Fate and Effects of Barium and

Radium-Rich Fluid Emissions from

Hydrocarbon Seeps on the Benthic Habitats

of the Gulf of Mexico Offshore Louisiana

US. DeDartment of the Interior

Minerals Management Service Gulf of Mexico OCS Region

OCS Study

MMS 2001 -004

Coastal Marine Institute

Cooperative Agreement Coastal Marine Institute

Louisiana State University
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

Authors

Paul Aharon

Dan Van Gent

Baoshun Fu

L. Max Scott

January 2001

Prepared under MMS Contract

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

OCS Study

MMS 2001 -004

Cooperative Agreement Coastal Marine Institute

Louisiana State University
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 radiumrich 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.

111
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 endosyrnbionts 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 (87Sr/86Sr) and stable oxygen isotopes (ö'80). 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/86Sr ratios and stable

l 80 and 34S 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 15x 10, 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

V
vi

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

Mississippi River.
TABLE OF CONTENTS

PAGE

List of Figures vii

List of Tables ix

Acknowledgments . xi

Introduction 1 1.1. Statement of the Problem 1 1.2. Objectives and Questions Addressed in the Study 3

1.3. Site Descriptions 5

Methodology 7

2.1 Pore Fluids 7

2.2. Barite Deposits 9

2.2.1. Imaging, Chemical and Stable Isotope Assays 9

2.2.2. Principles of Dating Methods 11

2.2.3. Sample Processing for Dating 15

2.2.4. Instrumentation and Analytical Methods 17

2.3.SeepFauna is

Results 23

3.1 Hydrocarbon-Rich Fluids 23

3.1.1. Radium and Barium Assays in Barite-Bearing Seeps 23

3.1.2 Chemical and Isotope Compositions 27

3.2 Barite Deposits Associated with Seeps 34

3.2.1. Morphology and Petrography 34

3.2.2. Elemental Chemistry 38

3.2.3. Sulfur, Oxygen and Strontium Isotope Assays 42

3.2.4. Radioisotope Assays 46

3.2.4.1. Soft and Fragmented Chimneys 48

3.2.4.2 Hardened Chimneys 54

3.2.4.3 Barite Crusts 54

3.2.5. Validation of the Ra-Series Dating Methods 59

3.2.6. 210Pb/226Ra Ages 63

3.2.7. 228ThI228Ra Ages 66

3.3. Radium and Barium Systematics in Chemotrophic and Heterotrophic Benthic Fauna

from Barite-Bearing Seeps 70

3.3.1. Mussels with Methanotrophic Endosymbionts 70

3.3.2. Heterotrophic Fauna 76

3.3.3. Barium-Radium Relationship in Fauna 76

vii
yin

Discussion 81

4.1. Sources of Fluids Advecting at Seeps 81

4.1.1. Seawater-Derived Fluids 81

4.1.2. Fluids Derived by Dissolution of Salt Diapirs 82

4.1.3. Fluids Derived by Advection of Deep-Seated Formation Waters 83

4.2. Radium and Barium in Barite-Bearing Seeps 89

4.2.1. Source/s of Ra and Ba in the Saline Fluids 89

4.2.2 The Residence Time of Fluids in Barite-Bearing Seeps 93

4.3. Ages and Growth Rates of Barite Deposits 94

4.3.1. Growth Rates of Chimneys 94

4.3.2 Initial 228Ra/226Ra in the Barites 96

4.3.3. 226Ra Activity in the Barites and Its Source/s 102

4.3.4. The Source of Sulfate in the Barites 107

4.3.5. The Source of Major (Ba) and Trace Elements (Sr and Ca) in the Barites 109

4.3.6. Depositional Model of the Barites in the Gulf of Mexico 111

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

Habitat 111

4.4.1. Fluxes of Radium/Barium Rich Fluids from Point Sources 111

4.4.2. Leakage of Radium/Barium Rich Fluids in the Benthic Habitat 116

4.4.3. The Impact of Radium/Barium Rich Fluid Venting on the Benthic

Fauna 119

4.5 Radiation Consequences of Radium Leakage from Seeps on the Benthic Fauna 121

4.5.1. Radium Metabolism and Radiation 121

4.5.2. Chemical and Radiation Flow of Radium Into Benthic Fauna 124

4.5.3. Radiation Risk Assessment 126

Conclusions and Recommendations 131

References 133
LIST OF FIGURES

PAGE

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 2

Figure 1.2. Occurrences of highly radioactive barite deposits associated

with bariumlradiumlmethane-rjch fluid emissions in

deepwater Gulf of Mexico 4

Figure 2.1. Flow chart of chemical and isotope analyses of pore fluids 8

Figure 2.2. Flow chart of imaging, chemical and isotope analyses

of barites 10

Figure 2.3. 226Ra, 228Ra and their parent and daughter radionuclides 13

Figure 2.4. Radioactivity decay and growth in the system 228Ra-228Th 14

Figure 2.5. Radioactivity decay and growth in the system 226Ra-2' 0Pb 16

Figure 2.6. Flow chart of chemical and isotope assays of fauna 19

Figure 3.1. Depth profiles of salinity, Ba, and 226Ra and 228Ra activities

in the pore fluids from core 3 566-1 taken at Garden Banks block 382 28

Figure 3.2. Salinity, Ba and Ra relationships in pore fluids from barite-bearing seeps 29

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

baritebearing seeps 30

Figure 3.4. Plots of Cl against Ca, Sr, Ba and 504 in pore fluids 32

Figure 3.5. Clö18O relationships in pore fluids from seeps 33

Figure 3.6. Barite chimney #2 from the Gulf of Mexico slope 35

Figure 3.7. Distinct morphologies of the barite chimneys displayed

in SEM photomicrographs from thin section. A: String-like

morphology. B: Dendritic-like morphology 36

Figure 3.8. Principal building block of the barite in SEM views.

A: Rosette-shaped barite. B: Three dimensional view of

the rosette. 37

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

Figure 3.10. Microprobe analyses from barite chimneys and crusts.

Positive linear relation between Sr and Ca compositions.

Inverse linear relation between Ba and Sr compositions,. 40

Figure 3.11. Sulfur and oxygen isotope compositions of GUM barites 44

Figure 3.12. Sulfur and oxygen isotope compositions profile

of chimney #9 45

Figure 3.13. 87Sr/86Sr ratios of Gulf of Mexico bantes and seawater 47

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

and GOM bante chimneys and crusts. 51

Figure 3.15. Chimney #9 from Garden Banks block 382 52

Figure 3.16. Chimney #10 from Garden Banks block 382 53

Figure 3.17. Dating profile of growth increments in barite chimney #6

from Garden Banks Block 382 using the 228Th1228Ra method 55

Figure 3.18. Dating profile of growth increments in barite chimney #8

from Garden Banks Block 382 using the 228Th1228Ra method 56

ix
xFigure 3.19. Dating profile of growth increments in barite chimney #11

from Mississippi Canyon block 929 using the 228ThI228Ra method 57

Figure 3.20. Relationship between the first and second 228Th/228Ra

dating of pure barites. 62

Figure 3.21. 2 10Pb/226Ra ages of pure barites 64

Figure 3.22. 210Pb/226Ra of barites before and after purification 65

Figure 3.23. Sulfide effects on the 210Pb/226Ra dating 68

Figure 3.24. 228ThI228Ra ages plotted against their 228ThI228Ra ratios of chimneys based

on pure barites. 69

Figure 3.25. Relationship between 228Th/228Ra ages of raw barites

and pure barites of chimneys. 71

Figure 3.26. Radium-226 activity concentrations of methanotrophic mussels

body tissues 74

Figure 3.27. Radium-226 activity concentrations of methanotrophic mussel calcareous shells75

Figure 3.28. Radium-226 activity concentrations of heterotrophic seep fauna 77

Figure 3.29. The relation of barium and 226Ra in hydrocarbon seep fauna 79

Figure 4.1. Na-Cl relationship in pore fluids type II 84

Figure 4.2. Plots of Cl against Na, Ca, Sr and Ba in pore fluids type III 85

Figure 4.3. Plot of 87Sr/86Sr against 1/Sr in pore fluids type III 86

Figure 4.4. Barium-sulfate relationship in formation waters from Miocene and Pleistocene

reservoirs offshore Louisiana 90

Figure 4.5. 226Ra-salinity relationship in formation waters and

discharged produced waters from the Gulf Coast region 92

Figure 4.6. Ages of barite chimneys and crusts in the Gulf of Mexico 95

Figure 4.7. Vertical age profile for chimney #6, Garden Banks block 382 97

Figure 4.8. Vertical age profile for chimney #8, Garden Banks block 382 98

Figure 4.9. Vertical age profile for chimney #11, Mississippi Canyon block 929 99

