron microprobe model JEOL-733,
equipped for WDS X-ray spectroscopy. The instrumental conditions for the
duration of the analyses were: 15 KV accelerating voltage, 2 nA beam current,
and 60 second counting intervals. Sulfates and carbonates of known chemical
compositions were used for the purpose of standardization using conventional



122

Table 4.5. Radium-226 Activities (dpm/g) in Marine Molluscans and Crustaceans

Organism Iyengar and Narayana Alam ct al. This Study‘ Rabalais et al.
(1990) (1999) (1991)
Mollusca
Soft parts 0.048 — 0.834 0.02 — 0.3 0.02 - 1.5 0.1~2.7
Shells 0.03 — 0.031 0.6 —0.8 BDL*- 2.3 NR?
Crustacea
Soft parts BDL*— 0.18 NR? 6.04 — 10.7 NR?
Exoskeletons 0.032 — 0.204 NR? 0.42 — 0.95 NR?

“BDL: below detection limit

*NR: did not report data
Barite Seeps



128

Table 4.6. **Ra and ”*Ra mean activities in barite deposits with 2-o random counting error

2Ra ZBRa

Location Number Mean dpm/g Mean dpm/g
Garden Banks 28 2796 + 414 1872 + 564
Mississippi Canyon 4 7146 + 3168 2568 + 1644

Table 4.7. External dose rates to seep fauna calculated from Microshield 5.0

Location Ra Ra Total
Dose Rate :wGy/h Dose Rate : uGy/h Dose Rate :uGy/h
Garden Banks 3.4 (0.34 mrad/h) 3.0 (0.30 mrad/h) 7.4 (0.74 mrad/h)
Mississippi Canyon 8.7 (0.87 mrad/h) 4.1 (0.41 mrad/h) 12.8 (1.28 mrad/h)



68

2.0 ; —
A Barite chimney
@ Barite crust
157 e 4
gs
=
a e
S 10p 1
7
J
a
<J
05 7 e 7
e
e e
e
0.0 + 1
0 10 20 30 40

Sulfide (% by weight)

Figure 3.23. Sulfide effects on the 2!0pp/226Ra dating. Note the positive
relationship between the sulfide concentration and the difference in

210pb/226Ra before and after barite purification.



26

Table 3.3. Salinity, Barium Concentration and Ra Isotope Activities in Pore Fluids from Barite-Bearing Seeps.

Sampling Sample Core Dive MMS Water Salinity Ba =Ra =Ra 228/226
Year Code Depth No. Block Depth (ppt), (mg/L). (@pnv/L)_—(apm/L) AR
(cm) No. (m)
1993, 3559-214 4 —«SSL-3559—«MC-929_—«640, 52 0.19 190.1226 —:106.828 0,560.09
1993, 3559-2-17 7 JSL-3559-« MC-929 640 65 0.12 27.2414
1993, 3559-220 +10 «ISL-3559 MC-929 «640 96 0.48 —_-| —-
1993 3559223 13. JSL-3559 MC-929 «640 130 1.98
1993 3559-232 22 JSL3559 MC-929 640 155 90.60 738.744 490.5226 0.6640.05
1993 3562-120 -6 —‘ISL-3562. MC-929 657 6 106.1420 63.345 0,600.12
1993 3566-115 -7-—JSL3566. | GB-382. 513, 134 548.80 17314482 1327.1290 0.7720.06)
1993 3566-118 10 -SJSL-3566 GB-382 513 127 547.80 13813458 1239.6865  0.90:0.06|
1993 3566-121 13, «JSL-3566 © GB-382. 513 139 833.00 24915488 2137.8291 0.8640.05
1993 3966-1-24 16 «JSL-3566  GB-382. 513, 135 925.80 28293474 245284165 — 0.87:0.06|
1993, 3566-127 19 -JSL-3566  GB-382. 513, 127 467.80 13142464 10602483 0.810.07|
1993 3566-130 22» -JSL-3566 © GB-382 513 119 290.50 701.8448 606.2846 0.8640.09|
1993, 3566-227 -1.—JSL-3566 © GB382— 513, 45 0.29 —
1993, 3566-230 -4 «—-JSL-3566 © GB-382513, 31 023° 427817 = |
1997 97-2897-1-1 -12-‘JSL-2897«GB-338. «510 154 616.30 24514271 94276 0.3840.03
1997 97-2897-1-2-15.‘JSL-2897GB338 510 154 756.12 1970.7465 828.8259 0.4240.03
1997 97-2897-1-3 18 «JSL-2897 GB-338 510 154 936.32 3655.9287 834.6252 0.2340.02
1997 97-2897-1-4 21. SL-2897 GB338.— 510 152 913.13 27573286 698.4257 0.25+0.02
1997 97-2897-1-5 24 ISL-2897 GB-338.= 510 152 89151 2270.7469 779.7443 0,340.02
1997 97-2897-1-6 27 «JSL-2897 GB-338.— 510 153 750.07 3250.0#85 695.5443 0.2140.01
1997 97-2897-1-7 -30-«JISL-2897 GB-338.— 510 150 797.37 21795474 664.7446 0.3130.02]
1997 97-2897-1-8 33 «JSL~2897- GB-338.— 510 150 689.50 27229485 958.2464 0.350.03
1997 97-28973-1 16 —‘JSL-2897 GB-338. 510 50 0.28 773.8461 491.3445 0,630.08
1997 97-2897-3-2 -19-«JSL-2897 GB-338.— 510 53 0.32 287.3427 172.5#18 _0.6020.09
1997 97-2897-3-3 © -22««JSL-2897 GB338 510 58 0.64 214.3249
1997 97-2897.3-4 25 ‘JSL-2897 GB-338. 510 60 157 607.1469 292.4498
1997 97-2897-44 ISL-2897 GB-338 510 B 0.24 224.9449
1997 97-2897-4-2 JSL-2897 GB-338 510 88 119.34 789.8442 241.1435
1997 97-2897-43, ISL-2897  GB-338 510 104 246.06 12613252 281.1437
1997 97-2897-4.4 JSL-2897 GB338 510 1s 436.36 16093458 316.1437
1997 -97-2897-4.5 JSL-2897  GB-338 510 122 732.19 202.5464 350.7450
1997 97-2897-4.6 JSL-2897 GB-338 510 132 883.74 2748.5482 589.2465
1997 97-2897-4-7 JSL-2897  GB-338 510 146 1125.48 30233479 819.6497
1997 97-2897-4-8 —--33.-—JSL-2897 GB338 510 144 1173.27 27784875 393.943
1997 97-2897-MV-L JSL-2897 GB-338 S10, 45.5 057 80.4417
1997 97-2897-MV-2 ISL-2897  GB-338 510 138 9.80 302288 = ——— |
1997 97-2897-MV-3 ISL-2897  GB-338 510 132 425 240.2425,
1997 97-2897-MV-4 ISL-2897  GB-338 510 146 17.48 390.5430 0.67+0.09
1997 97-2897-MV-5 JSL-2897  GB338 510 141 93.43 474.0432 0.5140.10
1997 _97-2897-MV-6 JSL-2897 _GB-338___510 140 162.55 840.2243 0,620.07

Note: Barium determinations by ICP carry an estimated error of 40.5%.



top

y = 25.31 + 9.32x R42 = 0.99

_
J
T

Distance (cm)

bottom

po

3.0
Age (yrs)

Figure 4.7. Vertical age profile for chimney #6, Garden Banks block 382.
Growth rate determined on the basis of slope is 9.32 cm/yr.

97



33

3 6 T

=
i}
3 B
AA
0
At

2 4
0 1000 2000 3000

23 ; ;
2a a j

deepwater

nr
-—_ eau)

>
T

4000

8180 (°/00 SMOW)

He

i
197
al Ate a
15 ¢ 7

13 1 n 1
550 560 570 580 590

Cl (mM/L)

Figure 3.5. Cl-8180 relationships in pore fluids from seeps. A: pore fluids type I,
Il and Ill. The measured 8180 values of the bathyal and abyssal Gulf of Mexico
waters are 1.4°/,, and -0.15°/,, (SMOW), respectively. B: The inverse

relationship observed between Cl and 8180 in pore fluids type I is attributed to
sublimation of gas hydrates (see text). Empty squares are pore fluids from core

2639-2 (GC-232) and solid circles are pore fluids from core 2647-2 (GC-184/185,
Bush Hill).



Table 3.8. Activities and ages of barite crusts,

[Sampte Sample "Ph "Ra "Th Ra Measured Measured Sampling
mame type  dpmv/g dpnvg ss dpmv/g-dpm/g Ss TM"PD/*Ra Ra Ra PD Ra Pb Ra
age (yr) age (yr)
Barite erusts from Garden Banks block 382
er-93-7-71 R 553 887 - : 0.62940.014 - 316424 — 29,742.4
cr93 P 416 1352 : - 0.35640.014 : 14114 12.2414
Cr-93-8-5/3/u R 1651475 - - 0.79740.004 - 50.341.2  48.4£1.2
C1-93-8-5/3/u Pp 816 1545 - - 0.53340.002 - 24,3203 22.440.3
Cr-93-8-5/3/1 R 362 1381 - - 0.629:40,006 - 3L.6H1.0  29.741.0
Cr-93-8-5/3/1 Pp 761 1502 - : 0,51 140.005 - 22.940.7 —21.040.7
Cr-93-8-1/24u R sl 672 - : 0,76840.011 - 46.242.9  44.242.9
Cr-93-8-7/2/0 P 449 1203 : : 0.37620.013 - 15141.3 13.2413
Cr-93-8-7/2/ R 382 269 : - 1.43 140.048 : - -
C1-93-8-7/2/1 P 640 1725 - - 0.37420.021 - 15.042.1 13.142.4
Cr-93-8-7/3 R 292 145 - - 2.038%0.070 - - -
cr-93-8-7/3 P 654 1271 - : 0.51940.015 - 23.4419 21.5419
Barite crusts from Mississippi Canyon block 929
C1-93-1-3/1 R 705 1934 : 262 —0,36840.006 0.13540.010 14,7406 12.840.6
Cr-93-1-3/1 P 1085-3797 - 180 0.28840.005 0.04740.009 —10.9+0.5 9,040.5
Cr-93-4.4/ 1/0 R 2443 34 - - 1,01340,005 - - -
Cr-93-4-4/1/u Pp 1451-2701 - - 0,54240,002 - 24,940.2 23.1403
Cr-93-4-4/1/1 R 1374689 - : 2.01340.005 - - -
C1-93-4-/1/1 P 2002___3940 - : 0,51340,001 < 23.040. 21.1401

Notes: R= raw sample.

pure barite
= below detection limit,
Measured age is obtained by the measured isotope ratio.

Sampling age = age corrected to the time of sample collection.

8s



29

108 ot oe oy ey

102

Salinity (ppt)

¥ =78.65 + 22.46 * LOG(x) R2 = 0.74

JOU Lvtieah  t l  l
10-2 10-1 100 101 102 108 104

Ba (mg/L)
103 5 ® . T T T T 7
S L 4
& . "here 5
Bm gz b =u oF |
3 "am ‘1 |
‘ 4
101 in L 1 1 L
0 2000 4000 6000
Ra (dpm/L)

Figure 3.2. Salinity, Ba and Ra relationships in pore fluids from barite-bearing
seeps.



Table 3.9, Results of repeat dating of chimneys.

Sample Date of Th Ra Measured Sampling
name Measurement (dpm/g) (dpm/g) TP Ra 2Th*Ra = Th/*Ra
age (yr) age (yr)

First dating
Ch#2-a Apr-95 819 1135 0.772+0.006 2.7240.06 0.93:40.06
Ch#2-b May-95 624 690 0.904+0.006 3.82+0.08 1.94+0.08
Ch#9-B/b Aug-95 708 739 0.9580.005 4,22+0.08 2,130.08
Chi 10-T/b Sep-96 784 793 0.988:+0.001 4.4540,02 1.25+0.02
Ch#10-B/b Sep-96 819 755 1,0860.003 5,3340,06 2.18:40.06

Second dating

Chi#f2-a Feb-98 955 860 1.110+0,005 5.5840.11 1,080.11
‘Ch#2-b Feb-98 684 570 1.200+0.003 6.6740.08 2.10£0.08
Ch#9-B/b Feb-98 647 535 1.210+0.006 6.8140.09 2.3140.09
Ch#10-T/b Feb-98 620 559 1.110+0.004 5,59+0,09 1.09+0.09
Ch#10-B/b Feb-98 727 599 1.214+0.003 6.87+0,09 2.0040.08

See Table 3.7 for explanation of measured and sampling ages.

19


98

25
= r
ao
Eg 1st
2 y = 21.30 -8.93x R42 = 0.95
5
g
2 10/7
a
st
£
£
=
i=)
2 0 1 L i
0 1 2 3

Age (yrs)

Figure 4.8. Vertical age profile for chimney #8, Garden Banks block 382.
Growth rate determined on the basis of slope is 8.93 cm/yr.



120

largely dependent on the effective ionic radius of the respective element.
According to Szabo (1967), the sequence of selectivity in Mollusca and
Crustacea is found to be as follows:

Element: Ra> Ba> Sr> Ca> Mg
(DF) (19) (18) (1.2) (1.0) (0.1)

where DF is the Discrimination Factor which, for example, when calculated
with respect to Ba is as follows:

DF(Ra/Ba)= [(CRa/CBa) org/(CRa/CBa) sw] (4.11)

where C= concentration of the respective chemical element in the
organism (org) or sea water (sw) as denoted by subscripts.

Fairly high levels of Sr and Ca are also known to be present in the pore
fluids of barium-radium-rich seeps (Table 3.1), and these elements may also
selectively compete for biological radium uptake by virtue of their respective
smaller discrimination factors.

Marine Mollusca have been extensively studied for radium and other

radionuclides. A comprehensive literature review of 226R, results from
marine animals worldwide has been completed by Iyengar and Narayana

(1990) for the International Atomic Energy Agency. 226R, activities in
molluscan soft parts were found to range from 0.005 dpm/g to 0.1 dpm/g wet

weight. These authors also found molluscan shell 226R, activities to range
from 0.03 dpm/g to 0.3 dpm/g. These values overlap with some of the lowest

226, activities measured in seep mussels studied here (Table 3.11).

Alam et al. (1999) studied radionuclide concentrations in marine mussel
Perna viridis and estuarine mussel Modiolus striatulus H. from the southern
coast of Bangladesh. For the whole soft tissues of three size groups of the
marine mussels, the averages were 0.3, 0.2, and 0.2 dpm/g, respectively. For the
shells, the averages were 0.8, 0.6, and 0.6 dpm/g, respectively. The three size
groups of the estuarine mussels yielded average values of the whole soft tissue
of 0.2, 0.1, and 0.1 dpm/g and 0.7, 0.5, and 0.06 dpm/g, respectively, for the
shells.

Rabalais et al. (1991) found that oysters deployed closest to nearshore Gulf
of Mexico oil well discharge points contained soft body total radium
concentrations ranging from 1.1 dpm/g to 4.3 dpm/g dry weight as compared
to 0.2 dpm/g (lower limit of detection) for control oysters. The author reported

that, in general, 226R, concentrations in most cases constitute approximately

2/3 of the total radium. The adjusted 226R4 concentration range would then be
0.7 dpm/g to 2.9 dpm/g. All mussel soft body tissues in Rabalais' study ranged
from 0.1 dpm/g (lower limit of detection) to 4.3 dpm/g. By comparison, mussel
soft tissues from barite seeps range from 0.1 dpm/g to 1.3 dpm/g while mussel
soft tissues from carbonate seeps range from below detection limit to 1.5 dpm/g
which are somewhat lower than that of the oysters deployed by Rabalais.

In a bioaccumulation study near produced water discharges in low-energy,
brackish environments of southeastern Louisiana (St. Pé, 1990), one of three

deployed oysters exhibited an elevated 226p4 activity (6.9 dpm/g) which
exceeds by far all the mussel soft tissues measured in this study (Table 3.11).



Figure 3.9. SEM view of the accessory pyrite associated with the barite

chimneys. A: Core and rim of euhedral pyrite. B: pyrite framboid containing
microcrystallites.

39



81

4. DISCUSSION
4.1. Sources of Fluids Advecting at Seeps

The geochemistry of pore fluids in marine sediments typically reflects the
seawater trapped in the sediments at the time of deposition and modified by
diagenesis. The solutes in pore fluids could deviate from the seawater values
because of exchange with the clay fraction of the sediments (Presley, 1969). In
seepage areas, however, the chemical fingerprints are complicated by the
availability of multiple fluid sources advecting through subsurface conduits,
complex processes involving fluid interaction with salt diapirs (Aharon et al.,
1992a), and microbial activity fueled by hydrocarbons (Aharon, 2000).
Extensive in situ diagenesis involving sediment-fluid interaction can be ruled
out from the onset because the cores from which the pore fluids were
extracted in this study represent surficial sediments less than 50 cm below the
seabed. The discussion that follows addresses the question of the fluid source/s
and the processes controlling the chemical compositions of pore fluids in the
GOM seeps.

4.1.1. Seawater-Derived Fluids

Similar salinities and concentrations of the conservative elements Cl and
Na in type I pore fluids and the ambient bathyal seawater (Table 3.1) suggest
that these are primarily seawater trapped in the sediment. This contention is
further confirmed by the 8751/86Sr ratios (0.70916) which are identical to the
measured deepwater value (0.70917, Table 3.1).

The marked $04 deficiency relative to seawater is attributed to its
consumption during microbial sulfate reduction within the seep sediments
using the carbon-derived hydrocarbons as electron donor (Aharon, 2000).
Microbial sulfate reduction using hydrocarbon substrates causes sulfate
depletion and simultaneous bicarbonate and hydrogen sulfide enrichments.
The occurrence of microbial sulfate alteration in seep sediments is confirmed
by the observed inverse linear correlation between dissolved sulfate and
hydrogen sulfide (Aharon and Fu, 2000). The slight depletion of Ca (Ef is down
to 0.6, Table 3.1 and Fig. 3.4 A), Mg (Ef is down to 0.9, Table 3.1) and Sr (Ef is
down to 0.7, Table 3.2 and Fig. 3.4 B) in pore fluids type I is attributed to
precipitation of authigenic carbonates because dissolved bicarbonate and
carbonate species are generated during the sulfate reduction (Aharon and Fu,
2000). Consequently, the increase in the carbonate alkalinity of the pore fluids
triggers carbonate precipitation which scavenges Ca, Mg and Sr. The predicted
carbonate deposition in seeps agrees well with the observed authigenic
carbonates which are ubiquitous at hydrocarbon seeps from the Gulf of Mexico
(Roberts and Aharon, 1994; Ferrell and Aharon, 1994), and elsewhere (e.g.,
North Sea, Hovland et al., 1987; Pacific northwest, Ritger et al., 1987).

The observed 180 enrichments in type I pore fluids of up to 1°/o0 relative
to a seawater source also require an explanation (Fig. 3.5). Two processes
relevant to seeps could shift positively the 8180 values of seawater-derived
fluids, namely (i) sublimation of gas hydrates, and (ii) microbial sulfate
reduction. Gas hydrates (clathrates) consist of gas (primarily methane) and
water mixed in about 1:6 molar proportion (Kvenvolden, 1988). Because the
solid-ice gas hydrates sequester more of the 180 isotope relative to the liquid
phase upon sublimation in the host sediment (Hesse and Harrison, 1981), the



1. INTRODUCTION
1.1. Statement of the Problem

Northern Gulf of Mexico offshore Louisiana is a mature oil and gas
exploration province where nearly 18,000 oil and gas wells have been drilled
since 1947. Deepwater exploration and development are predicted to increase

in the future such that by the first decade of the 218! century, 45% of all
domestic oil and gas will be produced from new offshore developments. A
variety of wastes are generated in drilling and production from oil and gas
wells but the discharges of drilling mud and produced water in the offshore
Gulf of Mexico has generated the most attention (Costlow et al., 1983; Boothe
and Presley, 1985; Clark and Patrick, 1987; Environmental Health Criteria 107,
1990; Rabalais et al., 1991).

Barite (BaSO4) is the principal constituent of drilling mud (about 63%) used
in oil and gas drilling operations. A previously estimated annual rate of

1.13x106 metric tons of barite discharged with the fluid muds in the Gulf of
Mexico caused sufficient concern on its impact on the marine environment to
Prompt special inquiries by the National Research Council (Costlow et al.,
1983), the National Academy of Sciences (Jarmul, 1984), and academic
institutions (e.g., Boothe and Presley, 1985). In addition to elevated barium,
discharged produced water is also anomalously enriched in radionuclides
(Bloch and Key, 1981; Kraemer and Reid, 1984; Raloff, 1991) with radium levels

exceeding by up to a factor of 3x104 the concentration in Gulf of Mexico waters
(Rabalais et al., 1991). The Minerals Management Service has also funded a
number of recent surveys aiming to assess the fate and effects of drilling
fluids and cuttings (e.g., OCS Study MMS 93-0021) and produced water
discharges in coastal environments (e.g., OCS Study MMS 91-0004, Rabalais et
al., 1991).

During routine submersible dives in 1993 followed by additonal targeted
dives in 1997, we discovered that in addition to hydrocarbons some of the seeps
occurring at upper bathyal depths (510 to 657 m) in Garden Banks-382 (27°
37.77 N; 92° 28.08' W), Garden Banks-338 (27937.79'; 92°28.12') and Mississippi
Canyon-929 (28° 01.46'N; 89° 43.63'W) are also issuing copious amounts of
barium and radium-rich fluids (Fig. 1.1). Upon their exit on the seafloor, these
fluids were likely to constitute the primary source for the extensive radium-
rich barite deposits of varied morphological shapes documented by us on the
seafloor (Fig. 1.2). At the time of the submersible observations, the impact of
the excess barium and radium on the benthic habitats was unknown but the
mud volcanoes blanketed with thick barite crusts (Fig. 1.2. A & B), the barite
chimneys occurring above point sources (Fig. 1.2 C) and the gullies filled with
highly radioactive coarse barite sands engulfing fields of dead
methanotrophic Bathymodiolus spp. mussels over extensive areas (Fig. 1.2 D)
suggested that the phenomena may be widespread on the seafloor along the
Louisiana slope.

The inference that hydrocarbon seeps may act as additional sources and
sinks for barium and radium in the benthic environment of the OCS Gulf of
Mexico throws a "monkey wrench" on the previous assessments concerning
the fate and effects of offshore drilling which have assumed that the only
external source for these toxic/radioactive elements is derived from the
drilling muds and produced waters, respectively (Costlow et al., 1983; Jarmul,
1984; Boothe and Presley, 1985; Clark and Patrick, 1987; Rabalais et al., 1991).



vi

considerable, matching in size the fluxes of Ba and Ra delivered to the Gulf of Mexico from the
Mississippi River.



36

Figure 3.7, Distinct morphologies of the barite chimneys displayed in SEM
photomicrographs from thin section. A: String-like morphology typical of the
dark-gray growth layers. B: Dendritic-like morphology typical of the light-
yellow growth layers.



Table 3.10. Sulfide concentrations of barites and their effect on °Pb/"*Ra dating.

Sample PpP*Ra = TM"°Pb/*Ra = AC’ Pb/”*Ra) _—éPyrite
name raw pure (wt%)
Barite chimneys
Chi#2-a 0.133 0.079 0.054 9.6
Ch#2-b 0.079 0.061 0.018 5.2
Ch#9-T/b 0.125 0.115 0.010 47
Chi#9-B/b 0.105 0.099 0.006 43
Ch#10-T/a 0.042 0.040 0.002 5.1
Ch#10-B/a 0.097 0.059 0.038 48
Barite crusts
Cr-93-1-3/1 0.368 0.288 0.080 89
Cr-93-4-4/1/u 1.013 0.542 0.471 28.5
Cr-93-4-4/1/1 2.013 0.513 1.500 30.5
Cr-93-7-7/1 0.629 0.356 0.273 16.9
Cr-93-8-5/3/u 0.797 0.533 0.264 9.0
Cr-93-8-5/3/1 0.629 0.511 0.118 20.1
Cr-93-8-7/2/u 0.768 0.376 0.392 22.2
Cr-93-8-7/2/1 1.431 0.374 1.057 317

67



76

below detection limit to 1.6 dpm/g. Mussel shell sampled from carbonate seeps

(control sites) average 0.8 +0.7 dpm/g 226p, (n=7) and range from below
detection limit to 2.3 dpm/g. Among shell samples, GOM-S-2 from GC-234

exhibits the highest 2265 value (2.3 dpm/g). This result is contrary to a priori
expectations, since the carbonate-bearing seeps were originally hypothesized
to be radium depleted relative to the barite-bearing seeps. A possible
explanation for this result is that in barite seeps, soluble radium venting from
hydrocarbon seeps rapidly coprecipitates with barium and seawater sulfate
ions to form barite, so that the seep water is radium depleted when it reaches
the benthic fauna. However, an unpaired, two-tailed t-test reveals that there is

no significant difference of 226p, activity at 95% confidence level between
mussel shells from barite and carbonate-bearing seeps.

3.3.2 Heterotrophic Fauna

226Ra data from heterotrophic seep fauna are summarized in Table 3.12 and
graphed in Figure 3.28. Body tissues of galatheid crabs 97-GL1/C1 and 97-

GLI/C2 (B) from barite seeps yield higher 22°Ra activities than any other
heterotrophic fauna samples (6.0 and 10.7 dpm/g, respectively). The GS-8 crab
from a carbonate seep is included for comparison and yields a much lower

value of 0.8 dpm/g 226R, Since galatheid crabs are presumably interloping
scavengers, there may be a component of radium transference from the
chemosynthetic barite community fauna; thus, radium is possibly accumulated

through the food chain. An alternate explanation for the observed high 226R4
activities is the accumulation of insoluble barite crystals in the GI tract of the
crabs associated with their scavenging behavior.

Skeletal elements of the galatheid crabs exhibit the lowest radioactivities
among all heterotrophic fauna elements. GS-8 crab from a carbonate seep

yields a low 226R4 activity of 0.1 dpm/g representing the lowest value of any
body component of the heterotrophic fauna group.

Three starfishes sampled from barite seeps exhibit uniformly high 226R 9
activity levels (average 2.5 dpm/g) compared to a single starfish collected
from a carbonate seep (0.8 dpm/g). The barite-seep starfishes were second
highest of all heterotrophic faunal groups.

226R, activity levels of two polychaetes sampled at barite seeps average 0.8
dpm/g. No ploychaetes were recovered from carbonate seeps, so no
comparison can be made with control fauna in this case. Soft body parts were
also unavailable for analyses because polychaetes soft tissues decompose
rapidly after harvesting.

3.3.3. Barium-Radium Relationship in Fauna

Representatives of hydrocarbon seep fauna used for barium analysis are

listed along with their respective 2269 values in Table 3.13 and data are
graphed in Figure 3.29. The fauna consist of external hard skeletons and whole

soft tissues, including galatheid crabs, starfishes, and mussels. The 226p 4



OCS Study
MMS 2001-004

Coastal Marine Institute

Fate and Effects of Barium and
Radium-Rich Fluid Emissions from
Hydrocarbon Seeps on the Benthic Habitats
of the Gulf of Mexico Offshore Louisiana

U.S. Department of the Interior
Minerals Management Service

Gulf of Mexico OCS Region

Cooperative Agreement
Coastal Marine Institute
Louisiana State University



102

228Ra/226Ra for the chimneys exhibit two negative trends (Fig. 4.11) where

high 228R9/226Ra ratios correspond to younger ages. One negative trend is for
the chimneys in Garden Banks and the other trend is for the chimneys in
Mississippi Canyon. Therefore, the 228Ra/226Ra may be used to estimate
whether the sample is old or young but not to obtain an accurate age.

4.3.3. 226Ra Activity in the Barites and Its Source/s

Relatively young barites are often radioactive because Ra is commonly
coprecipitated with Ba during the formation of barite. Radioactive barites
forming in different environments have been documented by previous
studies. For example, high activity of 226Ra has been reported in marine
barites sampled in cores taken from the Northeast Equatorial Pacific (Borole
and Somayajulu, 1977; Paytan et al., 1996). Radium-enriched barite sinters
were reported at a cold spring site in northwestern Canada (Cecile and Jones,
1979, Cecile et al., 1984). More recently, abundant radioactive barite-sulfide
deposits have been discovered at hydrothermal vents from deep-sea spreading
centers (Lalou & Brivhet, 1982, 1987; Grasty et al., 1988; Moore and Stakes, 1990;
Reyes et al., 1995). The overall ranges of radium activity in barites reported
previously, along with the values obtained in this study, are summarized in
Figure 4.12. Because the activity of 228Ra is strongly controlled by the age of
the sample, only 226Ra activities are used here for comparison.

The specific 226Ra activities of the Gulf of Mexico barites range from about
1,000 to 4,000 dpm/g and are comparable with the barite-rich hydrothermal
deposits found at the deep-sea spreading centers but are two to four orders of
magnitude higher than those of marine barites and sulfide-rich hydrothermal
deposits (Fig. 4.12). Some barites from Mississippi Canyon (e.g., sample Ch#11-
F/1, Table 3.6) contain by far the highest values (about 4,000 dpm/g) reported
to date. Barites deposited in the same area of the Gulf of Mexico seem to have
similar 226Ra activities, but there appears to be regional differences. For

example, the 226Ra activities of the chimneys and crusts in Garden Banks
range from 1100 to 1700 dpm/g, whereas the chimneys and crusts in

Mississippi Canyon range from 2700 to 4000 dpm/g. Therefore, the 226Ra
activities of barites in Mississippi Canyon are higher by a factor of two
relative to those in Garden Banks.

Two important questions need to be addressed concerning the 226Ra in the
Gulf of Mexico barites: (i) the source/s of 226Ra in the barites, and (ii) the

cause(s) of differences in 226Ra activities between barites from Garden Banks
and from Mississippi Canyon.

Seawater can be precluded as the source for 226Ra in the barites because it

contains trace amounts of 226Ra (0.001-0.01 dpm/L, Chung, 1980). The 226Ra in
marine barites is commonly attributed to uptake from pore fluids which are
generally enriched in 226Ra by factors of 100 to 1500 (1 to 15 dpm/L) over that
in near bottom water (e.g. Borole and Somayajulu, 1977; Cochran, 1979; Key,
1981;  Paytan et al., 1996). The enrichment of marine pore fluids in 226Ra are
ascribed to two processes related to the distinct chemical behavior of U and Th
in the seawater. First, alpha decay of 234U in the water column produces 230Th
which is scavenged by adsorption onto rapidly sinking particulate matter.

Second, the removal process results in 230Th activities in the sediment which



125

A. Chemical Flow of Radium into Fauna

-—SsrTC Ra-rich Fluid

scavenging . Methanotrophic
Heterotrophs Barite Mussels
B. Radiation Flow to Fauna
Barite
226Ra— 222Rn—P 210PphH
(external) (external)
Y a Y
a >
<4) (internal) Methanotrophic
Heterotrophs by ingestion Mussels

_——

Consequences

Impaired cell function
Cellular death
Genetic damage

Figure 4.17. Conceptual model depicting the chemical and radiation flow
derived from radium-rich barite deposits into fauna (methanotrophic mussels;
heterotrophs, including galatheid crabs, starfishes, and polychaetes)
inhabiting the barite-rich seeps (see text).



ABSTRACT

The primary goals of this study were to document products and processes related to barium
(Ba) and radium (Ra) -rich fluids seeping on the seafloor at bathyal depths and assess their
impact on the benthic habitats on the Louisiana upper slope of the Gulf of Mexico.

Our strategy for deriving an inventory of Ba and Ra sources and sinks involved sampling of
pore fluids at the points of exit, barite deposits consisting of chimneys and crusts, and fauna
inhabiting the seepage sites consisting of sessile epifaunal mussels (Bathymodiolus spp.)
harboring methanotrophic endosymbionts and vagrant heterotrophs such as galatheid crabs and
starfishes. In order to establish the source of the emissions we fingerprinted the fluids using
elemental chemistry tracers (Ca, Mg, Na, K, Sr, Ba, SO4, Cl, Br), radioisotopes of Ra (226Ra
and 228Ra), radiogenic Sr isotopes (87S1/86gr) and stable oxygen isotopes (8180). The origin
and nature of the barite deposits and their genetic link to the fluid emissions was established on
the basis of micro-chemical analyses of their Ba, Ca and Sr contents, 87Sr/8Sr ratios and stable
5180 and 834s compositions of the sulfate. Importantly, we exploited the anomalously high Ra
content of the barites and the radioactive decay properties of its isotopes in order to derive a
precise chronology of fluid emissions and barite deposition at seeps. Finally, we established the
Ba and Ra flow into the fauna inhabiting the seeps by determining their contents in both soft
tissue bodies and hard exoskeletons.

Our results indicate that the fluids advecting on the seafloor at barite-bearing seeps are
typically highly saline (salinity up to 155°/oo) and anomalously enriched in Ra and Ba relative
to bottom water values (by factors of 4.5x104 and 15x104, respectively). The chemical and
isotope fingerprints show striking similarities to produced waters from offshore fields and point
to a deep-seated source matched to Mesozoic-age formation waters. The barite deposits
surrounding the seeps serve as an important sink for the Ba and Ra in the advecting fluids
whereas their sulfate is derived primarily from seawater. Unlike the carbonate-bearing seeps
which yield ages back to 200,000 years (Aharon et al., 1997) thus recording long-term
hydrocarbon seepage, the barite deposits are remarkably young. Barite chimneys are 0.5 to 6.5
years old and their estimated growth rates vary from 4.4 cm/yr to 9.1 cm/yr, whereas the crusts
are 9.0 to 23.1 years old. The fauna around the barite-seeps have the ability to concentrate Ba and
Ra to varying degrees. In general, the heterotrophs appear to concentrate Ra preferentially
compared to the mussels, probably through ingestion of the highly radiaoctive barite particles.

On the basis of mass balances we established leakage rates of 250 L/yr and 2920 L/yr from
point sources in Garden Banks and Mississippi Canyon, respectively. The areal extent of the

barite-bearing seeps is not known with certainty but our estimates suggest they may be



103

10 T T T
4 Garden Banks
@ Mississippi Canyon
e :
2
2
oD
3
gZ
7) 6F 4
a
2
~~
2
a
4+ 4
2
0.0 0.2 0.4 0.6 0.8 1.0

228Ra/226Rg

Figure 4.11. 228Ra/226Ra AR versus 228Th/228Ra ages (yrs) of chimneys.
These relationships can be used to quantitatively estimate the ages of samples.