Figure 4.10. Initial 228Ra/226Ra versus age of barite chimneys 101

Figure 4.11. 228Ra/226Ra AR versus 228Th1228Ra ages of chimneys 103

Figure 4.12. 226Ra activities in barites formed in different depositional environments 104

Figure 4.13. Compositions of Ca and Sr in barites from GOM and barites

from other settings (hydrothermal, marine and continental settings). 110

Figure 4.14. Depositional model of GOM bantes 112

Figure 4.15. Dominant morphological truncated conical-shaped barite chimneys used in the

modeling of radium fluxes 114

Figure 4.16. A comparison of 226Ra activity ranges from literature

with results from barite seeps 123

Figure 4.17. Conceptual model depicting the chemical and radiation flow derived from

radium-rich barite deposits into fauna

inhabiting the barite-rich seeps 125
LIST OF TABLES

PAGE

Table 3.1. Chemical and isotope compositions of pore fluids from seeps and

ambient Gulf of Mexico deepwaters 24

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

been analyzed in this study 25

Table 3.3. Salinity, barium concentration and Ra isotope activities in

pore fluids from barite-b earing seeps 26

Table 3.4. Elemental chemistry of barite chinmeys and crusts from

the Gulf of Mexico slope 41

Table 3.5. 5, 0 and Sr isotope compositions of the barites and C and 0

isotope compositions of the coexisting carbonates 43

Table 3.6. Activities and ages of barite chimneys 49

Table 3.7. Ages of hardened chimneys 50

Table 3.8. Activities and ages of barite crusts 58

Table 3.9. Results of repeat dating of chimneys 61

Table 3.10. Sulfide concentrations of barites and their effect on

210Pb/226Ra dating 67

Table 3.11. Radium-226 activity concentration in chemosynthetic

(methanotrophic) Bathymodiolus spp. mussels from seeps 72

Table 3.12. Radium-226 content in heterotrophic fauna from seeps 73

Table 3.13. Relationship between barium and radium in selected

hydrocarbon-seep fauna 78

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

typelll 88

Table 4.2. 226Ra/Ba ratios in barites and fluids 106

Table 4.3. Fluxes of radium-rich fluids calculated from barite chimneys 115

Table 4.4 Annual discharge estimates based on data from barite-bearing seeps in the Garden

Banks and Mississippi Canyon

produced in this study. 117

Table 4.5. Radium-226 activities (dpmlg) in marine molluscans

and crustaceans 122

Table 4.6. 226Ra and 228Ra mean activities in barite deposits with

2- random counting error 128

Table 4.7. External dose rates to seep fauna calculated from

Microshield 5.0 128

xi
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 AdministrationlNational 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 RIV Edwin Link, R!V Seward Johnson and RIV Atlantis U 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.
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 21-. 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. 1 3x 1 06 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 3xl04 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 93-002]) and produced water

discharges in coastal environments (e.g., OCS Study MMS 910004, 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 (27037.79'; 92°28.l2') and Mississippi

Cariyon-929 (28° 01.46'N; 89° 43.63W) are also issuing copious amounts of barium and radiumrich fluids (Fig. 1.1). Upon their exit on the seafloor, these

fluids were likely to constitute the primary source for the extensive radiumrich 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).

1
2

,

- Breton - Sound II

Viosca Knoll /' I S

Mississippi Canyon (MC)

East Breaks Garden Banks (GB) Green Canyon (GC)

Green Knot

Atwater Valley

0 50 100

km Lund

94' 92' 90' 880 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.

Alaminos Canyon Keathley Canyon Walker Ridge

28'

26'
3

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:

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?

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

What are the timing and duration of emissions? are these emissions

altered or accelerated by anthropogenic activities?

What are the fluxes of barium and radium from naturally occurring

emissions on the benthic habitat?

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
4

S

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.

i: .
5

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 MC929 (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 babes consisting of

fine anoxic sediment. Profuse trains of gas bubbles venting into the water

column were noticed during the uprooting of the chimneys.
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 2.11 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 j.tm 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

(3180) of water.

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

having an accuracy of ±10/00. Dissolved sulfide and sulfate concentrations

were determined by the colorimetric method of Cline (1969) and barium

gravimetric method (Presley, 1969), respectively. The analytical error (1) 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 1 8 are within 0.1°/oo

based on standard repeats. Stable isotopes are reported in the delta (3) 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. 87Sr/86Sr ratios of the pore fluids were measured on a Finnigan MAT 262

six collector thermal ionization mass spectrometer (TIMS). The results are

7
8

Radioactive

isotopes

226Ra, 228Ra

Isotope analyses

Radiogenic

isotopes

87Sri86Sr

PORE-FLUIDS

Stable

isotopes

6180(1120)

Chemical analyses

Compounds

SO4

Elements

Ca, Ba, Sr, Mg,

Na, K, Cl, Br, etc.

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

1 I 1
9

normalized to 87Sr/86Sr of NBS-987 = 0.71023 and to 86SrI88Sr 0.1194 with an

estimated analytical error of ±lxlO5 (2o').

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 o-discrimination capabilities. The total activity of

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 (b)

in the 226R activity values is estimated to be ±5% 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 minicolumn of Dowex 50W-X12 cation exchange resin. The 228Ra was allowed to

decay to its first daughter product, 228Ac, over a period of 36 hours.

Subsequently, the 228Ac 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 ) 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 backscatter 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 KY 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
10

Petrographic

analyses

'Er SEM and

Microscopy

Barites

Isotope

analyses

Radioactive

isotopes

226Ra, 228Ra

Radiogenic

isotopes

1

87SrI86Sr

Stable

isotopes

+

13C and 18O(CaCO3);

34S and 18O(SO4)

Chemical

analyses

Microprobe

I

Ca, Sr, Mg. Ba,

Na, Fe, etc.

Figure 2.2. Flow chart of imaging, chemical and isotope analyses of barites.
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 HC1 to decompose calcite, dolomite, and iron oxide. Organic

phases were then removed by reaction with warm (about 50 °C) sodium

hypochiorite. Silicate minerals were neglected due to their low concentrations

(<1% by volume).

834S 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 Cu20 (mixture 1:4 by mass) at 900°C to SO2, which

was analyzed with the mass spectrometer. The overall error (± I c) of 834S

determinations is better than 0.20/00 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 CO2 some CO was produced, which

was converted into CO2 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 8 I 80 determinations is better than

0.30/00 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 BaCO3) 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 HC1 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 ±1x105 (2o. For carbon isotope analysis of carbonates in the crusts, powders of barite

crusts were dissolved at 50°C in 100% H3P03 under vacuum, and 613C and 8180

of the carbonates were determined on the cryogenically purified CO2 using an

automated Nier-type triple collector mass spectrometer. The isotope data are

reported iii the conventional 'd" notation in permil relative to the PDB

standard. Accuracy and precision for 813C and 8180 are within 0.10/00

2.2.2. Principles of Dating Methods

Barite deposits including chimneys and crusts were examined with a portable gamma-ray detector (siR meter) which indicated that they were unusually highly radioactive. These initial radioactivity measurements led to a
12

more detailed gamma-ray spectrometric investigation which revealed that

226Ra and 228Ra were unsupported by their parents 238U 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 228Th arid 210Pb, 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 13 13 226Ra >222Rn >214Pb >210Pb >206Pb

l.62x103y 3.82d 26.8mm

3 13 a a 3

228Ra >228Ac >228Th >220Rn >212Pb >208Pb

5.75y 6.13h l.91y 55,6s 10.6h

Since the longest-lived daughters of 226Ra and 228Ra decay series are

210Pb and 228Th, respectively, 210Pb and 228Th will accumulate with time.

Therefore, the 210Pb/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:

(A228Th)/(A228Ra)[?\.228Th/(2.228Th - 228Ra)I[1 -exp(?.228Ra - 228Th)t] (2.1.)

where A228Ra and A228Th are the measured activities of 228Ra and 228Th,

respectively; t is the age of the sample in years; and A228Ra and ?.228Th are the

decay constants for 228Ra and 228Th, respectively. The half lives of 228Ra and

228Th are 5,75 and 1.91 years (Broecker and Peng, 1982) and these translate to

decay constants of 0.1205 yr-1 for ?L2,2SRa and 0.3628 yr-1 for 2'228Th

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

I) 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 rio 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.
13

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

1982),

238U Series 232Th Series

2sU

4.5x109

yr

2.48x1O

Y%min

234Pa

1:18

V

234Th

24.1

days

230Th

7.52x104

yr

232Th

l.4xlO

yr

1.91

V4hr

228 Ac

6.13

Ra

1622

yr

5.75

yr

224Ra

3.64

day

222Rn

3.82

days

22Op

54,5

sec

2l8p0

3.05 mm

214Po

1.6x10

sec

2lOjc

138.4

days

216Po

0.158

sec

212Po

30x107

sec 214Bj

19.7 mm 210B.

5.01

days V

206Pb

stable

I 212Pb

10.6

hr

60.5 mm

I

I 208Pb

stable

214Pb

26.8 mm

213Pb

223

yr

208Tj

31

mm
14

g

101

t112(228Ra) = 5.75 yrs

t112(228Th) = 1.91 yrs

60

0 5 10 15 20 25 30 Ii

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 arid 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 L5. The useful range of the 228Th/228Ra dating methodology is 15

yrs or younger.
15

Since the useful range for 228Th/228Ra method is 15 years or less,

210Pb/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 228Th/228Ra method and can be described by the following equation:

(A2 1 0pb)I(A226Ra)[(2 lOpb)/(?L2l OPb - 226Ra)I[' -exp(?226Ra - 21 Opb)t] (2.2)

where, A226Ra and A210pb are the activities of 226Ra and 210Pb, respectively;

t is the age of the sample in years; and 2226Ra and 21Opb are the decay

constants for 226Ra and 210Pb, respectively. The half lives of 226Ra and 210Pb

are 1,620 and 22.3 years, respectively (Broecker and Peng, 1982) and these

translate to decay constants of 4,28xl04yr and 3.11xlO2yr1, respectively.

Because the half life of 226Ra is sufficiently greater than that of 210Pb, the

parent (226Ra) does not appreciably decay during the period of ingrowth of

the daughter (210Pb). The 210Pb/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 210Pb in the barite, and

(iii) the system must have remained closed with respect to 226Ra and 210Pb. 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 baritebearing 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 HC1 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 hypochiorite

(slightly acidified with HCI to produce Cl2 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 HC1 and heated gently for 4 hr
16

10

8

- t112(226Ra) = 1.62x103 yrs

t112(210Pb) = 22.3 yrs

0 20 40 60 80 100 120 140 160

20 40 60 80 100 120 140 160

Time (yrs)

Figure 2.5. Radioactivity decay and growth in the system 226Ra-210Pb, 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 210Pb activity is zero at t = 0. Lower:

Evolution through time of the 210Pb/226Ra activity ratio toward secular

equilibrium (210Pb/226Ra = 1). This ratio serves as an useful chronometer for samples whose age does not exceed 120 yrs.
17

to remove any Fe203 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. Na2CO3 was used

to dissolve barite and then 2N HC1 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 Na2CO3 and HC1 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 cm3

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 keY 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 212Pb at 300.09 keY 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 228Ac 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 (21 2Pb, 228Ac 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). 21 0Pb 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), 210Pb, 228Ac

(proxy for 228Ra), and 212Pb (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
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 (LSUOI (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 210Pb/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-1841185, also known as Bush Hill, GC-234, GC-272, MC709) and Alaminos Canyon. Major faunal groups assayed for 226Ra include the

symbiont-bearirig, 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
Body

Chemoautotrophs

(mussels)

6Ra, Ba

Shell

FAUNA

Body

Heterotrophs

(crabs, starfish, etc.)

{ 26Ra, Ba

Exoskeleton

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

19
20

powder, acid digested and subsequently assayed for 226Ra using a modified

standard quantitative technique described in EPA-600/4-80-032 (EPA, 1980).

This technique involves determination of 226Ra by measuring its first decay

product, 222Rn, via radioequilibration ingrowth and emanation into Lucas

Cells (EPA method 903.1, EPA, 1980). Since the half-life of 222Rn is 3.82 days, an

ingrowth period of approximately 24 days for each sample was sufficient for

achieving 95% secular equilibrium with the 226Ra parent nuclide before

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 lO-ml PYREX test tubes. Five ml of concentrated hydrochloric acid brought to boiling temperature on a block digester was used to dissolve

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 226Ra 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 226Ra, a 24-day ingrowth period was required. After

this ingrowth period, the samples were sparged with aged nitrogen gas in
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 226Ra per

sample unit weight. Picocuries of 226Ra per unit weight of sample is calculated

as follows:

A=(CSCB)I[CF* D I Wt]

Where,

A picoduries of radon per gram of sample

CS= Sample cpm

CB= Background cpm

CF cpm/pCi conversion factor

D= Decay Correction

1= Ingrowth Correction

Decay Correction (D) EXP (0.693*T1I 14/2)

Wt= Sample weight in grams Where,

T1 Time in days from the emanation time to midpoint of counting time

tl/2= Radiological half-life of 222Rn, 3.82 days

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

Where,

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

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

A reagent blank and an EPA 226Ra standard were processed exactly as

described above for each batch of approximately 10 environmental samples.

The calculated values of 226Ra were corrected for any background artifacts

indicated by procedural and reagent blanks of typically 0.01 dpm level.

(2.3)
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 ±30 ppm (Jo error).
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:

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).

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. Fluids (n6) 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 0/00) to

highly saline (up to 182 0/00), 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 (450/00 to

1 55°/oo).

3.1.1. Radium and Barium Assays in Barite-Bearing Seeps

This section report 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, GB382 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°/oo to 155°/oo, 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

23
24 FU U . -; - Hu - - iuu 'I;.--= I_._ - !!-- .,,
Table 3.2. Description of push cores taken in barite deposition sites whose pore waters have been analyzed in this study.

Push Core

No.

MMS

Block

Lat./Long. Water

Depth

(m)

Core

Length

(cm)

Description

93-3559-2 MC-929 28°0l.471/ 640 28 Flank ofa mud volcano draped with orange-stained barite crusts. Silty sediment

89°43.651 changing downcore color from orange (2cm), to black (4 cm) to grey (24 cm).

93-3562-I MC-929 28°0l.509/ 657 13.5 Base of dormant mud volcano draped with clusters of methanotrophic mussels.

89°43.572

93-3566-1 GB-382 27°37.771/ 513 30 Black, anoxic, silty sediment at the base of twin barite chimneys.

92°28.076

93-3566-2 GB-382 27°37.747/ 513 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.
26

Table 33. Salinity, Barium Concentration and Ra Isotope Activities in Pore Fluids from Barite-Bearing Seeps

Notc: Barium determinations by ICP carry an estimated error of ±0.5%

Sampling Sample Core Dive MMS Water Salinity Ba Ra Ra 228/226

Year Code Depth No. Block Depth (ppt) (rng/L) (dpmlL) (dpmlL) AR

(cm) No. (m)