‘Table 3.1, Chunical atdasutnpecomstaas of for Mus fom seeps and amtuem Gul of Menivo deepens

sample depth sullnty Br HS CH 80, Na K es Se Ws Sh 80 Sere
ome fem) ome, EE Cone, EE Cones HE Canes BE Cons. Cone, BE Cones Bt
Joute orMesi hoa water 600. depth
cuir 3 ee 20 48 2 104 522 ca 02 0494 o70or
Gul or Moxie bom wate at 1958 mt
seawater w = 506 wos yy La ino 520 Hos - a.
Sun seas Gecen Kut (1980 arms),
ra02M2n aa W462 A LA WK 66 WH 26 KD 26H MH AOS THD
paamve == m1 wid 67 446 LS WH 7429. 2H DAK HY MG PAO 2D
pama4es = a a8 wr 69 AT LM RIO 76 L262 BLT O72 tas
pam-ieuis = m2 io 29 425 14 MG DY M9 2S SA Lo 472 09 MS OL 28 Ee
pamias RR Tas SBA ATLA STUY 2d
2a LY eT CE 2 2?
are eas, Misioaryn Canju lock 25 (199 ee)
fssrzts 602A ISR 1S S25 LAL 2h LAS
jss9217 toss 20H OUD 12H 2A SRE a
s5992.20 too BOG OS AW US TRH 4H SK 0 TORY AS ITO ttass
fsss9227 140) HMA OY S82 SLD GSH TLLRL USDA TIRET
ssv2 an ass 2 4520 62 SON TN 69 RT HOTA SDS ALTA 1S 89D HY oH UT
Bare ares, Gdn Banks Bock 192 (L099 ers mr)
ses 13, IM 27M) ONS AR OT OO 2ST AY OT OHO HAD TOMA SIGHED Ot
soo 8 tO 724A OH HDA AG TUL LOS 76 HLM BIRT W2 ARS UH LOS
soo 1-28 80 2H «DOS OMBS 52D AZ NRT 7D._ IOLA WHT LO 7TH
seo 1-4 626TH ORD ATs HH 2.741 OM URE WH ME MMS OY oS
F866 1.27 IAG 272K BMY BOTH AHA L7H ANE ON DSHS MH ORY THAD LOLA 0.70887
[i565 B22 AIS AOS OH LORLALI1AoHOHAAY YAMS MES 215A IOWTH 1d La OTOH
isis 227 kw as 2 We 16 M8 TMS WN LBL SH MWS
cc re Ty YY
{Cluananythee comunity areas, Geen Cann Block 242 (1995 ele),
: 0 6) SN koh aM LF L286 OY LHe LS
Joss 1-3 re no Ok 4 Ln 9 LA US Oy SOR ORE
ss-1-4 mv sme M1 02 #8 1D MO 13 93 Oy Sod AR LD
lasts 1.8 an Ton wa 06 WO dake OD SLT DT
ls 16 4n + Wo) 52) S9 MS aE DATARS LTR O20
1a + 21 a9 sm 30 127129910 527A
a 210 + 7 OY SH OL Ake OST
«0 wo + + O85 A397 YS MAO OSA
40 = RHR OTTO SES 101
0 ne oe eT TO Te
Sr eS ee ST YY
laa 22 Ws 4m S58 19407 sow LN 2829 S02 ONY TS done
leo 24 w+ Aes 57H 1017906 OR SOA ORTH
lasw 24 an = 10089 SHY 12S aS LDS 12RD HSS? L&D
now 2s 4 = ANGI) SRLS O37! 1D 712 RAR HT THK ds
aav.2 6 4+ KH KS LOK Od R102 ROO OTR dss
e392 7 40 ISIS) SRL LOT 03K ORR TS 7 HO OO yt
aovv.2 8 ao Lew SH L7H od ak tO 12Y FR THT 8 ORAS 000
fctomesynetie commana teas, son Canyan Hck BRAAINS (1985 ai)
frst. v0 a eT re  ) eS
400 MD So) TD SADT TART a a es
40 9 STH 1027 a
Cr er ee Se N97 OS IR 9 DoF
20 Ah sO STS OS) HLTA oF a
Isl 4). BKM 5 LOS OTM ay suo 07 WPL
Ce ne ee ee |
an 4a + to 18 Of 42 10K OA SY Oe Oy S66 07-29 Ona
2140 i ne Ly RS 09 242
oo a0 YY eT Lyi 99 3402

‘Nove, Convene H.S, Ba and Sere Ln MSA, te compos are wn K/L, 8° a peril (SMOW) Ess yt
ise share Samples ron 3995 lives ao se ds ep sutple fro 19d pore Malay Mame fo 194 lives ae pore Avie

‘Tae charge balance an ney slower ha 25 era

retson a ino of the the each fer (80 ate goto kx

ve



83

whereas brines evaporated beyond the halite saturation level would have a
Na/Cl ratio lower than seawater. Viewed this way, the substantially lower Br/Cl

molar ratios of type IL pore fluids (0.34x10-3 to 1.19x10°3) relative to seawater

(1.52x10-3), coupled with their near 1.0 Na/Cl molar ratios (Fig. 4.1), support
the view that their Cl and Na were derived from dissolution of subsurface salt
diapirs by seawater convective circulation (Jensenius and Munksgaard, 1989).
The Ca and SO4q enrichments in type II pore fluids relative to seawater (Figs. 3.4
A and C) are attributed to dissolution of anhydrite and gypsum disseminated in
the halite diapir. The enrichment of K is interpreted to result from dissolution
of sylvite (KCl) in the salt, and the deficiency of Mg relative to seawater (Table
3.1) is attributed to the dolomitization as justified by Aharon et al. (1992a).

The 87sx/86Sr ratios of type II pore fluids, ranging from 0.70797 to 0.70897
(n = 3), are significantly lower than the seawater value of 0.70917. These lower
ratios can be explained by the scavenging of Sr with lower 87Sr/86Sr ratios
during salt dissolution. According to Land et al. (1988), most Sr in salt domes
occurs in anhydrite inclusions and has a wide range of 875,/86sr ratios (from
0.7067 to 0.7095) that are more radiogenic than the presumed coeval Jurassic
(Callovian) seawater due to the extensive water-rock interaction during salt
deformation. The 87Sr/86Sr ratio of the salt under the Green Knoll has not
been measured and therefore it is difficult to quantify the contribution of Sr
derived from salt dissolution. However, by using a minimum value of 0.7070 for
the 87sr/86sr ratio of Jurassic salt (Burke et al, 1982), the maximum amount of
Sr derived from salt dissolution which is required to achieve the observed

8751/86Sr ratio in type II pore fluids can be estimated from a simple mass
balance equation:

A (7Sr/86Sr)sait + (1-A) (8781/8 6Sr) oy, = (87s1/86Sr)p¢ (4.1)

where A is the fractional contribution of Sr from salt dissolution; salt, sw and
pf subscripts are the 87Sr/86Sr ratios of the dissolved salt (0.7070), seawater

(0.70917) and pore fluids, respectively. Using the least radiogenic 87Sr/86sr
ratio of 0.70797 measured in the pore fluids, the equation above yields a 55% Sr
contribution from salt dissolution.

4.1.3. Fluids Derived by Advection of Deep-Seated Formation Waters

The linear correlations of Na, Ca, and Sr with Cl in type III pore fluids, and
the regression lines passing through the seawater values (Figs. 4.2 A-C),
confirm that these pore fluids were formed by mixing of two distinct end
members represented by modern seawater at the low end and by Na, Ba, Sr and
Ca-rich (but SO4 depleted) unidentified high end member fluid. This
interpretation is further supported by the linear relationships between

87§1/86sr ratios and the reciprocal of Sr concentrations (Fig. 4.3) because a

straight line is expected when fluids are formed by mixing of two component
end-members with different strontium isotope signatures (Faure, 1986). The
non-linear correlation observed between Ba and Cl (Fig. 4.2 D) indicates that
Ba was not conservative during the mixing and was removed from the fluids.
This observation is explained by barite precipitation when the Ba-rich end

members mixed with the SO4-rich seawater and is confirmed by the



78

Table 3.13. Relationship between barium and radium in selected hydrocarbon-seep fauna.

Sample ID Biota! Type? Ba (ppm)* Ra-226 (ppt)*
Barite Seeps
Soft Tissues

GOM-S-50 body MM BBC 1 0.18 £ 0.22
GOM-S-47 body MM BBC 2 0.01 + 0.34
GOM-S-48 body MM BBC 5 0.57 £ 0.15
97-SF3 starfish body SF BBH 281 0.914021
97-SF2 starfish body SF BBH 1174 0.78 + 0.20
97-SF1 starfish body SF BBH 3619 1.77 £ 0.25
97-GLI crab 2 body GC BBH 472 4.824 1.27
97-GL]I crab 1 body Gc BBH 4319 2.72 £1.15
Exoskeleton

GOM-S-44 Shell MM BCE 496 0.72 £ 0.27
GOM-S-47 Shell MM BCE 96 0.12 + 0.16
97-GL1 crab] Shell GC BHE 113 0.43 + 0.17
97-GLI crab 2 Shell GC BHE 138 0.19 + 0.15

Carbonate Seeps
Soft Tissues

J-2 body MM CCB 3 0.67 + 0.27
MAC-2 body MM CCB 4 0.05 £ 0.15
GOM-S-8 body MM CCB 5 0.29 + 0.19
GOM-S-9 body MM CCB 6 0.41 + 0.17
929-4 body MM CCB 8 0.08 + 0.16
GOM-S-44 body MM CCB 8 0.39 + 0.18
GOM-S-1 body MM CCB 15 0.60 + 0.36
GOM-S-2 body MM CCB 28 0.50 + 0.13
GS1 starfish SF CHB 7 0.344 0.18
Exoskeleton

MAC-2 Shell MM CCE 1 0.16 + 0.16
GOM-S-8 Shell MM CCE 1 0.48 + 0.15
GOM-S-2 Shell MM CCE 1 0.63 + 0.36
GOM-S-9 Shell MM CCE 469 0.214019
GS-8 crab GC CHE 15 0.03 + 0.14

? MM- methanotrophic mussels Bathymodiolus spp. ; GC- galatheid crab; SF- starfish; PY- polychaetes

? BBC- chemotrophic soft tissues from barite seeps; BBH- heterotrophic soft tissues from barite seeps;
BCE- chemotrophic exoskeletons from barite seeps; BHE- heterotrophic cxoskeletons from barite
seeps; CCB- chemosynthetic soft tissues from carbonate seeps; CHB- heterotrophic soft tissues from
carbonate seeps; CCE- chemosynthetic exoskeletons from carbonate seeps; CHE- heterotrophic
exoskeletons from carbonate seeps

3 Values reported in parts per million

“Values reported in parts per trillion with 2-0 random counting error



deposits. Radium and barium in growth increments from the barite chimneys
were used as an archived history and record of episodic venting whose
chronology was derived from the radium-decay series isotopes. The
youthfulness of all the barite deposits may be taken as an indication of
possible accelerated emissions from the seeps, implicating unusual
nonuniform disturbances of the subsurface hydrology increasing formation
water ascent to the seafloor through normal faults from deep reservoirs.

In order to resolve the fifth question, external gamma ray and internal
alpha radiation (derived by ingestion of radioactive barite) exposure and dose
to epifauna inhabiting the barite seeps, consisting of vagrant heterotrophs
(galatheid crabs, starfishes) and sessile methanotrophic mussels, was estimated
from measured radium-decay series specific concentrations by means of a
computerized dose calculation program (Microshield 5.0).

1.3. Site Descriptions

The barite-bearing seeps which form the focus of this study have been
documented and sampled during submersible dives in GB-382, GB-338 and MC-
929 (Fig. 1.1). The sites are characterized by intense gaseous hydrocarbon and
fluid mud venting occurring from numerous cone-shaped mud volcanoes (Fig.
1.2 A &B). Subsurface seismic data indicate that shallow salt diapirs are present
under the three sites and seepage occurs through normal faults associated
with the diapirs (Fu, 1998). These seeps are in the process of evolving from a
state of rapid venting, which produces mud-prone features, to slow seepage
which produces mineral-prone features (Roberts and Carney, 1997). Vast
quantities of mud have been expelled from the Garden Banks circular
expulsion centers, now encrusted with barite deposits, producing mudflows
and fluid debris flows clearly recognizable on side-scan sonar data. A thin (<4
m thick) hemipelagic sediment drape occurs over much of the seafloor which
is thinning toward young flow channels suggesting a very young age
(Roberts and Carney, 1997).

Dense communities of methanotrophic mussels (Bathymodiolus sp.), neritid
gastropods and galatheid crabs inhabit the flanks of these mud volcanoes.
Other chemosynthetic biota such as tube worms, clams, and Beggiatoa mats
that are common elsewhere on the Gulf of Mexico slope in association with
hydrocarbon seeps (Carney, 1994) are conspicuously absent from these sites.
The chimneys (Fig. 1.2 C) are engulfed in black-colored haloes consisting of
fine anoxic sediment. Profuse trains of gas bubbles venting into the water
column were noticed during the uprooting of the chimneys.



48

19.7 min) and 210pp (half life 22.3 yr). The presence of 214pp with its short
half life attests to the presence of 226Ra in the barite chimneys and crusts.

Except for 232Th at 59 keV, all the gamma-ray peaks occurring in the
thorium standard spectrum are also observed in the barite chimneys (Fig.

3.14). These gamma rays are mainly caused by 228 Ac (half life 6.13 hr), 212Pb
(half life 10.6 hr), 212Bi (half life 60.6 min) and 2987) (half life 3.05 min). The
presence of the short lived 228 Ac daughter isotope is indicative of the

presence of 228Ra parent isotope in the barite chimneys.

In contrast to the chimneys, all the peaks in the thorium decay series
observed in the thorium standard are absent in the typical barite crusts (Fig.
3.14). The absence of all the daughter nuclides of the Th decay series in the
barite crusts is attributed to their older ages. As shown later, most barite crusts

yield measured ages >15 years. Within this time frame, 95% of 228Ra in the
barites would have been decayed due to its relatively short half life (5.75 yr).

3.2.4.1. Soft and Fragmented Chimneys

Sixteen fragments from four barite chimneys (three from GB-382 and one
from MC-929) were chosen for dating. A raw sample and a purified barite was
prepared and analyzed for each of the 16 chimney fragments as indicated in
Table 3.6. Since these samples represent chimneys which disintegrated during
sampling, and the exact position of each fragment is uncertain, the ages
yielded by the fragments do not represent the time when the chimneys started
forming, but only reflect the age of the particular part of the chimney.
Description of the chimneys and results follow.

Chimney #2, recovered from GB-382 (Fig. 1.1), was broken during sampling
on the seafloor. The fragment used for dating of about 2 cm long, 4 cm wide
and 3 cm thick, exhibited two sublayers: an inner dark-gray layer and an
outer light-gray layer (Table 3.6). Microscope and SEM analyses indicate that
both sublayers are dominated by barite but the inner layer (Ch#2-a) contains
relatively more sulfide than the outer layer (Ch#2-b). The raw samples yield

2287Th/228Ra ages of 1.26 +0.06 years and 2.11 +0.07 years for for Ch# 2-a and
Ch#2-b, respectively. After barite purification, the ages were shifted to 0.93
+£0.06 and 1.94 40.08 years, respectively. Raw samples Ch#2-a and Ch#2-b yield
210pp/226Ra ages of 2.52 +2.39 and 0.55 £2.23 years (see section 3.2.5 for a
discussion of the reasons for large errors when dating chimneys using the

210pp/226Ra method), respectively, and substantially younger ages for the
purified barites (Table 3.6).

Chimney #9 and #10, recovered from GB-382, were well preserved during
sampling at the seafloor but began to fall apart when unwrapped in the
laboratory. The paired conical-shaped chimneys are of similar size; 18 to 19 cm
in height and 7 to 10 cm in bottom diameter. Six subsamples from different
sections of each chimney were used for dating and the results are listed in

Table 3.6 and graphed in Figures 3.15 and 3.16. The 228Th/228Ra ages of the
purified barites show a consistent trend in the vertical profiles with the ages
becoming progressively younger from bottom (2.1 to 2.2 years old) to top of
chimneys (1.3 to 1.2 years old). Generally the exterior samples yield older ages
relative to the inner samples by amounts varying from 20 to 137% (Figs. 3.15
and 3.16) suggesting that the chimneys buildup started with the outer frame
and then progressively infilled the inner conduits.



90

1000 T T
100F 3
=> wb ]

& oe

a ot 3

e
fi > |

01 ; sl 1
Jl 1 10 100 1000 10000

SO, (ppm)

Figure 4.4. Barium-sulfate relationship in formation waters from Miocene and
Pleistocene reservoirs offshore Louisiana. Chemistry data of formation waters
from Miocene strata (empty circles) and Pleistocene strata (empty squares) are
from Carothers et al. (1986) and Land et al. (1988). Note the similarity of the
pore fluids type III to the formation waters. Solid circle and solid triangle
refer to pore fluids in Mississippi Canyon and Garden Banks.



5 1 r r
4b 4
7
_
2
G 3h 4
Of
aS
Ss z
4
2 z =
F *t
g 7
1- r z 4
zz
0 1 fi fi
0.6 0.8 1.0 12 14
228Th/228Ra

Figure 3.24. 228Tn/228Ra ages plotted against their 228Th/228Ra ratios of

chimneys based on pure barites.

69



30

Figure 3.3. Ra-Ba and 226Ra-228Ra relationships in pore fluids from barite-

bearing

6000

seeps.

g 4000
i
z=
ey
2000
0 1 4 L
0 400 800 1200
Ba (mg/L)
4000 T T T
® iH
by
3000 4
_~ AE AG
= ee
a EH
= 2000+ HH 4
S
& a
a a
1000 + 4
0 1 1 1
1000 2000 3000
228Ra (dpm/L)



42

presence of iron oxides. This inference is consistent with the observation that
most rosette cores show a brownish color under optical microscope. The above
results indicate that the geochemistry of chimneys is similar to the crusts. in
the barites support the inference that Sr and Ca are isomorphically replacing
Ba in the lattice and do not form separate mineral phases.

The Na content in the barite is also relatively high, ranging from 1.1 to 3.2
mol% (mean of 1.9 mol% + 0.6 for n= 10, Table 3.4) for the chimneys and from
1.1 to 6.9 mol% (mean of 3.65 mol% + 2.4 for n= 10, Table 3.4) for the crusts.
However, it is uncertain whether Na replaces Ba in the barite lattice or forms a
separate authigenic mineral phase. In contrast to the other chemical elements
reported here, the Fe content in the barites is relatively low, ranging from 0 to
0.7 mol% (mean of 0.3 mol% + 0.22 for n=10, Table 3.4) for the chimneys and 0
to 0.3 mol% (mean of 0.2 mol% + 0.1 for n=10, Table 3.4) for the crusts. High Fe
contents, of up to 22 mol%, were generally found in the center of rosette cores
in the chimneys. However, the poor stoichiometry of the analyses with higher
Fe contents suggest that the anomalously high Fe may have been caused by the
presence of iron oxides. This inference is consistent with the observation that
most rosette cores show a brownish color under optical microscope. The above
results indicate that the geochemistry of chimneys is similar to the crusts.

3.2.3. Sulfur, Oxygen and Strontium Isotope Assays

Sulfur, oxygen and strontium isotopes are commonly used to decipher the
origin of barites (Sakai, 1971; Claypool et al., 1980; Cecile et al., 1983; Kesler et
al., 1988; Fu and Aharon, 1995), This is because sulfur and oxygen isotopes are
reliable fingerprints of the source of sulfate whereas the strontium isotopes
are excellent tracers of fluid sources and transport history. The purpose of
measuring these isotopes in the barites from the Gulf of Mexico is to use them
as tracers in order to elucidate the origin of the barites and outline a
depositional model for these highly unusual occurrences based on the data in
hand.

Sulfur, oxygen and strontium isotope compositions of barite, along with the
carbon isotope compositions of carbonates in the crusts are listed in Table 3.5.
As previously indicated, these barites were recovered from two areas, Garden
Banks block 382 and Mississippi Canyon block 929. Barite chimneys from MC-
929 and GB-382 have similar 834S and 6180 compositions which range from 20.3
to 21.60/00 (CDT) and from 9.6 to 11.79/00 (SMOW), respectively (Table 3.5 and
Fig. 3.11). These values are similar to, or slightly higher than, those of modern
seawater sulfate (8534S = 20.30/00 CDT and 8180 = 9.70/00 SMOW, Cecile et al., 1983;
Faure, 1986). Generally, samples from the inner side of chimneys tend to yield
relatively higher §34S and 8180 values than samples from the outer part of
chimneys (Table 3.5 and Figure 3.12). In contrast, 8348 and 8180 values of
barite crusts range from 26.1 to 62.39/00 CDT and 12.4 to 21.99/00 SMOW,
respectively, which are anomalously enriched in both 34§ and 180 relative to
the modern seawater sulfate (Table 3.5 and Fig. 3.11). Most barite crusts exhibit
two sublayers: the upper layer is light gray and the lower layer exhibits a
black or deep gray color. 8534S and §180 values of the lower layers are
generally higher than those of the upper layers (Table 3.5). chimneys (Table
3.5 and Figure 3.12). In contrast, 34S and 8180 values of barite crusts range
from 26.1 to 62.30/00 CDT and 12.4 to 21.99/00 SMOW, respectively, which are

anomalously enriched in both 34S and 180 relative to the modem seawater
sulfate (Table 3.5 and Fig. 3.11). Most barite crusts exhibit two sublayers: the



where A is the fractional contribution of Sr from seawater; sw, f and br

subscripts are the 8757/86Sr ratios of seawater (0.70917), end member fluids
(0.70854 and 0.70843 for Garden Banks and Mississippi Canyon, respectively)

and barites, respectively. Using the highest 8751/86sr ratios in the barites
(0.70861 and 0.70849 for Garden Banks and Mississippi Canyon, respectively,
Table 3.5), the equation above yields the maximum amounts of Sr from
seawater of 11% and 8% for the barites in Garden Banks and Mississippi
Canyon, respectively.

4.3.6. Depositional Model of the Barites in the Gulf of Mexico

The isotope compositions of barites presented above allow us to delineate a
model of their origin (Fig. 4.14). In general, the GOM barites formed by the
mixing of sulfate-rich seawater with Ba, Sr, and Ca-rich formation fluids
advecting with the hydrocarbons. The chimneys formed at or above the
sediment/water interface whereas the crusts formed below the
sediment/water interface. The latter precipitated from residual seawater
sulfate left over from a pool of pore water sulfate consumed by sulfate-
reducing bacteria in a closed or semiclosed system (Fig. 4.14).

Since the chimneys formed in oxidizing environments at or above the
sediment/water interface, they preserve the sulfur and oxygen isotope

compositions of seawater sulfate. Slightly heavier 8345 and 8180 values
measured in some samples from the inside of the chimneys may have resulted
from weak bacterial sulfate reduction in the micro-environments.

The sulfur and oxygen isotope compositions of the barite crusts clearly
place their formation in anoxic environments below the sediment/water
interface where extensive sulfate reduction occurred. Therefore, it is
reasonable to suggest that the extensive bacterial sulfate reduction in the pore
fluids preceded the Ba, Sr, and Ca-rich fluids venting through the sediments.
Prior to the formation of the crusts, the pore fluids must have been enriched
in H2S, HCO37 with light 813C and impoverished in S04 with heavier 8348 and
8180 due to sulfate reduction using an organic carbon substrate. When the Ba,
Sr, and Ca-rich fluids vented through the sediments and mixed with the pore
fluids, light 5!3C-calcite and heavy 8345 and 5180 barites co-precipitated due to
the supersaturation of the fluid with respect to both calcite and barite. This
model explains why most calcite and barite in the crusts are interlocked
showing mosaic textures. Pyrite could have precipitated either before or after
the fluid venting through the sediments, depending on whether the source of
ferrous iron was from sediments or from the venting fluids. Sulfate reduction
using thermogenic methane is inferred on the basis of 813C of carbonates in
the crusts (-21 to -36°/00 PDB, Table 3.5). This is consistent with the
submersible observations that intensive gaseous hydrocarbon venting occurs
in barite depositional areas.

4.4, The Impact of Ra and Ba-Rich Venting Fluids on the Benthic
Habitat

4.4.1. Fluxes of Radium/Barium-Rich Fluids from Point Sources

The fluxes of the radium and barium-rich fluids from which the Gulf of
Mexico barite precipitated are important. The following approach is used to



47

GOM BG MC
seawater fluids fluids

GB MC
barites | barites

ti y
Zam

0.7093 0.7091 0.7089 0.7087 0.7085 0.7083

8757 86Sr

Figure 3.13. 87Sr/86Sr ratios of Gulf of Mexico barites and seawater. Notes: MC
and GB represent Mississippi Canyon and Garden Banks, respectively. The
87S1/86Sr ratios of MC and GB end member fluids are from Table 3.1.



133

6. REFERENCES

Aharon, P. (2000) Microbial processes and products fueled by hydrocarbons at
submarine seeps In Microbial Sediments R.E. Riding and S.M. Awramik
(Eds.) Springer-Verlag Berlin, Heidelberg. pp. 270-281.

Aharon, P., Graber, E.R. and Roberts, H.H. (1992) Dissolved carbon and 8l3c
anomalies in the water column caused by hydrocarbon seeps on the
northwestern Gulf of Mexico slope. Geo-Marine Lett. 12: 33-40.

Aharon, P, H.H. Roberts, and R. Snelling (1992a) Submarine venting of brines
in the deep Gulf of Mexico slope: Observations and geochemistry: Geology
20: 483-486.

Aharon, P., Schwarez, H.P. and Roberts, H.H. (1997) Radiometric dating of
submarine hydrocarbon seeps in the Gulf of Mexico. Geol. Soc. Am. Bull.
109: 568-579.

Aharon, P., and B. Fu. (2000) Microbial sulfate reduction rates and sulfur and
oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of
Mexico. Geochim. Cosmochim. Acta 64: 233-246.

Alam, M.N., Chowdhurry, M.I., Kamal, S., Matin, A.K., and Ferdousi, G.S. (1999)
Radionuclide concentrations in mussels collected from the southern coast
of Bangladesh. J. Environ. Radioactivity 47: 201-212.

Bacg, Z.M. and P. Alexander (1961) Comparative Radiosensitivity of Living
Organisms. In: Fundamentals of Radiobiology, 2nd Ed., Pergamon Press,
Oxford, pp. 299-309.

BEIR V Report (1990) Health effects of exposure to low levels of ionizing
radiation. National Academy Press, Washington D.C., pp. 71-134.

Bence, A-E. and A.L. Albee (1968) Empirical correction factors for the electron
microanalysis of silicates and oxides: J. Geol. 76: 382-403.

Berner, R.A. (1971) Principles of chemical sedimentology: McGraw-Hill, N.Y.,
240pp.

Bertine, K.K. and J.B. Keene (1987) Submarine barite-opal rocks of
hydrothermal origin: Science 188: 150-152.

Bloch, S. and Key, R.M. (1981) Modes of formation of anomalously high
radioactivity in oil field brines. Am. Assoc. Petrol. Geol. Bull. 65(1): 154-159.

Boothe, P.N. and Presley, B.J. (1985) Long term fate of drilling mud barite on
the Texas-Louisiana continental shelf and slope. EOS, Am. Geophys. Union
66 (51): 1329.

Borole, D.V. and B.L.K. Somayajulu (1977), Radium and lead-210 in marine
barite: Marine Chem. 5: 291-296.

Boulegue, J., A.M. Kersabiec, and F. Vidot (1990) Trace metals (Ba, Sr, Mn, Cu) in
interstitial waters, Leg 116: Proc. Ocean Drill. Progr. Sc. Res. 116: 117-125.
Breit, G.N., E.C. Simmons and M.B. Goldhaber (1985) Dissolution of barite for the
analysis of strontium isotopes and other chemical and isotopic variations

using aqueous sodium carbonate: Chem. Geol. 52: 333-336.

Broecker, W.S. and Peng, T.H. (1982) Tracers in the Sea. LDGO Press, Palisades,
New York, pp. 169.

Buerkert, T.M. (1997) Barium in water and foraminiferal shells: Indicators of
present and past oceanographic conditions in the Gulf of Mexico.
Unpublished PhD Dissertation, Louisiana State University, pp. 96.

Burke, W.H., R-E., Denison, E.A., Hetherington, R.B.. Koepnick, H.F., Nelson and

J.B., Otto (1982) Variations of seawater 875/865, throughout Phanerozoic
time: Geology 10: 516-519.



Table 3.5. $, O and Sr isotope compositions of the barites and C and O isotope compositions of the coexisting carbonates.

43

Type of Sample Description of 8"0 (SO) 5*S (SO) CaCO; °C (CaCO) 80 (CaCO,) “Sr/*7Sr
sample sample Che SMOW) (Yn CDT) ___(%) (/.. PDB) (hoo PDB)
Barite chimneys from Garden Banks block 382
Cha2a inner part, deep gray 99 205 - - - -
Chi2-b outer part, light gray 100 205 - - - -
Chita inner part, deep gray 108 215 - - - -
Ch#4-b middle part, gray 96 20.6 - - - -
Chittc outer part, redish 97 204 - - - -
Chi5-a inner part, gray 10.2 207 - - - -
Ch#5-b outer part, redish 9.6 203 - - - -
Ch#9-T/a top, inner part, gray 109 2.6 - - - -
Ch#9-Tf top, center part, yellow 102 aut - - - 0.70858
‘Ch#9-Tre _top.outer part, orange 97 203 - - - 0.70861
Barite Chi#9-M/a middle, inner part, yellow =, 11.7 23 - - - -
Ch#9-M/b middle, inner part, gray 10.9 20.4 - - - -
Ch#9-M/c middle, center part, yellow 98 20.5 - - - -
chimneys Ch#9-M/d middle, outer part, red 98 20.1 - - - -
Ch#9-B/a bottom, inner part, gray 10.7 213 - - - 0.70859
Ch#9-B/) bottom, center part, yellow 98 209 - - - 0.70858
Ch#9-B/e bottom, outer part, red 95 202 - - - -
Ch#10-TYa top, inner part, gray 10.0 20.3 - - : -
Ch#10-T top, outer aprt, red 99 20.3 - - - -
Ch#10-M/a middle, inner part, gray 99 204 - - - -
Ch#10-Mib middle, outer part, red 98 203 - - - -
Ch#10-B/a bottom, inner part, gray 10.3 20.5 - - - -
Ch#10-B/> bottom, outer part, red 99 20.3 - - - -
Barite chimneys from Mississippi Canyon
Ch#ll-a inner part, gray 10.7 20.3 - - - 0.70848
Ch¥11-b outer part, orange 98 20.1 : : - 0.70849
Barite crusts from Garden Banks block 382
Cr-93-7-7/1 light gray 12.9 28.1 - - - -
Cr-93-8-5/3/1 lower part, deep gray 13.5 28.9 - - - -
Cr-93-8-5/3/u upper part, gray 127 265 38.0 213 44 0.70857
bBarite Cr-93-8-7/2 lower part, black 155 33.6 52.2 21.0 42 -
Cr-93-8-7/2/u upper part, gray 124 26.1 163 24.1 2.6 -
‘crusts Barite crusts from Mississippi Canyon block 929
Cr-93-1-3/1 lower part, black 21.9 62.3 : - - -
Cr-93-1-3/1/u upper part, deep gray 215 615 63.6 36.2 28 0.70845
Cr-93-4-4/10 lower part, dark 143 33.0 568 29.5 29 -
93-93-4-4/1/u_ upper part, gray 12.6 28.7 26.3 312 31 -



72

Table 3.11. Radium-226 activity concentration in chemosynthetic (methanotrophic) Bathymodiolus spp.
mussels from seeps. Seep designation “C” denotes carbonate seep and “B” denotes barite seep.

Sample Biota’ Type Year | Dive No.* GOM Block | Seep* | Ra-226°
LD. dpm/g
GOM-S-1 MM Shell 1995 | ISL-2634 GC-234 Cc 0.54 0.4
GOM-S-1 MM Body 1995 | JSL-2634 GC-234 Cc 1.3208
GOM-S-2 MM Shell 1995 | JSL-2634 GC-234 Cc 2340.8
GOM-S-2 MM Body 1995 | ISL-2634 GC-234 Cc 11203
GOM-S-8 MM Shell 1995 | ISL-2647 GC-185 c 11403
GOM-S-8 MM Body 1995 | ISL-2647 GC-185 Cc 0.64 0.4
GOM-S-9 MM Shell 1995 | ISL-2647 GC-185 Cc 05+ 0.4
GOM-S-9 MM Body 1995 | ISL-2647 GC-185 Cc 0.9 + 0.4
J2 MM Body 1991 | JSL-3120 GC-272 Cc 1540.6
J6 MM Shell 1991] JSL-3120 GC-272 Cc 0.2+0.4
J6 MM Body 1991 | JSL-3120 GC-272 c 0.9+0.4
L-5 MM Body 1991 | JSL-3116 GC-185 ¢ BDL
MAC-2 MM Shell 1990 | Alvin-2209 Alaminos fal 0.4404
Canyon
MAC-2 MM Body 1990 | Alvin-2209 Alaminos C 0.1403
Canyon

709-1 MM Shell 1993 | JSL-3560 MC-709 c 0.4404
709-1 MM Body 1993 | JSL-3560 MC-709 Cc 0.4404
GOM-S-44 MM Shell 1997__| JSL-2897 GB-338 B 1.62 0.6
GOM-S-44 MM Body 1997 __ | JSL-2897 GB-338 B [09404
GOM-S-46 MM Shell 1997 __[ JSL-2897 GB-338 B 13403
GOM-S-46 MM Body 1997 | JSL-2897 GB-338 B [0820.4
GOM-S-47 MM Shell 1997 | JSL-2897 GB-338 B BDL.
GOM-S-47 MM Body 1997__| JSL-2897 GB-338 B [04+08
GOM-S-48 MM Shell 1997 | JSL-2897 GB-338 B [08+04
GOM-S-48 MM Body 1997 | JSL-2897 GB-338 B 13403
GOM-S-49 MM Shell 1997 | JSL-2897 GB-338 B [04404
GOM-S-49 MM Body 1997 | JSL-2897 GB-338 B_ [01403
GOM-S-50 MM Shell 1997 | JSL-2897 GB-338 B OL 0.3
GOM-S-50 MM Body 1997 __ | JSL-2897 GB-338 B 0.4405
929-4 MM Shell 1993 | JSL-3559 MC-929 B BDL
929-4 MM. Body 1993 [ JSL-3559 MC-929 B [0.2204

'MM- methanotrophic mussels Bathymodiolus spp.

? Year of collection

3 Submersible dive number; JSL = Johnson-Sea-Link

* Seep type according to predominant geologic products; C = carbonate-dominated seep; B = barite-
dominated seep

* Radium-226 determination with 2-o counting error; BDL = below detection limit



82

water fraction of the hydrate would contribute to the elevated 38!80 and
simultaneous freshening (i.e., lower salt content) of the pore fluids. For

example, a 1°/oo positive shift in pore fluids relative to the overlying seawater
(Fig. 3.5 B) can be achieved by mixing a 10% gas hydrate-derived fluid having

a 8180 of 10°/00 (assuming multiple thawing-freezing in a closed system with a
starting gas hydrate 5180 value of 2.69/00, Davidson et al., 1983) with seawater-

derived fluid having a typical bathyal 8180 value of 0.3°/00 (Table 3.1). Under
these circumstances the gas hydrate sublimation will also cause a pore fluid
freshening of about 10% relative to bathyal seawater.

Microbial sulfate reduction is another process common in seeps which may

alter the 6180 values of the pore fluids. According to Fritz et al. (1989),
bacterial reduction of sulfate is accompanied by oxygen isotope exchange

reactions between sulfate and water. Because solute S04 is enriched in 180

(8180 = 9.79/00 SMOW, Faure, 1986) relative to seawater (H20), the microbial
breakdown of the S-O bonds affords isotope exchange between the sulfate-

oxygen and water-oxygen and may cause an !8Q-enrichment in the pore
fluids. However, it can be estimated that the sulfate reduction effect on the

8180 of pore fluids is small (at most 0.02°/00) compared with gas hydrate
sublimation effects because the amount of oxygen in pore fluid SOq is
substantially smaller (480 times) than oxygen in water.