1993 3559-2-14 -4 JSL-3559 MC-929 640 52 019 190.1±26 106.8±8 0.56±0.09

1993 3559-2.17 -7 JSL-3559 MC-929 640 65 0.12 272±14

1993 3559-2-20 -10 JSL-3559 MC-929 640 96 0.48

1993 3559-2-23 -13 JSL-3559 MC-929 640 130 1.98

7387±44 490.5±26 0.66±0.05 1993 3559-2-32 -22 JSL-3559 MC-929 640 155 90.60

1993 3562-1-20 -6 JSL-3562 MC-929 657 64 106.1±20 633±5 0.60±0.12

1993 3566-1-15 -7 JSL-3566 GB-382 513 134 548.80 173L4±82 1327.1±90 0.77±0.06

1993 3566-1-18 -10 JSL-3566 08-382 513 127 54780 1381.3±58 12396±65 0.90±0.06

1993 3566-1-21 -13 JSL-3566 GB-382 513 139 838.00 2491.5±88 2137.8±91 086±0.05

1993 3566-1-24 -16 JSL-3566 08-382 513 135 925.80 2829.3±74 2452.8±165 0.87±0.06

1993 3566-1-27 -19 JSL-3566 GB-382 513 127 467.80 1314.2±64 1060.2±83 0.81±007

1993 3566-1-30 -22 JSL-3566 GB-382 513 119 290.50 701.8±48 606.2±46 086±009

1993 3566-2-27 -1 JSL-3566 08-382 513 45 0.29

1993 3566-2-30 -4 JSL-3566 GB-382 513 51 0.23 42.7±17

1997 97-2897-1-1 -12 JSL-2897 08-338 510 154 61630 2451.4±71 942±76 0.38±0.03

1997 97-2897-1-2 -15 JSL-2897 GB-338 510 154 756.72 19707±65 828.8±59 042±0.03

1997 97-2897-1-3 -18 JSL-2897 08-338 510 154 936.32 3655.9±87 834.6±52 0.23±0.02

1997 97-2897-1-4 -21 JSL-2897 GB-338 510 152 913.13 27573±86 698.4±57 015±0.02

1997 97-2897-1-5 -24 JSL-2897 GB-338 510 152 891.51 22707±69 779.7±43 0.34±0.02

1997 97-2897-1-6 -27 JSL-2897 GB-338 510 153 75007 3250.0±85 6955±43 0.21±0.01

1997 97-2897-1-7 -30 JSL-2897 GB-338 510 150 797.37 2179.5±74 664.7±46 0.31±0.02

1997 97-2897-1-8 -33 ISL.-2897 GB-338 510 150 689.50 27229±85 958.2±64 035±0.03

1997 97-2897-3-1 -16 JSL-2897 GB-338 510 50 028 773.8±61 49L3±45 0.63±008

1997 97-2897-3-2 -19 JSL-2897 GB-338 510 53 0.32 287.3±27 172.5±18 060±0.09

1997 97-2897-3-3 -22 JSL-2897 GB-338 510 58 0.64 214.3±49

292.4±28 1997 97-2897-3-4 -25 JSL-2897 08-338 510 60 157 607.1±69 0.48±007

1997 97-2897-4-1 -12 JSL-2897 GB-338 510 78 0.24 224-9±49

2411±35 0.30±005 1997 97-2897-4-2 -15 JSL-2897 GB-338 510 88 119.34 789.8±42

1997 97-2897-4-3 -18 JSL-2897 GB-338 510 104 246.06 1261.3±52 281.1±37 022±003

1997 97-2897-4-4 -21 JSL-2897 GB-338 510 115 43636 1609.3±58 3161±37 0.20±002

1997 97-2897-4-5 -24 JSL-2897 08-338 510 122 732.19 20025±64 350.7±50 0.18±0.03

1997 97-2897-4-6 -27 JSL-2897 GB-338 510 132 88374 2748.5±82 5891±65 0.21±002

1997 97-2897-4-7 -30 JSL-2897 GB-338 510 146 1125.48 30233±79 819.6±97 0.27±0.03

1997 97-2897-4-8 -33 JSL-2897 GB-338 510 144 117327 2778.4±75 3939±43 0.14±002

1997 97-2897-MV-I JSL-2897 08-338 510 45.5 057 80.4±17

1997 97-2897-MV-2 JSL-2897 GB-338 510 138 9.80 3022±28

1997 97-2897-MV-3 JSL-2897 GB-338 510 132 4.25 240.2±25

263.6±30 1997 97-2897-MV-4 JSL-2897 08-338 510 146 1748 390.5±30 0.67±009

1997 97-2897-MV-S JSL-2897 GB-338 510 141 93.43 4740±32 241.9±44 0.51±0.10

1997 97-2897-MV-6 JSL-2897 GB-338 510 140 162.55 840.2±43 522.6±51 ft62±007
27

633 to 2453 dpm/L. These values are considerably higher than the ambient Gulf of

Mexico bottom water (13.2x02 dpm/L for 226Ra and 0.45x102 dpmfL 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 (i.e., 7.9 x103 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 0/00, 926

mg/L, 28.3x 102 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 0/oo, 291 mg/L,

7.Ox 102 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 2.5xl03 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

(RaIBa4.82 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/238U 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 (RaJBa6.6 dpm!mg). The 228Ra/226R a

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

(n2) 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/(Cj/(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 va1e of <1 indicates removal of the constituent from the original seawater.
28

0

-5

-20

-25

110 120 130 140 2000

S

7000 0 1500 3000 0 1500 3000

Salinity (°/oo) Ba (!tMIL) 226Ra (dpmlL) 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.
101

io

102 --

i0

I IIIIIJ I I I I 1111 I I I 111111 I I I III q I I II III I I I liii!.

$

.

S

y = 78.65 + 22.46 * LOG(x) R2 = 0.74

I IIIiiiil I IIIIiiII I iiiiiitl I I IIIIl I 1111111

Ru

. . Ru . U

29

0 2000 4000 6000

Ra (dpm/L)

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

seeps.

j-2 10-1 10 101 102 io iO

Ba (mgfL)
30

6000

0

4000

3000

2000

1000

0

Figure 3.3. Ra-Ba and

bearing seeps.

400

Ba (mg/L)

800 1200

3000

228Ra (dpmfL)

226Ra-228Ra relationships in pore fluids from barite-
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, Sr/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 nonconservative 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 87Sr/86Sr ratios (0.70916 ±lx105, n3) are identical

to the overlying seawater ratio (0.70917) thus supporting a seawater origin. In

contrast, the 6180 values that range from 0.3 to 1.30/00 (SMOW) are generally

higher than the ambient deepwater at comparable depth (0.30/00, SMOW, Table

3.1 and Fig. 3.5 A) and their inverse relation with Cl concentrations (Fig. 3.5 B)

is attributed to ifl 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 in

depth (Fig. 1.1). These fluids are highly saline (salinity range from 116°/oo to

1820/00, 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 SO4 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 (n3), are

significantly lower than the seawater (0.70917) whereas the 3180 values (-1.4

to 0.10/00 SMOW) are similar to the ambient bottom water (0.20/0O SMOW,

Table 3.1 and Fig. 3.5 A).

Pore fluids Type III are clearly distinguished from the other two types of

pore fluids by their high salinity (45°/oo to 155°/oo) coupled with

anomalously high enrichments in Ca, Sr and Ba (Ef=8.2, 37.2, and 3.4xl04,

respectively, Table 3.1 and Figs. 3.4 A, B, D) and strong SO4 depletions (Ef=0.02,

Table 3.1, Fig. 3.4 C). The 6180 values (maximum value up to l.7°/oo SMOW), are

generally higher than the ambient Seawater (Fig. 15 A) but their 87Sr/86S r

ratios (0.70843 to 0.70862 for n8) 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; Eu and Aharon, 1997).
32

100

80

60

40

20

0

0

50

40

30

' 20

C

10

1000 2000 3000

Cl (mMIL)

I

A

1000 2000 3000

Cl (mMIL)

4000

4000

Figure 3,4. Plots of Cl against Ca, Sr, Ba

types of pore fluids can be discerned on

SO4/Cl molar ratios. The empty circle r

solid circle, empty triangles, and solid

I, H and III, respectively.

1000

10000

10

0

I I . 1000 2000 3000 4000

Cl (mMJL)

Cl (mMJL)

and SO4 in pore fluids. Note that three

the basis of CaJC1, Sr/Cl, Ba/Cl and

epresents deepwater Gulf of Mexico, the

triangles represent the pore fluids type

0 £
2.3

2.1

1.9

1.7

1.5

1.3

1000 2000 3000

Cl (mMJL)

Figure 35. Cl-18O relationships in pore fluids from seeps. A: pore fluids type I,

H and III. The measured 18O values of the bathyaj and abyssal Gulf of Mexico

waters are l.401oo and O.l50Ioo (SMOW), respectively. B: The inverse

relationship observed between Cl and ö180 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).

4000

33

550 560 570 580 590
34

3.2 Barite Deposits Associated with Seeps

3.2.1. Morphology and Petrography

Barite chimneys and crusts were collected with the submersible JohnsonSea-Link in Mississippi Canyon (MC-929) and in Garden Banks (GB-382; GB338)(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 m 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 stringlike or dendritic-Jike barites.

Although the overall concentration of pyrite is low (generally I 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
35

Figure 36. 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.
36

Figure 3,7. Ditinet morphologies of the harite chimneys displayed in SEM

photorncrographs from thin section. A: String-like morphology typical of the

dark-gray growth layers. B: Dendritic-like morphology tipical of the light- yellow growth layers.
A

B

Figure 38. Principa.1 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
38

varying from 1 jim to 10 jim (Fig. 3.9 A); (ii) euhedral cubic grains; (iii)

anhedral to subhedral grains of up to 20 jim across, and (iv) framboids varying

in diameter up to 15 jim and containing microcrystallites of about 1 jim 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 ilnienite

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 jim.