The effects of gas hydrate sublimation on the pore fluids type I can be

gauged in Figure 3.5 B on the basis of 8!80-chloride inverse relations

indicating that 180 enrichments are accompanied by chloride dilution.
Observations indicate that gas hydrates are common in the bathyal seeps
where type I pore fluids were acquired (MacDonald et al., 1994) and their
sublimation have the potential to alter the oxygen isotope fingerprints of seep
fluids.

4.1.2 Fluids Derived by Dissolution of Salt Diapirs

The similarity of 5180 values in type II pore fluids with the 8180 of ambient
bathyal water (Fig. 3.5 A) suggests that the latter is the source for these pore
fluids. However, the observed enrichments of Cl, Na, Ca, K and SO4 (Table 3.1
and Fig. 3.4) indicate that source(s) of solutes other than seawater must be
involved in the formation of these pore fluids. Two distinct sources for the
elemental enrichments in seep fluids can be discerned: (i) salt diapir
dissolution and/or (ii) residual Jurassic seawater (e.g., Louann brine)
evaporated at least to the point of saturation with halite and buried along with
its cogenetic evaporite (Aharon et al., 1992a). Generally, these two sources can
be distinguished on the basis of their Br/Cl and Na/Cl molar ratios as indicated
below.

Because Cl is preferentially partitioned over Br into Na, K and Mg-halogen
salts during their precipitation, the residual brine will become progressively
enriched in Br (White, 1965; Rittenhouse, 1967; Kharaka et al, 1987). Therefore
brines formed by subaerial evaporation of sea water will have higher Br/Cl
ratios relative to brines formed by dissolution of halite (Rittenhouse, 1967;
Carpenter, 1978; Egeberg and Aagaard, 1989). The Na/Cl ratio is another
measure to gauge the effect of salt dissolution. Seawater dissolving halite but
remaining unmodified by other chemical reactions will have a Na/Cl molar
ratio close to 1.0 and substantially higher than normal seawater (0.857)



Figure 3.8. Principal building block of the barite in SEM views. A: Rosette-
shaped barite from thin section. Note the pitted nature of the core. B: Three
dimensional view of the rosette from a chimney fragment.

37



123

mW Oyster Soft Parts (3)

Mussel Soft Parts (4)

Crustacean Exoskeletons
(4)

7) Baa Mussel Shells (4)
6 | RRBBEA] Mussel Soft Parts (1)

8 FA snusset shets 2)

4+ FB snusser sott Parts (2)

3 BJ Crustacean Exoskeletons
+ (1)

2a
f

4

ICrustacean Soft Parts (1)

Mussel Shells (1)

Crustacean Soft Parts (4)

° p=

2

6
Ra-226 ° (dpm/g)

Figure 4.16.-A comparison of ”°Ra activity ranges from literature with results

from barite seeps. Wet weights from Iyengar and Narayana (1990) were

converted to a dry weight basis by assuming a soft tissue water content of 90%.

(1)- Iyengar and Narayana (1990)
(2)- Alam et al. (1999)

(3)- Rabalais et al. (1991)

(4)- Barite Seeps, this study



107

4.3.4, The Source of Sulfate in the Barites

Sulfur (8348) and oxygen (8180) isotope compositions are commonly used as
indicators of the source of SO42- because they preserve the identity of the
sulfate in the fluids from which the barites precipitated (Church, 1970; Sakai,
1971; Cecile et al., 1983). The following are the most common sources of sulfate
attributed to marine barites.

(1) Barites precipitated from normal seawater have similar oxygen and
sulfur isotope compositions to those of the coeval seawater sulfate which
varied through the geological time (Cecile et al., 1983). The 5345 and 8180
compositions of modern seawater sulfate are constant within narrow limits
and are represented by a mean 5348 value of 20.3°/o00 CDT and a mean 8180
value of 9.7°/00 SMOW, tespectively (Cecile et al., 1983; Faure, 1986). The
constancy and oceanic uniformity are due to the rapid mixing rate of the
oceans and the long residence time of sulfate in seawater (i.e., 12.3 Ma, Drever,
1982). In modern pelagic sediments most large barites (>5 wm), interpreted to
be precipitated from seawater (Church, 1970), match or are slightly heavier in
both oxygen and sulfur isotope compositions relative to modern seawater
sulfate.

(2) Barites formed in anoxic environments display heavier 8348 and 5180
values than those of the coeval seawater sulfate. The enrichments of 348 and
186 of barites are ascribed to the reduction of sulfate ions by anaerobic
bacteria such as Desulfovibrio (Berner, 1971). During bacterial reduction,

sulfates containing 160 and 32s isotopes are preferentially metabolized (Rafter
and Mizutani, 1967; Sakai, 1971; Mizutani and Rafter, 1973; Claypool et al., 1980;
Cecile et al., 1983). Thus, barites precipitated from a residual sulfate reservoir
will display heavier 834S and 6180 values than those of the coeval seawater
sulfate. This process has been used to explain the heavier 8345 and 5180 values
of barite off the California coast (Goldberg et al., 1969; Sakai and Krouse, 1971),
barite concretions from the Japan Sea (Sakai, 1971), and barite nodules and
rosettes of Late Devonian age in Nevada (Rye et al., 1978).

(3) Barites can also form by oxidation of sulfides. Since most sulfides have

highly negative 6345 values, barites formed by this process often exhibit light
sulfur isotope compositions. The 8180 of the barites formed by this process
varies depending on the 8180 values of dissolved oxygen (O2) and the water
(H20) from which the barites precipitated. This is because three of the four
sulfate oxygens are derived from dissolved oxygen (O02) and the forth one from
water (H2O) during the oxidation of sulfides (Lloyd, 1968). Smaller barites (<5
um) in modem pelagic sediments usually show lighter 6345S values than
modern seawater sulfate and were explained by this sulfide oxidation process
(Church, 1970).

(4) Barites formed by mixing of barium-rich hydrothermal fluids with
sulfate-rich cold seawater are common in submarine hydrothermal vent sites

(e.g. Peter and Scott, 1988; Kusakabe et al., 1990). In this case, the 33458 of
barites are controlled by the mixing of the following sources: (i) seawater

sulfate (834§ = 20.3°/00 CDT); (ii) sulfides and sulfate leached from basalt (8348
of about 0°/00 CDT), and (iii) residual seawater sulfate after thermal chemical
sulfate reduction occurred at high temperatures (8348 > 20.3°/00 CDT).



116

1.2x106 and 1.43x106 dpm/yr in Garden Banks and 3.5x106 dpm/yr in
Mississippi Canyon. The highest total radioactivity (A226Ra + A228Ra)

measured in pore fluids from barite areas are 5.2x103 and 1.2x103 dpm/L (Table
3.3) in Garden Banks (GB-382) and Mississippi Canyon(MC-929), respectively.
Thus, leakage of 230 to 270 L/yr of fluids in GB-382 and 2920 L/yr of fluids in
MC-929 from a single point source are required to account for the radium
present in the barite chimneys (Table 4.3).

4.4.2. Leakage of Radium/Barium-Rich Fluids in the Benthic
Habitat

In the following we detive estimates of Ba and Ra fluxes into the benthic
habitat of the Louisiana upper slope using the preceding fluid fluxes from
point sources amounting to a mean of 250 L/yr in Garden Banks and 2,920 L/yr
in Mississippi Canyon. Here we assume that pore fluids in barite-bearing seeps
are saturated with respect to barite and therefore in chemical equilibrium
with the sediments.

The annual discharge estimates of Ba and Ra from barite-bearing seeps are
listed in Table 4.4. Annual fluxes from the Mississippi River are also listed for
comparison purposes. With the exception of the areal coverage of seeps, to be
discussed below, all other data are self-explanatory. Although leakage of
hydrocarbons from the deep reservoirs on the upper Louisiana slope has been
known for over two decades (see MacDonald et al., 1993 for a review), their
areal extent is still debated. Using the imagery data reported by MacDonald et
al. (1993) we estimate that about 5% of the upper Louisiana slope area of 15,552
km? is covered by bottom hydrocarbon seeps, and only 0.1% by barite-bearing
seeps (i.e., 1/50 of seeps, amounting to 16 km2). On the basis of submersible
observations we also estimate a density of 5 point sources per square
meter(Fig. 1.2 B).

Viewed this way, the annual fluxes of Ba using either Garden Banks or
Mississippi Canyon fluid rates yield comparable values (i.e., 2.3x10!0 and
2.1x1010 g/yr, respectively) and are of the same order of magnitude as the total
annual Ba discharge from the Mississippi River (4.1x1010 g/yr). Calculations
of the total Ra fluxes (ie., 226Ra + 228Ra) yield a similar picture, namely close
agreement between Garden Banks and Mississippi Canyon rates (1.2x10!4 and
2.8x1014 dpm/L) which amount to fluxes having the same order of magnitude
as the Mississippi River (1.5x10!4 dpm/L). The considerably slower fluid flow
rates in seeps relative to the River are compensated by the much greater
concentration of the Ba and Ra elements in the seepage fluids (Table 4.4).
Another major difference between the seep sources and the river source is
that the former delivers the Ba and Ra directly to the benthic habitat whereas
the latter gets dispersed from the surface. A future comprehensive survey of
Ba and Ra concentrations in bottom waters is required in order to estimate the
proportion scavanged by the barites and the leftover leakage into the bottom
waters.

Although our flux estimates carry the uncertainty of the areal coverage of
barite-bearing seeps, we feel that the values are likely to be conservative. The
assumptions regarding the parameters of the model can be tested by future
empirical observations and the veracity of the estimates can now be
challenged by future studies.



121

Table 4.5 summarizes the comparison data. The data taken from Iyengar and
Narayana (1990) was originally reported on a wet-weight basis. Soft tissues
were assumed to contain 90% water in order to adjust the data to a dry weight
basis. The Crustacea soft body parts from the barite seeps stand out with

respect to their high 226Ra content relative to all other comparable fauna
(Fig. 4.16).

Rabalais et al. (1991) found that oysters deployed closest to nearshore Gulf
of Mexico oil well discharge points contain soft body total barium
concentrations occasionally exceeding 3200 ppm. The highest barium content
found in mussel soft bodies from this study was 28 ppm (Table 3.11). However,
one galatheid crab whole soft body collected from a barite seep site yielded a Ba

value of 4319 ppm and also one of the highest 226p4 levels (2.72 dpm/g).

Again, these high values are likely to reflect barite crystals harbored in the GJ
tract of the galathean crab and not necessarily soft tissue concentrations of
barium and radium.

4.5 Radiation Consequences of Radium Leakage from Seeps on the
Benthic Fauna

4.5.1. Radium Metabolism and Radiation

Radium is a skeletal seeker because of its chemical similarities to calcium.
Therefore its presence in the body long after initial uptake is due to
sequestration in the skeleton of vertebrates or calcareous exoskeleton of
invertebrate animals, such as molluscs. Concomitantly, radium is absorbed
from the GI. tract (International Commission on Radiological Protection,
1973), which accounts for its potential oral radio-toxicity (Raabe, 1994).

Additionally, 226p a5 ability to radioactively decay into a noble gas, 222Rn,
become airborne, diffuse throughout the entire organism, and subsequently

transmutate into short lived alpha-emitting daughters (2718p and 214p 9, Fig.
2.3) increases the potential for internal cellular injury. These radioactive
decay products diffuse throughout the entire organism via radon gas and
subsequently irradiate sensitive cells remotely located from the original site of
deposition in radium contaminated organisms. The primary mode of potential
cellular damage resulting from internal deposition of radium is alpha particle

radiation resulting in cellular energy deposition of about 24 MeV (3.84x10°!6
Joules) per atom of 226p, and its three short lived alpha emitting daughters

(where the assumption is made that no 222Ry gas escapes from the
contaminated organism). Under conditions of chronic intake, radium becomes
very uniformly distributed in calcareous body parts (International
Commission on Radiological Protection, 1973).

In addition to potential cellular damage due to internally deposited radium,
a potentially significant hazard is posed by external gamma rays emitted by
authigenic radium and its radioactive daughters. The gamma radiation from
radium and its daughters is sufficiently penetrating so that cells may suffer
electromagnetic ionizing radiation damage in addition to that caused by
internally deposited radionuclides.

Biological radiation damage depends on the absorption of energy from
ionizing radiation, whether deposited by an external or internal source. The
amount of cellular damage from radiation is approximately proportional to the
concentration of absorbed energy per unit mass of tissue. For this reason, the



DISCLAIMER

This report was prepared under contract between the Minerals Management Service (MMS) and
the Louisiana State University, Coastal Marine Institute. This report has been technically
reviewed by the MMS, and it has been approved for publication. Approval does not signify that
the contents necessarily reflect the views and policies of the MMS, nor does mention of trade
names or commercial products constitute endorsement or recommendation for use. It is,
however, exempt from review and compliance with the MMS editorial standards.

REPORT AVAILABILITY

Extra copies of this report may be obtained from the Public Information Office (Mail Stop 5034)
at the following address:

U.S. Department of the Interior
Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)
1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394

Telephone: (504) 736-2519 or
1-800-200-GULF

CITATION
Suggested citation:

Aharon, P., D. Van Gent, B. Fu, and L. M. Scott. 2001. Fate and effects of barium and radium-
rich fluid emissions from hydrocarbon seeps on the benthic habitats of the Gulf of
Mexico offshore Louisiana. OCS Study MMS 2001-004. Prepared by the Louisiana
State University, Coastal Marine Institute. U.S. Department of the Interior, Minerals
Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 142 pp.

ABOUT THE COVER
Cartoon showing composite images of submarine barite deposits and associated fauna from gas

seeps occurring on the Gulf of Mexico slope. Original sketch by Paul Aharon; final drawing by
Mary Lee Eggart.

iii



140

Rittenhouse, G. (1967), Bromide in oilfield waters and its use in determining
possibilities of origins of these waters: Am. Assoc. Petrol. Geol. Bull. 51:
2430-2440.

Roberts, H.H., P. Aharon, R. Sassen, R. Carney, J. Larkin, and R. Sassen (1990)
Sea floor responses to hydrocarbon seeps, Louisiana Continental Slope: Geo-
Marine Lett. 10: 232-243.

Roberts, H.H. and P. Aharon (1994) Hydrocarbon-derived carbonate buildups of
the northern Gulf of Mexico continental slope: A review of submersible
investigations. Geo-Marine Lett. 14(2):135-148.

Roberts, H.-H. and Carey, R.S. (1997) Evidence of episodic fluid, gas, and
sediment venting on the Northern Gulf of Mexico continental slope. Econ.
Geol. 92: 863-879.

Robinson, P. (1980) Determination of calcium, magnesium, manganese,
strontium, sodium and iron in the carbonate fraction of limestones and
dolomites: Chem. Geol. 28: 135-146.

Rye, R.O., D.R. Shawe, and F.G. Poole (1978) Stable isotope studies of bedded
barite at East Northumberland Canyon in Toquina Range, central Nevada: J.
Res. U.S. Geol. Surv. 6: 221-229.

Sakai, H. (1971) Sulfur and oxygen isotopic study of barite concretions from
banks in the Japan Sea off the Northeast Honshu, Japan: Geochem._J. 5: 79-
93.

Sakai, H., and H.R. Krouse (1971) Elimination of memory effects in 189/160
determination in sulfate: Earth Planet. Science Lett. 11: 369-373.

Schroll, E. (1987) Die Standardgesteinsreferenzprobe Dolomit/Topla TU-EA-ADT.
Bundesversuchs und Forschungsanstalt Arsenal Report, 23 pp.

Somayajulu, B.L.K. and T.M. Church (1973) Radium, thorium, and uranium
isotopes in the interstitial water from the Pacific Ocean sediments: J.
Geophys. Res. 78: 4529-4531.

Stakes , D. and W.S. Moore (1991) Evolution of hydrothermal activity on the
Juan de Fuca Ridge: Observations, mineral ages, and Ra isotope ratios: J.
Geophys. Res. 96(B13): 21,739-21,752.

Stuben, D., N.E., Taibi, GM. McMurtry, J. Scholten, P. Stoffers and D. Zhand
(1994), Growth history of a hydrothermal silica chimney from the Mariana

backare spreading center (southwest Pacific, 18°13'N): Chem. Geol. 113: 273-
296.

Stueber, AM., P. Pushkar, and E.A. Hetherington (1984) A strontium isotopic
study of Smackover brines and associated solids, southern Arkansas:
Geochim. Cosmochim. Acta 48: 1637-1649.

St. Pé, K.M. (1990) An assessment of produced water impacts to low-energy,
brackish water systems in Southeast Louisiana. Louisiana Department of
Environmental Quality, Water Pollution Control Division, Baton Rouge,
Louisiana, 204 pp.

Szabo, B.J. (1967) Radium content in plankton and seawater in the Bahamas.
Geochim. Cosmochim. Acta 31: 1321-1331.

Tanner, A.B. (1964) Physical and chemical controls on distribution of radium-
226 and radon-222 in ground water near Great Salt Lake, Utah Jn
Proceedings of International Symposium on the Natural Radiation
Environment (e.d., J.C. Adams and W.M. Lowder): University of Chicago
Press, Chicago, Illinois, p. 253-276.

Tivey, M.K. and J.R. Delaney (1986) Growth of large sulfide structures on the
Endeavour Segment of the Juan de Fuca Ridge: Earth Planet. Science Lett,
77: 303-317.



100

between the initial 228Ra/226Ra and the ages of the chimneys (Fig. 4.10) leads
to the following three observations: (i) the initial 228Ra/226Ra ratios are
variable; (ii) there appears to be regional differences in the initial
228Ra/226Ra between Garden Banks (0.9 to 1.2) and Mississippi Canyon (0.6 to
0.8), and (iii) a broad negative relationship exist between the initial
228Ra/226Ra and the ages of the chimneys.

Regional and temporal variations of the initial 228Ra/226Ra (from 0.2 to
1.0) have also been found in the hydrothermal sulfides and barite-enriched
deposits occurring in the mid-oceanic hydrothermal vents (Stakes and Moore,
1991; Kadko and Moore, 1988). It is generally accepted that Ra in the
hydrothermal deposits is primarily leached out from the basalts and the

variations of initial 228Ra/226Ra are caused by differences in either fluid
residence time, depth of generation of hydrothermal fluids, or temperature
fluctuations near the basalts (Stakes and Moore, 1991; Kadko and Moore, 1988).

Concerning the cause(s) of the regional difference in initial 228Ra/226Ra
found in the Gulf of Mexico barites, two possibilities are proposed. First, fluids
in different areas may have originated from different sources. Viewed this
way, the fluid source under Garden Banks area may have higher 228Ra/226Ra
than under Mississippi Canyon area implying different Th/U ratios in the
respective sources. Second, the subsurface fluid sources may have the same
228Ra/226Ra under the two areas, but the fluid migration time is different.
Because of its short half life (5.57 yr), the 228Ra concentration in the fluid

decreases exponentially with migration time, but 226Ra concentration can be
considered to remain constant due to its long half life (1,620 yr). Hence, the

longer the fluid migration time, the lower the 228Ra/226Ra in the seepage

fluids would be and in turn would result in a lower initial 228Ra/226Ra values
in the barite. If migration time is the controlling factor, the average initial

228Ra/226Ra value of 1.09 in Garden Banks and 0.71 in Mississippi Canyon will
correspond to fluids in Mississippi Canyon travelling 3.5 years longer than the
fluids in Garden Banks. The fluid migration time could be controlled by either
the length of the pathway and/or the migrating rate which is reflected by the
seepage rate. Therefore, the fluids in Mississippi Canyon are either derived
from a deeper source, their migration rate is lower than the fluids in Garden
Banks, or the source in the former is overlain by a thicker sediment sequence
resulting in a longer fluid migration time.

As previously indicated, the chemistry, isotopes and radioactivity C26Ra
and 228Ra) of the end member fluids in the two areas are different, suggesting
that fluids in the two areas may be derived from different sources.
Alternatively, different growth rates of chimneys (8.9 to 9.3 cm/yr in Garden
Banks and 4.4 cm/yr in Mississippi Canyon) may reflect distinct fluid
migration times.

Another implication concerning the initial 228Ra/226Ra is that the
resence of 228Ra in the barites indicates that the maximum migration time
Pp ° 8
for the fluids in both areas should be less than 20 years, otherwise 91% of the
228Ra would have decayed due to its short half life (5.75 yrs).

228Ra/226Ra could also be used to date barites if the initial ratio is known.
However, because the initial 228Ra/226Ra of the barites vary, 228pa/226Ra can
not be used to calculate the exact age. A plot of the ages versus the



60

indicates that 238U jis absent or at a very low concentration in the barites.

Furthermore, the gamma peak at 67.7 keV from 2309Tp occurring in the
uranium standard spectrum is also absent in the barite spectra. These features

confirm that 226Ra in the barites is unsupported (orphan).

The gamma peak of 232Th at 59 keV is present in the thorium standard
spectrum but it is absent in both the barite chimney and crust spectra (outside

the energy range of Fig. 3.14). This observation indicates that 228Ra in the
barites is unsupported (orphan) as well.

In order to determine whether or not initial 228Th was present in the
barites, the dating of five barite chimney samples were repeated two years

after the initial dating measurements. If significant initial 228Th were
present, the 228Th/228Ra ages measured at about 2 years interval should yield

a large discrepancy because of the short half life of 228Th (1.91 yr). The
experimental data are summarized in Table 3.9 and are plotted in Figure 3.20. It
is noted that the two sets of ages of the same samples agree within a 13%
margin of error (error calculated on the basis of data in Table 3.9) and
converge along a line with a slope of 1 (Fig. 3.20) thus confirming that the

barites contain a negligible initial 228Th. Therefore all the 228Th in the
barites can be considered to be supported by the decay of 228Ra and the
228Th/228Ra ages are regarded as valid.

As shown later, some 210pp, unsupported by the decay of 226Ra, do occur in
the raw barite samples but most of the excess 210pb is hosted in the sulfide

rather than in the barite phase. Therefore it is assumed that the initial 219Pp
in the sulfide-free purified barites is insignificant. This assumption is
supported by the observation that all the young chimneys yield concordant

210pp/226Ra and 228TH/228Ra ages (Table 3.6), although the 210pp/226Ra ages
of the chimneys have only qualitative values.

The third assumption above states that the 228Th derived by decay of 228Ra,

and the 210pPb derived by decay of 226Ra do not migrate after the formation of
the barites. In most cases the migration of daughter isotopes from minerals is

attributed to alpha recoil (Kigoshi, 1971) but the decay of 228Ra and 228A ¢
(228 Ac is used to determine the 228Ra activity) is by beta emission which
causes the recoil energy associated with the 228Th production to be
insignificant (Stakes & Moore, 1991). In contrast, the decays of 226Ra and
228th are by alpha emission and if the system is leaking because of alpha
recoiling, the most probable leakage is likely by gaseous 222Rn in the 226Ra
decay series and by gaseous 229Rn in the 228Ra decay series. Leakage of 222Rn
and 229Rn could definitely affect both 228Th/228Ra and 210pp/226Ra dating
methods. This is because in the 210pp/226Ra method, both 2!4Pp which is used
to calculate the 226Ra activity and 210Pp are the daughters of 222Rn, whereas
in the 228Tp/228Ra method the activity of 228Th is calculated from the activity
of 212Pb which is the daughter of 220Rn.

A test of the potential loss of 229Rn and 222Rn in the barites was made by
monitoring for 10 days the gamma-ray emission of a sealed barite sample. The
results showed a leakage of less than 0.05% of Rn which indicates an



46

upper layer is light gray and the lower layer exhibits a black or deep gray
color. 834§ and §180 values of the lower layers are generally higher than
those of the upper layers (Table 3.5).

The 87sr/86Sry ratios of barite chimneys and crusts range from 0.70845 to
0.70861 which are significantly lower than the ambient Gulf of Mexico

deepwater (0.70917, Table 3.1); small regional variations in 87Sr/86Sr ratios
exist between MC and GB sites. In general, barite chimneys and crusts from

Garden Banks have higher 87s1/86s¢ ratios (0.70857 - 0.70861, n = 5) than
chimneys and crusts from Mississippi Canyon (0.70845 - 0.70849, n = 3) Table
3.5 and Fig. 3.13).

The 813C compositions of carbonates occurring in the barite crusts range
from -21.0 to -36.2°/00 PDB (Table 3.5), which are substantially depleted in 13¢
relative to the ambient seawater (0.69/00).

3.2.4, Radioisotope Assays °

Barite deposits including chimneys and crusts associated with hydrocarbon
seeps in the Gulf of Mexico (Fig. 1.1) were examined with a portable gamma-
ray detector (uR meter) which showed that they were unusually radioactive.
These initial radioactivity measurements led to a more detailed gamma-ray

spectrometric investigation which revealed that 226Ra and 228Ra were

unsupported by their parents 238U and 232Th, respectively, and therefore can
be considered orphans. The apparent ages of the barite samples, therefore, can
be determined using the Ra decay series isotopes. Here we report the results of
the 228Th/228Ra and 210pp/226RQ daughter/parent assays which led to
estimation of the chronology of these barite deposits and their growth rates.
Some of the preliminary results shown here have already been published
(Van Gent et al., 1995).

The isotope activities and ages obtained from soft and hardened barite
chimneys are listed in Tables 3.6 and 3.7 and the ones from the barite crusts
are reported in Table 3.8. All errors reported here are at the 95% confidence
level (2 o). The radioactivity unit used here is dpm/g (disintegration per
minute/gram of material).

Two age nomenclatures are used here and in the following sections:
measured age and sampling age. The measured age is based on the direct
measurement data which represents the age from the formation of barite to
the time of dating. Because samples were measured several years after
collection, the sampling age which is corrected to the time of collection (July,
1993) is reported here in addition to the measured age. It must also be stated
that an additional time-lag error was introduced during the calculation of the
sampling age. This time-lag error depends on the amount of time that has
elapsed between the time of collection and the time of measurement.
Consequently the age error reported here for the sampling age is the
‘maximum. Except when specified, ages reported below represent sampling
ages.

Representative gamma spectra of barite chimneys and crusts, along with
the gamma-ray spectra of uranium and thorium standards for the energy
range from 750 keV to 1050 keV are shown in Figure 3.14. It follows that all the

gamma-ray peaks of 226Ra daughter nuclides occurring in the uranium
standard also show up in both the barite chimneys and crusts. These gamma-

ray peaks are primarily caused by 714Pb (half life 26.8 min), 214Bi (half life



237 MCC
GB-Cr (n23)
(n=6) Pay
207 Saas Seen
Essa ee
Raa ae
15} a Ena
ze ened eee
2 eaves
2 Sa
So peed
< pRaa
10 SS
MC-Ch
5k
GB-Ch
0

Figure 4.6. Ages of barite chimneys and crusts in the Gulf of Mexico. Notes: All
the ages are corrected to the sampling time (July 20, 1993). The ages of chimneys
are obtained from ??8Th/228Ra method on pure barites and hardened chimneys.
The ages of crusts are obtained from 2!0Pp/226Ra method on pure barites.

GB-Ch: Chimneys in the Garden Banks.
GB-Cr: Crusts in the Garden Banks.

MC-Ch: Chimneys in the Mississippi Canyon.
MC-Cr: Crusts in the Mississippi Canyon.

95



20

powder, acid digested and subsequently assayed for 226, using a modified
standard quantitative technique described in EPA-600/4-80-032 (EPA, 1980).

This technique involves determination of 2226p, by measuring its first decay
product, 222Ry via radioequilibration ingrowth and emanation into Lucas

Cells (EPA method 903.1, EPA, 1980). Since the half-life of 7?7Rn is 3.82 days, an
ingrowth period of approximately 24 days for each sample was sufficient for

achieving 95% secular equilibrium with the 226R,

emanation analysis was carried out.

Following radium analysis, selected acid-digested whole soft tissue and
calcareous exoskeleton samples from each site were subjected to stable barium
analysis. This was carried out by means of standard ICP-AES elemental analysis
in order to estimate seep fauna barium concentrations and compare the results
with the corresponding radium values for each selected sample.

Hydrochloric and nitric acid tissue digestion methods adapted from Horwitz
(1980), for fish and other marine products, were utilized for biological sample
dissolution in this study. Previously oven dried samples sealed in moisture
proof vials were weighed (approximately 0.5 gram each sample) and placed
into acid washed 10-ml PYREX test tubes. Five ml of concentrated hydrochloric
acid brought to boiling temperature on a block digester was used to dissolve

parent nuclide before

the calcareous mussel shells. Sample dissolution for counting 226Ra and stable
barium of all whole soft tissue and chitinous exoskeleton was accomplished by
utilizing 5 ml of concentrated nitric acid and 1 ml of 30% hydrogen peroxide
brought to boiling temperature with approximately 0.5 g of each sample.
Organic exoskeleton samples, such as powdered crab shells, required
special care when adding acid so as not to lose sample due to initial exothermic
foaming. Acid was added drop-wise into test tubes containing such samples,
while all exothermically produced foam dissipated before the entire volume of
acid was added for complete digestion. The samples were carefully brought to
boiling temperature in a block digester and refluxed until the samples
achieved optical clarity. In some instances, it was difficult to completely
dissolve some of the body tissue and chitinous components because these body
parts contain a considerable quantity of organic oils and waxes which were
not completely soluble in inorganic acids. When this difficulty was
encountered, the waxy residue was vacuum filtered onto a Gelman 0.45 micron

membrane filter, and the filtered liquid was collected for 226R4 and Ba assays.
All acid digested samples were cooled, diluted to 10 ml with double-distilled
deionized water, and sealed in radon bubblers for approximately three weeks
before radon emanation.

Acid-digested samples were processed following the guidelines of EPA
method 903.1 (EPA, 1980). Evacuated 150 ml Lucas scintillation cells were
attached to an aged nitrogen gas cylinder and allowed to come to atmospheric
pressure with the nitrogen gas. Each cell was subsequently counted for 1000
minutes on a Randam Model SC-5 Scintillation Counter, and the resulting
values were used for background subtraction. Acid-digested samples were
transferred to clean dry radon bubblers. A radon bubbler consists of three
main components: scintillation cell, desiccant column, and bubble chamber.
The sample was sealed airtight in the bubble chamber, and the start date and

time of radon ingrowth was recorded. In order for 222Rn to achieve near

secular equilibrium with 226R, a 24-day ingrowth period was required. After
this ingrowth period, the samples were sparged with aged nitrogen gas in



16

Activity (dpm)

ty/2(226Ra) = 1.62x103 yrs
ty/2(21 Opp) = 22.3 yrs

Va

210pp/226Ra Activity Ratio

—)

20 40 60 80 100 120 140 160

Time (yrs)

Figure 2.5. Radioactivity decay and growth in the system 226Ra-210pp. Upper:
Decay of parent isotope 226Ra and ingrowth and decay of daughter isotope
210Pb as a function of time. For illustrative purposes an activity of 100 dpm
was assigned to 226Ra whereas the 2!9Pp activity is zero at t = 0. Lower:
Evolution through time of the 219Pp/226Ra activity ratio toward secular

equilibrium (2!9Pp/226Ra = 1). This ratio serves as an useful chronometer for
samples whose age does not exceed 120 yrs.



a 15 7
&
y = 29.84 - 4.38x R*2 0.94
~ 10
&
g
=|
a 57
=I
2
3 0 n 1 1 1
3 4 5 6 7

Age (yr)

99

Figure 4.9, Vertical age profile for chimney #11, Mississippi Canyon block 929.

Growth rate determined on the basis of slope is 4.38 cm/yr.



28

5h 4 5 L 4 L 4
2 -of 4 of L dof 4
Ss
3
Ey
A -15t 7 of r to oF 4
-20F 7 7 r 7 + 7
-25.
110 120 130 140 2000 7000 0 1500 3000 0 1500 3000
Salinity (°/oo) Ba (uM/L) 226Ra (dpm/L) 228Ra (dpm/L)

Figure 3.1. Depth profiles of salinity, Ba, and 226Ra and 228Ra activities in the
pore fluids from core 3566-1 taken at Garden Banks block 382.



109

crusts (Fig. 3.11). For example, the 8345 of barite crusts are as high as 62.3°/00
(CDT, Table 3.5), which indicates that 95% of seawater sulfate has been reduced

based on the values of 8348S and the concentration of sulfate in pore fluids. In

contrast, the highest 8345 value in chimneys is 21.69/00 (CDT) (Table 3.5)
suggesting that only about 5% of seawater sulfate has been reduced.

4.3.5. The Source of Major (Ba) and Trace Elements (Sr and Ca) in
the Barites

The mechanism for the formation of barite from normal seawater is
problematic because modern oceans are commonly undersaturated with
respect to barite due primarily to the low concentration of Ba (e.g., the
average concentration of Ba in seawater is about 14 ppb, while the saturation
value is 44 to 50 ppb, Church and Wolgemuth, 1972). Two barium sources have
been proposed to explain the occurrence of barite in normal marine
sediments. One source of Ba is from dissolution of biologically-derived
inorganic and organic compounds in sulfate-rich micro-environments
(Church, 1970; Dehairs et al., 1980). The other source is from barium-rich
brine seeps (Hanor, 1982).

Our direct observations during submersible dives indicate a close
association of barite deposits with gaseous hydrocarbon seeps. In addition,
high concentrations of Ba (up to 1,173 ppm) were measured in the pore fluids
sampled from barite-bearing seeps (Table 3.3) and all these pore fluids are
supersaturated relative to barites. Therefore it is evident that the most likely
source of Ba for the GOM barites is the fluids advecting to the seafloor and
derived from formation waters ascending through fault conduits.

Sr and Ca are generally the next most abundant elements after barium in

the barite due to their similar ionic radii (1.13 A for Sr2+, 0.99 A for Ca2+ and

1.35 A for Ba2+). However, compared with barites from hydrothermal and
marine settings, the GOM barites are anomalously enriched in Sr and Ca by a
factor of about 15 and 28, respectively (Fig. 4.13). This suggests that the Sr and
Ca of the GOM barites must be derived from a source other than the Gulf of
Mexico seawater. As shown in the results, in addition to Ba the pore fluids
(pore fluids type III) sampled from barite deposition sites are also highly
enriched in Sr (up to 250 ppm) and Ca (up to 3000 ppm) relative to modern GOM
bottom waters (7.4 ppm and 389 ppm for Sr and Ca, respectively, Aharon et al,
1992). It is therefore self-evident that the Sr and Ca are derived from venting
fluids. This interpretation is strongly supported by the similar 87Sr/86Sr ratios
in the barites to those in the venting fluids. As shown in Figure 3.13, 87Sr/86gr
ratios of barites (including chimneys and crusts) from Garden Banks (0.70857-
0.70861) are slightly higher than those from Mississippi Canyon (0.70845-
0.70849). However, their values are very close to the saline end member fluids
in each area (The 8751/86Sr ratios for the saline end member fluids in Garden
Banks and Mississippi Canyon are 0.70854 +0.00001 and 0.70843 +0.00001,
respectively). The slightly higher 87Sr/86sr ratios in barites than the end
member fluids may result from the addition of Sr from seawater (0.70917). The
contribution of Sr from seawater in the formation of barite can be estimated
from the following simple mass balance equation:

A (8781/86Sr)sw + (1-A) 8781/86Sr)¢ = (8781/86 Sr)p, (4.5)



131

5. CONCLUSIONS AND RECOMMENDATIONS

1. Three chemical types of pore fluids associated with hydrocarbon seeps
occur in the deepwater Gulf of Mexico. Two are derived from the ambient
seawater and their chemical compositions are altered by microbial
interactions and by dissolution of sub-seafloor salt diapirs. The third type,
highly enriched in Ba and Ra, is derived from deep-seated formation waters
and its advection on the seafloor through fault conduits associated with salt
diapirism impacts adversely the ambient benthic habitats.

2. Formation-water-derived fluids emerging on the seafloor are
anomalously enriched in Ra and Ba relative to seawater by factors of up to

4.5x104 and 15x104, respectively. Their migration time from the source to the
seafloor is established to be < 20 years on the basis of the relatively short-lived

228Ra isotope content.

3. Barite (BaSO4) deposits consist of chimneys, crusts blanketing mud
volcanoes, and unconsolidated sands infilling gulleys. The chimneys are
dominated by the mineral celesto-barite (i.e., Sr-rich barite), whereas the
crusts consist of both barite and calcite. The Gulf of Mexico seep barites are
anomalously enriched in Sr, Ca and Ra by comparison with barites from
normal marine settings elsewhere.

4, Sulfur, oxygen and strontium isotope compositions indicate that the Gulf
of Mexico seep barite deposits form by the mixing of sulfate-rich seawater
with Ba-, Sr-, and Ca-rich formation fluids seeping with gaseous
hydrocarbons, primarily thermogenic methane. The chimneys are erected
vertically above the seafloor whereas the crusts drape mud volcanoes and/or
form pavements at the sediment/water interface.

5, Both 226Ra and 228Ra are “orphans” in the sense that their respective

parents 238U and 232Th are practically absent in the fluids and barites. The
“orphan" property of Ra affords radiometric dating of the barites by

210pp/226Ra and 228Th/228Ra daughter/parent isotope pairs. The assays
indicate that chimney ages range from 0.5 to 6.5 years whereas the crust ages
range from 9.0 to 23.1 years. The growth rates of the chimneys vary from 4.4
cm/yr to 9.1 cm/yr. These results indicate that the Ba-Ra-rich fluid expulsion
on the seafloor, triggering barite deposition, is a rapid and recent event. At
this point we cannot discern whether or not the intensified emissions are
related to disturbances in subsurface hydrology triggered by exploration and
production from deepwater platforms. This question must be addressed
responsibly in future studies.

6. Fluid fluxes from point sources in the Garden Banks and Mississippi
Canyon, estimated on the basis of material mass balance, are 250 L/yr and 2920
L/yr, respectively. Assuming that the Ba-Ra-rich seeps cover about 0.1% of the
Louisiana upper slope, the annual fluxes of Ba and Ra are estimated to be of the
same order of magnitude as their annual discharges from the Mississippi
River. Although our flux estimates carry the uncertainty of the areal coverage
of barite-bearing seeps, the values reported here are likely to be conservative.
The assumptions regarding the parameters of the model can be tested by
future empirical observations and the veracity of our estimates can be



52

Ch#9-T/b = 1.23 yr

Figure 3.15. Chimney #9 from Garden Banks block 382. The ages
shown here are based on the 228Th/228Ra_ assays.



Table 4.2. ”*Ra/Ba ratios in barites and fluids,

Sampling Barite deposits End member fluids
area Sample TMRa =Ra/Ba | Sample | TMRa Ba TMRa/Ba
name__| (dpm/g BaSO,) |_(dpm/mgBa) |_name_| (dpnvL) |_(mg/L) | (dpm/mg Ba)

Chi2-a 1452 2.469  |3566-1-21 | 2492 919 2.712
IChi#2-b 1184 2.014
Chi#9-Tia 1279 2.175
Ch#9-Mfa 1246 2.119
Ch#9-Bla 1339 2277
Ch#9-T% 1229 2.090
ch#9-Mib 1346 2.289
Ch#9-Bfo 1174 1,997
Garden Banks Ch#10-T/a 1125 1,913
Ch#10-M/a 1332 2.265
Chi#10-B/a 1250 2.126
Block 382 Ch#10-T/ 1345 2,287
Chi#10-M/> (21 2,060
Ch#10-Brb 1302 2.214
93-7-7/1 1352 2.299
93-8-5/3/ 1545 2.628
93-8-8/3/ 1502 2.554
93-8-7/2hu 1203 2.046
93-8-772A 1725 2.934
93-8-7/3 1271 2.162

[Average 2,24640,245

Chit -F/ 3045 6.709 [2559-232 | 739 80 9.238
Mississippi Canyon |Ch#l 1-F/2 3697 6.288
93-1-3/ 3797 6.458
Block 929 93-4-4/1 fu 2701 4,504
93-4-4/1/ 3940 6.701

[Average 6.150+0,888

Note: Pore fluids 2559-2-32 and 3566-1-21 are representatives of end member fluids in Mississippi Canyon and Garden Banks,
respectively (see text). Their Ra activities are from Table 3.3.

90L



70

in 228Th/228Ra dating method is obtained from the activity of 2!2Pb which, if
unsupported, could also be derived from the decay of 220Rn in the advecting
fluids. In order to test the possible sulfide effect on the 228Th/228Raq dating,
2287h/228Ra ages of the raw chimney barites are plotted in Figure 3.25 against
those of their paired purified barites. Because the ages of the raw chimney
barites and purified barites are concordant, it can be concluded that (i) raw
(untreated) chimneys contain a very small amount sulfide (< 2% by volume)
and therefore unsupported 2!2pb is insignificant, and (ii) barite purification

scheme does not impair the 2287H/228Ra ages. On the basis of this observation,
subsamples sectioned from the hardened chimneys were dated without
purification and their 228Th/228Ra ages are likely to represent "true"
chimney's ages (Table 3.7).

It can be concluded that the 228Th/228Ra ages obtained from the chimneys
are (i) highly reliable; (i) the 228Th/228Ra age errors are small (<8%, Table 3.6
and 3.7), and (iii) the three age profiles (Figs. 3.17, 3.18, and 3.19) show a
systematic decrease in the ages from the bottom to the top of the chimneys
which is consistent with the expectation of older ages at the bottom and
younger ages at the top.

3.3. Radium and Barium Systematics in Chemotrophic and
Heterotrophic Benthic Fauna from  Barite-Bearing Seeps

The data listed in Tables 3.11 and 3.12 show that the range of 226R 4
concentrations among seep fauna samples is large. Methanotrophic
endosymbiont-bearing mussel samples (Bathymodiolus spp. ) consist of body
and calcareous shells separated from one another; galatheid crab samples are
composed of soft tissues and chitin exoskeletons. In the case of polychaetes,

only the chitinous exoskeleton parts were available for 226R, measurements.
Starfish carbonate skeleton and soft tissue samples were analyzed as whole
entities because of the inordinate difficulties associated with completely
separating soft tissue from calcareous parts.

3.3.1. Mussels with Methanotrophic Endosymbionts

Radium-226 results for Bathymodiolus spp. paired soft tissues and
calcareous shells are summarized in Table 3.11 and the results are depicted in
Figures 3.26 and 3.27 for comparative purposes.

226R, of soft tissue mussels exhibit a broad range of radioactivity from the
lower limit of detection (0.1 dpm/g) to 1.5 dpm/g. Mussel tissues from barite-

bearing seeps averaged 0.6 +0.4 dpm/g (n=7) 226, and ranged from 0.1 to 1.3
dpm/g. Mussel tissues from carbonate-bearing seeps, serving as controls,

averaged 0.8 +0.5 dpm/g (n=9) 226p4 and ranged from 0.0 to 1.5 dpm/g. An
unpaired, two-tailed t-test reveals that there is no significant difference
between mussel soft tissues from barite and carbonate-bearing seeps at 95%
confidence level.

Not unlike the body tissues, the 226K, in mussel shells also spans a broad range
of radioactivity, from below detection limit to 2.3 dpm/g. Mussel shells sampled

from barite seeps average 0.6 +0.7 dpm/g 226R, (n=7) and range from



22

Elemental barium concentrations of acid-digested soft tissues and hard
skeletal parts were obtained following the procedures described by Robinson
(1980) and were measured with a Perkin-Elmer ICP-AES 6500. The elemental
concentrations were corrected for systematic variations with the Topla
reference dolomite (Schroll, 1987). Replicate analyses of the reference
solution indicate an average Ba concentration of 55 £3.0 ppm (lo error).



88

Table 4.1. Chemical compositions of the two possible saline end member
fluids of pore fluids type IIT.

Components Saline end member fluids at:
Garden Banks Mississippi Canyon

salinity 139 155
Cl 2792 3520

Br 26 3.2
SO, 0.5 0.5
Na 2485 3338

K eT 14.8

Ca 84.4 16
Mg 43.2 59.5

Sr 3.2 17

Ba 6.1 0.6

Si 1 0.6
DIC 24 24
xe) -3.8 0.8

"Sr/*Sr 0.70854 0.70843

80 (HO) 17 0.1

Note: Concentrations are in mM/L, salinity is in permil, 54°C is in
permil (PDB) and 8" is in permil (SMOW).



ay ~’|__ Breton
le” T~ Sound.
can
~~. jiosta Knoll
. ver| |Eugene ship | s.
[Gatveston! High Island | Cameron | Island] Shoal [Timbalier s
£
st 29 Mississippi Canyon (MC) 28
us ;
Pp es
|
yj ©] Green Knoll
[cast Breaks Garden Banks (GB) Green Canyon (GC) Atwater Valley
°
50 100
|
Alaminos Canyon Keathley Canyon Walker Ridge km turd |}...
9a? 90° 38°

Figure 1.1. Base map of the study area on the Northern Gulf of Mexico slope and
basin showing the location of hydrocarbon seep sites where cores, pore fluids,
barites and fauna were acquired during submersible dives. Circles indicate
carbonate-bearing seeps; triangles indicate barite-bearing seeps.



71

5 T T T T
e 1
2
2
s 3b 4
i)
S slope=1
[-4
A 2b 4
=~
a
a Ip 7
0 f \ f fi
0 1 2 3 4 5
228Th/228Ra ageph (yr)

Figure 3.25. Relationship between 228Th/228Ra ages of raw barites (rb) and
pure barites (pb) of chimneys. Note that all the data converge along the 1:1
line suggesting that barite purification does not significantly affect the

228Th/228Ra dating.



77

14.0

®

Bs

Ps

6.0

Ra-226 Activity (dpmig)

GS-8 Sk (C)

97-GL1\C2 SK (B)

97-GL1IC1 SK (
GS-8 ST (C)

Figure 3.28.- Radium-226 activity

SK denotes hard skeletal component;

| mraz |
Content

GS-1 ST (C)
97-SF2 ST (B)
97-SF3 ST (B)
97-SF1 ST (B)

s
Z
G
g
S
if

87-GLnc1 st (a) &
97-GL1\c2 ST (8) BR
97-TW1 SK (B)

concentrations of heterotrophic seep fauna.
ST marks soft tissue component; B and C

marks barite and carbonate-bearing seeps, respectively.



Figure 3.19.

Figure 3.20.

Figure 3.21.
Figure 3.22.
Figure 3.23.
Figure 3.24.

Figure 3.25.
Figure 3.26.

Figure 3.27.
Figure 3.28.

Figure 3.29.

Figure 4.1.
Figure 4.2.

Figure 4.3.
Figure 4.4.

Figure 4.5.

Figure 4.6.
Figure 4.7.
Figure 4.8.
Figure 4.9.

Figure 4.10.
Figure 4.11.

Figure 4.12.
Figure 4.13,

Figure 4.14.
Figure 4.15.

Figure 4.16.

Figure 4.17.

Dating profile of growth increments in barite chimney #11
from Mississippi Canyon block 929 using the 228Thy228Ra method................57

Relationship between the first and second 228Th/228Ra
dating of pure barites. ........

210pp/226Ra ages of pure barite:
210pp/226Ra of barites before and after purification.
Sulfide effects on the 210pp/226Ra dating...

2287 p/228Ra ages plotted against their 228K ratios of of hime based

on pure barites........... wee we 69
Relationship between 228 TH/228Ra ag ages soft raw / barites

and pure barites of chimneys. .. seeceestecseecseeesere TA
Radium-226 activity concentrations Sof ‘methanotrophic mussels |

body tissues 74

Radium-226 activity concentrations of ‘methanotrophic mussel ‘calcareous ‘shells75
Radium-226 activity concentrations of heterotrophic seep fauna.
The relation of barium and 226Ra in hydrocarbon seep fauna.
Na-C] relationship in pore fluids type IL......
Plots of Cl against Na, Ca, Sr and Ba in pore fluids type II

Plot of 87Sr/86Sr against 1/Sr in pore fluids type I...

Barium-sulfate relationship in formation waters from Miocene and Pleistocene
90

reservoirs offshore Louisiana.

226Ra- salinity relationship in formation waters and.
discharged produced waters from the Gulf Coast region...
Ages of barite chimneys and crusts in the Gulf of Mexico
Vertical age profile for chimney #6, Garden Banks block
Vertical age profile for chimney #8, Garden Banks block 382
Vertical age profile for chimney #11, Mississippi Canyon block 929
Initial 228Ra/226Ra versus age of barite chimneys...
228Ra/226Ra AR versus 228Th/228Ra ages of chimneys
226Ra activities in barites formed in different depositional environments
Compositions of Ca and Sr in barites from GOM and barites

from other settings (hydrothermal, marine and continental settings).
Depositional model of GOM barites....
Dominant morphological truncated conical. -shaped barite chinmeys u used in the

modeling of radium fluxes sesceeseeeneeees 114
A comparison of Ra activity ranges from literature

with results from barite seeps .. 123
Conceptual model depicting the ‘chemical and radiation flow derived ‘from.
radium-rich barite deposits into fauna

inhabiting the barite-rich Seeps -....-..secseeceeeeseeesesseeseetenseeaneeseees 125



105

are in excess of the amount which can be supported by the coexisting 234y
(Cochran, 1979). Consequently, the 226Ra resulting from the radioactive decay

of excess 230Th in the sediment ends up in the pore fluids by leaching and
alpha-recoil processes (Kigoshi, 1971). Concerning the Gulf of Mexico barites,

normal marine pore fluids can be ruled out as the source of 226Ra because
most barites, especially the barite chimneys, form above the water/sediment
interface, and because 226Ra activities in pore fluids from non-seep sediments
are low (<16 dpm/L).

Basalt is generally considered as the source of 226Ra for hydrothermal
deposits in deep-sea spreading centers (MacDonald et al., 1980; Dymond et al.,
1983; Tivey and Delaney, 1986; Kadko and Moore, 1988; Stakes and Moore, 1991;
Kim and McMurtry, 1991; Stuben et al., 1994). This mechanism, however, can
be dismissed here because there are no surficial basalts and no evidence of
hydrothermal venting has been reported from the Gulf of Mexico.

Because Ra-rich fluid seepage is pervasive in the study areas (Fu et al.

1996), it is reasonable to propose that the 226Ra in the barites is derived from

fluids which originated as deep-seated formation waters. The 226Ra activity of
barites in Mississippi Canyon is higher than in Garden Banks (Table 3.6). By

contrast, the 226Ra activity in the pore fluids from Mississippi Canyon are
generally lower than in Garden Banks (Table 3.3). Therefore, the difference of

226Ra activity in the pore fluids can not explain the 226Ra activity variation
in the barites between the two areas. In order to understand the process

causing the difference of 226Ra activity in the barites, the homogeneous
distribution law of Henderson and Kracek (1927) is applied in which the ratio
of the concentrations of Ra to Ba in the precipitated crystals is proportional to
the ratio of the concentrations in the final solution:

(Ra/Ba)crystal = D (RaBa)s olution (4.4)
where D is the homogeneous distribution coefficient of Ra and Ba;
(Ra/Ba)crystal and (Ra/Ba)sojution are 226Ra/Ba ratios in units of dpm/mg in
barites and fluids, respectively.

It is obvious from equation 4.4 that the radioactivity in the barites are
controlled by (i) the partition coefficient D, and (ii) the Ra/Ba ratio, instead of
the absolute Ra concentration in the fluids. The coefficient D in the two areas
is considered to be constant because of the similar temperature and pressure
(water depths are 510 and 640 m in Garden Banks and Mississippi Canyon,
respectively). Therefore, the difference in 226Ra activities of the barites
between Garden Banks and Mississippi Canyon should be controlled by the
Ra/Ba ratio in the fluids. The calculated Ra/Ba ratio for the most concentrated
fluids in Mississippi Canyon and Garden Banks are 9.2 and 2.7 dpm/mg,

respectively (Table 4.2). Thus, the 226Ra activities of barites in Mississippi
Canyon are predicted to be 3.4 times that in Garden Banks. This value is close to
the observed value of 2.7 for the barites (Table 4.2). Therefore, the barite

226Ra activity differences between Mississippi Canyon and Garden Banks are
attributed to the different Ra/Ba ratios of the fluids in the two areas.



75

35 - oo ~

3.0

25 TMRa-226 Content!

Ra-226 Activity (dpmig)

GOM-S-47 (8)
GOM-S-50 (8)
GOM-S-49 (B)
GOM-S-48 (B)
GOM-S-46 (B)
GOM-S-44 (8)
929-4 (B)
J-6 (C)
MAC-2 (C)
709-1 {C)
GOM-S-9 {C)
GOM-S-1 (C)
GOM-S-8 (C) j
GOM-S-2 (C)

Figure 3.27.- Radium-226 activity concentrations of methanotrophic mussel

calcareous shells. Error bars accompanying data represent the 95% confidence
interval for random counting error. “B” and “C” denote mussel shells collected
from barite and carbonate seeps, respectivel



=
s
4
x
3
Zz 08
i -]
aQ
a
Ao
0.7
0.6
0.5

Figure 4.10. Initial 228Ra/226Ra versus age of barite

Garden Banks

Age (yrs)

chimneys.

101



86

0.7092 T T T T T
y = 0.70854 + 5.44*109 X
0.7090 R2=0.99 \ 4
DN
<S
=]
‘0.7088 5 7
x
* y = 0.70842 + 6.45*10°9 X
0.7086 R?=099 ;
0.7084 t ; t ;

1/Sr

Figure 4.3. Plot of 87Sr/86Sr against 1/Sr in pore fluids type III. The empty
circle refers to seawater, solid triangles and solid circles represent the pore
fluids from Garden Banks and Mississippi Canyon, respectively.



54

In general, purified barites yield 219Pp/226Ra ages comparable to the

228Th/228Ra ages in chimney #9 but yield discordant ages in chimney #10,
including some “negative” ages. The negative values in conjunction with the

large errors clearly indicate that the 210pp/226Ra ages of chimneys are
unreliable.

Chimney #11 is the only one recovered from MC-929. The conical-shaped
chimney is 11 cm in height and its base and top diameters are 10 cm and 3 cm,
respectively. Two fragments (Ch#11-F/1 and Ch#11-F/2) from the outside of
the chimney (sampled before hardening) were dated. The whole chimney was
later hardened with resin and the dating profile based on the hardened

samples was also obtained (see below). The purified barites yield 228Th/228Ra
ages of 4.56 40.07 and 2.42 40.04 years for base and top of chimney (Table 3.6)

while the corresponding 210pp/226Ra ages are 2.06 +2.40 and 0.92 £1.13 years,

respectively. The effects of barite purification on the 2!0pp/226Ra age
determinations can also be gauged for these samples (Table 3.6).

3.2.4.2 Hardened Chimneys

Three barite chimneys (chimneys# 6 and #8 from GB-382 and chimney #11
from MC-929) were resin-hardened and sliced vertically. Subsamples at 2 to 3
cm intervals were sectioned along the growth bands from each chimney and
were subjected to radioisotope analyses for the purpose of obtaining
228Th/228Ra ages. The analytical data for these chimneys are listed in Table
3.7 and graphed in Figures 3.17, 3.18, and 3.19.

Chimneys #6 and #8, recovered in pair from GB-382, are 15 and 20 cm tall
and the base and top diameters of the cones are 7 to 8 cm and 4 cm,

respectively. The 228Th/228Ra ages of five distinct growth bands from
chimney #6 and eight from chimney #8 decrease systematically from the base
to the top as follows: 2.48 +0.04 and 2.1140.02 years at the bases to 1.62 +0.01 and
0.42 +0.02 years at the top, respectively (Figs. 3.17 & 3.18).

Chimney #11, the only specimen recovered from MC-929, is a cone-shaped,
18 cm tall chimney with 14 cm and 5 cm diameters at the base and top,
respectively. The 228Tp/228Ra ages of five growth bands decrease
systematically from 6.44 +0.23 years at the base to 4.00 + 0.01 years at the top
(Fig. 3.19).

3.2.4.3, Barite Crusts

Barite crusts occur either on the flanks of mud volcanoes or as tee-pees
covering the seafloor. Unlike the chimneys which are predominantly barite
(>95% by volume), the crusts also contain variable amounts of carbonates and
sulfides. Six crust samples, four from GB-382 and two from MC-929, were
recovered for radioisotope assays and both raw and purified barites were
analyzed. With the exception of Cr-93-1-3/1, all other crusts contain 228Ra and
228TH activity levels below our detection limit and therefore only the
210pp/226Ra ages were determined (Table 3.8).

The light-gray Cr-93-7-7/1 crust was recovered from the flank of an active
mud volcano in GB-382. An age of 29.6742.40 years was obtained for the raw
sample and a younger age of 12.17£1.39 years for the purified barite.

The Cr-93-8-5/3 crust, occurring as a teepee draping the seafloor, was
recovered from GB-382. The crust is about 7 cm thick and displays two distinct



50

Table 3.7. Ages of hardened chimneys.

Measured Sampling Initial
Sample ="Th/"Ra *RalRa > ThRa Th/*Ra > RalTMRa
age (yr) age (yr)

|Chimneys from Garden Banks block 382
Chi#6-1 1.01140.002 0.58440.001 4.64440.035 2.47740.035 1.022+0.012
Chi##6-2, 0.97240.001 0.681£0.001 4.32240.008 2.154+0.008 1.127+0.006
Chi#6-3 0.943£0.001 0.676£0.001 4.10340.018 1.93640.018 1.108+0.008
Chi#t6-4 0.928£0.002 0.70540.001 3.99240.006 1.778+40.006 1.141+0.005
Chi#6-5 0.908£0.001 0.68440.001 3.84840.008 1.623£0.008 1.087+0.005

Ch#8-1 0.99740.001 0.57840.001 4.52340.016 2.106+0.016 0.997+0.007
Chi#8-2, 0.944+0.001 0.577+0.002 1.693+0.010 0.94740.020
Ch#8-3 0.829+0.002 0.611+40.001 1.32440.011 0.959+0.008
Ch#8-4 0.82840.001 0.73140.001 0.906£0.012 1.091+0.006
Ch#8-5, 0.82240.001 0.79020.001 0.77040.010 1.173+0.006
Ch#8-6 0.786+0.001 0.803+0.002 3.073+0.008 0.656+0.008 1.163+0.007
Ch#8-7 0.743+0.001 0.826+0.001 2.83240.016 0.415+0.016 1.162+0.007

Chimney from Mississippi Canyon block 929
Ch#11-1  1.35740.004 —0.18640.0019.775+0.232 6.44240.232-—_0.605:0.040
Ch#11-2 —-'1.31240.001 —0.24140.001 -8.61940.018 —5.28640.018 —_0.68140.009
Ch#11-3  1.281£0.002 —0.23640.001—-7.98320.018 —4.65040.018 + 0.617£0.008
Ch#11-4 —— 1.25740.002 -0.31240.0027.54740.010 4.21440.010 (0.750.012
Ch#11-5 _1.244+0.001__0.322+0.001___7.33040.004 _3.99740.004 __0.779+0.006

Notes: Measured age is based on the direct measurement data which represents the age from the formation
of barite to the time of dating. Sampling age is corrected to the time of sample collection (July, 1993).



12

more detailed gamma-ray spectrometric investigation which revealed that
226Ra and 228Ra were unsupported by their parents 238y and 232Th, and
therefore can be considered "orphans". The apparent ages of the barite

samples, therefore, can be determined using the Ra decay series isotopes.

The radionuclides used in this study for dating barites are 228Ra, 226Ra and
their radioactive daughters 2287h and 210Pp, respectively (Fig. 2.3). Because
Ra and Ba are chemically similar (both occupy the same column in the
periodic table), Ra commonly coprecipitates with Ba during the formation of

barites. Subsequently, both 226Ra and 228Ra, uptaken by the barites, decay
with time as follows:

a a
226Rq ----- 3222 Rn cece S214 pp eevee S210pp >206p pH
1.62x103y 3.82d 26.8min 22.3y
B B a oa B
228Rq anne $228 A¢ ---->228Th nn 5220py -.--->212pp -..-->208pp
5.75y 6.13h 1.91y 55.6s 10.6h

Since the longest-lived daughters of 226Ra and 228Ra decay series are
210pb and 2287h, respectively, 210pp and 228Th will accumulate with time.

Therefore, the 219pp/226Ra and 228Th/228Ra daughter/parent ratios in the
barites can be used for age determinations.

The 228Th/228Ra method is based on the incorporation of 228Ra into the

barite, where it decays to 228Th. The 228Th/228Ra present in the barite is
dictated by the decay rates of these two isotopes and the time elapsed since

228Ra was incorporated. Therefore the age of the barite can be determined
according to the following equation:

(A228 7 p)(A228R4)=14228TH/A228 TH - 4228Rq)I[1-exp(A228p, -42287p)t] (2-1.)

where A228pq and A2287} are the measured activities of 228Ra and 228Th,
respectively; t is the age of the sample in years; and A228 Raq and A2Q287pH are the

decay constants for 228Ra and 228Th, respectively. The half lives of 228Ra and
2287h are 5.75 and 1.91 years (Broecker and Peng, 1982) and these translate to
decay constants of 0.1205 yr for A228p,q and 0.3628 yr for 4228 7h>
respectively.

Because the half lives of both the parent (228Ra) and daughter (228Th) are

comparable, the 228Th/228Ra will first reach transient equilibrium (a ratio of
1) and will increase to approach a constant value with time. As shown in
Figure 2.4, this constant value will reach 1.5 after 15 years. Therefore, the

useful range of the 228Th/228Ra dating method does not exceed 15 years.
The successful application of the 228Th/228Ra method depends upon the
following assumptions: (i) the 228Ra must be orphan, namely no parents of
228Ra exist in the barite, (ii) there must be no initial 228Th in the barite, and
(iii) the system must have remained closed with respect to both 228Ra and

228Th. As will be shown in the discussion, all these assumptions have been
validated for the Gulf of Mexico barites.



Table 3.4. Elemental chemistry of barite chimneys and crusts from the Gulf of Mexico slope.

Barite chimneys

Barite crusts

1 2 3 4 3S 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
(WL%)
BaO 45.61 5655 54.91 61.56 55.34 59.67 S144 53.14 53.42 54.45] 56.78 57.23 62.33 53.76 62.04 59.17 55.20 49.99 58.99 54.56)
SrO 14.42 4.77 6.97 1.82 TAT 4.06 9.66 TAL 8.70 8.20) 4.26 5.77 17s 7.25 2.08 2.91 600 8.04 3.25 7,34
CaO 121 0.68 0.36 0.33 0.33 0.37 1.02 1.29 0.79 0.57 0.79 0.67 0.43 0.28 0,50 0.64 = 0,92 13) 085 0.97
Na,O. 0.67 0.61 0.37 0.53 0.47 0.30 0.76 0.89 0.35 0.42 O17 O18 O18 0.20 0.84 0.77 0.67 0.94 0.76 0.30]
FeO 0.14 0.16 0.13 0.04 0.23 6,00 0.02 0.08 0.06 0.02 0.01 0.02 0.02 0.05 0.02 610 O10 0.06 0.04 0.02
SO, 37.57 34.83 34.82 34.83 35.65 35.12 36.05 35.97 36,53 35.47] 34.55 35.71 3459 34.03 33.78 35.72 35.98 37.52 35.71 36.01
Total 99.62 97.59 97.56 99.12 99.20 9951 98.97 9847 99.86 99.13] 96.56 99.58 99.30 95.57 99.26 99.31 98.87 97.86 99.60 99.20)
‘Atoms (based on 4 oxygens)
Ba 0.633 0.846 «0.82! 0.923 0.810. 0.89 0.741 0.770 0.765 0.798] 0.865 0.841 0.937 0.817 0.908 0.882 0.809 0,739 0.869 0.793
sr 0.296 0,106 0.154 0.040 0,155 0.09 0.206 0,152 0.184 0.178} 0.096 0.125 0,039 0.163 0.045 0,064 0.130 0.176 0.071 0.158
Ca 0.046 0,028 0.015 0.014 0,013 0.015 0.040 0.051 0.031 0.023] 0.033 0.027 0.018 0.012 0.020 0,026 0,037 0.053 0.034 0,039
2Na 0.023 0.023 0,014 0.020 0.017 0.011 0.027 0.032 0,012 0.015] 0.011 0.013 0.013 0.015 0.061 0.057 0.049 0.069 0.055 0.022
Fe 6.004 0.005 0.004 0.001 0.007 0.000 -—:0,00! 0,003 0.002 0,001] 0.000 0.001 0.001 0.002 0.001 0.003 0.003 0.002 0.001 0,001
Total 1.003 1.007, 1,008 0.998 1.003 1.002 LOIS 1.007 0,995 1.014) 1.005 1,007 1.008 «1.008 1.034 1.032 1.027, 1.039 1.031 1,012
8 0,999 0.998 __0.997__ 1.001 0.999 0.999 0.995 0,998 1.002 0.995} 1.008 1.005 0.996 0.990 0.947_—*.0191.009_—:1.063_—*1.008 1.003

Notes: The data are based on the analyses of chimney #2 and crust Cr-93-1-3/1. The overall error (+16) of the chemical analyses reported here are estimated, on the basis of standard analyses,
to be (in weight % units): 0.39 for SO,; 0.32 for Ba; 0,09 for SrO; 0.06 for CaO; 0.09 for Na,O; 0.06 for FeO.

Iv



Figure 3.12. Sulfur and oxygen isotope compositions profile of chimney
#9. Note: sulfur and oxygen isotope compositions of the barite increase
laterally from the outer part to the inner part of the chimney.

45



74

2.5

2.0

Ra-226 Activity (dpm/g)

0.5

0.0

929-4 M(B)
709-1 (C)
L-5 (C)
MAC-2 (C)
GOM-S-1 (C)
J-2 (C)
GOM-S-2 (C)

ao
e
2 9
= =
ce} ce}
Lo} oO

GON-S-47 (B)
GON-S-46 (B)
GOM-S-50 (B)
GOM-S-44 (B)
GOM-S-48 (B)
GONM-S-49 (8)

Figure 3.26.- Radium-226 activity concentrations of methanotrophic mussels
body tissues. Error bars accompanying data represent the 95% confidence
interval for random counting error. “B” and “C” denote mussels collected from

barite and carbonate seeps, respectively.



137

Jensenius, J. and N.C. Munksgaard (1989) Large scale hot water migration
systems around salt diapirs in the Danish central trough and their impact
on diagenesis of chalk reservoirs: Geochim. Cosmochim. Acta 53: 79-88.

Kadko, D., R. Koski, M. Tatsumoto, and R. Bouse (1985) An estimate of
hydrothermal fluid residence times and vent chimney growth rates based
on 210pp/Pb ratios and mineralogical studies of sulfides dredged from the
Juan de Fuca Ridge: Earth Planet. Science Lett. 76: 35-44.

Kadko, D. and W.S. Moore (1988) Radiochemical constraints on the crustal
residence time of submarine hydrothermal fluids: Endeavour Ridge:
Geochim. Cosmochim. Acta 52: 659-668.

Kalogeropoulos, S.I. and P. Mitropoulos (1983) Geochemistry of barite from
Milos island: Neues Jahrbuch fuer Mineral. Monatsh. |: 13-21.

Kesler, S-E., L.M. Jones and J. Ruiz (1988) Strontium and sulfur isotope
geochemistry of the Galeana barite district, Nuevo Leon, Mexico: Econ. Geol.
83: 1907-1917.

Key, R.M. (1981) Examination of abyssal sea floor and near-bottom water
mixing processes using Ra-226 and Rn-222. Unpublished Ph.D Dissertation,
Texas A & M University, 227 pp.

Kharaka, Y.K., A.S., Maest, W.W., Carothers, L.M., Law, P.J., Lamothe, and T.L.,
Fries (1987) Geochemistry of metal-rich brines from central Mississippi
Salt Dome basin, U.S.A.: Applied Geochem. p. 543-562.

Kigoshi, K. (1971) Alpha-recoil thorium-234: Dissolution into water and the
uranium-234/uranium-238 disequilibrium in nature: Science 173: 47-48.

Kim, K.H. and G.M. McMurtry (1991) Radial growth rates and 210pb ages of
hydrothermal massive sulfides from the Juan de Fuca Ridge: Earth Planet.
Science Lett. 104: 299-314.

Kraemer, T.F. and Reid, D.F. (1984) The occurrence and behavior of radium in
saline formation water of the U.S. Gulf coast region. Chem. Geo]. (Isotope
Geosc. Sec.) 2: 153-174.

Krest, J.M. (1995) ?8Ra and *?°Ra as indicators of sea water-sediment
interactions in the Mississippi and Atchafalaya mixing zones. Unpublished
MS Thesis, University of South Carolina, pp. 60.

Kronfeld, J., Minster, T. and Ne'eman, E. (1993) 238U-series disequilibrium in
the Upper Cretaceous oil shales of Israel as the primary source for the Dead
Sea 226Ra anomaly. Terra Nova 5: 563-567.

Kronfeld, J. (1995) Oil shale brines and radium anomalies (Israel). Geol. Soc.
Am. Abstracts with Programs 27: A97.

Kusakabe, M., S. Mayeda, and E. Nakamura (1990) S, O and Sr isotope systematics
of active vent materials from the Mariana backarc basin spreading axis at
18°N: Earth Planet. Science Lett. 100: 275-282.

Kvenvolden, K.A. (1988) Methane hydrate: A major reservoir of carbon in the
shallow geosphere. Chem. Geol. 71: 41-51.

Lalou, C. and E. Brichet (1982) Ages and implications of East Pacific Rise sulfide
deposits at 21°C: Nature 300: 169-171.

Lalou, C. and E. Brichet (1987) On the isotopic chronology of submarine
hydrothermal deposits. Chem. Geol. 65: 197-207.

Land, L.S., and D.R. Prezbindowski (1981) The origin and evolution of saline
formation water, Lower Cretaceous carbonates, south-central Texas:
Hydrol. 54: 51-74.



129

study, there is a need to know the extent to which radon is present in
chemosynthetic ecosystems before assessing the radon radiation hazards.



2. METHODOLOGY

Laboratory practices employed in this study can be conveniently grouped
under three headings: (i) chemical and isotope methods applicable to pore
fluids in seep sediments, particularly from barite seeps, to enhance our
understanding of the source and nature of the Ba and Ra-rich fluid venting;
(ii) petrographic, mineralogic, chemical and isotope methods applicable to
barite deposits associated with seeps to provide insights on the mode, age and
rate of barite deposition, and (iii) radiological and chemical methods
applicable to chemoautotrophic and heterotrophic seep fauna to derive the
uptake of radionuclides by both soft tissues and hard exoskeletons and to
evaluate the consequences of radionuclide radiation on the benthic fauna. In
the following sections the laboratory methods used in the fluid assays will be
described first.