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
Figure 39. SEM view of the accessory pyrite associated with the harite chimneys. A: Core and rim of euhedral pyrite. B: pyrite framboid containing

m i C r ocr v St a I I it es

39
40

I

4..- U

.I.

I

1_.lI I

_'% .. . o ' I o io

100

A

20 30

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.

I________

40

6

5 .

U

. chimney

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

Notes: The data are based on the analyses of chimney #2 and crust Cr-93-l-3/l The overall error (±lrr) of the chemical analyses reported here are estimated, on the basis of standard analyses,

to be (in weight % units): 039 for SO; 032 for BaO; 009 for SrO; 0.06 for CaO; 0.09 for NaO; 0.06 for FeO.

2 3 4

Barite chimneys

5 6 7 8 9 10 1 2 3 4

Barite crusts

5 6 7 8 9 10

(Wt%)

BoO 4561 5655 54.91 61.56 55.34 59,67 51.44 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 7.17 4.06 9.66 7.1! 8.70 8.20 4.26 5.77 1.75 7.25 2.08 2.91 6.00 8.04 3.25 7.34

CaO 1.21 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 1.31 0.85 0.97

Na20 0.67 0.61 037 0.53 0,47 0.30 0.76 0.89 0.35 0.42 0.17 0.18 0.18 0.20 0.84 0.77 0.67 0.94 0.76 0.30

FeO 0.14 0.16 0,13 0.04 023 000 0.02 0.08 0.06 0.02 0.01 0.02 0.02 0.05 0.02 0.10 0.10 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 34.59 34.03 33.78 35.72 35.98 37.52 35.71 36.01

Totul 99.62 97.59 97.56 99.12 99,20 99.51 98.97 98.47 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 0.821 0.923 0.8 10 0.89 0.741 0.770 0.765 0.798 0.865 0.841 0.937 0.8 17 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 0.03! 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 0.004 0.005 0.004 0.001 0.007 0.000 0.00! 0.003 0.002 0.001 0.000 0.00! 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 1.015 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

S 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 1.019 1.009 1.063 1.008 1.003
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; Pu 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 MC929 and GB-382 have similar ö34S and ö180 compositions which range from 20.3

to 21.6°/oo (CDT) and from 9.6 to 11.7°/oo (SMOW), respectively (Table 3.5 and Pig. 3.11). These values are similar to, or slightly higher than, those of modern

seawater sulfate (34S = 20.3°/oo CDT and 18Q 9.7°/oo SMOW, Cecile et al., 1983;

Faure, 1986). Generally, samples from the inner side of chimneys tend to yield

relatively higher ö34 and 18 values than samples from the outer part of

chimneys (Table 3.5 and Figure 3.12). In contrast, 34 and 3180 values of

barite crusts range from 26.1 to 62.3°/oo CDT and 12.4 to 21.9°/oo SMOW,

respectively, which are anomalously enriched in both 34 and 1 8 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. 634S and 31 80 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, 334S and l 8 values of barite crusts range

from 26.1 to 62.3°/oo CDT and 12.4 to 21.90/oo SMOW, respectively, which are

anomalously enriched in both 34S and 180 relative to the modern seawater

sulfate (Table 3.5 and Fig. 3.11). Most barite crusts exhibit two sublayers: the
Table 3.5. S. 0 and Sr isotope compositions of the barites and C and 0 isotope compositions of the coexisting carbonates.

43

Type of Sample Description of 6'O (SO4) S (SO4) CaCO3 5'3C (CaCO5) '5O (CaCO3) 57SrP7Sr

sample sample (/,,, SMOW) ("I,,, CDT) (%) ('Y PDB) (/,,, PDB)

Barite chinmeys from Garden Banks block 382

Ch#2-a inner part, deep gray

Ch#2-b Outer part, light gray

Ch#4-a inner part, deep gray

Ch#4-b middle part, gray

Ch#4-c Outer part, redish

Ch#5-a inner part, gray

Ch#5-b outer part, redish

Ch#9-T/a top, inner part, gray

Ch#9-T/b top, center pan, yellow

Ch#9-T/c top,outer part, orange

Sante Ch#9-M/a middle, inner part, yellow , Ch49-M/b middle, inner part, gray

Ch#9-MJc middle, center part, yellow

chimneys Ch#9-M/d middle, outer part, red

Ch#9-B/a bottom, inner part, gray

Ch#9-B/b bottom, center part, yellow

Ch#9-B/c bottom, outer part, red

Ch#l0-T/a top, inner part, gray

Ch#lO-T/b top, outer aprt, red

Ch#10-M/a middle, inner part, gray

Ch#10-M/b middle, outer part, rod

Ch#10-B/a bottom, inner part, gray

Ch#1O-B/b bottom, outer part, red

Barite chimneys from Mississippi Canyon

Ch#1 1-a inner part, gray Ch#1 1-b outer part, orange

Barite crusts from Garden Banks block 382

Cr-93-7-7/1 light gray

Cr-93-8-5/3/l lower part, deep gray

Cr-93-8-5/3/u upper part, gray

Sante Cr-93-8-7/2/l lower part, black

Cr-93-8-7/21u upper part, gray

crusts Barite crusts from Mississippi Canyon block 929

Cr-93-1-3/1/l lower pat-t, black

Cr-93-1-3/lIu upper part, deep gray

Cr-93-4-4/1/1 lower part, dark

93-93-4-4/l/u upper part, gray

9.9 20.5

10U 2ft5

10.8 21.5

9.6 20.6

97 20.4

10.2 20.7

9.6 20.3

10.9 21.6

10.2 21.1 0.70858

9.7 20.3 0.7086 1 11.7 21.3

10.9 20.4

9.8 20.5

9.8 20.1 10.7 21.3 0.70859

98 20.9 0.70858

9.5 20.2

10.0 20.3 9.9 20.3 9.9 20.4

9.8 203

10.3 20.5

9.9 20.3

10.7 20.3 0.70848

9.8 20.1 0.70849

12.9 28.1

13.5 28.9

12.7 26.5 8.0 27.3 4.4 0.70857

15.5 33.6 52.2 -21.0 4.2

114 26J 16.3 -24.1 2.6

21.9 62.3 - 21.5 61.5 63.6 -36.2 2.8 0.70845

14.3 33.0 56.8 -29.5 2.9

12.6 28.7 26.3 -31.2 3.1
44

70

60

50

40

30

20

10

5 10 15 20 25

18O (°/oo SMOW)

Figure 111. Sulfur and oxygen isotope compositions of GOM barites (n = 34). Sulfur and oxygen isotope compositions of the chimneys are similar to those of modern seawater sulfate. In contrast, the Crusts are enriched in both 34S and

180 relative to seawater sulfate.

r

. I

o seawater - A chimney - Crust -
18 0. / 102 /10.9 1 Ch#9-T I I I

334s 0. 20.3] 21.1/ 21.6/

18o 9.8 /

10.9 /11.7

I I / Ch#9-M

34s 0. 20.1 / 20.4 / 21.3

Ch#9-B

18 0. 9.5 / 9.8 / 10.7 A

o34s 0. 20.2 /

20.9 /

21.3 /

F

19cm

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
46

upper layer is light gray and the lower layer exhibits a black or deep gray

color. 34S and 1 80 values of the lower layers are generally higher than

those of the upper layers (Table 3.5).

The 87Sr/86Sr 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 87Sr/86Sr 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 ö 1 3 compositions of carbonates occurring in the barite crusts range

from -21.0 to -36.2°/oo PDB (Table 3.5), which are substantially depleted in 13C

relative to the ambient seawater (0.60/00),

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 (fiR 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 210Pb/226Ra 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 ). 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 gammaray peaks are primarily caused by 214Pb (half life 26.8 mi, 214Bi (half life
GOM

seawater

GB

barites

BG MC

fluids fluids

MC

barites

01 0'.

47

0.7093 0.7091 0.7089 0.7087 0.7085 0.7083

87Sr/86Sr

Figure 3.13. 87Sr/S6Sr ratios of Gulf of Mexico barites and seawater. Notes: MC

and GB represent Mississippi Canyon and Garden Banks, respectively. The

87Sr/86Sr ratios of MC and GB end member fluids are from Table 3.1.
48

19.7 mm) and 210Pb (half life 223 yr), The presence of 214Pb 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 228Ac (half life 6.13 hr), 212Pb

(half life 10.6 hr), 212Bi (half life 60.6 mm) and 208Tl (half life 3.05 mm). The

presence of the short lived 228Ac 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

228Th/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 ±0.08 years, respectively. Raw samples Ch#2-a and Ch#2-b yield

210Pb/226Ra ages of 2.52 ±2.39 and 0.55 ±2.23 years (see section 32.5 for a discussion of the reasons for large errors when dating chimneys using the

210Pb/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.
Table 3.6. Activities and ages of barite chinineys.