2.1 Pore Fluids

Figure 2.1 is a flow chart summary of the analytical assay types employed
in this study for pore fluids.

Push cores, up to 40 cm long, were taken with the submersible robot arm
during dives in the Gulf of Mexico at seep sites indicated in Figure 1.1.
Immediately after recovery, the cored sediments were taken to the laboratory
on the support surface ship where continuous pore fluid profiles were
acquired with a squeezer modified from Jahnke (1988). The pore fluids were
passed into plastic disposable syringes inserted into the core barrel at 3-4 cm
depth intervals and were filtered through a 0.25 um filter capping the
syringe. After filtering, each pore fluid sample was divided into three
aliquotes. One was used for determination of salinity, dissolved sulfide and
sulfate concentrations immediately after recovery. The second was acidified
with few drops of concentrated HNO3 to pH<1 and stored in 5 ml size glass
bottles with rubber stoppers and aluminum vacuum caps for subsequent
elemental analysis. Mercuric chloride was added to the third to prevent
bacterial activity and this aliquot was used for the analysis of oxygen isotopes
(8180) of water.

Salinity was measured by a hand-held refractometer (model A366 ATC)

having an accuracy of +1°/o0. Dissolved sulfide and sulfate concentrations
were determined by the colorimetric method of Cline (1969) and barium
gravimetric method (Presley, 1969), respectively. The analytical error (10)
based on replicate analyses of standards was 4.5% for sulfide and 0.5% for
sulfate determinations. 5180 of water were determined with a Nier-type triple
collector gas source mass spectrometer using the technique described by

Graber and Aharon (1991). Accuracy and precision of 5180 are within 0.19/00
based on standard repeats. Stable isotopes are reported in the delta (5) notation
in permil relative to SMOW standard for oxygen isotopes (Graber and Aharon,
1991).

Dissolved K, Na, Ca, Mg, Sr and Ba were determined using an inductively
coupled plasma spectrometer (ICP-AES); Cl and Br were measured using an Ion
Chromatograph (IC). Replicate analyses of standards and samples yield
precisions of £0.5% for the major and minor cations, +3% for Cl and +3.5% for

Br. 8751/86Sr ratios of the pore fluids were measured on a Finnigan MAT 262
six collector thermal ionization mass spectrometer (TIMS). The results are



132

challenged by future studies. Additional local and regional surveys should also
address the question of Ra and Ba dispersion from the seeps into the water
column in view of data suggesting an "unexplained" increase in the
downstream Ra concentrations in the Gulf of Mexico (Reid, 1979; Key, 1981).

7. The uptake of Ba and Ra by the fauna inhabiting the barite-bearing
seeps was found to be proportional to the Ba/Ra ratio in the pore fluids.
Mussels harboring methanotrophic endosymbionts average 0.6 dpm/g and 0.5

dpm/g 226Ra for soft tissues and calcareous shells, respectively, which are
significantly higher than those reported for shallow marine mussels. Vagrant.
heterotrophic, fauna living around the seeps yield 226Ra and Ba up to 10.7
dpm/g and 4319 ppm, respectively. The considerably higher levels of Ra and

Ba in the heterotrophs are probably acquired through ingestion of barite
particles.

8. The largest possible radiation dose to fauna from Ra-rich barites (0.0128
mGy/day) is substantially lower than the threshold of detrimental radiation
effects for aquatic animals. However, the cumulative radiation dose and
consequential genetic effects over a mean lifespan of 25 years for mussels
could be substantial. These estimates have not considered the radiation

exposure derived from 222Rn gas dissolved in the surrounding bottom waters
because of absence of data. There is a need to determine the level of radon in
the ambient seeps in order to properly assess the impact of radon radiation
hazards.



‘Table 3.6. Activities and ages of barite chinmeys.

Sample Sample Pb Ra TMTh TM Ra TMPb/*Ra ThA Ra Raf Ra Measured Sampling Measured ‘Sampling Initial
name type. dpmv/g_dpm/g_dpmvg_—dpnvig pH Ra PP Ra —Th/TMRa “TheRa Ral Ra
age (yr) age (yr) age (yr) age (yr) ratio

[Chimneys from Graden Banks block 382

ch#2- R 144 10B8B = GH3_— 774 0.13340.032 ——0.85140.004 0. 7180.001 4.60142.388 251842388 «3.34120.056 ——1.25840.056 —_1.07440.017
Chif2-0 P 113 1452 B19 1135 0,07940,058 0.722+0.006 0.789+0.001 2.63644.066 0,55344.066 2.71 640,064 0,933:0.064 1.094+0.020
ch#2-b R 102 1310 -726-——760-——0,.07940.0320.95640.004 ——0.5860.00)2.63042.254 0.54742.254 4, 19640.066 —2.11340.066 0.97140.019
Ch#2-b P 7 1184 624 690 0,06130.040 0,9040.006 0.5884 0.001 2.01342.292, -0.07042.292. 3: 82140.076 1.938+0.076 0.93340.020
Ch#o-Ta R 191 1305, 848 906 0.14640.041 0.936+0.005 0,694+0,002 5,07543,084 1,96543,084 4.05240,080 0,942+0.080 1,13140.028
Ch#d-Tha P 170 1279 TB 8d2 0,13340.032, 0,92440,005 0.658+0.001 4.58242.376 1.47242.376 3.9630.070 0.853:10.070 1,06140.021
|Ch#9-Mia R 4h NB 739 TO 0.12040.029 0.93540.006 0,67240,002 4.12422, 116 1,01442,116 4.04140,091 0,93 140.091 1,093:t0.031
Ch#9-Mia Pp 153 1246, 769 820 0.123£0.038 0,93740,004 0.658:40.001 4.21742.786 1.10742.786 4.054+0.062, 0.94440.062, 1.073+0.019
ChH9-Bia R 175 140L «876838 0.12540.027 —1.04540.005 0.59880.0024.29441.980 —0.99441.980 4.94240.094 —1.64240.094 —_1.085:40.032
}Ch#9-Bia P TS 1339, 789 765 0,0860.041 1,032:40.005, 0.57140.001 2,87542,878 -O.A25+2,878 4.82540.091 1,52540,091. 1,02140,026
ChH9-TYb- R ISL 1208, m9 755 0.125+0.032 0.99340.004 0.624£0.001 4,.28522.350 0.98542.350 4.49310.066 1,193:40.066 1.07340.020
lchwo-t7 P 1411229707708 .11540.031 —0.99880.004 —0.57680.002 3.92142.258 0,62142.258 4.53440.072 —*1.28440.072 —0.99580.024
chs9-mirs R 1531275818 BAT 0.12040,028 —0.96640.006 0.640.001 4.11442.040 1,03482.040 —4,2800.084 ——1.20080.084—_1.11240.026
|Ch#9-M/b P 18 ‘1346 788 822 0.08840.037 0.958:+0.006 0.61)40.001 2.97942.604 -0.10142,604 4.21240.094 1.13240.094 1.015+0,026
Ch#9-B/b R 122 1170 756 772 0,105:0,039 0,97940.005 0,666+0.001 1.485+1.398 4.37640.074 2.29340.074 1.12940.025,
Ch#9-B/b P 16 1174 8 739 0,09940,042, 0,95840.005 0,635:40.00) 1.28741.500 421740,081 2.13440.081 1,05640.024
Ch#l0-Ta R 52 1236 784 1044 0,042:0.039 0.75040.003 0.853+0.001 38741335 ~0.69641.335 2.8710,032 0.71840.032, 1,205+0 012,
Chit 0-Ta Pp 44 1125, 649 876 0,04040.045, 0.741£0.004 0.78540.002 T.31041.186 -0.77341.186 2,819+0.042 0.666+0.042 1,10340.017
|Chi#]0-Méa R 136 1410 856 1168 0.097+0.036 0.733+0.002 0,83620,001 3,102+1,281 1,20941.281 2.77740,026 0,694+0.026 1,16940.010
chitio-Mia P 12} 13327541041 0.09240.036 —0.72480.003 0. 78840,001 «3.23281.273 —LOISKI.273—-2.72980.034 —0.64640.034—1.09520.012
Chit] 0-B/a R 104 1086, 687 832 0,097+0.045, 0.82640.001 0.77440.002 +3.27141.599 1,18841.599 3.30940.018 1,22640.018 1,12340.011.
}Ch#10-Béa. P 7 1250 709 967 0.059+0.039° 0.73320.001 0,78140,001 1,9561,354 O.12741.354 2TTEHOOL2 0,99640.012 1,091:40.006
}Ch#LO-Tyb- R 159 1307 B54 873 0,122+0.038 0.97840.002 0,668:40.001 4.19242.810 0.97242.810 4.37240.036 1,15240,036 1.13240,013
Ch#10-T/b P 156 1345, 84 793 0.116+0.032 0,9880,001 0,590:40,001 3,95342,328 0,73342,328 4.45140.018 1,25040,018 1.01 140,008
}Ch#10-M/b R 150 WL 752 TB .128:+0,042 1,034£0.001 0.621:40.003 4.38523.106 1,1653,106 4.843+0.018 1.623:40,018 1.11440,016
Chal0-M/b P 147 121 680 669 0,12140,039 1,01740.002 0.552+0.002 4,128+2.850 0,97842.850 4,69140.036 1.54140,036 0.972+0.015
Ch#l0-B/b R 167 1297 980 908 0.12940.041 1.07940.001 0,70040.001 4.42443,038 1,274£3,038 5,26540.021 2.11540,021 1,32140.027
Ch#10-B/b P 160 1302 B19 755 0.1230.045, 1.0860.003 0.580£0.003 4.20643.302, 1.05643,302 5,33040.062 2.18040,062 1.10240,028
Chimneys trom Mississippi Canyon block 929

}Civ#] 1-F/L R 229 2464 134 112 1.02040,002, 0.45620.00) 3.16741,101 1.0841.101 4.7) 640,028 2.63340.028 0,80440.028
|Chit] 1-F/L P ag 3045, 1670, 1680 0.08940.032 0,9940.003 0.43040.00) 3.000.131 O.9ITELI31 4.50140.044 2.41840.044 0,729+0,009
Chit] 1-F/2 R 45t 2769 1049, B19 0.16340.036 1.2810.002. 0.296+0.002 5 704£2.764 2.45442.764 7.991:0.084 4.74 140.084 0,775+0,026
chur 1-472 P 56236971286 1010.15 240.032 ___1.27340.002___0.27340.001___ 531142400 __2.06142.400___7.84140.074 __4.56440.074 __0.70140.018

Note: R= raw samples,
P= pure barite.

Measured age is obtained by the mesured isotope ratio.
Sampling age = ages corrected to the time of sample collection.

Error = 26,

6r



TABLE OF CONTENTS

List of Figures
List of Tables .
Acknowledgments... Aeeeeeeneestesitsnesssentesceseeseesnessssneereeseeresneeasesrese Kh

1. Introduction

1.1. Statement of the Problem 0... sees

1.2. Objectives and Questions Addressed in the Study.........
1.3. Site Descriptions.....ssccsscscseccsssescessecsssessssveessuecessssecssstessonuessaneessnscsunessenssecsnseessaseetessneees D

2. Methodology

2.1 Pore FIUIdS 0. eessessteetese cece eeseeeeeeeseneenenennennneeeees

2.2. Barite Deposits
2.2.1. Imaging, Chemical and Stable Isotope Assays.
2.2.2. Principles of Dating Methods...
2.2.3. Sample Processing for Dating...
2.2.4. Instrumentation and Analytical Methods

2.3. Seep Fauna.

3, Results ....-soeescsecssstessssnresessseessnecconssecossccosnvetssssusssssecssusessuuessasecennnecanaeeseneesaneseensecesinecessnneecssee 23

3.1 Hydrocarbon-Rich Fluids ..
3.1.1. Radium and Barium “Assays in Barite-1 “Bearing Seep
3.1.2 Chemical and Isotope Compositions ....

3.2 Barite Deposits Associated with Seeps
3.2.1. Morphology and Petrography
3.2.2. Elemental Chemistry.
3.2.3. Sulfur, Oxygen and Strontium Isotope Assays ..
3.2.4. Radioisotope Assays...

3.2.4.1. Soft and Fragmented ‘Chimney:

3.2.4.2 Hardened Chimneys.

3.2.4.3 Barite Crusts ....
3.2.5, Validation of the Ra-Series Dating Methods.

3.2.6. 210pp/226Ra Ages
3.2.7. 228Th/228Ra Age:

3.3. Radium and Barium Systematics in Chemotrophic and Heterotrophic Benthic Fauna
from Barite-Bearing Seeps...........
3.3.1. Mussels with Methanotrophic Endosym! ion
3.3.2. Heterotrophic Fauna ..
3.3.3. Barium-Radium Relationship i in Fauna

ARSS



66

Another question that needs to be addressed is the exact source of excess
210pb in the raw barite crusts. Microscope and SEM analyses indicate that the
most abundant minerals other than barite in the crusts are carbonates and
sulfides. Since Pb has two oxidation states (+2 and +4) and could react with
sulfide to form PbS, it is not unreasonable to assume that sulfide in the crusts
may act as a Pb sink which accounts for the excess 2!9Pb in the raw samples.
This inference is confirmed by the observed relationship between the
concentration of sulfides and the degree of discrepancy in 719pPp/226Ra data
before and after barite purification (Table 3.10 and Fig. 3.23) indicating that
the higher the sulfide concentration in the sample, the larger the discrepancy
of 210pp/226Ra before and after barite purification. This positive relationship
strongly suggests that the excess of 2!0Pb in the raw samples stems from the
Pb association with sulfides in the crusts.

The excess 210Pp added to the sulfide coexisting with the barite is likely to
have originated from the decay of 222Rn emanating from the venting fluids.
This contention is supported by the study of LaRock et al., 1996) which
indicates that Pb coprecipitates with sulfide if the hydrogen sulfide
concentrations are high. As it will be discussed later, microbial sulfate
reduction plays an important role in the formation of the barite crusts.
Because hydrogen sulfide is one of the two main by-products of microbial
sulfate reduction, the hydrogen sulfide concentration remains high during
the formation of the barite crusts. In addition, the advecting fluids in the
barite-bearing seeps are likely to be enriched in 222Rn due to the high
concentration of 226Ra in the seeping fluids (Table 3.3, and Fu et al., 1996).
Thus when radon gas diffuses through the crusts, 219Pp will rapidly deposit
because the half life of 222Rn is 3.82 days and all the intermediate isotopes
have half life are less than an hour. This 210Pp will react with the hydrogen
sulfide resulting from the microbial sulfate reduction and precipitate as
pyrite. Because the 219Ppb in the sulfides had not originated from the decay of
226Ra in barites, the exogenous or excess 210Pb in the sulfide coexisiting with
barite will make the 210pp/226Ra of the raw sample larger than that of
purified sample and hence yield a spurious age.

The purification of barites outlined previously removes carbonates,
organic matter and fine sulfides from the raw sample and effectively removes
exogenous 210pp. Thus the 2!9pp/226Ra dating method can be accurately
applied to the crust barites and yield true ages after emoval of the sulfides.
Contrary to the systematics of the impure barite crusts, the similarity of
210pp/226Ra between raw and purified barite chimneys 1s attributed to the
lower concentration of sulfides in the chimneys (Fu et al, 1994),

3.2.7, 228pn/228Ra Ages

The 228Th/228Ra ages of purified barite chimneys range from 0.5 to about 5
years (Fig. 3.24) indicating that the chimneys are of young age and some
formed just several months before they were recovered in July 1993. As

previously reported, excess 210pp present in the sulfide-bearing crusts tend to
yield spurious 210pp/226Ra ages. Any sulfides in the chimneys could also
affect the 228Th/228Ra age determinations because the activity of 228Th used



21

order to emanate the radioequilibrated radon into evacuated Lucas cells. This
step required about 15 minutes of sparging in order for the gas in the Lucas
scintillation cell to reach atmospheric pressure. Following the sparging
process, the scintillation cell stopcock was closed, and a 3-hour equilibration
period was allowed to pass in order for radon to reach transient equilibrium
with its short-lived alpha emitting daughters, thus increasing the efficiency
of the counting. The scintillation cell was then removed from the radon
bubbler and placed inside the counting chamber of a Randam Scintillation Cell
Counter. This instrument contains a counting chamber optically coupled to a
photomultiplier. Alpha particles striking the zinc sulfide coating on the inside
of the Lucas cell give off visible flashes of light which are detected and
amplified into a signal by the photomultiplier and recorded on a digital
readout.

After a three-hour equilibration period, scintillation cells were counted
1000 minutes to reduce counting error to acceptable levels. Total counts on the
digital readout and counting time were recorded for each sample when the
1000 minute counting time had elapsed. The original background counts were
carried out with nitrogen gas in the Lucas cell at atmospheric pressure
because the alpha background inherent to the cells is higher in a vacuum
than in nitrogen gas at atmospheric pressure. The background counting was
done under the same counting conditions as those used for the samples and
standards. Based on the sample weight and the total counts per 1000 minutes,
an activity per unit weight was obtained. The data results were calculated
using recommended formulas found in the EPA-600/4-80-032 procedure (EPA,

1980). Radioactivity was originally calculated in units of picocuries 2260p, per

sample unit weight. Picocuries of 226R, per unit weight of sample is calculated
as follows:

A=(Cg-Cp)/[CF* D* I* Wt] (2.3)
Where,

A= picocuries of radon per gram of sample

Cs= Sample cpm

CBp= Background cpm

CF= cpm/pCi conversion factor

D= Decay Correction

I= Ingrowth Correction

Decay Correction (D)= EXP (-0.693*Ty/ t1/2)

Wt= Sample weight in grams

Where,

T,= Time in days from the emanation time to midpoint of counting time

t1/2= Radiological half-life of 222Ry, 3.82 days

I= [1- EXP (-0.693*T2/t1/2)].

Where,

T2= Time in days from the sealing time to starting point of emanation time

t1/2= Radiological half-life of 727Rn, 3.82 days.

A reagent blank and an EPA 2260p, standard were processed exactly as
described above for each batch of approximately 10 environmental samples.

The calculated values of 22°Ra were corrected for any background artifacts
indicated by procedural and reagent blanks of typically 0.01 dpm level.



64

25 T T

157

10/7

Age (yrs)

Chimneys

5 1 1 1 L 1
0.0 0.1 0.2 0.3 0.4 0.5

210Pp/226Ra Ratio

Figure 3.21. 210pb/226Ra ages of pure barites. Bars represent 20

0.6

errors.



141

Urabe, T. and M. Kusakabe (1990) Barite silica chimneys from the Sumisu Rift,
Izu-Bonin Arc: possible analog to hematitic chert associated with Kuroko
deposits: Earth Planet. Science Lett. 100: 283-290.

Van Gent, D.L., B. Fu, P. Aharon, and L.M. Scott (1995) Radium decay series
dating of barite deposition in the Gulf of Mexico: Initial results. In
Environmental Issues and Solutions in Petroleum Exploration, Production,
and Refining. Sept. 25-27, New Orleans, LA. IPEC Proc. (ed. K.L. Sublette),
Penwell Publ. p. 821-829.

White, D.H. (1965) Saline waters in sedimentary rocks: Am. Assoc. Petrol. Geol.
Mem. 4: 342-366.

Wolgemuth, K. and Broecker, W. S. (1970). Barium in seawater. Earth Planet.
Science Lett. 8: 372-378.

Zierenberg, R.A., W.C. III Shanks and J.L. Bischoff (1984) Massive sulfide
deposits at 21°N, East Pacific Rise: Chemical composition, stable isotopes, and
phase equilibrium: Geol. Soc. Am. Bull. 95: 922-929.



Sample Age Initial

name (yts) 228Ra/226Ra
Chil 1-5 4.0040.01 0.78+0.01
Chit |-4 4.21+0.01 0.78+0.01
Ch#lI-3 4.6540.02 0,620.01
Chil 1-2 §.2940,02 0.68:0.01
Chiff l-f 6.4440.23 0,610.04

Figure 3.19, Dating profile of growth increments in barite chimney #11 from Mi
228Th/228Ra method, Ages were corrected to July 20, 1993 when the chimney wa

ippi Canyon block 929 using

sampled.

¢



14

H H
t1/2(228Ra) = 5.75 yrs
ty2(228Th) = 1.91 yrs]

Activity (dpm)

2287 n/228Ra Activity Ratio

0 5 10 15 20 25 30
Time (yrs)

Figure 2.4. Radioactivity decay and growth in the system 228Ra-228Th. Upper:
Decay of parent isotope 228Ra and ingrowth and decay of daughter isotope
228Th as a function of time. For illustrative purposes an activity of 100 dpm

was assigned to 228Ra whereas the 228Th activity is zero at t = 0. The parent and
daughter reach transient equilibrium at about 4.3 yrs from t = 0. Lower:

Evolution through time of the 228Th/228Ra activity ratio approaching a

maximum of 1.5. The useful range of the 228Th/228Ra dating methodology is 15
yrs or younger.



31

Enrichment factors for the various chemical constituents are listed in
Table 3.1. Three types of pore fluids (Fig. 3.4) can be discerned on the basis of
molar Ca/Cl, S1/Cl, Ba/Cl and SO4/Cl ratios as indicated below.

Pore fluids pertaining to type I have the conservative elements (e.g., Na
and Cl) at concentrations similar to the overlying seawater (Ef = 1, Table 3.1)
but their Ca, Sr and SO4 concentrations are depleted relative to seawater (Ef < 1,
Table 3.1 and Figs. 3.4 A-C). These observations suggest that type I pore fluids
are essentially derived from a contemporaneous seawater whose non-
conservative elements, such as Ca, Sr and SO4, were partly removed by
processes occurring within the seep sediments. The Ba concentrations of these
pore fluids are consistently higher than bottom water (Ef >1, Table 3.1 and Fig.
3.4 D) and therefore are analogous to pore fluids in normal marine sediments

(Boulegue et al., 1990). The 8751/86 ratios (0.70916 +1x10°-5, n=3) are identical
to the overlying seawater ratio (0.70917) thus supporting a seawater origin. In

contrast, the 5!80 values that range from 0.3 to 1.3°/00 (SMOW) are generally

higher than the ambient deepwater at comparable depth (0.3°/00, SMOW, Table
3.1 and Fig. 3.5 A) and their inverse relation with Cl concentrations (Fig. 3.5 B)
is attributed to in situ dissociation of gas hydrates. Pore fluids of type I occur at
bathyal depths in hemipelagic sediments blanketed by clusters of
chemosynthetic mussels (Bathymodiolus spp.) and patchy Beggiatoa mats and
are often associated with gas hydrates. These fluids represent normal seawater
whose chemical composition was modified by bacterial processes and mineral
deposition.

Pore fluids type II are from brine-issuing sites on Green Knoll at 1920 m
depth (Fig. 1.1). These fluids are highly saline (salinity range from 116°/o0 to

182°/o0, Table 3.1) and show substantial enrichments in Na and Cl relative to
the ambient abyssal seawater. The Ca contents are higher than both seawater
and pore fluids type I but lower in comparison with pore fluids type III (Fig.
3.4 A). The Sr and Ba contents are similar to the pore fluids type I but are much
lower than those of pore fluids type III (Figs. 3.4 B & C). An outstanding feature
is the marked enrichment in S04 relative to seawater (Ef = 1.4 to 1.5, Table 3.1

and Fig. 3.4 C). The 87Sr/86Sr, ranging from 0.70797 to 0.70897 (n=3), are
significantly lower than the seawater (0.70917) whereas the 5180 values (-1.4

to 0.19/00 SMOW) are similar to the ambient bottom water (-0.2°/o00 SMOW,
Table 3.1 and Fig. 3.5 A).

Pore fluids Type II are clearly distinguished from the other two types of
pore fluids by their high salinity (45°/00 to 155°/00) coupled with

anomalously high enrichments in Ca, Sr and Ba (Ef=8.2, 37.2, and 3.4x104,
respectively, Table 3.1 and Figs. 3.4 A, B, D) and strong SOQ4 depletions (Ef=0.02,

Table 3.1, Fig. 3.4 C). The 3180 values (maximum value up to 1.79/00 SMOW), are

generally higher than the ambient seawater (Fig. 3.5 A) but their 87s1/86sy
ratios (0.70843 to 0.70862 for n=8) are below the seawater value (0.70917). Pore
fluids type III are unique to seeps associated with extensive barite deposits and
are derived from deep-seated formation waters (Fu et al., 1994; Fu and Aharon,
1997).



51

syuno> jauueys

° ° 2
T T a T T a T T wo T
> = |Z g g = g
5 5 5
2 ° E &
S  100T-ed. = “t, ‘
Ls nef te LE toordez f Jo Lb E TO0T-"d pez 42 +38
5 = r--} = =) = ia
FE 696-0Ve07
696-¥ er
Lepr Vee fT te L ig 78 +
an aL an
peo-td,
viz TI6-?V gz¢
TI69V,
L 8z 4 J 4 L
3 3 5
098-Eggz
a — 45 =
léeg- . 7 B
6£8-OV ez Mpyz 2 & Op8-V
8zz
SOL?V, 4 L
+ 8z 73 4
g 3 3 eeu Vet
89L" yz
4 —]
cy = " a & a & ag ow t

850 900 950 1000 1050

800

Energy (keV)

Figure 3.14. Gamma-ray spectra of uranium, thorium ore standards and

GOM barite chimneys and crusts.


17

to remove any Fe2O3 produced during the oxidation step. After this final step,
the purified barite is dried and ready for analysis.

Because of their friable nature, most chimneys were hardened at room
temperature using Araldite GY-506 water-insoluble resin to preserve their
morphologies. Continuous suites of subsamples were sectioned from the base to
the top of the hardened chimneys at intervals of 2 to 3 cm and each subsample,
herein called hardened chimney sample, was also subjected to nuclide
analysis.

In order to check the effects of sulfides on the age determination, sulfide
concentrations of the chimneys and crusts were determined. NagCO3 was used
to dissolve barite and then 2N HCI acid was used to dissolve the carbonates. The
residues are dominated by organic matter, sulfides and detrital minerals. The
amount of organic matter in the barites was estimated by combustion of the
residue and measuring the CO2 pressure. Since the SEM (scanning electron
microscopy) and microscope analyses reveal that the chimneys and crusts
contain only a negligible amount of detrital minerals (<1% by volume), the
sulfide concentrations were roughly estimated by subtracting the organic
matter from the residue after NazCO3 and HCI treatment.

2.2.4. Instrumentation and Analytical Methods

All the samples were non-destructively analyzed by using a high purity
intrinsic germanium gamma detector (HPGe) in order to quantify the activities
of the radionuclide occurring in the barites. The detector consists of a 98 cm?
coaxial intrinsic germanium crystal with a 1 cm in diameter and 4 cm long
well. The detector was coupled with a 8192 multiple channel analyzer (Ortec
Buffer card). The resolution is 2.1 keV at 1332 keV.

For raw and pure barite samples, about 1 gm subsample powders were
spread out evenly over a filter paper and made to conform as nearly as
possible to the original mixed gamma planar geometry. The filter paper was
placed on the detector head inside the shielding assembly. The hardened
chimney samples (about 3x5 cm) were put in a plastic bag and placed with the
bag directly on the spectrometer window. For all types of samples, gamma
spectra were allowed to collect for up to 300,000 seconds.

The gamma ray from the decay of 212Pp at 300.09 keV was used as a proxy
nuclide to calculate the activity of 228Th. To obtain the activity of 228Ra, the
peak at 911.21 keV from 228 Ac was used. Likewise, the 226Ra activity was
calculated by using the energy at 351.92 keV from the decay of 214pb. This is
because these daughters (212pp, 228 Ac and 214Pb) are in secular equilibrium
with their parents (228Th, 228Ra and 226Ra), therefore, AD (activity of
daughter) is equal to AP (activity of parent). 210Pp activity was directly
measured from its 46.52 keV gamma ray peak.

Gamma spectra collected from the samples were subjected to an analysis
scheme which allowed quantification of 214Pb (proxy for 226Ra), 210pp, 228A ¢
(proxy for 228Ra), and 212ppH (proxy for 228Th). GammaVision© Software by
Ortec was utilized for peak search, identification, and quantification of the
radionuclides. Nuclide libraries were created and optimized for accurate
quantification of the isotopes by minimization of the possibility of interfering
peaks. Since the 226Ra series is pervasive throughout nature, a 300,000 sec
background (bare detector) count was stripped from each spectrum prior to



139

Paytan, A., W.S. Moore, and M. Kastner (1996), Sedimentation rate as

determined by 226Ra activity in marine barite_Geochim. Cosmochim. Acta
60(22): 4313-4319.

Peter, JM. and $. Scott (1988) Mineralogy, composition, and fluid-inclusion
microthermometry of seafloor hydrothermal deposits in the southern
trough of Guaymas basin, Gulf of California: Can. Mineral. 26: 567-587.

Presley, B.J. (1969) Chemistry of interstitial water from marine sediments.
Unpublished Ph.D dissertation, University of California at Los Angeles, 225

PP.

Prichard, H.M., Gesell, T.F. and Mewyer, C.R. (1980) Liquid scintillation analysis
for radium-226 and radon-222 in potable water: In Liquid Scintillation
Counting-Recent Applications and Development (eds. Peng, C.T., Horrocks,
D.L. and Alpen, E.L.), Academic Press, N.Y., v. 1, p. 347-355.

Raabe, O.G. (1994) Dose reconstruction from bioassay measurements of long-
lived residual activity Jn Internal Radiation Dosimetry (ed. Raabe, O.G.),
Medical Physics Publishing, Madison, W.I. p. 495-515.

Rabalais, N.N., McKee, B.A., Reed, D.J. and J.C. Means (1991) Fate and effects of
nearshore discharges of OCS produced waters. Volume I. Executive
Summary. OCS Study/MMS 91-0004. U.S. Dept. of the Interior, Minerals
Management Service, Gulf of Mexico OCS Regional Office, New Orleans,
Louisiana, 48 pp.

Rabalais, N.N., B.A. McKee, D.J. Reed, and J.C. Means (1991) Fate and effects of
nearshore discharges of OCS produced waters: Volume III. Appendices. OCS
study/MMS 91-0006. U.S. Dept. of the Interior, Minerals Management
Service, Gulf of Mexico OCS Regional Office, New Orleans, Louisiana, 225 p.

Rafter, T.A. and Y. Mizutani (1967) Oxygen isotopic composition of sulfate, Part
2. Preliminary results on oxygen isotopic variation in sulfates and the
relationship to their environment and to their 834 values: New Zealand J.
Science 10: 816-840.

Raloff, J. (1991) NORM: The new hot waste. Science News 140: 264-267.

Reid, D.F. (1979) The near-surface distribution of radium in the Gulf of Mexico
and Caribbean Sea: Temporal and spatial variability and hydrographic
relationship: Ph.D dissertation. Texas A & M University, 245 p.

Reid, D.F., W.S. Moore and W.M. Sackett (1979) Temporal variation of 228Ra in
the near-surface Gulf of Mexico: Earth Planet. Science Lett.

43: 227-236.

Reid, D.F. (1984) Radium variability produced by shelf-water transport and

mixing in the western Gulf of Mexico: Deep-Sea_Res. 31(12): 1501-1510.

Reyes, A. O. (1993) 228Th/228Ra ages of hydrothermal chimney samples from
the Endeavour Segment of the Juan de Fuca Ridge. Unpublished Master
Thesis, University of South Carolina, 118 pp.

Reyes, A.O., W.S. Moore, and D.S. Stakes (1995) 228Th/228Ra ages of a barite-
rich chimney from the Endeavour Segment of the Juan de Fuca Ridge:
Earth Planetary Science Letters 131: 99-113.

Ritger, S$., B. Carson, and E. Suess (1987) Methane-derived authigenic
carbonates formed by subduction-induced authigenic carbonates along the
Oregon/Washington margin: Geol. Soc. Am. Bull. 98: 147-156.

Ritcey, G. M. (1990) Weathering processes in uranium tailings and the
migration of contaminants. Jn The Environmental Behaviour of Radium.
IAEA Technical Report Series No. 310 (2): 27-82.



84

4000 T T T
3000
y= - 375.33 + 1.07%

5 2000+
3
Zz

1000; 4

0 1 1 1
0 1000 2000 3000 4000

Cl G@nM/L)

Figure 4.1. Na-Cl relationship in pore fluids type II. The almost 1:1 slope
indicates that Cl and Na are derived primarily from dissolution of halite in the
salt domes.



38

varying from 1 wm to 10 um (Fig. 3.9 A); (ii) euhedral cubic grains; (iii)
anhedral to subhedral grains of up to 20 um across, and (iv) framboids varying
in diameter up to 15 um and containing microcrystallites of about 1 um in
diameter (Fig. 3.9 B). Some of the framboids exhibit coccolith structures
suggesting that, in addition to barite, coccolith tests may have also been
replaced by pyrite. Very small (generally < 1% in volume) amounts of
accessory minerals such as Mg-calcite, quartz, feldspar, rutile and ilmenite
have also been identified by the SEM analysis in the barites. These minerals
often occur in the mud matrix as anhedral grains ranging in size from 0.5 to
10 um.

The crusts are porous, friable and loosely cemented. Most crusts display two
layers with the upper part exhibiting a orange to white color. This surface was
partly covered by sediments (fine sand) before it was retrieved from the
seafloor. In contrast, the lower part shows deep-gray to black and was buried
in the black-colored anoxic sediments before sampling.

Microscopic and SEM analyses reveal that the crusts are dominated by
barite and calcite with minor amounts of pyrite and kerogen. The calcite, up to
75% by weight, exhibits subhedral to euhedral crystals. Generally, the lower
deep-gray parts of the crusts contain more calcite than the orange-colored
upper parts. Barites in the crusts exhibit fine-grained massive morphology,
euhedral crystals, or individual rosette barite. String-like and dendritic-like
barites which are common in the chimneys are rarely found in the crusts. The
barite and calcite in the crust are often interlocked and exhibit a mosaic
texture, which indicates that calcite and barite precipitated simultaneously.
Pyrite, up to 8% by volume, often occurs in association with calcite as ovoids,
anhedral to euhedral cubic grains and framboids (Fig. 3.9).

3.2.2. Elemental Chemistry

Representative electron microprobe analyses of Ba, Sr, Ca, Na, and Fe in the
barite chimneys and crusts are listed in Table 3.4. The Sr concentration ranges
from 4.0 to 29.6 mol% (mean of 15.5 mol% + 6.5 for n= 10) and 3.9 to 17.6 mol%
(mean of 10.7 mol% + 5.1 for n= 10) for chimneys and crusts, respectively,
making Sr the most abundant minor element. The observed relative high
abundance of Sr replacing Ba in the barite lattice can be attributed to their
nearly matching ionic radii (1.18 A for Sr2+ and 1.34 A for Ba2+). The Ca
content ranges from 1.3 to 4.6 mol% (mean of 2.8 mol% + 1.3 for n= 10) for the
chimneys and from 1.2 to 5.3 mol% (mean of 3.0 mol% + 1.2 for n= 10) for the
crusts. The positive correlation observed between Ca and Sr contents (Fig. 3.10
A) and the inverse linear relationship observed between Sr and Ba (Fig. 3.10 B)
in the barites support the inference that Sr and Ca are isomorphically
replacing Ba in the lattice and do not form separate mineral phases.

The Na content in the barite is also relatively high, ranging from 1.1 to 3.2
mol% (mean of 1.9 mol% + 0.6 for n= 10, Table 3.4) for the chimneys and from
1.1 to 6.9 mol% (mean of 3.65 mol% + 2.4 for n= 10, Table 3.4) for the crusts.
However, it is uncertain whether Na replaces Ba in the barite lattice or forms a
separate authigenic mineral phase. In contrast to the other chemical elements
reported here, the Fe content in the barites is relatively low, ranging from 0 to
0.7 mol% (mean of 0.3 mol% + 0.22 for n=10, Table 3.4) for the chimneys and 0
to 0.3 mol% (mean of 0.2 mol% + 0.1 for n=10, Table 3.4) for the crusts. High Fe
contents, of up to 22 mol%, were generally found in the center of rosette cores
in the chimneys. However, the poor stoichiometry of the analyses with higher
Fe contents suggest that the anomalously high Fe may have been caused by the



719

4
rt si
L td [o}
8 ap (end-member fluid)
. fe} a
~ 1
£
e
= 20 oe
a Ko utc (eng-member fluia)
2
s A
1

7

OxebDeocmo
SARS BARES

.
a

0 1 2 3
Ra-226 (ppt)

Figure 3.29.- The Relation of Barium and ?°Ra in Hydrocarbon Seep Fauna. End
member pore fluid values are similar to barite exoskeleton in Ra and Barium
content. Radium-226 values are given in ppt (parts per trillion) for order of
magnitude comparisons with barium concentrations. Dpm/g Ra values may be
obtained by multiplying parts per trillion by a factor of 2.22 dpm/ppt.

BBC- chemotrophic soft tissues from barite seeps

BBH- heterotrophic soft tissues from barite seeps

BCE- chemotrophic exoskeletons from barite seeps
BHE- heterotrophic exoskeletons from barite seeps
CCB- chemosynthetic soft tissues from carbonate seeps
CHB.- heterotrophic soft tissues from carbonate seeps
CCE- chemosynthetic exoskeletons from carbonate seeps
CHE- heterotrophic exoskeletons from carbonate seeps
EMF-end-member pore-fluid Ba and Ra concentrations



56

Sample Age Initial
name (yrs) 228Ra/226Ra

Ch#8-7 | 0.42+0.02 1.1640.01

Ch#8-6 0.66+0.01 1.16£0.01

Ch#8-5 0.7740.01 1.17+0.01

Ch#8-4 0.91+0.01 1.09£0.01

Ch#8-3 1.32+0.01 0.96£0.01

Ch#8-2 1.6940.01 0.95+0.02

Ch#8-1 2.1140.02 1.00£0.01

Figure 3.18. Dating profile of growth increments in barite chimney# 8 from
Garden Banks block 382 using ??8Th/228Ra method. Ages were corrected to July
20, 1993 when the chimney was sampled.



80

concentrations of the fauna range from 0.01 ppt (parts per trillion) in a
mussel shell from a carbonate seep to as high as 4.8 ppt in a galatheid crab

from a barite seep. Parts per trillion 226R, is used in this case to demonstrate
orders of magnitude differences between barium and radium. Parts per trillion

2226p, may be converted to dpm/g by multiplying by a factor of 2.22.

Figure 3.29 suggests a log-linear relationship between Ba and 226R, in
fauna from barite seeps. In this region of the figure, Ba increases

logarithmically whereas the 226Ra increases linearly. These results seem to be

consistent with the pore fluid end members 226RaBa values from Mississippi
Canyon and Garden Banks barite seeps which are also shown.



Table 4.3. Fluxes of radium-rich fluids calculated from barite chimneys.

115

Chimney Topradius Bottomradius Growthrate Density Ra, RaFlux Fluid flux
name (cm) (cm) (cm/yr) (gm/cm*) (dpm/gm)__(dpm/yr) (Liyr)
|Garden Banks block 382
Chit6 2.0 4.0 93 2.6 2000 1.4x10° 270
Ch#8 2.0 35 89 2.6 2132 1.2x10° 230
Mississippi Canyon block 929
Ch#l1 25 7.0 44 2.6 3987 3.5x10° 2920

Note: Ra, is the mean total initial radium activity in dpm/gm scavenged by the barite (see text).



118

226Ra and 228Ra activities of the saline fluids advecting on Garden Banks
and Mississippi Canyon are highly anomalous as they exceed by a factor of up

to 2.8x104 and 5.5x109, respectively, the values reported from the Gulf of
Mexico ambient deep water (Table 3.3). This observation suggests that Ra in the
fluids venting on the sea floor may diffuse freely into the water column and
may contribute to the reported radium enrichment in the Gulf of Mexico
relative to the inflowing Caribbean Sea waters (Reid, 1979, 1984: Reid et al.,
1979; Key, 1981).

The studies conducted by Reid (1979) and Key (1981) have indicated that: (i)

the top 100 m of central Gulf of Mexico waters have a mean specific 226Ra
activity higher by up to 0.11 dpm/L relative to the Caribbean Sea, (ii) the

average near-bottom 226Ra for the Gulf of Mexico (0.132 40.002 dpm/L) is
statistically indistinguishable from the inflowing Caribbean Sea waters (0.123
+0.008 dpm/L), but the detailed analyses hint that Gulf of Mexico samples
might be slightly higher than that of Caribbean, and (iii) the increase of

226Ra activity with depth is greater in the Gulf of Mexico than in the

Caribbean Sea. The excess 226Ra in the Gulf of Mexico was attributed by these
authors (op. cit.) to either deep (about 600 m) injection of Ra produced after
the water influx from the Caribbean Sea, or to the relatively long residence
time of deep water in the Gulf of Mexico allowing for accumulation of Ra
diffusing upward from the sediment. In a subsequent time-series study, Reid
(1979, 1984) and Reid et al. (1979) also found a significant increase of both
226Ra and 228Ra activities from 1973 to 1976 in the upper 100 m of the Gulf of
Mexico (i.e., from 0.072 +0.007 to 0.091 +0.007 dpm/L for 226Ra and from 0.051
+0.006 to 0.062 +0.003 dpm/L for 228Ra), that was attributed to a Ra flux from
sediments on the northern Gulf of Mexico shelf.

The impact of these Ra-rich fluids leakage from seeps into the overlying
water column of the Gulf of Mexico is not presently known with certainty
because of the absence of Ra and daughter isotopes determinations in the
water column above seeps. However, gullies filled with highly radioactive
coarse barite sediment and occurrence of Ra-rich barite chimneys over
extensive areas of the slope suggest that the phenomena might be widespread
on the sea floor (Fig. 1.2). Therefore, previously unknown leakage of Ra-rich
fluids on the sea floor may be one of the major factors controlling the Ra (and
Ba) budget in both bottom and surface waters of the Gulf of Mexico. Fluid
venting is likely to be periodic and with variable intensity because fault-
related seeps are not continually active (Roberts et al., 1990). Hence leakage

periodicity may have contributed to the temporal variability of 226Ra and

228Ra in the near-surface Gulf of Mexico observed by Reid (1979, 1984) and
Reid et al. (1979). The knowledge of the impact of radioactive fluids leakage on
benthic marine habitats is presently in its infancy. But the observation of
extensive dead fields of methanotrophic mussels surrounding the seeps in GB-
382 and GB-338 and the presence of mussels engulfed in highly radioactive
barite sands (Fig. 1.2 D) suggest that harmful effects of excessive radioactivity
on the benthic fauna are likely (see below). Further investigations along the
line of this study should be important to the overall assessment of the Ra (and
Ba) budget in the Gulf of Mexico in general, and for evaluation of the
environmental impact of natural radium discharges on the offshore habitats
in particular.

It needs to be stated that recent studies by Moore (1996, 1997) indicate that

submarine groundwater discharge may be responsible for the 226Ra



Table 3.2. Description of push cores taken in barite deposition sites whose pore waters have been analyzed in this study.

Push Core MMS Lat./Long. Water Core Description
No. Block Depth | Length
(m) (cm)

93-3559-2 MC-929 28°O1.471/ 640 28 Flank of a mud volcano draped with orange-stained barite crusts. Silty sediment
89°43.651 changing downcore color from orange (2 cm), to black (4 cm) to grey (24 cm).

93-3562-1 MC-929 28°01.509/ 657 13,5 Base of dormant mud volcano draped with clusters of methanotrophic mussels.
89°43.572

93-3566-1 GB-382 2P37.ITY 513 30 Black, anoxic, silty sediment at the base of twin barite chimneys.
92°28.076

93-3566-2 GB-382 27°37.740 S13 8 White sand to silt-size barite (probably transported) infilling a small depression,
92°28.108

97-2897-1 GB-338 27°37,794¢ 510 35 Shoulder of an active mud volcano draped with orange-stained barite crusts. Silty
92°28.123 sediment with downcore change of color from black (6 cm) to grey mud (12 cm) to

black mud with vertical stripes (18 cm).

97-2897-3 GB-338 27°37.794/ 510 15.5 Flat black surface next to an incipient mud volcano. Upper part is missing (removed
92°28.123 during coring). Downcore change from black silt (3 cm) to grey mud (12.5 cm).

97-2897-4 GB-338 27°37.794 510 36 Chimney site after sampling the chimney. Uniform grey mud. Profuse gas release
92°28.123 during punching of the core.

St



xiii

ACKNOWLEDGMENTS

We thank C. W. Wheeler for assistance with analyses, drafting and document layout, Mary
Lee Eggart for skillful drafting of the cover and R. Carney for a critical reading of an early draft
of the report.

Samples analyzed in this project were acquired during submersible dives in 1990, 1993, 1995
and 1997. These submersible activities were funded by grants from National Oceanic and
Atmospheric Administration/National Undersea Research Program. We thank the pilots of the
submersibles Johnson-Sea-Link and Alvin, and the captains and crews of the surface support
vessels R/V Edwin Link, R/V Seward Johnson and R/V Atlantis Il for assistance with diving and
collection of samples.

We thank our colleagues H. H. Roberts, P. LaRock, R. Carney, B. K. Sen Gupta and J.
Larkin at Louisiana State University for sharing with us the excitement of deep-sea exploration
during cruises and dives, and Mary Boatman and Gail Rainey from the Mineral Management
Service in New Orleans for constructive reviews of an early draft of the report.



134

Carney, R.S. (1994) Consideration of the oasis analogy for chemosynthetic
communities at Gulf of Mexico hydrocarbon vents: Geo-Marine Lett.
14: 149-159.

Carothers, W.W., Cocker, J.D., Law, L.M. and Kharaka, Y.K. (1986) Application of
stable isotopes to the origin and migration of oil-field waters in Pleistocene
reservoir rocks, offshore Texas. In: Selected Papers in the Hydrologic
Sciences (ed. Subitzky, S.), US Geol. Survey Water Supply Paper 2310: 117-
127.

Carpenter, A.B. (1978) Origin and chemical evolution of brines in sedimentary
basins, in K.S. Johnson and J. Russell, eds., 13th Annual Forum of Geology
of Industrial Minerals: Oklahoma Geological Survey Circular 79: 60-77.

Cech, IL, M. Lemma, C.W. Kreitler and H.M. Prichard (1988) Radium and radon in
water supplies from the Texas Gulf Coastal aquifer: Water Res. 22(1): 109-
121,

Cecile, M.P., and L.D. Jones (1979) Note on a radioactive barite sinter, Bonnet
Plume map-area, District of Mackenie: In Current Research, part B,
Geological Survey of Canada, paper 79-1B, p. 416.

Cecile, M.P., M.A. Shakup, and H.R. Krouse (1983) The isotopic composition of

western Canadian barites and the possible derivation of oceanic sulfate 3345

and 31809 age curves: Can. J. Earth Sciences 20: 1528-1535.

Cecile, M.P., W.D. Goodfellow, L.D. Jones, H.R. Krouse and M.A. Shakur (1984)
Origin of radioactive barite sinter, Flybye springs, Northwest Territories,
Canada: Can. J. Earth Sciences 21: 383-395.

Cember, H. (1996) Introduction to Health Physics. 3rd. ed. McGraw-Hill New
York, NY, pp.733.

Chilton, A,B, Shultis, J.K., and RE. Faw (1984) Principles of Radiation Shielding.
Prentice-Hill, Inc., Englewood Cliffs, NJ, pp. 175-177.

Chung, Y. (1980) Radium-barium-silica correlations and a two-dimentional
radium model for the world ocean. Earth Planet. Science Lett. 49: 309-318.

Church, T.M. (1970) Marine barite. Unpublished PhD Dissertation: University
of California, San Diego, 100 p.

Church, T. M. and K. Wolgemuth (1972) Marine barite saturation. Earth Planet.
Science Lett. 15: 35-44.

Church, T.M. (1979) Marine barite. In: Marine Minerals, (ed. Burns, R.G.),
Mineral. Soc. Am. Short Course Notes, Washington DC, v. 6, ch. 7, p. 175-210

Clark, J.R. and Patrick, J.M. (1987) Toxicity of sediment-incorporated drilling
fluids. Marine Pollution Bull. 18 (11): 600-603.

Claypool, G.E., W.T. Holser, I.R. Kaplan, H. Sakai, and I Zak (1980) The age curves
of sulfur and oxygen isotopes in marine sulfate and their mutual
interpretation: Chem. Geol. 28: 199-260.

Cline, J.D. (1969) Spectrophotometric determination of hydrogen sulfide in
natural waters. Limnol. Oceanogr. 14: 454-458.

Cochran, J.K. and S. Krishnaswami (1977), Ra-226 and Ra-228 in sediment pore
water: EOS, Am. Geophys. Union Trans. 58(12): 1153.

Cochran, J.K., (1979) The geochemistry of 226Ra and 228Ra in marine deposits:
Unpublished Ph.D dissertation, Yale University, 260 p.

Cochran, J.K. (1982) The oceanic chemistry of the U and Th series nuclides. In
M. Ivanovich and R.S. Harmon (eds), Uranium Series Disequilibrium:
Application to environmental problems: Claredon Press, Oxford, p. 384-430.

Cook, L.M (1980) The uranium district of the Texas Gulf Coastal Plain: In Natural
Radiation Environment III (ed T.F. Gessel and W.M. Lowder), . U.S.
Department of Energy, Washington, D.C, pp. 1602-1922



The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for
most of our nationally owned public lands and natural resources. This includes fostering sound use of
our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the
environmental and cultural values of our national parks and historical places; and providing for the
enjoyment of life through outdoor recreation. The Department assesses our energy and mineral
resources and works to ensure that their development is in the best interests of all our people by
encouraging stewardship and citizen participation in their care. The Department also has a major
responsibility for American Indian reservation communities and for people who live in island territories
under U.S. administration.

The Minerals Management Service Mission ,

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary
responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf
(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute
those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program
administers the OCS competitive leasing program and oversees the safe and environmentally sound
exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The
MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and
accurate collection and disbursement of revenue from mineral leasing and production due to Indian
tribes and allottees, States and the U.S. Treasury.

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being
responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected
parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for
all Americans by lending MMS assistance and expertise to economic development and environmental
protection.



40

6 T T T
Sr A . @)
Z
e 7 ayn" 1
= q ene 4
= 3 Ls .
Roy = =
s ar We . 1
o t = 4 K = chimney/ |
A crusts
1
30 40
|
g
s 4
= = chimney
3 A crusts
-
60 1 ! 1
0 10 20 30 40

Sr (mol %)

Figure 3.10. Microprobe analyses from barite chimneys and crusts. A: Positive
linear relation between Sr and Ca compositions. B: Inverse linear relation
between Ba and Sr compositions, suggesting that Sr replaces Ba in the barite
lattice.



Table 3.1.
Table 3.2.
Table 3.3.
Table 3.4.
Table 3.5.
Table 3.6.
Table 3.7.

Table 3.8.
Table 3.9.

Table 3.10.

Table 3.11.

Table 3.12.
Table 3.13.

Table 4.1.

Table 4.2.
Table 4.3.
Table 4.4

Table 4.5.

Table 4.6.

Table 4.7.

LIST OF TABLES
PAGE
Chemical and isotope compositions of pore fluids from seeps and
ambient Gulf of Mexico deepwaters ... 24

Description of push cores taken in barite deposition sites Whose | pore ‘waters have

been analyzed in this study. sees 25
Salinity, barium concentration and Ra isotope activities in

pore fluids from barite-bearing seeps... - cesseceeseeseseeseneeaeencereneeneene 20)
Elemental chemistry of barite chimneys and crusts from

the Gulf of Mexico slope... Al

S, O and Sr isotope compositions ‘of the barites and Cc and: Oo.

isotope compositions of the coexisting carbonates .
Activities and ages of barite chimneys.............
Ages of hardened chimneys ....
Activities and ages of barite crust
Results of repeat dating of chimneys ..
Sulfide concentrations of barites and their effect on
210pp/226Ra dating...
Radium-226 activity concentration in chemosynthetic
(methanotrophic) Bathymodiolus spp. mussels from seep:
Radium-226 content in heterotrophic fauna from seeps ....
Relationship between barium and radium in selected
hydrocarbon-seep fauna .. .
Chemical compositions of ‘the two possible saline end- member fluids of pore fluids
type Il...
26Ra/Ba ratios in barites and fluids ..
Fluxes of radium-rich fluids calculated : from barite chimneys
Annual discharge estimates based on data from barite-bearing seeps in the Garden

Banks and Mississippi Canyon

produced in this study. .. seeeeee 117
Radium-226 activities emg) in marine molluscans |

and crustaceans... - 122
226Ra and 28am mean activities in barite deposits with

2-c random counting error... .-- 128

External dose rates to seep fauna calculated ‘from

Microshield 5.0 .. 128



34

3.2 Barite Deposits Associated with Seeps
3.2.1. Morphology and Petrography

Barite chimneys and crusts were collected with the submersible Johnson-
Sea-Link in Mississippi Canyon (MC-929) and in Garden Banks (GB-382; GB-
338)(Fig. 1.1). The chimneys are 5 to 8 cm in diameter and generally exhibit
porous and friable upright columnar structures rising 10 to 30 cm above the
seabed (Figs. 1.2 C and 3.6 A). Most chimneys have central orifices up to several
centimeters in diameter through which gas and fluid brines escape into the
water column. Some chimneys have one to several small nodules which
protrude laterally. Juvenile Bathymodiolus sp. methanotrophic mussels were
found attached to the base and side of some chimneys. When sliced, the
interiors of the chimneys exhibit growth layers which alternate between
dark-gray and light (Fig. 3.6 B). The orange-white barite crusts of 2-10 cm
thick are typically draping dormant mud volcanoes containing a brine-filled
caldera (Fig. 1.2 A) or occur as thin pavements covering the seafloor.

Microscopic and SEM analyses reveal that the chimneys consist principally
of high reflectance barite and contain minor amounts of pyrite-like iron
sulfide (hereafter named pyrite), Mg-calcite, kerogen, and iron oxides.
Detritalminerals such as quartz, feldspar, rutile and ilmenite are also present
in trace amounts in the fine mud matrix.

The chimneys display two distinct morphologies that are rarely intermixed:
(i) string-like (Fig. 3.7 A), and (ii) dendritic-like (Fig. 3.7 B). The string-like
barite, up to 1.2 mm in length, forms, in association with pyrite, the gray-dark
(growth) layers whereas dendritic-like barite, ranging in length from 0.2 to 5
mm, forms the light-yellow (growth) layers which contain little or no pyrite.
Growth directions of both morphological types are often oriented
perpendicular to the growth layers.

Interlocking, spherical to ellipsoidal-shaped rosette barite constitutes the
basic building block of both morphological types. These rosette barites,
ranging in size from 20 to 40 um in diameter, are composed of euhedral to
subhedral tabular crystals which radiate from a central core (Fig. 3.8 A). The
core, occupying a space equal to between 1/4 to 1/2 of the rosette diameter, is
brown to yellow in transmitted light suggesting the presence of iron oxides
and/or organic matter in trace amounts. In cross polarized light, most rosette
barites display pseudo-uniaxial extinction crosses.

SEM examination of polished thin sections reveal that the cores of rosette
barites exhibit a high degree of structural variability. Most rosette barites
Possess porous cores which are possibly of a biogenic origin. When these core
structures are well-preserved they have been identified as the relics of
coccolith tests replaced by barites. Less commonly the cores are either empty
or are filled by cryptocrystalline barite, Mg-calcite, quartz, and feldspar.

A three-dimensional view of the rosette-barite morphology has been
obtained from examination of chimney fragments under SEM (Fig. 3.8 B). In
this view, the individual rosettes display spherical shapes that show a single
axial symmetry and multiple tabular crystals oriented around the central axis.
The rosettes are often in contact with each other and coalesce to form string-
like or dendritic-like barites.

Although the overall concentration of pyrite is low (generally 1 to 2% by
volume), it is the next most abundant mineral in the chimneys after the barite.
Pyrite grains of various sizes and shapes often occur in association with
string-like barite or in the mud matrix. It can occur as (i) ovoids of sizes



114

r

Figure 4.15. Dominant morphological truncated conical-shaped barite
chimneys used in the modeling of radium fluxes (see text).



13

238U Series 232Th Series
TOU STU
4.5x109 2.48x10:
ye yt
Wapy
1.18
min
Bay 20Th 232TH 23TH
24.1 1752x104 14x10 14 1.91
days ye ye a Bi
228 Ac
{ | 6.13
qty
ORa yy Ra’ 224R_
1622 5.75 3.64
yr yr day
| y
22Rn 220Rn
3.82 54.5
days sec
| Ad
218Po 214Po 210Po9 216P9 212 Po
3.05 yon 138.4 0.158 3.0x107]
min sec lays: sec sec
214; Tope 212p;
19.7 5.01 60.5
fin days eonin
214 pp’ ae 206pp 212pp 208 ph
26.8 3 stable 10.6 stable
min yr br
DOT}
3.1
min

Figure 2.3. 226Ra, 228Ra and their parent and daughter radionuclides; the half-
lives are listed below the radionuclides (adapted from Broecker and Peng,

1982).



PORE-FLUIDS

a

Isotope analyses

aoYTM

NN

Chemical analyses

a

Radioactive] | Radiogenic| | Stable [Compounds | | Elements |
isotopes isotopes isotopes
226R,228Ra | | 87sr86sr] | 8180420) HyS5 SOq a aac

Figure 2.1. Flow chart of chemical and isotope analyses of pore fluids.



viii

A, Discussion «0.2.2... secseesceeceneceseeceneceneccnsessneceseceneeesaceaeeneeeneesnsennsnanenssensneesseesreeseesseeesreeeneeseeeeeess OL

4.1. Sources of Fluids Advecting at Seeps
4.1.1. Seawater-Derived Fluids...
4.1.2. Fluids Derived by Dissolution of Salt Diapirs........
4.1.3. Fluids Derived by Advection of Deep-Seated Formation Waters

4.2. Radium and Barium in Barite-Bearing Seep:
4.2.1. Source/s of Ra and Ba in the Saline Flui
4.2.2 The Residence Time of Fluids in Barite-Bearing Seeps.

4.3. Ages and Growth Rates of Barite Deposits .
4.3.1. Growth Rates of Chimneys...
4.3.2 Initial 228Ra/226Ra in the Barites ...........
4.3.3. 226Ra Activity in the Barites and Its Source/s
4.3.4. The Source of Sulfate in the Barites .
4.3.5. The Source of Major (Ba) and Trace Elements (Sr and Ca) in in the: Barites 109
4.3.6. Depositional Model of the Barites in the Gulf of Mexico... aesseee

4.4. The Impact of Ra- and Ba-Rich Venting Fluids on the Benthic
Habitat........
4.4.1. Fluxes of Radium/Barium Rich Fluids from Point Sources .

1

116

4.4.2. Leakage of Radium/Barium Rich Fluids in the Benthic Habitat....................
4.4.3. The Impact of of Radium/Barium Rich Fluid id Venting ot on the Benthic
Fauna .. we sossteesecneeneeeeee LIQ
4.5 Radiation Consequences of Radium Leakage from Seeps on the Benthic Fauna 121

121
124
126

4.5.1. Radium Metabolism and Radiation ...
4.5.2. Chemical and Radiation Flow of Radium | Into Benthic Fauna.
4.5.3. Radiation Risk Assessment.......

5. Conclusions and Recommendations ............e.-se-cseesesceeessessnsensessueenstenneeeoeesenseseteeateeenteeeeeeeeee LL

6. References .........ssecceeccceeccsecsneeeseseeseeensesnessaecsnvesseessnecscesonesseesseesetseansesessusenecanesanessneesnetsaneenens LOD



126

sand particles. The extent to which barite dissolves in the digestive tracts of
these animals is not certain. However, in vitro barite remains resistant to acid
or akaline conditions and is sparingly soluble.

Figure 4.17 B is a conceptual model of ionizing radiation energy flow to
seep fauna. Radium coprecipitated in the barite radioactively decays to its
respective daughter products by emissions of alpha and beta particles. The

226R9 decay chain of short lived daughters produces the majority of its gamma

ray emissions up through the radionuclide 210py. The 228Ra decay series emits

a nearly equal amount of gamma rays as 226Ra decay chain if activities are
equivalent (dpm/dpm). The external gamma rays easily penetrate through the
shells of the mussels and the heterotrophs in near proximity to the barite
chimneys and barite sand infilling the gulleys (Fig. 1.2). The gamma rays
account for the majority of the dose received by the mussels and heterotrophs.
Radiation consequences are primarily from externally emitted penetrating
gamma rays from the radium incorporated in barite deposits. The most likely
consequences include impaired function of affected cells, possible cellular
death, and stochastic genetic damage. Impaired cell function may or may not
lead to cell death. If enough cells are affected, a threshold is reached where
effects are manifested at the whole organism level with consequences ranging
from temporary mild organismal impairment to death of the affected
individual. These types of effects are termed non-stochastic effects and are
proportional in severity to the radiation dose received (Cember, 1996). This
effect parallels morbid effects from other types of noxious agents such as
poisonous chemical exposure. For example, a small amount of cyanide will not
have any significant effects until a certain cellular threshold level is reached.

Genetic damage (mutations) at the DNA level may also result in impaired
cell function and cell death as well as eventual organismal death. These type of
effects are termed stochastic effects and are independent of the radiation dose
received (Cember, 1996). However, the probability of an effect increases with
radiation dose and may manifest itself in one organism but not in another,
even though both received identical radiation doses.

Genetic damage to germinal cells has been shown to propagate into future
generations as mutations, usually being of a deleterious nature. However, most
expressive mutations are lethal if they are dominant, and are therefore
usually not passed on to successive generations as dominant alleles (Cember,
1996).

4.5.3. Radiation Risk Assessment

External dose rates to sessile epifauna (Bathymodiolus spp. mussels) located
in radium-rich barite seeps were calculated using the computer program
MICROSHIELD Version 5.0. The MICROSHIELD program uses the “point kernel”
technique to estimate dose rates from external gamma rays generated by
radioactive geometric volumes. Radioactive sources were specified as the

measured radioactivities of 226R,, 228Ra and their radioactive decay products
in conical geometric volumes approximating the actual shape and dimensions
of barite chimneys found in the barite seep sites (Figure 1.2).

The point kernel technique of dose estimation is not trivial. This method of
dose estimation involves dividing up volumetric sources (in this case, a
truncated cone) into differentially small sources which can be treated
independently as point sources. The resulting differential doses to the target
(mussel) become an integrand or “kernel” inside an integral, which then



104

GOM GOM
4000 MC-Ch MC-Cr

3000
2
a
=
& 2000 _ GoM
¥ GB-Cr
a
1000

Figure 4.12. 226Ra activities in barites formed in different depositional
environments.

Gulf of Mexico
GOM GB-Cr: Barite crusts from Garden Banks.

GOM GB-Ch: Barite chimneys from Garden Banks.
GOM MC-Cr: Barite crusts from Mississippi Canyon.
GOM MC-Ch: Barite chimneys from Mississippi Canyon.
M-B: Marine barites (Borole and Somayajulu, 1977; Paytan et al., 1996).
HY-B: Barite-rich hydrothermal deposits (Grasty et al., 1988; Moore and Stakes,
1990; Reyes, 1993; Reyes et al., 1995). HY-S: Sulfide-rich hydrothermal
deposits (Finkel et al., 1980; Lalou and Brichet, 1982; Kadko et al., 1985:

Kim and McMurtry, 1991).



63

insignificant loss of radon decay products in the barites. Therefore, the barites
can be considered a closed system for all practical purposes.

In summary, all three assumptions for both 228Tn/228Ra and 210pp/226R a
dating methods are satisfied and the barite ages reported here are considered
reliable.

3.2.6. 210pp/226Ra Ages

The 210pp/226Ra method has been applied here to derive the ages of barite
chimney fragments and barite crusts (Tables 3.6 and 3.8). The 210pp/226Rq
ages of purified barites, plotted in Figure 3.21, show that the ages of chimneys
range from 0 (some chimneys yielding negative ages) to 2.5 years, whereas
the age of the crusts range from 9.0 to 22.5 years old. Because the half life of
210ppH is 22.3 years, serious questions arise as to the accuracy of this technique
when applied to young barite chimneys (<5 yrs). As shown in Figure 3.21,
large analytical errors (up to 77%) in age determinations occur when
calculating the chimney ages. In addition, some chimneys yield negative ages
( e.g. -0.13 yr for Ch# 10-B/a) which are impossible. All these features are
attributed to the low activity of 210pp in the chimneys due to their young ages
which makes it harder for the instrument to detect the nuclide and hence
causes serious statistical errors. Additionally, the gamma peak at 46.52 keV
measured for 210Pp is significantly attenuated because of self absorption at
low energy. Therefore, the 210pb/226Ra pair is not an appropriate technique
for quantitatively dating young chimneys. These disadvantages, however, do
not preclude the method's qualitative value in estimating the approximate age
of chimneys; all the chimneys that have low 210pp/226Ra are also very young
which is confirmed by the 228Th/228Ra dating method (Tables 3.6 and 3.8).

Whereas the 210pp/226Ra method is inaccurate for determining the age of
young chimneys, the following observations suggest that the 2!0pp/226Rq
ages of purified crust barites are reliable. First, all the crust samples exhibit
strong 210pp peaks due to their relative older ages (measured ages >15 yr)
which greatly reduce the analytical errors (<10%). Second, most barite crusts
yield 210pp/226Ra ages older than 15 years and this is consistent with the
observation that most of the crusts are lacking 228Th and 228Ra (Table 3.8).

One interesting aspect of the 2!0pp/226Ra dating method is that the barite
purification procedure appears to affect its age determination. It is noted that
the purified barite ages of both chimneys and crusts tend to be younger than
those of their matching raw samples (Table 3.6 and 3.8). A comparison of
210pp/226Ra data of raw and purified barites in Figure 3.22 indicates that all
the chimney samples converge along the line with a slope of 1 whereas the
crusts deviate significantly from the line. This observation indicates that the
raw crust barites yield ages much older than those of the purified barites and
hence produce anomalous and biased results. These discrepancies suggest that
some 210pp in the raw samples are not supported by indigenous 226Ra decay.
The inference is confirmed by the fact that some raw barite crusts possess
210pp/226Ra >I (e.g. 2.01 for Cr-93-4-4/1-1; 2.04 for Cr-93-8-7/3; and 1.43 for Cr-
93-8-7/2), which are impossible if the only source of 219Pb is from the decay
of the parent 226Ra indigenous to the sample.



18

analysis. Each spectrum was subjected to an internal energy calibration
secondary to that based on the mixed gamma standard. After peak search and
identification of primary gamma peaks based on the energy calibration
obtained from the mixed gamma standard were achieved, a range of peaks
from naturally occurring radioisotopes inherent to the counting system
shielding was used to obtain a customized energy calibration for each
spectrum. This methodology significantly improved quantitative results since
peak fitting algorithms are greatly dependent on the accuracy of the internal
energy calibration. This technique is extremely useful for correcting minor
energy calibration drift during long counting periods.

Potential problems such as self-absorption of the gamma rays by the
sample were obviated by utilizing an "infinitely thin" planar counting
geometry. This was accomplished by carrying out an efficiency calibration
using a mixed gamma standard in the form of 1.0 gm of sandy loam. The sandy
loam mixed gamma standard was spread out in a 4.5 cm diameter Whatman
filter paper to a linear thickness less than 2 mm. The filter paper with mixed
gamma standard was placed on the center of the detector head, and counted for
1 hour. The net area under 15 primary gamma and X-ray peaks was used to
calculate fractional efficiency. The 15 peak centroid positions were used to
calibrate the counting system at corresponding energies ranging from 31.8
keV to 1836.1 keV. Fractional efficiency was calculated by dividing net counts
per second under each primary gamma peak by the absolute number of
gamma photons emitted per second by the corresponding radionuclide. A
continuous energy vs. efficiency function was derived by fitting a 5th order
polynomial equation to the 15 data points obtained from the 1 hour count of
the mixed gamma standard. The total uncertainty of the fit was 2.18%. This
function was subsequently utilized to quantify gamma emitting radionuclides
endogenous to 1 gm subsamples of barites.

Uranium and thorium ore reference materials (LSUO1 (U) and LSUO2 (Th))
were also measured for comparison. A 1.0 gm sample of uranium ore standard
was ground to a fine powder and used for quality assurance of the analysis

scheme. The 2!0pp/226Ra for the ore standard as determined by the counting
system and analysis scheme was not significantly different from 1.0 at the 95%
confidence level throughout the experiment thus validating the methodology.

2.3. Seep Fauna

The flow chart illustrating the type of materials and methods employed in
the analyses of seep fauna is shown in Figure 2.6.
Macrofauna were collected from barite-bearing seeps in Garden Banks (GB-
382; GB-338) and Mississippi Canyon (MC-929), and carbonate-bearing seeps
from Green Canyon (GC-184/185, also known as Bush Hill, GC-234, GC-272, MC-

709) and Alaminos Canyon. Major faunal groups assayed for 226R, include the
symbiont-bearing, chemoautotrphic, Bathymodiolus spp. mussels, and
heterotrophic fauna living in the proximity of seeps including starfishes,
galatheid crabs, and polychaetes. These samples were obtained during
submersible dives from 1990 to 1997 on the outer continental shelf of the Gulf
of Mexico (Fig. 1.1). The carbonate-bearing seep fauna were hypothesized a
priori to contain less radium activity and barium levels relative to the
radioactive barite-rich seep fauna.

All available seep fauna whole soft tissues and separated calcareous parts
were oven-dried at 90 degrees centigrade, ground into a fine homogenous



138

Land, L.S., Macpherson, G.L. and Mack, L.E. (1988) The geochemistry of saline
formation waters, Miocene, offshore Louisiana. Gulf Coast Assoc. Geol. Soc.
Trans. 38: 503-511.

Land, L.S. and G.L. Macpherson (1989) Geochemistry of formation water, Plio-
Pleistocene reservoirs, offshore Louisiana: Gulf Coast Assoc. of Geol. Soc.
Trans. 39: 421-430.

Land, L.S., Macpherson, G.L. and Mack, L.E. (1988) The geochemistry of saline
formation waters, Miocene, offshore Louisiana. Gulf Coast Assoc. Geol. Soc.
Trans. 38: 503-511.

LaRock, P. J.H. Hyun, S. Boutelle, W.C. Burnett, and C.D. Hull (1996) Bacterial
mobilization of polonium: Geochim. Cosmochim. Acta 60(22): 4321-4328.

Lloyd, R.M. (1968) Oxygen isotope behavior in the sulfate-water system: J.
Geophys. Res. 73: 6099-6110.