Note: R = raw samples.

P = pure barile.

Measn red age is obtained by he mese red isotope ratio.

Sampling age = ages cosrected to the time of sample collection.

Error = 2.

Sample

name Sample

type

°Pb

dpm/g

°Ra

dpm/g

mTh

dpmlg

tmRa dpni/g

mPb/mRa tmTh/mRa Ra/5°Ra Measured

SoPbIuRa

age (yr)

Sampling

coPb/Ra

age (yr)

Measured

mTh/mRa

age (yr)

Sampling

°Th/mRa

age (yr)

Inilial

50RImRa

ratio

Chimneys from Graden Banks block 382

Cli#2-a R 144 8088 643 774 0.133±0.032 0.831±0.004 0.718±0.001 4.601±2.388 2.518±2.388 3.341±0.056 1.258±0.056 1.074±0.017 Ch#2-a p 113 1452 819 1135 0.079±0.058 0.722±0.006 0.789±0.001 2.636±4.066 0.553±4.066 2.716±0.064 0.933±0.064 1.094±0.020 Ch82-b R 102 1310 726 760 0.079±0.032 0.956±0.004 0.586±0.00] 2.630±2.254 0.547±2.254 4.196±0.066 2.113±0.066 0.971±0.019 Ch82-h P 71 1184 624 690 0.061±0.040 0.904±0.006 0.588±0.001 2.013±2.292 -0.070±2.292 3821±0.076 1.938±0.076 0.933±0.020 Ch#9-TJa R 191 1305 848 906 0.146±0.041 0.936±0.005 0.694±0.002 5.075±3.084 1.965±3.084 4.052±0.060 0.942±0.080 1.131±0.028 Ch#9-TIn P 170 1279 778 842 0.133±0.032 0.924±0.005 0.658±0.00 I 4.582±2.376 1.472±2.376 3.963±0.070 0.853±0.070 1.061±0.021 Ch#9-M/n R 141 1178 739 791 0.120±0.029 0.935±0.006 0.672±0.002 4.124±2.1 16 1.014±2.116 4.041±0.091 0.931±0.091 1.093±0.031 Ch#9-M/n P 153 1246 769 820 0.123±0.038 0.937±0.004 0.658±0.001 4.217±2.786 1.107±2.786 4.054±0.062 0.944±0.062 1.073±0.019 Ch#9-B/a R 175 1401 876 838 0.125±0.027 1.045±0.005 0.598±0.002 4.294±1.980 0.994±1.980 4.942±0.094 1.642±0.094 1.085±0.032 Ch89-J3/a P 115 1339 789 765 0.086±0.041 1.032±0.005 0.571±0.001 2.875±2.878 -0.425±2.878 4.825±0.091 1.525±0.091 1.021±0.026 CIs#9-T/b R 151 1208 749 755 0.125±0.032 0.993±0.004 0.624±0.001 4.285±2.350 0.985±2.350 4.493±0.066 1.193±0.066 8.073±0.020 Ch119-]fh P 141 1229 707 708 0.115±0.031 0.998±0.004 0.576±0.002 3.921±2.258 0.621±2.258 4.534±0.072 .234±0.072 0.995±0.024 Ch#9-M/h R 153 1275 818 847 0.120±0.028 0.966±0.006 0.664±0.001 4.114±2.040 1.034±2.040 4.280±0.084 1.200±0.084 1.112±0.026 Ch#9-MJb P 118 8346 788 822 0.088±0.037 0.958±0.006 0.611±0.001 2.979±2.604 -0.101±2.604 4.212±0.094 1.132±0.094 1.015±0.026 Ch#9-B/b R 122 1170 756 772 0.105±0.039 0.979±0.005 0.666±0.001 3568±1.398 1.485±1.398 4.376±0.074 2.293±0.074 1.129±0.025 Ch#9-l3/b P 116 1174 708 739 0.099±0.042 0.958±0.00.5 0.635±0.00] 3.370±1.500 1.287±1.500 4.217±0.081 2.134±0.081 1.056±0.024 Ch#10-T/n R 52 1236 784 1044 0.042±0.039 0750±0.003 0.853±0.00] 1.387±1.335 -0.696±1.335 2.871±0.032 0.718±0.032 1.205±0 012 Ch#10-T/a P 44 1125 649 876 0.040±0.045 0.741±0.004 0.785±0.002 1.310±1.186 -0.773±1.186 2.819±0.042 0.666±0.042 1.103±0.017 Ch#10-M/a R 136 1410 856 1168 0.097±0.036 0.733±0.002 0.836±0.001 3. 102±1.281 1.209±1.281 2.777±0.026 0.694±0.026 1.169±0.010 Ch#10-M/a P 121 1332 754 1041 0.092±0.036 0.724±0.003 0.788±0.001 3.232±1.273 1.019±1.273 2.729±0.034 0.646±0.034 1.095±0.012 CIs#10-B/a R 104 1086 687 832 0.097±0.045 0.826±0.001 0.774±0.002 3.271±1.599 1.188±1.599 3.309±0.018 1.226±0.018 1.123±0.011 Cli#l0-10/a P 73 1250 709 967 0.059±0.039 0.733±0.00] 0.781±0.001 1.956±1.354 -0.127±1.354 2.776±0.012 0.996±0.012 1.091±0.006 ChIIIO-T/b R 159 1307 854 873 0.122±0.038 0.978±0.002 0.668±0.001 4.192±2.810 0.972±2.810 4.372±0.036 1.152±0.036 1.132±0.013 Ch#10-T/h P 156 1345 784 793 0.] 16±0.032 0.988±0.001 0.590±0.001 3.953±2.328 0.733±2.328 4.451±0.018 1.250±0.018 1.011±0.008 Ch#I0-M/b R 150 117] 752 728 0.128±0.042 1.034±0.001 0.621±0.003 4.385±3.106 1.165±3.106 4.843±0.018 1.623±0.018 1.114±0.016 Cb810-MJb P 147 1211 680 669 0.121±0.039 1.017±0.002 0.552±0.002 4.128±2.850 0.978±2.850 4.691±0.036 1.541±0.036 0.972±0.015 Ch#l0-B/b R 167 1297 980 908 0.129±0.041 1.079±0.001 0.700±0.001 4.424±3.038 1.274±3.038 5.265±0.021 2.115±0.021 1.321±0.027 CIi#I0-R/b P 160 1302 819 755 0.123±0.045 1.086±0.003 0.580±0.003 4.206±3.302 .056±3.302 5.330±0.062 2.180±0.062 1.102±0.028 Chimneys Irons Mississippi Canyon block 929 Ch#t 1-171 R 229 2464 1134 1112 0.094±0.031 1.020±0.002 0.456±0.001 3.167±1.101 1.084±1.101 4.716±0.028 2.633±0.028 0.804±0.028 ChIC 1-P/I P 348 3945 1670 1680 0.089±0.032 0.994±0.003 0.430±0.001 3.000±1.131 0.917±1.131 4.501±0.044 2.418±0.044 0.729±0.009 CbS] 1-F/2 R 451 2769 1049 819 0.163±0.036 1.281±0.002 0.296±0.002 5704±2.764 2.454±2.764 7.991±0.084 4.741±0.084 0.775±0.026 Ch#1 1-172 P 562 3697 8286 1011 0.152±0.032 1.273±0.002 0.273±0.001 5.311±2.400 2.061±2.400 7.841±0.074 4.564±0.074 0.701±0.018
50

Table 3.7. Ages of hardened chimneys.

Chimneys from Garden Banks block 382

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).