MacDonald, K.C., K. Becker, F.N. Speiss and R.D. Ballard (1980) Hydrothermal
heat flux of the 'black smoker’ vents on the East Pacific Rise': Earth Planet.
Science Lett. 48: 1-7.

MacDonald, I.R., Guinasso, N.L., Ackleson, S.G., Amos, J.F., Duckworth, R., Sassen,
R. and Brooks, J.M. (1993) Natural oil slicks in the Gulf of Mexico visible
from space. J. Geophys. Res. 98 (C9): 16,351-16,364.

MacDonald, I.R., Guinasso, N.L., Sassen, R., Brooks, J.M., Lee, L. and Scott, K.T.
(1994) Gas hydrate that breaches the sea floor on the continental slope of
the Gulf of Mexico. Geology 22: 699-702.

Macpherson G.L. (1989) Lithium, boron and barium in formation waters and
sediments, northwestern Gulf of Mexico sedimentary basin. Unpublished
Ph.D dissertation, University of Texas at Austin, 286 p.

Manker, J.P. and Rickman, D.A. (1994) Toxic metal concentrations and
distributions in sediments along the Louisiana/Texas coast. Geol. Soc. Am.
Abstr. with Progr., Southeastern Sec. 26 (14): 26.

McGee, D.T., P.W. Bilinski, P.S. Gary, D.S. Pfeiffer, and J.L. Sheiman (1994)
Geological models and reservoir geometries of Auger Field, deepwater Gulf
of Mexico: GCS-SEPM Foundation 15th Annual Res. Conf., Submarine Fans
and Turbidite Systems, p. 245-256.

Mendelssohn, I-R., Flynn, K.M. and Wilsey, B.J. (1990) The relationship between
produced water discharges, and plant biomass and species composition in
three Louisiana marshes. Oil and Chem. Poll. 7(4): 317-335.

Mizutani, Y. (1971. An improvement in the carbon reduction method for the
oxygen isotopic analysis of sulfates. Geochem. J. 5: 69-77.

Mizutani, Y. and T.A. Rafter (1973) Isotopic behavior of sulfate oxygen in the
bacterial reduction of sulfate: Geochem. J. 6: 183-191.

Moore, W. and D, Stakes, (1990), Ages of barite-sulfide chimneys from the
Mariana Trough: Earth Planet. Science Lett, 100: 265-274.

Moore, W.S. (1996) Large groundwater inputs to coastal waters revealed by

226Ra enrichments: Nature 380: 612-614.

Moore, W.S. (1997) High fluxes of radium and barium from the mouth of the
Ganges-Brahmaputra River during low river discharge suggest a large
groundwater source: Earth Planet. Science Lett. 150: 141-150.

Nix, E.R., Fisher, C.R., Vodenichar, J. and Scott, K.M. (1995) Physiological
ecology of a mussel with methanotrophic endosymbionts at three
hydrocarbon seep sites in the Gulf of Mexico. Mar. Biol. 122: 605-617

Orr, J.C. (1988) 222Rn, 226Ra, and 228Ra as tracers for the evolution of warm
core rings. Unpublished Ph.D dissertation. Texas A & M University, 211 p.



Therefore, in order to establish a technical basis for decisions concerning
possible impact of deepwater drilling offsetting the subsurface hydrology and
industrial discharges on the deepwater marine habitats, it was necessary to
evaluate the contribution of barium and radium derived from seeps, their
removal into the sediment and their uptake by the chemosynthetic
communities associated with the seeps.

In contrast to the shelf area, where 50 years of drilling and production of
hydrocarbons occurred (Clark and Patrick, 1987; Mendelssohn et al., 1990;
Jackson et al., 1994; Manker and Rickman, 1994), the deepwater habitats along
the slope are relatively pristine. However, as the oil and gas industry is
moving its exploration and production main activities to deepwaters, their
present pristine status is likely to change. It is therefore of importance to
explore the natural processes occurring in deepwaters ahead of the
anticipated anthropogenic complications.

1.2. Objectives and Questions Addressed in the Study

The primary objectives of this study were to document products and
processes related to naturally occurring barium and radium-rich fluid
emissions from hydrocarbon seeps, and assess their impact on the offshore
habitats in deepwater Gulf of Mexico. The specific questions we have addressed
are as follows:

(1) What are the levels of barium and radium in the pore fluids underlying
seeps, in the barite deposits forming around the seeps, and the chemosynthetic
communities?

(2) What are the nature and origin of barite deposits and the processes
promoting their formation?

(3) What are the timing and duration of emissions? are these emissions
altered or accelerated by anthropogenic activities?

(4) What are the fluxes of barium and radium from naturally occurring
emissions on the benthic habitat?

(5) Do recently discovered radium-rich fluid emissions pose a significant
internal and/or external radiation threat to the existing seep communities?

The first and second questions were addressed by a direct approach
involving (i) specific determinations of barium, radium and selected chemical
elements and their isotopes in pore fluids squeezed from sediment cores taken
with the submersible robot arm from barite-bearing seep sites; (ii)
measurements of barium, radium and selected chemical elements and their
isotopes in solid barites, and (iii) determinations of radium and barium in both
soft tissues and hard skeletons from methanotrophic mussels and
heterotrophic fauna living in the proximity of the barite seeps. These results
were compared and contrasted with radium and barium determinations from
pore fluids and fauna from carbonate-bearing seeps which appear to
represent natural, unperturbed, background radioactive and chemical
emission sites. We found that pore fluids contain important information
concerning their source and migration pathways and their chemical
fingerprinting allowed us to identify their subsurface source/s with a fair
degree of certainty.

The third and fourth questions are much more difficult to resolve
definitively within the time-limited scope of this study. An indirect approach
to the problem was taken, which involved inferences from spatial and
temporal distribution patterns of barium and radium within the barite



136

Goldberg, E.D., B.L.K. Somayajulu, J. Galloway, LR. Kaplan, and G. Faure (1969)
Differences between barites of marine and continental origins: Geochim.
Cosmochim. Acta 33: 287-289.

Graber, E.R. and Aharon, P. (1991) An improved microextraction technique for
measuring dissolved inorganic carbon (DIC), 5!3C (DIC) and 8180 (H20) from
milliliter-size water samples. Chem. Geol. (Isotope Geosc. Sec. ) 94: 137-144.

Grasty, R.L, C.W. Smith, J.M. Franklin and IR. Jonasson (1988) Radioactive
orphans in barite-rich chimneys, Axial Caldera, Juan de Fuca Ridge: Can.
Mineral. 26: 627-636.

Hannington, M.D. and R. Scott (1988) Mineralogy and geochemistry of a
hydrothermal silica-sulfide-sulfate spire in the caldera of axial seamount,
Juan de Fuca ridge: Can. Mineral. 26: 603-625.

Hanor, J.S. (1966) The origin of barite: Unpublished Ph.D Dissertation, Harvard
University, 257 pp

Hanor, J.S. (1982) Problems concerning the geochemistry and genesis of
ratiform barite-pyrite ore deposits in Arkansas, in McFarland, J. D., ed.,
Contributions to the Geology of Arkansas: Ark. Geol. Comm. Misc. Publ.

18: 27-32.

Hanslik, E. and A. Mansfield (1990) Removal of radium from drinking water in
The environmental behaviour of radium. IAEA Technical Report Series No.
310(2): 229-268.

Henderson, L.M. and F.C. Kracek (1927) The fractional precipitation of barium
and radium chromates: J. Am. Chem. Soc. 47: 738-749.

Hesse, R. and W.E. Harrison (1981) Gas hydrates (clathrates) causing pore-
water freshening and oxygen isotope fractionation in deep-water
sedimentary sections of terrigenous continental margins: Earth Planet.
Science Lett. 55: 453-462.

Hofmann, R. and A. Baumann (1984) Preliminary report on the Sr isotopic
composition of hydrothermal vein barites in the Federal Republic of
Germany: Mineralium Deposita 19: 166-169.

Horwitz, W. (1980) Fish and other marine products. In: Official methods of
analysis of the association of official analytical chemists, 13th ed.
Association of Official Analytical, Chemists. p. 285-290.

Hovland, M., M. Talbout, H. Qvale, S. Olausson, and L. Aasberg (1987) Methane
related carbonate cements in pockmarks of the North Sea: J Sedim. Petrol.
57: 881-892.

International Atomic Energy Agency (IAEA) (1992) Effects of Ionizing
Radiation on Plants and Animals at Levels Implied by Current Radiation
Protection Standards. Technical Reports Series No. 332, IAEA, Vienna.

International Commission on Radiological Protection. (1973) Alkaline Earth
Metabolism in Adult Man. Publication 20; Pergamon Press, New York

Tyengar, K. and R. Narayana. (1990) Uptake of radium by marine animals In
The environmental behaviour of radium. International Atomic Energy
Agency. Technical Report Series No. 310(1): 467-485.

Jackson, T.J., Wade, T.L., McDonald, T.J., Wilkinson, D.L. and Brooks, J.M. (1994)
Polynuclear aromatic hydrocarbon contaminants in oysters from the Gulf
of Mexico (1986-1990). Env. Poll. 83 (3): 291-298.

Jahnke, R.A. (1988) A simple, reliable, and inexpensive pore-water sampler:
Limnol. and Oceanogr. 33(3): 483-487.

Jarmul, D. (1984) Drilling mud poses minimal risk. Geotimes 29 (1): 14.



23

3.0 RESULTS
3.1. Hydrocarbon-Rich Fluids

Fluids occurring in hydrocarbon seep sites at bathyal depths (Fig. 1.1) were
assayed for isotopes and elemental chemistry according to the methods
described in methodology. The fluids analyzed here can be conveniently
grouped according to their mode of acquisition and their association with
specific geologic deposits as follows:

(i) Pore fluids (n=34, Table 3.1) acquired from 40 cm long cores taken by the
Johnson-Sea-Link submersible in carbonate-bearing seep sediments (Fig. 1.1)
and squeezed on the support surface ship using a modified Jahnke device
(Jahnke, 1988).

Gi) Pore fluids (n=34, Tables 3.1, 3.2 and 3.3) acquired from 40 cm long cores
taken by the Johnson-Sea-Link submersible in barite-bearing seep sediments
(Fig. 1.1) and squeezed on the support surface ship as above.

Gii) Fluids (n=6) from the crater of a barite-draped mud volcano drawn into
the submersible through a thin probe (Tables 3.2 and 3.3).

Our geochemical surveys show that pore fluids from hydrocarbon seeps in
the Gulf of Mexico offshore Louisiana can be clearly distinguished on the basis
of their level of Ba and radioactivity ascribed to 226Ra and 228Ra. The pore
fluids from carbonate-bearing seeps (Table 3.1), ranging in salinity from

slightly higher than normal Gulf of Mexico deepwater values (35.0 °/oo) to

highly saline (up to 182 °/o0), show universally low Ba concentrations (0.2 to
3.3 ppm) and Ra activity levels (<16 dpm/L, not reported in the Table) which
are compatible with values previously reported in pore fluids from open ocean
marine sediments (Cochran, 1982). Recent Ba assays in the water column of the
Western Gulf by Buerket (1997) indicate levels of 7.9 ppb at 700 m depth and a
maximum of 9 ppb at lower bathyal depths which are 3 orders of magnitude
lower than the Ba concentrations in pore fluids from carbonate-bearing seeps
(Table 3.1). In contrast, pore fluids from barite-bearing seeps (Table 3.3)
display highly anomalous Ra activity levels that are a factor of about 200 to
40,000 times higher than that of the ambient Gulf of Mexico bottom water
(0.132 dpm/L, Key, 1981), Ba levels higher by up to a factor of 130,000 and

salinities substantially higher than normal marine seawater (45°/o0 to
155°/o00).

3.1.1. Radium and Barium Assays in Barite-Bearing Seeps

This section reports the anomalously high Ra isotopes and elemental Ba in fluids
advecting at barite-bearing seeps in cores retrieved by submersibles in MMS blocks
MC-929, GB-382 and GB-338 (Fig. 1.1). These fluids belong to type III according to the
clasification of their chemical fingerprinting reported below. Table 3.2 lists the exact
locations of the material studied and core-sediment descriptions. The objectives here
are to delineate the activity of the specific isotopes of 226Ra and 228Ra and Ba
concentrations in order to provide insights on the source and origin of the fluids
advecting on the seafloor in barite-bearing seeps. The following patterns emerge
from the data listed in Table 3.3.

The fluids yield a wide range of salinities, from 45.5°/00 to 155°/00, which are
higher than the ambient Gulf of Mexico deepwater at 600 m by factors of 1.2 to 4.1.

The 226Ra activities range from 27.2 to 3656 dpm/L and 228Ra activities range from



92

106 y

105 F q
3 [ 1
2 104 F 4
=
z “
Zz
& 103 & q

§ #
© formation waters
102 + discharged produced waters
+ A pore fluids
10! ,
102 102 103

Salinity /oo)

Figure 4.5. 226Ra-salinity relationship in formation waters and discharged
produced waters from the Gulf Coast region. Formation water data (open
circles) are from Kraemer and Reid (1984). Discharged produced waters data

(crosses) are recalculated from Rabalais et al. (1991) assuming that 226Ra
activities represent 2/3 of the total radium reported by these authors. Note the
overlapping of the pore fluid data (solid triangles) with the field defined by
the formation water.



135

Costlow, J.D. and 12 co-authors (1983) Drilling discharges in the marine
environment. National Academy Press, Washington D.C., pp. 180.

Davidson, D.W., D.G. Leaist, and R. Hesse (1983) Oxygen-18 enrichment in the
water of a clathrate hydrate: Geochim. Cosmochim. Acta 47: 2293-2295.

Dehairs, F., R. Chesselet, and J. Jedwab (1980) Discrete suspended particles of
barite and the barium cycle in the open ocean: Earth Planet. Science Lett.
49: 528-550.

Dinnel, S.P. and Wiseman, W.J. (1986) Fresh water on the Louisiana and Texas
shelf. Cont. Shelf Res. 6 (6): 765-784.

Drever, J.I., 1982, The geochemistry of natural waters: Prentice Hall, New
Jersey, 437 pp.

Dymond, J., R. Cober, L. Gordon, P. Biscaye and G. Mathieu, (1983) 226Ra and

222Rn contents of Galapagos Rift hydrothermal waters-the importance of
low-temperature interactions with crustal rocks: Earth and Planet. Science
Lett. 64: 417-429,

Egeberg, P.K., and P. Aagaard (1989) Origin and evolution of formation waters
from oil fields on the Norwegian shelf: Applied Geochem. 4: 131-142.

Environmental Health Criteria 107 (1990) Barium. World Health Org., Geneva,
148 pp.

EPA (1980) EPA/600/4-80-032 & EPA 903.1: Prescribed Procedures for
Measurement of Radioactivity in Drinking Water. U.S. Government
Printing Office, 133 pp.

Faure, G. (1986) Principles of Isotope Geology: John Wiley & Sons, New York,
531 pp.

Ferrell, R-E. and P. Aharon (1994) Mineral assemblages occurring around
hydrocarbon vents in the northern Gulf of Mexico: Geo-Marine Lett. 14: 74-
80.

Finkel, R.C., J.D. Macdougall, and Y.C. Chung (1980) Sulfide precipitates at 21°C

on the East Pacific Rise: 226Ra, 210pp and 210po: Geophys. Res. Lett. 7: 685-
688.

Fritz, P., G.M. Basharmal, R.J., Drimmie, J. Ibsen and R.M. Qureshi (1989) Oxygen
isotope exchange between sulfate and water during bacterial reduction of
sulfate: Chem. Geol. 79: 99-105.

Fu, B., Aharon P., Byerly G.R., and Roberts, H.H. (1994) Barite chimneys on the
Gulf of Mexico slope: initial report on their petrography and geochemistry:
Geo-Marine Lett. 14: 81-87.

Fu, B. and Aharon, P. (1995) Oxygen and sulfur isotopes and elemental
chemistry of barite deposits associated with hydrocarbon seeps in the deep
Gulf of Mexico. Geol. Soc. Am. Abstracts with Programs 27: A97.

Fu, B. , P. Aharon, D.L. Van Gent, and L.M. Scott (1996) Anomalously high 226p,
in fluids advecting to the sea floor: A new radioactive source in the Gulf of
Mexico. Gulf Coast Assoc. Geol. Soc.Trans. 46: 125-131.

Fu, B. and P. Aharon (1997) Origin and depositional model of barite deposits
associated with hydrocarbon seeps on the Gulf of Mexico slope, offshore
Louisiana: Gulf Coast Assoc. Geol. Soc. Trans. 47: 13-20.

Fu, B. (1998) A Study of Pore Fluids and Barite Deposits from Hydrocarbon
Seeps: Deepwater Gulf of Mexico. Unpublished PhD Dissertation, Louisiana
State University, 243 pp.

Gofman, J.W. (1981) Radiation and Human Health. A comprehensive
investigation of the evidence relating low-level radiation to cancer and
other diseases. Sierra Club, San Francisco. pp. 908.



4000 ) T T T
GQ 3000F ——y=101.3140.92x
R7=0.99
= 2000F 1
s
4 1000+ 7
0 1 1 1
0 1000 =2000 §=3000 »3=— 4000
Cl (mM/L)
4 © T T T
~ 0.75+1.38*10-3x, 4
: ar |
s GL |
1 IE-0,1945.37*10-%e
0 1 R?=0.99
0 1000 =2000 =. 3000 = 4000
Cl (mM/L)

Figure 4.2. Plots of Cl against Na, Ca, Sr and Ba in pore fluids type III. The

Ca (mM/L)

Ba (uM/L)

85

100 ® T T 1
80F 1
607 Ss y |
4p < J
[ y=1.9042.26"10% |
20 R=0.99
0 1 F f
0 1000 ©2000 ©3000» 4000
Cl (mM/L)
7000 T —_—T
6000 LO a 1
5000 q
4000F a q
3000. a q
2000 A 7
1000 q
0
0 1000 2000 3000 4000
Cl (mM/L)

linear correlations in A, B and C suggest the mixing of seawater with saline

end member fluids. The non-linear correlations between Ba and Cl in D result
from barite precipitation. Empty circles refer to seawater, solid triangles and
solid circles represent pore fluids from Garden Banks and Mississippi Canyon,
respectively.



mT Ft yar ss DPT fe 38
- ae, tg 2103_
<

0.2

Figure 1.2. Occurrences of highly radioactive barite deposits associated with
barium/radium/methane-rich fluid emissions in deepwater Gulf of Mexico
(Garden Banks blocks #338 and #382 at 510 to 522 m depth and Mississippi
Canyon block#929 at 640 m depth). Field of view in the photographs is 0.5 m. A:
Dormant mud volcano with orange-white barite crust blanketing its flanks.
Note the brine-filled caldera and the presence in the foreground of living
methanotrophic Bathymodiolus spp. mussels. B: Cup-shaped barite deposits.
Note the high density of fluid emission sites. C: Cylindrical-shaped barite
chimney. Note the galatheid crab in the foreground and the incipient growth
of chimneys in the backround. D: Dead methanotrophic mussel shells engulfed
in coarse-grained barite sand infilling gulleys.



91

excess of the amount which can be supported by the coexisting 234U
(Cochran,1979). Consequently, the 226Ra resulting from the radioactive decay
of excess 230Th in the sediment ends up in the pore fluids by leaching and

alpha-recoil processes (Kigoshi, 1971). 232Th in the water column is also
scavenged by the falling particulate matter and is buried in the marine

sediments (Broecker and Peng, 1982). Consequently the 232Th within the
sediments decay to 228Ra which enters the pore fluids by leaching and alpha-

recoil processes resulting in a higher concentration of 228Ra in pore fluids
than the overlying water column (Cochran,1979).

Although there is a scarcity of 226Ra and 228Rq analyses of pore fluids in
deep-sea sediments, the few available data tend to support the model outlined

above. For example, determination of 22Ra and 228Ra activities in pore fluids
from deep-sea cores in the eastern equatorial Pacific by Somayajulu and
Church (1973) and Cochran and Krishnaswami (1977) yielded a range of values

from 1 to 15 dpm/L for 226Ra and about 0.43 dpm/L for 228Ra. These values are
higher by factors of 8 to 40 and 150, respectively, relative to the ambient
Pacific bottom water (0.36 dpm/L for 226Ra and 3x10-3dpm/L for 228Ra).

The 226Ra and 228Ra activities of the pore fluids analyzed in this study,
ranging from 27.2 to 36.6x102 dpm/L for 226Ra and 63.3 to 24.5x102 dpm/L for
228Ra, are by far the highest values reported to date for marine sediments
(Table 3.3). Clearly, source(s) of 226Ra and 228Ra other than decay of excess
230Th and 232Th in the sediments must be involved in the interaction with
these fluids.

Barium in seawater is typified by surface water depletions (~7 ppb) and
deep water enrichments (~25 ppb) and has a nutrient-like profile that suggest
Ba transport in biological debris (Wolgemuth and Broecker, 1970). Barite
continuously forms in the water column and is an ubiquitous minor phase in
deep-sea sediments (Church, 1979). The fallout of pelagic barite into
unsaturated sediments causes partial dissolution of the barite and Ba
enrichment in the normal marine pore fluids (40 to 50 ppb) (Church, 1979).
The Ba concentrations in the pore fluids from barite-bearing seeps are 839
ppm and 83 ppm in the Garden Banks and Mississippi Canyon end-member
fluids (Table 4.1), respectively, which are higher by orders of magnitude
relative to normal oceanic pore fluids reported above. These pore fluids were
interpreted to represent a mixture of advecting, highly saline, fluids sourced
in the deep subsurface with seawater on the basis of elemental and isotope
chemistry.

It is reasonable to assume that the anomalously high 226Ra, 228Ra
activities and Ba concentrations in the pore fluids are derived from deep-
seated highly saline formation waters advecting to the seafloor. This
interpretation regarding the source of Ra in the pore fluids is supported by
the significant positive relationship between salinity and Ra observed in the
fluid samples (Fig. 3.2) which is remarkably similar to that derived for
formation waters produced by onshore and offshore oil and gas rigs, and
produced waters discharged in the nearshore marine habitats (Fig. 4.5).
Similarly, the identification of the Ba source in deep-seated formation waters
is supported by the observed overlap between the pore fluid and produced
waters Ba and SOq data (Fig. 4.4).



96

in coordinates of 228Th/228Ra ages vs. the distance from the bottom of the
chimneys (Figs. 4.7, 4.8 and 4.9). In these plots a constant linear growth is
assumed in each sampling interval (2 to 3 cm) and the age is assigned to the
midpoint of each interval. The growth rates determined by this method are 9.3
cm/yr for chimney #6, 8.9 cm/yr for chimney #8, and 4.4 cm/yr for chimney
#11. These growth rates indicate that the buildup of a chimney is very rapid.
The growth rates obtained here are similar to the growth rate of about 6 cm/yr
for barite-rich hydrothermal chimneys at Juan de Fuca Ridge (Grasty et al.,
1988; Kim and McMurtry, 1991; Reyes et al., 1995), but greater than the value of
1.2 cm/yr calculated for a hydrothermal sulfide chimney at the same ridge
(Kadko et al., 1985).

Regional differences in growth rates may occur between sites. The two
chimneys from Garden Banks (chimney #6 and chimney #8) have similar
growth rates and yield an average value of 9.13 +0.28 cm/yr whereas the
growth rate obtained from chimney #11 in Mississippi Canyon is about 4.4
cm/yr. The growth rate in Garden Banks is thus twice that in Mississippi
Canyon. Because these barite chimneys formed by the mixing of sulfate-rich
seawater with the Ba/Ra-rich seeping fluids, the difference in growth rate
may be controlled either by distinct Ba concentrations in the seeping fluids
and/or by different fluid seepage rates. Indeed, the end member fluid in
Garden Banks contains a higher concentration of Ba than Mississippi Canyon
(Table 4.1), and this could explain the faster growth rate of chimneys in
Garden Banks. Alternatively, the chimney growth rates may be controlled by
the fluid seepage rate. The higher the fluid seeping rate, the more Ba could be
vented out resulting in more barite precipitation within a certain period of
time. The higher growth rate found in the chimneys from Garden Banks may
therefore suggest that the fluid seepage rate in Garden Banks is higher than
in Mississippi Canyon.

4.3.2 Initial 228Ra/226Ra in the Barites

The 228Ra isotope decays much faster than 226Ra and therefore the latter
can be assumed to have remained constant in the short life span of the barites.

Thus the initial 228Ra/226Ra can be calculated according to the following
equation:

(228Ra/226Ra), = 228Ra/226Ra)ge-At (4.3)

where (228Ra/226Ra), is the measured ratio in the barite, t is the age as
determined by the 2?8Tn/228Ra dating method, (228Ra/226Ra)q is the initial

ratio, and % is the decay constant of 228Raqa (228Ra = 0.121/yr). The decay
constant of 226Ra is ignored in the equation because of its long half life (1,620
yrs).

The initial 228Ra/226Ra of the chimneys determined by eq. 4.3 above are
listed in Tables 3.6 and 3.7. The initial 228Ra/226Ra for the crusts were not
calculated because most crusts contain 228Ra below the detection limit. The

initial 228Ra/226Ra of the chimneys in Garden Banks range from 0.9 to 1.2
(mean = 1.09 40.08, n = 24) whereas the ratios in Mississippi Canyon range from
0.6 to 0.8 (mean = 0.71 +0.08, n = 7). The broad negative relation occurring



112

5
3
Zz "Ep
Cy _
“ pore fluid sulfate with
seawater signature
. F
Cg +8047" = HyS + C037 + HO )
A
bo CTH COs? = Ca03 ¥
g Ba** + S04” = BaSO,
g ‘x $4 4 ¥
£ residual sulfate
3 after reduction
Venting fluids | | Gaseous | | Venting fluids
(Ba2*, sr2*, wen (Ba2*, s+,
Ra2*, Ca2*y NI Ra2*, Ca**)
tp co ly

Figure 4.14, Depositional model of GOM barites. Note that chimney and crust are

not shown to scale.



59

sublayers; an upper light-gray (Cr-93-8-5/3/u) overlying a black-colored (Cr-
93-8-5/3/l) layer buried within the sediment at the time of sampling. Raw
samples Cr-93-8-5/3/u and Cr-93-8-5/3/] yield ages of 29.72 +1.02 and 48.39
+1.20 years, respectively. Purification of samples caused a significant
reduction of the 210pp/226Ra ages to 22.42 £0.27 years and 20.99 +0.66 years for
Cr-93-8-5/3/u and Cr-93-8-5/3/l, respectively.

Crust Cr-93-8-7/2 was collected from the wall of a small depression in the
GB-382. The crust is about 8 cm thick and exhibits an upper light-gray colored
layer (Cr 93-8-7/2/u) overlying a black-colored lower layer (Cr-93-8-7/2/1).
The raw sample Cr-93-8-7/2/u yields an age of 44.23 £2.92 years which is about
30 years older than the purified barite age of 13.22 £1.33 years. The measured
210pp/226Ra activity ratio of the Cr-93-8-7/2/1 raw sample is 1.43 indicating
the presence of excess 210pp unsupported by the decay of the measured 226Ra.
The measured 210pp/226Ra activity ratio after purification was reduced to 0.37
which yields a corresponding age of 13.1142.13 years. Crust Cr-93-8-7/3 was
recovered from a different part of the same depression where Cr-93-8-7/2 was
sampled. The measured 210pp/226Ra of the raw sample is 2.04 which indicates
an excess of unsupported 210pb. The measured 210pp/226Ra atfter purification
decreased to 0.52 and yields an age of 21.50 +1.97 years.

Crusts Cr-93-1-3/1 and Cr-93-4-4/1 were sampled from the flank of a mud
volcano in MC-929 and were serving as substrate for methanotrophic mussels.
Microscope and SEM analyses indicate that Cr-93-1-3/1 contains a large
proportion of carbonates (about 50% by volume) in addition to barite. This is
the only crust measured in the study containing detectable 228Ra but no
2287h/228Ra age was obtained due to low concentrations of 228Th. The
210pp/226Ra age of the raw Cr-93-1-3/1 sample is 12.79 +0.61 years and 9.00
+0.45 years for the purified barite.

Crust Cr-93-4-4/1 is about 10 cm thick and consists of an upper light gray
layer (Cr-93-4-4/1/u) and a lower dark layer (Cr-93-4-4/1/1). No 210pp/226R a
ages were obtained for the raw samples because their measured 210pp/226R q
were >1 (Table 3.8). The measured 219pp/226Ra after purification were reduced
to 0.54 and 0.51 which gave corresponding ages of 23.05 +0.25 and 21.07 £0.13
years for Cr-93-4-4/1/a and Cr-93-4-4/1/l, respectively.

3.2.5. Validation of the Ra-Series Dating Methods

The accuracy of 228Th/228Ra and 210pp/226R, ages in barites depends
upon the validity of the following assumptions: (i) 226Ra and 228Ra in the
barites must be orphans; (ii) there must be no initial 228Th and 210Pb in the
barites (ie., they must be fully supported by their respective 228Ra and 226Ra
parents), and (iii) the system must have remained closed with respect to the
radioactive nuclides, 228Ra, 2287p, 226Ra, and 210pp since the formation of the
barites.

In order to check the validity of the first assumption above, gamma-ray
spectra of uranium and thorium standards were compared with those of barite
chimneys and crusts (Fig. 3.14). It is important to note that the gamma peak at
1001 keV from 234Pa in the uranium standard spectrum is absent in both the
chimney and crust spectra. As 234Pa reaches equilibrium with 238U within

few months (the intermediate isotope 234Th has a 24.1 day half life), this



25

2p b?26Ra), 4

T T T
Slope = 1:1

‘

@ Barite crust
A Barite chimney

Figure 3.22. 219pp/226Ra of barites before (rb) and after (pb) purification.

0.5 1.0 15 2.0

71PbP26Ra) pp

2.5

65

Note that all the barite chimneys are on the 1:1 line whereas most crust data

are above the 1:1 line. This indicates that raw crust samples tend to give

spurious ages.



10

Barites

_— | —-

Petrographic Isotope Chemical
analyses analyses analyses
SEM and Radioactive | |Radiogenic Stable Microprobe
Microscopy isotopes isotopes isotopes
: Ca, Sr, Ms
226pa,?°8pa | | 871/865, | | 513C and 5180(CaCO3); eS Mg.) Ba,
834g and 5180/04)

Figure 2.2. Flow chart of imaging, chemical and isotope analyses of barites.



93

The enrichment of Ra in the saline waters can be explained by the process
of cations competition with Ra for ion exchange sites resulting in a greater
proportion of Ra ions remaining in solution (Tanner, 1964). However, the
exact source of radium in highly saline formation waters is still poorly known.
For example, on the basis of radium-salinity relationship in formation waters
from the US Gulf Coast region, Kraemer and Reid (1984) argued that Ra
enrichment in these waters is derived from interaction of the fluids with the
reservoir matrix. Specifically, the most likely sources are the clay minerals
and the sand-sized quartz which contain adsorbed U and Th. In contrast,

Kronfeld et al. (1993) and Kronfeld (1995) suggest on the basis of 226Ra
anomalies determined in saline hot springs and borings from the Jordan Rift
Valley that high Ra levels in formation fluids result from leaching of U-rich
source rocks. The close association of high concentrations of Ra with wells
developed on the flanks of piercement-type salt domes found in parts of the
Texas Gulf Coast caused Cech et al. (1988) to speculate that the salt domes may be
one of the Ra sources. Due to a scarcity of available geochemical and
subsurface geophysical data in the study area, the source for the Ra in the
venting fluids is presently unknown. The high radium levels in the fluids
advecting to the sea floor are likely to be related to a number of factors,
including: (i) leaching of salt dome and salt sheets, (ii) leaching of deep-
seated, kerogen-rich source rocks, (iii) desorption from clay minerals and
silicate-rich sands, and (iv) leaching from subsurface uranium deposits (Cook,
1980). The interaction of rocks, salt domes, and uranium-bearing deposits as
the fluids migrate through fault conduits has not been well documented in the
literature and further study is needed to better understand the mechanisms
controlling the distribution of Ra, and particularly the role of salt domes.

4.2.2 The Residence Time of Fluids in Barite-Bearing Seeps

Two time intervals can be discerned concerning the advecting fluids. One is
the fluid traveling time (FTT) which is the time elapsed since the fluid left the
subsurface source until trapped in the sediment. The other is the fluid
residence time (FRT) which is the time period since the fluid was trapped in
the sediments until it was sampled.

The high concentration of 228Ra in fluids from barite-bearing seeps (Table
3.3) suggests that the total time, including FTT and FRT, must be less than 20

years. This is because 91% of the 228Ra would have decayed away within 20
years due to its short half life (5.75 yr). Because the Ra isotope composition in
the subsurface source is unknown, it is impossible to derive the exact FTT.
However, the FRT can be estimated according to the following equation:

(228Ra/226Ra), = (228Ra/226Ra)ge At (4.2)
where (228Ra/226Ra), is the measured activity ratio of the fluid in GB-382
(Table 3.3); t is the residence time of the fluid in the sediments; (228Ra/226Ra)o

is 228Ra/228Ra activity ratio of the fluid at the time it was trapped in the
sediment; and % is the decay constant of 228Ra (A = 0.121 /yr). In this
calculation it is assumed that 228Ra decays exponentially with time but that
226Ra remains constant due to its long half life (1,600 yr.).



15

Since the useful range for 2287n/228Ra method is 15 years or less,
210pp/226Ra method was also applied to date samples older than 15 years. This
technique uses the ingrowth of the 210pb which is a decay product of 226Ra in

the barite. The technique mimics the 2287n/228Ra method and can be
described by the following equation:

(A210pp)/(A226Rg)=[A210pp)/(A210pp - 4226 pq) I[l-exp(4226pq -4210pp)t] (2.2)

where, A226p,q and A210pp are the activities of 226Ra and 210pp, respectively;
t is the age of the sample in years; and 4226p, and A210pp are the decay

constants for 226Ra and 210pp, respectively. The half lives of 226Ra and 210pp
are 1,620 and 22.3 years, respectively (Broecker and Peng, 1982) and these

translate to decay constants of 4.28x10-4yr-! and 3.11x10-2yr-1, respectively.
Because the half life of 225Ra is sufficiently greater than that of 219Pb, the
parent (226Ra) does not appreciably decay during the period of ingrowth of

the daughter (210pp). The 210pp/226Ra will then approach a secular
equilibrium (a ratio of 1) at about 120 years (Fig. 2.5), which is the upper age
limit of the technique. This dating technique is therefore useful for dating
barites younger than 100 years.

Similar assumptions as for the 228Th/228Ra method apply here, namely: (i)
the 226Ra must be orphan, (ii) there must be no initial 210pp in the barite, and

(iii) the system must have remained closed with respect to 226Ra and 210Pp. As
will be discussed later, all these assumptions are satisfied for the Gulf of Mexico
barites.

2.2.3. Sample Processing for Dating

The samples used for dating are chimneys and crusts associated with barite-
bearing seeps in the Gulf of Mexico. Whereas the chimneys are dominated by
barite (>95% by volume), the crusts are composed of barite coexisting with
various amounts of carbonate and sulfide. Three types of samples (raw barite
sample, pure barite and hardened chimney sample) were analyzed in this
study.

For the raw barite samples, the original barite chimney and crust
fragments were air dried. Then, about 30 gm sample was taken from each
fragment and ground to a fine powder. Pure barite was also prepared for each
raw sample by chemical removal of carbonates, organic matter and sulfides
using methods modified from Church (1970) and Cecile et al. (1983).