Ch#6-1 1.011±0.002 0.584±0.001 4.644±0.035 2.477±0.035 1.022±0012

Ch#6-2 0.972±0.001 0.681±0.001 4.322±0.008 2.154±0.008 1.127±0.006

Ch#6-3 0.943±0.001 0.676±0.00 1 4. 103±0.018 1.936±0.018 1.108±0.008

Ch#6-4 0.928±0.002 0.705±0.001 3.992±0.006 1.778±0.006 1. 141±0.005

Ch#6-5 0.908±0.00 1 0,684±0.001 3.848±0.008 1.623±0.008 1.087±0.005

Ch#8-1 0.997±0.001 0.578±0.001 4.523±0.016 2.106±0.016 0.997±0.007

Ch#8-2 0.944±0.001 0.577±0.002 4.110±0.010 1.693±0.0 10 0.947±0.020

Ch#8-3 0.829±0.002 0.611±0.001 3.741±0.011 1.324±0.011 0.959±0.008

Ch#8-4 0.828±0.001 0J3 1±0.001 3.323±0.012 0.906±0.0 12 1.09l±0006

Ch#8-5 0.822±0.001 0.790±0.001 3.287±0.010 0.770±0.0 10 1. 173±0.006

Ch#8-6 0.786±0001 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 2.832±0.016 0.415±0.016 1. 162±0.007

Chimney from Mississippi Canyon block 929

Ch#1 1-1 1.357±0.004 0.186±0.001 9.775±0.232 6.442±0.232 0.605±0.040

Ch#1 1-2 I 312±0.001 0.241±0.001 8 .6 19±0.0 18 5.286±0.018 0.681±0.009

Ch#1 1-3 1.281±0.002 0.236±0.001 7.983±0.0 18 4.650±0.0 18 0.617±0.008

Ch#11-4 1.257±0.002 0.312±0.002 7.547±0.010 4.214±0.010 0.775±0.012

Ch#1 1-5 1.244±0.001 0.322±0.001 7.330±0.004 3.997±0.004 0.779±0.006

Measured Sampling Initial

Sample 22Th/Ra 22RaP2'Ra 2zThPzRa 2RaP26Ra

age (yr) age (yr)
©© 208 Ti 860 228AC 911 - © Channel counts 934 ©© Lr2l4Bi 964 3Pa-1001 969 214B1 964 © © ©© 234Pa-1001 - ©© -. 2. 1 1© I 1 ©© C) CD ©© -1 C)CD -to ©© ©00©©001Ji ,28Ac-795 I I 768 - 214B1 786 214Bi 806 - 214Bi 839 840
52

Ch#9-T/b = 1.23 yr

Ch#9-M/b 1.13 yr

Ch#9-B/b = 2.13 yr

Figure 3.15. Chimney #9 from Garden Banks block 382. The ages

shown here are based on the 228Th/228Ra assays.
Ch#10-T/b = 1.25 yr

Ch#10-M/b = 1.54 yr

Ch#10-BIb = 2.18 yr

Ch#10-/a=0.67yr

/ 4/4 Ch# 10-B/a =

0.65yr 18cm

Figure 3.16. Chimney #10 from Garden Banks block 382. The ages

shown here are based on the 228Th/228Ra assays.

53
54

In general, purified barites yield 210Pb/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 210Pb/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#1l-F/1 and Ch#1l-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 ±0.07 and 2.42 ±0.04 years for base and top of chimney (Table 3.6)

while the corresponding 2'0Pb/226Ra ages are 2.06 ±2.40 and 0.92 ±1.13 years,

respectively. The effects of barite purification on the 21 °Pb/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 G13-382 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 2.11±0.02 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 228Th/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).

3.2.43 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-l-3/l, all other Crusts contain 228Ra and

228Th activity levels below our detection limit and therefore only the

210Pb/226Ra ages were determined (Table 3.8).

The light-gray Cr-93-7-7/l crust was recovered from the flank of an active

mud volcano in GB-382. An age of 29.67±2.40 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
Initial

228RaJ226Ri

1.09±0.01

1.11±0.01

.lj±u.uI

1.02±0.01

Figure 3.17. Dating profile of growth increments in barite chiurney# 6 from Garden Banks Block 382 using the

228Th/228Ra method. Ages were corrected to July 20, 1993 when the chimney was sampled.

Sample

name

Age

(yrs)

Ch#6-5 1.62±0.01

Ch#64 1.78±0.01

Ch#6-3 1.94±0.02

Ch#6-2 2.15±0.0!

Ch#6-1 2.48±0.04
56

Ch#86

Ch#8-5

Ch#8-4

ft 66±0. 0 1 1.16±0,01

Sample Age Initial

name (yrs) 228RaI226Ra

Ch#87 0.42±0.02 1.16±0.01

Ch#81 2.1 1±0.02 1.00±0.01

Figure 3.18. Dating profile of growth increments in harite chimncy# 8 from

Garden Banks block 382 using 228Th/228Ra method. Ages were corrected to July

20, 1993 when the chimney was sampled.
S ample

1 aie

Ch#1 -5 4.00±ftW

Initial

228Ra/226Ra

Figure 3.19. Dating roiiIe of growth increments in harite chimney #11 froni Mississippi Canyon block 929 using

22Th/228Ra method. Ages were corrected to July 20, 1993 when the chimney was sampled.
Table 3.8. Activities and ages of barite crusts,

Sample Sample 21Pb zaRa 2Th Ra Measured Measured Sampling

name type dpm/g dpm/g dpmlg dpmlg zuIPb/zaRa Ra/Ra 5tPbPRa 2"PbfRa

age (yr) age (yr)

Brite crusts from Garden Banks block 382

Notes: R= raw sample.

P = pure bante

- = below detection limit.

Measured age is obtained by the measured isotope rS Ii0. Sampling age = age corrected to the time of sample collection.

Cr-93-7-7/l R 553 887 - 0.629±0.014 31.6±2.4 29.7±2.4

Cr-93-7-7/1 P 476 1352 0.356±0.014 14. 1±1.4 12.2±1.4

Cr-93-8-5/3/u R 1165 1475 - 0.797±0.004 - 50.3±1.2 48.4±1.2

Cr-93-8-5/3/iu p 816 1545 - 0.533±0.002 - 24.3±0.3 22.4±0.3

Cr-93-8-51311 R 862 1381 - - 0.629±0.006 - 31.6±1.0 29.7±1.0

Cr-93-8-513/1 P 761 1502 - 0,511±0.005 - 22.9±0.7 21.0±0.7

Cr-93-8-7/2ju R 511 672 - 0.768±0.011 - 46.2±2.9 442±2 9

Cr-93-8-7/2Ju p 449 1203 0.376±0.013 15.1±1.3 13.2±1.3

Cr-93-8-7/2/l R 382 269 - 1.431±0.048

Cr-93-8-71211 P 640 1725 - 0.374±0.021 15.0±2,1 13.1±2.1

Cr-93-8-713 R 292 145 - 2.038±0.070 - Cr-93-8-7/3 p 654 1271 - 0.519±0.015 - 23.4±1.9 21.5±1.9

Barite crusts from Mississippi Canyon block 929

Cr-93-l-3/1 R 705 1934 262 0.368±0.006 0.135±0.010 14.7±0.6 12.8±0.6

Cr-93-l-311 P 1085 3797 180 0.288±0.005 0.047±0.009 10.9±0.5 9.0±0.5

Cr-93-4-41!fu R 2443 2434 - 1.013±0,005 - - Cr-93-4-4/I/u p 1451 2701 - 0,542±0,002 - 24.9±0.2 23.1±0.3

Cr-93-4-4/1/1 R 1374 689 - 2.013±0.005

Cr-93-4-4/1/1 P 2002 3940 0.513±0.001 - 23.0±0.1 21.1±0.1
59

sublayers; an upper light-gray (Cr-93-8-5/3/u) overlying a black-colored (Cr- 93-8-5/3/1) layer buried within the sediment at the time of sampling. Raw

samples Cr-93-8-5/3/u and Cr-93-8-5/3/l yield ages of 29.72 ±1.02 and 48.39 ± 1.20 years, respectively. Purification of samples caused a significant

reduction of the 210Pb/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/l). 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

210Pb/226Ra activity ratio of the Cr-93-8-7/2/l raw sample is 1.43 indicating

the presence of excess 20Pb unsupported by the decay of the measured 226Ra.

The measured 210Pb/226Ra activity ratio after purification was reduced to 0.37 which yields a corresponding age of 13.11±2.13 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 210Pb/226Ra of the raw sample is 2.04 which indicates

an excess of unsupported 20Pb. The measured 210Pb/226Ra after purification decreased to 0.52 and yields an age of 21.50 ±1.97 years.

Crusts Cr-93-1-3/l and Cr-93-4-4/l 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-i-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

228Th/228Ra age was obtained due to low concentrations of 228Th. The

210Pb/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/l). No 210Pb/226Ra

ages were obtained for the raw samples because their measured 21 0Pb/226R a

were >1 (Table 3.8). The measured 210Pb/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/l/u and Cr-93-4-4/1/l, respectively.