Purification of a 1 gm sample was carried out by mixing it with 60 ml of 6N
double-distilled HCl and gently heated for 4 hr. This step removes carbonates
(calcite, aragonite and dolomite) and iron oxide. The sample is then thoroughly
washed by centrifugation to remove all leachate. Immediately following this
step, the sample is treated with warm (about 50°C) sodium hypochlorite
(slightly acidified with HCl to produce Clg gas) for 4 hours which removes the
organic matter and oxidizes the fine sulfides to iron oxides. Next, the residue is
washed and then leached with 0.02 M hydroxylamine hydrochloride in 25%
acetic acid in order to remove transition metal oxyhydroxides. Finally, the
residue is again reacted with 6 N double-distilled HCl and heated gently for 4 hr



44

70 T T T

607 1

8345 (°/00 CDT)
s

aft
*y ” |
© seawater
20r whe A chimney |]
m@ crust
10 1 1 1
5 10 15 20 25
5480 (°/00 SMOW)

Figure 3.11. Sulfur and oxygen isotope compositions of GOM barites (n = 34).
Sulfur and oxygen isotope compositions of the chimneys are similar to those of
modem seawater sulfate. In contrast, the crusts are enriched in both 34S and
180 relative to seawater sulfate.



89

(3) Anomalously high levels of Sr (mean 18.5 mM/L) and Ca (mean 610
mM/L) are generally assigned to Mesozoic formation waters (Macpherson,
1989). Albitization of detrital plagioclase in the Mesozoic reservoirs is
considered the principal source of Sr and Ca in Mesozoic formation waters
(Macpherson, 1989).

The source/s of the two end member of type III fluids can now be discussed
in light of the chemical and isotopic distinctions between the Cenozoic and

Mesozoic-derived formation waters. Based on the relationship between 5180 of
formation waters and depth in the sediment (Land and Macpherson, 1989), the

8180 of pore fluids are expected to be heavier than measured (Table 4.1) if they
are derived from Mesozoic strata. The 5180 values for the possible end-member

fluids are 1.7 and 0.19/00 (SMOW) in Garden Banks and Mississippi Canyon,
respectively (Table 4.1) suggesting a Cenozoic-age component in a mixed
source,

The relatively high concentrations of Sr and Ca in the fluids (Table 4.1)
suggest a partial contribution from a Mesozoic-derived source. Alternative
sources for Sr and Ca, such as dissolution of marine carbonates and/or
dissolution of anhydrite associated with the salt diapirs, are considered
unlikely because the measured levels of SO4 are incompatible with these
sources. Deficiency of SO4 (0.5 mM/L in Garden Banks and Mississippi Canyon,
Table 4.1) argues against Ca and Sr being sourced in carbonates and/or
sulfates, If all the Sr and Ca in the end member fluids were derived from
Mesozoic strata, the maximum "leakage" of Mesozoic-sourced Sr and Ca-rich
formation fluids are estimated to be between 14% and 17% in Garden Banks and

between 9% and 12% in Mississippi Canyon. The 87Sr/86Sr ratios, which are
0.70854 and 0.70843 for Garden Banks and Mississippi Canyon respectively, are
compatible with the values measured in Mesozoic formation waters (Stueber et
al., 1984) thus supporting our contention of a Mesozoic source for the Sr (and
Ca).

The lower contents of Sr and Ca but higher Cl and Na in the fluids of
Mississippi Canyon relative to those of Garden Banks can be explained by a
lower contribution from Mesozoic formation waters but a higher proportion
from salt dissolution. The higher Ba in Garden Banks end-member fluid could
be caused by the more intensive dissolution of K-feldspar in Garden Banks
than in Mississippi Canyon. This is consistent with the observation that the
saline end-member fluids in Garden Banks contain more Si than in Mississippi
Canyon (Table 4.1).

4.2, Radium and Barium in Barite-Bearing Seeps

4.2.1. Source/s of Ra and Ba in the Saline Fluids

Pore fluids of deep-sea sediments are generally enriched in 226Ra and

228Ra by factors of 5 to 100 and 60 to 250 over bottom water, respectively
(Somayajulu and Church, 1973; Cochran and Krishnaswami, 1977; Cochran,

1979; Key, 1981). The enrichment of 226Ra in the pore fluids is attributed to two
principal factors related to the distinct chemical behavior of U and Th in

seawater. First, alpha decay of 234U in the water column produces 230Th which
is scavenged by adsorption onto rapidly sinking particulate matter. Second,

the removal process results in 239Th activities in the sediment which are in



27

63.3 to 2453 dpm/L. These values are considerably higher than the ambient Gulf of

Mexico bottom water (13.2x0-2 dpm/L for 226Ra and 0.45x10-2 dpm/L for 228Ra, Key,
1981; Orr, 1988). Barium concentrations range from 0.12 mg/L to as high as 1173.3

mg/L which is up to 15x104 times higher than the upper bathyal waters in the Gulf
of Mexico (ie., 7.9 x10-3 mg/L, Buerket, 1997).

Typical depth profiles for 226Ra and 228Ra activities and Ba and salinity in
pore fluids from a core in Garden Banks (GB-382) are shown in Figure 3.1. The

salinity, Ba, 226Ra and 228Ra activities reach maximum values of 139 °/oo, 926
mg/L, 28.3x102 dpm/L and 24.5x102 dpm/L at 13 to 16 cm depth below sediment
surface, respectively, followed by a decrease of values of 119 °/oo, 291 mg/L,
7.0x102 dpm/L and 6.0x102 dpm/L at 22 cm depth.

The 226Ra and 228Ra activities analyzed here, of up to 3.7x103 dpm/L and 2.5x103
dpm/L, respectively, are by far the highest values reported so far from fluids
advecting through marine sediments. The positive relationships observed between

salinity and Ba (Fig. 3.2 A) and between salinity and 226Ra (Fig. 3.2 B) are remarkably
similar to that derived from formation waters produced by offshore oil and gas rigs
in the OCS Gulf of Mexico (Rabalais et al., 1991). We therefore propose that these

highly anomalous Ba, 226Ra and 228Ra values in the fluids from barite-bearing seeps
are derived from deep-seated and highly saline formation waters advecting to the
seafloor and mixing with modern seawater at the points of exit.

The data points in Figures 3.3 A and 3.3 B form two distinct mixing arrays between
subsurface fluid sources and the ambient Gulf of Mexico bottom waters. One mixing
line is characterized by a Ra/Ba ratio of 2.86 dpm/mg and a 228Ra/226Ra ratio of 0.84.
The other mixing line is formed by points having a higher excess of Ra over Ba

(Ra/Ba=4.82 dpm/mg) and a higher 226Ra excess over 228Ra (228Ra/226Ra=0.31). The
separation between the two types of fluids are likely to be related to segregated
sources in the subsurface having different Ra activities and Ba concentrations and
distinct. 232Th/238y parent ratios.

The Ra/Ba ratios of both fluid groups are anomalously low compared with
seawater. For example, the Ra/Ba ratios of the seep fluids are about 0.4 to 0.21
times the ratio in surface ocean water (Ra/Ba=6.6 dpm/mg). The 228Ra/226Rq
ratios of the fluids are 0.84 and 0.31 (derived from the slopes in Fig. 3.3 B)
which are substantially higher than the ratio in the ambient Gulf of Mexico

deepwater by a factor of 25 and 9, respectively (228Ra/226Ra=0.034, Orr, 1988).

3.1.2 Chemical and Isotope Compositions

Chemical and isotope compositions of pore fluids representative of seeps
(n=47) and from the Gulf of Mexico bottom waters at bathyal and abyssal depths
(n=2) are listed in Table 3.1. In order to ascertain the origin of the solutes in
the pore fluids, we calculated enrichment factors by using the relation Ef =
(Ci/Cw)f/(Ci/(Cw)sw (Aharon et al., 1992), where Ci and Cw are the

concentrations of constituent "i" and of water "w" and the the subscripts "f"
and "sw" refer to fluids and the Gulf of Mexico appropriate deepwater,
respectively. An Ef value of >1 indicates a net addition of the constituent to the
source water, whereas an Ef value of <1 indicates removal of the constituent
from the original seawater.



127

gives total target dose due to the entire radioactive volumetric source. The
mathematical concept is a special case of a Green’s function. A rigorous
treatment of this method is found in Chilton et al. (1984).

Table 4.6 lists the mean activities of 226Ra and 228Ra radionuclides in the
barite deposits measured in this study. For calculation purposes, the irradiated
target (mussel) was assumed to be at a position of 1 cm from the geometric
surface of the cone (barite chimney); Both chimney and mussel were
immersed in a water medium.

Internal dose contributions from 72°Ra incorporated into mussel soft
tissues would normally be calculated based on alpha emssion energy
transmitted from radium and its short-lived daughters to the unshielded tissues

of the whole organisms at 24 MeV per 226R, atom deposited. Actual calculations

based on measured 726Ra levels in barite-seep mussels reveal that the internal
dose rate is insignificant compared to external dose rates received by the
mussels. This inference is supported by the observation that there is no

significant difference of internal 226R, between control mussels (collected at
carbonate-bearing seeps) and radium-rich, barite-bearing seep mussels (Table
3.11).

The average absorbed external dose rates generated by radium-rich barite
chimneys and crusts are listed in Table 4.7. Accordingly, the estimated hourly
radiation dose rates are 6.4 Gy and 12.8 uGy for Garden Banks and Mississippi
Canyon, respectively. Age dating data have shown that a Bathymodiolus spp.
mussel of an average size lives for at least 25 years (Nix et al., 1995). Assuming
that the biota spend 100% of their time near a chimney, the maximum external
lifetime dose based on the highest total Ra value in the barite deposits back to
the time of collection is estimated to be 11.2 cGy/y (11.2 rad/y) or 280 cGy (280
rad) per average mussel lifetime of 25 years.

About 10 mGy/day threshold is necessary before detrimental effects to
aquatic organisms is noticeable (IAEA, 1992). The largest possible dose to the
mussels (without regard to possible radon gas dissolved in bottom water) is
estimated to be 0.0128mGy/day or 112 mGy/year which is much smaller than
the cited threshold. Therefore, radiation exposures from radiation sources both
external and internal to the fauna may pose an insignificant radiation hazard
to the methanotrophic mussels. The effect on gametes and subsequent
generations of mussels may be insignificant as well.

It should be noted for purposes of comparison that the 25 year cumulative
dose of 2800 mGy (280 rad) is a potentially lethal dose if received all at once in
an acute exposure, and is greater than the mean dose received by Hiroshima
survivors (Gofman, 1981). This dose would result in approximately 8.4% excess
cancers in an irradiated population of humans (BEIR V, 1990). This dose rate
and lifetime dose would definitely be of concern both somatically and
genetically if applied to a human population. However, due to the complexity
of radiation interactions between species, it is impossible to speculate at this
time what the long term effects on seep mussels would be.

A potentially significant source of radiation exposure to seep organisms

could be contributed from dissolved 222Ry in the surrounding bottom waters.
Laboratory tests of sampled barites reveal that barite can emanate (or lose) as
much as 3% of its radon gas in secular equilibrium with radium to the

environment. Internally deposited 222Ry and its alpha emitting decay
products in biological tissues is potentially damaging. Since no direct
measurements of radon gas dissolved in water could be carried out during this



11

electron microprobe techniques. Corrections for matrix effects used in the
final computations followed Bence and Albee (1968).

For sulfur, oxygen and strontium isotope analyses of barite samples were
purified with a modified procedure described by Church (1970). Samples were
first treated with 6 N HCl to decompose calcite, dolomite, and iron oxide. Organic

phases were then removed by reaction with warm (about 50 °C) sodium
hypochlorite. Silicate minerals were neglected due to their low concentrations
(<1% by volume).

834 was measured by graphite reduction of BaSO4 to BaS, which was
subsequently dissolved in water and precipitated as Ag2S with 0.1 N AgNO3.

Ag2S was then oxidized with Cu2O (mixture 1:4 by mass) at 900°C to SO2, which

was analyzed with the mass spectrometer. The overall error (+ 1) of 8348
determinations is better than 0.2°/00 and the reference standard is troilite
from the Canyon Diablo meteorite (CDT).

8180 of SO4 were analyzed in the manner described by Rafter and Mizutani
(1967) and Mizutani (1971). Intimate mixtures of BaSO4 and _ spectrographically
pure (99.999%) graphite (1:2 by mass) were decomposed in resistance-heated
platinum boats near 1000 °C. In addition to COZ some CO was produced, which
was converted into COQ using a high-voltage electrical discharge between

platinum electrodes in a trap assembly immersed in liquid nitrogen. Isotope
measurements were made on a Nier-type triple collector gas source mass
spectrometer. Accuracy and precision of 8180 determinations is better than
0.3°/00 based on repeat standard analyses and the results are reported relative
to the standard mean ocean water (SMOW).

For strontium isotope analysis pure barite was decomposed with sodium
carbonate (Sr free Na2CO3) using the method modified from Breit et al. (1985).

0.1 g of barite, 1 g of sodium carbonate and 200 ml of triple-distilled water were

combined in a Teflon beaker. The beaker was covered and heated at 95°C for 4
hours. After cooling the solid residue (SrCO3 and BaCOQ3) were separated by
filtration. The residue is rinsed thoroughly with triple-distilled water to
remove sodium and sulfate. The SrCO3 and BaCO3 obtained this way were

dissolved with 6 N HCl and Sr was then separated using standard ion

chromatography techniques. 87Sr/86Sr ratios were measured on a Finnigan
MAT 262 six collector thermal ionization mass spectrometer (TIMS). The results

are normalized to 87Sr/86Sr of NBS-987 = 0.71023 and to 86sr/88sr = 0.1194 and

carry an estimated analytical error of 1x10-5 (20).
For carbon isotope analysis of carbonates in the crusts, powders of barite

crusts were dissolved at 50°C in 100% H3P03 under vacuum, and 513¢ and 8180
of the carbonates were determined on the cryogenically purified CO? using an

automated Nier-type triple collector mass spectrometer. The isotope data are
reported in the conventional "d" notation in permil relative to the PDB

standard. Accuracy and precision for 313C and 8180 are within 0.19/00 .
2.2.2. Principles of Dating Methods
Barite deposits including chimneys and crusts were examined with a

portable gamma-ray detector (R meter) which indicated that they were
unusually highly radioactive. These initial radioactivity measurements led to a



Sample Age Initial
name (yrs) 2228p a/226R4
Ch#6-5 1.6240.01 1.0940.01
Chit6-4 1.7840.01 (1440.01
Ch#6-3 1,9440.02 L1Lt0.01
Ch#o-2 2,150.01 {130.01
Ch#6- | 2.48:40.04 1,02+0.01

Figure 3.17, Dating profile of growth increments in barite chimney# 6 from Garden Banks Block 382 using the
2287 h/228Ra method. Ages were corrected to July 20, 1993 when the chimney was sampled.

ge


124

basic unit of radiation dose is expressed in terms of absorbed energy per unit
mass of tissue. The SI unit for radiation absorbed dose is the gray (Gy) and is
defined as an absorbed dose of one joule per kilogram of tissue (Cember, 1996).
The gray is universally applicable to all types of ionizing radiation-irradiation
due to external fields of gamma rays, x-rays, neutrons as well as charged
particle emissions due to internally deposited radioisotopes.

The “rad" (radiation absorbed dose) is still widely in use in the U.S. and will
be briefly defined for comparison purposes here. Before the introduction of
the SI units, the radiation dose was measured by a unit called the "rad". One rad
is an absorbed radiation dose of 100 ergs per gram of tissue. One Gy is equal to
100 rad. Although the gray is the newer unit and will eventually replace the
rad, the rad nevertheless continues to be widely used. Since one centigray
(cGy) equals one rad, it is convenient to utilize this unit division of the Gy.

Somatic sensitivity to acute (delivered all at once) radiation dose varies
greatly among phylogenetic groups and tends to increase with phylogenetic
heirarchy. For mammals, the acute dose LD59 is approximately 400-500 cGy. For
molluscs, the acute dose LDsq is approximately 2000 cGy (Bacq and Alexander,
1961).

Genetic effects (mutations caused in chromosomes) manifested by
tadiation-produced mutations in germinal cells being carried on to future
generations remains a topic of much debate among experts (BEIR V, 1990) .
There are no reliable data available for calculating a genetically deleterious
radiation dose to molluscs in natural environments. Therefore, no attempt can
be made to estimate genetic effects from radiation measurements conducted
during this study.

4.5.2. Chemical and Radiation Flow of Radium Into Benthic Fauna

Figure 4.17 represents a conceptual model for chemical and radiation flow
derived from radium into seep fauna as a synthesis of interelationships
suggested by our observations.

226Ra and 228Ra resulting from the 238y and 232Th decay series,
respectively (Fig. 2.3) are highly mobile in concentrated brines and are
transported along with soluble barium via formation waters through
subsurface faulting zones to the seafloor where it coprecipitates with barium
sulfate and consolidates into radioactive barite deposits. About 2% (Ritcey,
1990) to 30% (our own experiments) of radium does not coprecipitate with
BaSO4 under ideal and realistic conditions, respectively, and is available in
soluble form for intake by seep fauna (Fig. 4.17 A). Because the Bathymodiolus
spp. mussels are methanotrophic symbiont-bearing and not filter-feeders,
radium cannot be ingested and subsequently absorbed through the digestive
tract. However, water circulation through the gills would provide a mode for
internal absoption of radium from the point source effluent. The radium would
then be available for incorporation into soft tissues and/or calcareous shells
by replacement of Ca in the carbonate lattice. Radium in the barite would also
dissolve slightly into interstitial pore fluids along with barium. It seems
apparent that pore fluid constituents such as radium and barium are
incorporated at their respective pore fluid concentrations in mussel shells
(Fig. 3.29).

Heterotrophic fauna such as galatheid crabs would also be capable of
internalizing radium via gill circulation and absorption of radium through
the large surface area provided by the gills. An additional source of radium to
scavenging heterotrophs is radium intake into the G.I. tract via fine barite



108

Therefore, the 8234S values of hydrothermal vent barites depend on the
proportions of the above sources in the mixture of the fluid from which the
barites precipitated (Zierenberg et al., 1984; Bertine and Keene, 1987; Kusakabe
et al., 1990). The 8180 values of hydrothermal barites are controlled primarily
by the ambient temperature which determines both the degree of sulfate
reduction and the extent of oxygen isotope exchange between dissolved sulfate
and water (Lloyd, 1968; Bertine and Keene, 1987; Kusakabe et al., 1990; Urabe
and Kusakabe, 1990).

Concerning the GOM barites, it is important to note that both 8345 and 5180
of the chimneys, listed in Table 3.5, are isotopically similar to modem sea
water sulfate. Therefore it is apparent that seawater sulfate has been the chief
contributor to the formation of the barite chimneys. Modern seawater can be
dismissed as a source for the crusts because of their substantially heavier 5348

and 6180 values relative to modern seawater sulfate. The possibility that the
crusts formed by oxidation of sulfides is also ruled out because this process
should yield lighter rather than heavier 8345 values. Sulfate reduction at high
temperature could cause enrichments of 345 and 180 in barites because
sulfates containing 160 and 325 isotopes are preferentially reduced there
(Lloyd, 1968). Enrichment of 180 in barites can also be attributed to isotope

exchange between dissolved sulfate and water, accelerated at higher
temperatures, and cause the oxygen isotope composition of sulfate to shift
toward the equilibrium sulfate-seawater value of 38°/00 SMOW (Lloyd, 1968:
Zierenberg et al., 1984; Bertine and Keene, 1987; Kusakabe et al., 1990; Urabe
and Kusakabe, 1990). Regarding the GOM barites, however, the temperature
factor seems to be insignificant in controlling the 8348S and 8180 of the barites
because our direct observations indicate relatively cold (8.6°C) and practically
invariable bottom-water temperatures.

The observed enrichments of 348 and 180 in the GOM barite crusts (Fig.
3.11) are likely caused by bacterial reduction of seawater sulfate. This
interpretation is corroborated by the following two lines of evidence. First,
SEM analyses reveal that the crusts consist of up to 8% by volume pyrite.
During bacterial sulfate reduction to H2S, the gas may escape from the system
or react with ferrous iron to form pyrite. Therefore the presence of pyrite in
the crusts is a good indicator of sulfate reduction. Second, the 3136 of calcite in
the crusts ranges from -21 to -36°/00 PDB (Table 3.5). These light isotope values
suggest that the carbonate-carbon is derived from the microbial degradation
of organic phases (most likely thermogenic methane). As previously discussed,
reduced carbon is oxidized to produce HCO3~ during microbial sulfate reduction

which is precipitated subsequently as authigenic carbonate with light 5!3c

inherited from the fossil fuel- derived carbon. Thus, the light 813C values of
calcite strongly indicate the occurrence of sulfate reduction during the
formation of the barite crusts.

The enrichments in 345 and 180 relative to the seawater in samples from
imer side of chimneys (Table 3.5 and Fig. 3.12) suggest that sulfate reduction
also occurred during the formation of the chimneys interior. However, the
sulfate reduction rate and intensity occurring in the chimneys must be lower
than that occurring in the crusts. This is because the enrichments in 34S and

186 relative to the seawater in chimneys are much smaller than those in the



110

6 T T T
:
5r . 7
:
=
3 4
&
a 4
1S)
= GOM 7
© other settings

30 40
Sr (mol %)

Figure 4.13. Compositions of Ca and Sr in barites from GOM and barites from
other settings (hydrothermal, marine and continental settings). Note:
Compared with barites from other settings, the GOM barites are enriched in Sr
and Ca by a factor of about 15 and 28, respectively. The data for hydrothermal
barites are from Hofmann and Baumann (1984), Kalogeropoulos and
Mitropoulos (1983), Hannington and Scott (1988), and Bertine and Keene (1987)
and the data for marine and continental barites are from Church (1970) and
Hanor (1966).



Figure 1.1.

Figure 1.2.

Figure 2.1.
Figure 2.2.

Figure 2.3.
Figure 2.4.
Figure 2.5.
Figure 2.6.
Figure 3.1.

Figure 3.2.
Figure 3.3.

Figure 3.4.

Figure 3.5.
Figure 3.6.
Figure 3.7.

Figure 3.8.

Figure 3.9.

Figure 3.10.

Figure 3.11.
Figure 3.12.

Figure 3.13.
Figure 3.14.

Figure 3.15.

Figure 3.16.
Figure 3.17.

Figure 3.18.

ix

LIST OF FIGURES

PAGE

Base map of the study area on the Northem Gulf of Mexico
slope and basin showing the location of hydrocarbon seep sites
where cores, pore fluids, barites and fauna were acquired during
submersible dives... sents
Occurrences of highly radioactive barite deposits associated

with barium/radium/methane-rich fluid emissions in

deepwater Gulf of Mexico.....

Flow chart of chemical and isotope analyses of pore fluids .........---.-ssecseesneneee 8
Flow chart of imaging, chemical and isotope analyses

of barites
226Ra, 228Ra and their parent and daughter radionuclide:
Radioactivity decay and growth in the system 228Ra-228Th
Radioactivity decay and growth in the system 226Ra-210pp
Flow chart of chemical and isotope assays of fauna saeesseatessesnecenees
Depth profiles of salinity, Ba, and 226Ra and 228Ra activities

in the pore fluids from core 3566-1 taken at Garden Banks block 382...
Salinity, Ba and Ra relationships in pore fluids from barite-bearing seeps.
Ra-Ba and 226Ra-228Rq relationships in pore fluids from

barite-bearing seeps...
Plots of Cl against Ca, ‘Sr, Ba ‘and ‘$04 in pore fluids

C1-8180 relationships in pore fluids from seeps .
Barite chimney #2 from the Gulf of Mexico slop:
Distinct morphologies of the barite chimneys displayed

in SEM photomicrographs from thin section. A: String-like

morphology. B: Dendritic-like morphology .. 36
Principal building block of the barite in SEM views.

A: Rosette-shaped barite. B: Three dimensional view of

the rosette. . 37

SEM view of the accessory pyrite associated with the barite chimneys. A: Core
and rim of euhedral pyrite. B: pyrite framboid containing microcrystallites. .....39
Microprobe analyses from barite chimneys and crusts.

A: Positive linear relation between Sr and Ca compositions.

B: Inverse linear relation between Ba and Sr composition:
Sulfur and oxygen isotope compositions of GOM barites..
Sulfur and oxygen isotope compositions Profile

of chimney #9..... sees seseeseescaeesesteseseecenennes
8751/86Sr ratios of Gulf of Mexico barites: and: seawater...

Gamma-ray spectra of uranium, thorium ore standards

and GOM barite chimneys and crusts.
Chimney #9 from Garden Banks block 382
Chimney #10 from Garden Banks block 382.
Dating profile of growth increments in barite chimney #6

from Garden Banks Block 382 using the 228Th/228Ra method ........--esnseeee-55
Dating profile of growth increments in barite chimney #8

from Garden Banks Block 382 using the 228Th/228Ra method .....ccessssssesssseee 56

40
44



87

occurrence of barite sands in the cores from which the type III pore fluids
were acquired.

In order to decipher the origin of type IIL pore fluids it is essential to
derive the chemical composition of the saline end member. The two regression

lines in the plot of 8757/8657 against 1/Sr for the pore fluids type III (Fig. 4.3)
suggest two distinct saline end-members in Garden Banks and in Mississippi
Canyon barite-bearing seeps. It is obvious that the two intercepts in Figure 4.3

should approximately represent the lowest possible 87Sr/86$r ratios for the
two end-member fluids. The two intercept values are 0.70854 and 0.70842 which
are identical to those of the highest salinity pore fluids (Table 3.1) in Garden
Banks and Mississippi Canyon, respectively.

Lower Br/Cl (0.93x10-3 and 0.90x10-3 in Garden Banks and Mississippi
Canyon, respectively, Table 4.1) and higher Na/Cl molar ratios (0.901 and 0.948
in Garden Banks and Mississippi Canyon, respectively) in the possible saline

end member fluids relative to seawater (1.52x10-3 and 0.857, respectively)
suggest that the elevated Cl and Na levels in the saline fluids were acquired by
subsurface salt dissolution. Dissolution of salt, however, could not by itself
explain the anomalously high levels of Ca, Sr and Ba in the fluids (Table 4.1).
The presence of shallow salt diapirs under the sampling areas in Garden Banks
382 and Mississippi Canyon 929, the occurrence of massive barite deposits (Fu
et al, 1994; Fu and Aharon 1997), and direct observations of gaseous
hydrocarbon and fluid venting during sampling, suggest that these end
member fluids were derived from deep-seated formation waters dissolving the
Jurassic-age salt during upward transport. The positive relation between pore
fluids 226Rq and salinity (Fig. 2.3 B) and its similarity to that found in produced
waters from oil and gas fields in the Gulf Coast region (Fu et al, 1996; see
below), and the similar negative relationship between pore fluids SO, and Ba

similar to that found in the formation waters offshore Louisiana (Fig. 4.4)
confirms the contention that these hydrocarbon-rich seeping fluids were
sourced in deep-seated reservoirs.

Formation waters in the offshore Gulf of Mexico may be transported to the
seabed in seeps from two distinct sources, either by dewatering of Cenozoic-
age clastic sediments, or by injection from deep-seated Mesozoic-age brines.
The two principal types of formation waters differ sufficiently in their
chemical constituents and isotope compositions in order to allow a more
specific chemical fingerprinting of the fluids advecting in seeps. The
chemical and isotope distinctions between the two principal formation waters
can be summarized as follows.

(1) Mesozoic and Cenozoic-sourced formation waters are distinguished on

the basis of their 5180 values which are controlled by the temperature and
extent of interaction between water and host sediments (Land and
Prezbindowski, 1981; Land and Macpherson, 1989). Most formation waters in

Mesozoic strata of the Gulf of Mexico exhibit relatively heavy 8180 values (ie,

>6°/00 SMOW) because extensive oxygen isotope exchange occurred between
water and rocks at the high temperatures typical of sediments buried at more
than 6 km depth (Land and Prezbindowski, 1981; McGee et al, 1994).

(2) Ba-rich formation waters in the Gulf of Mexico are primarily derived
from Cenozoic reservoirs and are produced from dissolution of detrital
Potassium-Feldspar containing 0.2 to 0.5 weight percent Ba (Macpherson,
1989).



32

10000

Ca (mM/L)

10 f . 1
0 1000 2000 3000 4000
Cl (mM /L)
10000 © 4
1000 1,
g 3 a0}
$s 4 10
= E
S
g z z
RT
1 1 1 1
0 1000 2000 3000 4000 “0 1000 2000 3000 4000
Cl (mM/L) Cl (MIL)

Figure 3.4. Plots of Cl against Ca, Sr, Ba and SO4 in pore fluids. Note that three
types of pore fluids can be discerned on the basis of Ca/Cl, Sr/Cl, Ba/Cl and
SO4/Cl molar ratios. The empty circle represents deepwater Gulf of Mexico, the
solid circle, empty triangles, and solid triangles represent the pore fluids type
I, UW and III, respectively.



119

enrichments to coastal waters and such fluxes may contribute to the 226Ra in
the global ocean. The results of this study indicates that natural discharges of

Ra through faults intersecting the seafloor may be another source of 226Ra in
the ocean and further studies are needed to assess this problem.

4.4.3, The Impact of Radium/Barium-Rich Fluid Venting on the
Benthic Fauna

A total of 42 faunal samples were assayed for 226R, activities which range
widely from below detection limits to 10.7 dpm/g (Tables 3.11 and 3.12). A
simple student-t test reveals that only soft-body galatheid crabs from barite
seeps are significantly different from the rest of the seep faunal elements.
This result can be explained by a number of conjectures. The most likely
explanation is that the scavenging behavior of these crabs results in the
ingestion of radium-rich barite material and consequent deposition of
resistent particles in the GI tract of the crabs. Although it is beyond the scope
of this study, future investigations should include separation of GI tracts from
whole soft tissue in all seep fauna, including methanotrophic mussels, so that
this relationship can be clearly delineated.

The observation that calcareous shells and soft tissue bodies of the
methanotrophic mussels from barite and carbonate seeps are statistically
indistinguishable with regard to their 226Ra content leads to the following
plausible explanations.

The high concentrations of Ba present in the venting fluids of barium-
radium-rich sites may cause rapid precipitation of BaSO4 when the venting

fluids come into contact with seawater sulfate. The accompanying soluble

226R, is consequently rapidly co-precipitated with highly insoluble barite;
thus, the seep water is possibly relatively Ra depleted (up to 98% removal
efficiency according to Hanslik and Mansfield, 1990) when it reaches the
mussels surrounding the barite deposits. In contrast, natural hydrocarbon
Seep emissions contain barium concentrations well below the Kgp for barite;

Hence, available 226p, does not coprecipitate with BaSOq because it does not
precipitate in these sites. “Free” radium present in carbonate seeps emission
fluid is, therefore, possibly available for uptake by fauna at carbonate-seep
sites. It was not possible to test this conjecture because all available fluids
consisted of interstitial pore fluids and did not necessarily represent
epibenthic venting fluids issuing directly from the vents.

The extremely high concentrations of Ba in the venting fluids of barium
seeps may competitively interfere with the biological uptake of relatively low
concentration Ra in the venting fluids. Barium is present in the radium-rich

barite formations at g/g concentration levels, while coprecipitated 226R, is
Present at the parts per trillion (picograms/g) level. This is representative of
the relative abundances of soluble elemental barium and radium in aqueous
venting fluids (Fig. 3.29). If the biokinetics of uptake can be assumed to follow
first order processes, the rate of uptake before equilibrium levels are reached
will be proportional to the elemental concentration of each ionic species.

Given a relative abundance ratio of 226Ra/Ba of approximately 10°12,

biouptake of Ba should be greatly enhanced over that of 226R, in agreement
with our observations (Fig. 3.29). In addition, the discrimination factor (DF)
for radium with respect to barium in marine organisms is different and



94

The initial 228Ra/226Ra activity ratio of the barite chimneys could be used
as a measure of (228Ra/226Ra)g for the advecting fluids. Thus the average

initial 228Ra/226Ra activity ratio of chimneys are 1.09 +0.07 and 0.71 +0.08 for
the fluids in Garden Banks and Mississippi Canyon, respectively (Table 3.6).
The FRT calculated by this method range from 0.6 to 2.9 years indicating that
the fluid seepage in the study areas is recent. This conclusion is consistent
with our observations of pervasive active fluid seepage during the submarine
sampling and the dating of barite chimney-ages of <7 years (Table 3.6).

4.3. Ages and Growth Rates of Barite Deposits

The following observations emerge from Figure 4.6 summarizing the
radiometric ages of barite chimneys and crusts acquired in this study: (i) in
general, the crusts are older than the chimneys; (ii) crusts from both Garden
Banks and Mississippi Canyon areas have a similar age range, from 9.0 to 23.1
years old, and (iii) the chimneys from Mississippi Canyon yield ages from 2.5
to 6.5 years which are slightly older than the chimneys from Garden Banks
which range from 0.4 to 2.5 years. The interpretation that barite deposition in
the Garden Banks and Mississippi Canyon sites is a relatively recent event is
substantiated by the fact that very little or no sediment cover was observed on
top of these barite deposits.

Concerning the age difference between the chimneys and the crusts, the
question that needs to be addressed is why all the chimneys are younger than
the crusts. Two possibilities are proposed here. First, based on the growth rates
(see below), the construction of a chimney should be rapid and most chimneys
were probably built within 5 years (assuming the height is less than 30 cm
and average growth rate is about 7 cm/yr). Because of the friable nature of
these chimneys and the possible ocean bottom current erosion, the chimneys
may collapse within a short period of time after their formation. Therefore,
the life span of a chimney from its initial formation to its toppling was
probably very short. This may be the main reason why all of the chimneys
dated here are very young (< 8 yrs). Second, it is noted that the surface of most
crusts is older than the bottom (Table 3.8) allowing to postulate a time-
sequence for their formation. During the early stage of fluid seepage, the Ba-,
and Ra-rich fluids mix with sulfate-rich seawater and precipitate barite which
forms the upper part of the crust. The later seeping fluids are probably
trapped below the fledgling crust surface and the Ba and Ra-rich fluids react
with the dissolved sulfate in the pore fluids and precipitate barite which forms
the lower part of the crusts. Because of sampling depth limitation with the
submersible robot arm, only surficial crusts were collected. Therefore, the
barite precipitation events deciphered here should represent the earliest
events occurring in the formation of the crusts which may explain why the
crusts are older than the chimneys.

Alternatively, the observed age gap between chimneys and crusts may
have resulted from sampling biases. A more systematic sampling is needed in
future studies to fully understand the complete history of the barite deposits.

4.3.1. Growth Rates of Chimneys

The 228Th/228Ra age profiles obtained from hardened chimneys #6, 8, and
11 (Figs. 3.17, 3.18, and 3.19) are used to reconstruct the growth history of the
chimneys. Growth rates were obtained from the slopes of the regression lines



35

°o

al
m
=
=
=
in
ay
m
a
a

Coe
|
Fy

Figure 3.6, Barite chimney #2 from the Gulf of Mexico slope. A: Side view of the
brown-reddish stained chimney. Note the protruding nodule on the middle left
and the top orifice of the chimney: B: Transverse section through a chimney
showing the alternating dark-gray and light-yellow growth layers.