3.2.5. Validation of the Ra-Series Dating Methods

The accuracy of 228Th/228Ra and 2'0Pb/226Ra 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 (i.e., they must be fully supported by their respective 228Ra and 226R a parents), and (iii) the system must have remained closed with respect to the

radioactive nuclides, 228Ra, 228Th, 226Ra, and 210Pb 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
60

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

Furthermore, the gamma peak at 67.7 keY from 230Th 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 22STh 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 210Pb, unsupported by the decay of 226Ra, do occur in

the raw barite samples but most of the excess 2 1 0Pb is hosted in the sulfide

rather than in the barite phase. Therefore it is assumed that the initial 21 OP b

in the sulfide-free purified barites is insignificant. This assumption is

supported by the observation that all the young chimneys yield concordant

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

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

and the 210Pb 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 228Ac

(228Ac 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

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 226R a

decay series and by gaseous 220Rn in the 228Ra decay series. Leakage of 222Rn

and 220Rn could definitely affect both 228Th/228Ra and 210Pb/226Ra dating

methods. This is because in the 210Pb/226Ra method, both 214Pb which is used

to calculate the 226Ra activity and 210Pb are the daughters of 222Rn, whereas

in the 228Th/228Ra method the activity of 228Th is calculated from the activity

f 212Pb which is the daughter of 220Rn,

A test of the potential loss of 220Rn 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
Table 3.9. Results of repeat dating of chimneys.

See Table 3.7 for explanation of measured and sampling ages.

Sample Date of 2Th 2Ra Measured Sampling

name measurement (dpm/g) (dpm/g) 228Th/2Ra 2Th/2Ra 22ThJ22Ra

age (yr) age(yr)

First dating

Ch#2-a Apr-95 819 1135 0.772±0.006 2.72±0.06 0.93±0.06

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 0.958±0.005 4.22±0.08 2.13±0.08

Ch#10-T/b Sep-96 784 793 0.988±0.001 4.45±0.02 1.25±0.02

Ch#l0-B/b Sep-96 819 755 1.086±0.003 5.33±0.06 2.18±0.06

Second dating

Ch#2-a Feb-98 955 860 1.110±0.005 5.58±0.11 1.08±0.11

Ch#2-b Feb-98 684 570 1.200±0.003 6.67±0.08 2.10±0.08

Ch#9-B/b Feb-98 647 535 1.210±0.006 6.81±0.09 2.3 1±0.09

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 2.00±0.08
62

3.0

2.5

2.0

1.5

1.0

0.5

05 1.0 1.5 2.0

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.

2.5 30
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 228Th/228Ra and 210Pb/226Ra dating methods are satisfied and the barite ages reported here are considered

reliable.

3.2.6. 210Pb/226Ra Ages

The 210Pb/226Ra method has been applied here to derive the ages of barite

chimney fragments and barite crusts (Tables 3.6 and 3.8). The 210Pb/226Ra

ages of purified barites, plotted in Figure 3.2], 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

210Pb 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 210Pb 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 2 1 0Pb 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 210Pb/226Ra are also very young

which is confirmed by the 228Th/228Ra dating method (Tables 3.6 and 3.8).

Whereas the 210Pb/226Ra method is inaccurate for determining the age of

young chimneys, the following observations suggest that the 210Pb/226Ra

ages of purified crust barites are reliable. First, all the crust samples exhibit

strong 210Pb peaks due to their relative older ages (measured ages >15 yr) which greatly reduce the analytical errors (<10%). Second, most barite crusts

yield 210Pb/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 1 °Pb/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

21 °Pb/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 210Pb in the raw samples are not supported by indigenous 226Ra decay. The inference is confirmed by the fact that some raw barite crusts possess

210Pb/226Ra >1 (e.g. 2.01 for Cr-93-4-4/l-l; 2.04 for Cr-93-8-713; and 1.43 for Cr93-8-7/2), which are impossible if the only source of 210Pb is from the decay

of the parent 226Ra indigenous to the sample.
64

0.0 0.1 0.2 0.3 0.4 0.5 0.6

210Pb/226Ra Ratio

Figure 3,21. 210Pb/226Ra ages of pure barites, Bars represent 2 errors.
2.5

1.5

LO

0.5

0.0

00

Figure 3,22. 210Pb!226Ra of barites

Note that all the barite chimneys are

are above the 11:1 line. This indicates

Spurious ages.

Slope = 1:1

Bari'te crust

A Barite chimney

0.5 1.0 1.5 2.0 25

(210PbI226Ra)b

before (rb) and after (pb) purification. on the 1:1 line whereas most Crust data

that raw Crust samples tend to give

65

2.0 0
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 1 0Pb in the raw samples.

This inference is confirmed by the observed relationship between the

concentration of sulfides and the degree of discrepancy in 210Pb/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 210Pb/226Ra before and after barite purification. This positive relationship

strongly suggests that the excess of 2 1 0Pb in the raw samples stems from the Pb association with sulfides in the crusts.

The excess 210Pb 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, 210Pb 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 2 1 °Pb will react with the hydrogen

sulfide resulting from the microbial sulfate reduction and precipitate as

pyrite. Because the 210Pb 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 210Pb/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 210Pb. Thus the 210Pb/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

2'0Pb/226Ra between raw and purified barite chimneys is attributed to the

lower concentration of sulfides in the chimneys (Fu et al, 1994).

3.2.7. 228Th/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 210Pb present in the sulfide-bearing crusts tend to

yield spurious 210Pb/226Ra ages. Any sulfides in the chimneys could also

affect the 228Th/228Ra age determinations because the activity of 228Th used
Table 3.10, Sulfide concentrations of barites and their effect on 210Pb/226Ra dating.

67

Sample

name

210Pb/226Ra

raw

210Fb/226Ra

pure

A(2°Pb/226Ra) Pyrite

(wt%)

Dante chimneys

Ch#2-a 0.133 0.079 0.054 9.6

Ch#2-b 0.079 0.061 0.018 5.2

Ch#9-Tfb 0.125 0.115 0.010 4.7

Ch#9-BIb 0.105 0.099 0.006 4.3

Ch#10-T/a 0.042 0.040 0.002 5.1

Ch#10-B/a 0.097 0.059 0.038 4.8

Barite crusts

Cr-93-l-3/l 0.368 0.288 0.080 8.9

Cr 93 4 4/1/u 1.013 0.542 0.471 28.5

Cr-93-44/1/l 2.013 0.513 1.500 30.5

Cr-93-7-7/1 0.629 0.356 0.273 16.9

Cr-93-8-5131u 0.797 0.533 0.264 9.0

Cr-93-8-5/311 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/l 1.431 0.374 1.057 31.7
68

2.0

1.5

1.0

0.5

0.0

A Barite chimney

Barite crust

S

0 10 20 30 40

Sulfide (% by weight)

Figure 3.23. Sulfide effects on the 210Pb/226Ra dating. Note the positive

relationship between the sulfide concentration and the difference in

210Pb/226Ra before and after barite purification.

S

S

S
5

4

3

2

1

0'

0.6

r

I

I

0.8 1.0 1.2 1 4

228Th/228Ra

Figure 3.24. 228Th/228Ra ages plotted against their 228Th/228Ra ratios of chimneys based on pure barites.

69
70

in 228Th/228Ra dating method is obtained from the activity of 212Pb 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/228Ra dating,

228Th/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 21 2Pb is insignificant, and (ii) barite purification

scheme does not impair the 228Th/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 3.1 1 and 3.12 show that the range of 226R a concentrations among seep fauna samples is Jarge. 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 226Ra 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.

226Ra 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 baritebearing seeps averaged 0.6 ±0.4 dpm/g (n=7) 226Ra 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 (n9) 226Ra 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 226Ra 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 226Ra (n=7) and range from
228Th/228Ra agepb (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.

71
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.

1 MM- methanotrophic mussels Bathymodiolus spp.

2 Year of collection

Submersible dive number; JSL Johnson-Sea-Link

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

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

Sample

I.D.

Biota1 Type Year2 Dive No,3 GOM Block Seep4 Ra-2265

dpmlg

GOM-S-1 MM Shell 1995 JSL-2634 GC-234 C 0.5 ± 0.4

GOM-S-1 MM Body 1995 JSL-2634 GC-234 C 1.3 ± 0.8

GOM-S-2 MM Shell 1995 JSL-2634 GC-234 C 2.3 ± 0.8

GOM-S-2 MM Body 1995 JSL-2634 GC-234 C 1.1 ± 0.3

GOM-S-8 MM Shell 1995 JSL-2647 GC-l85 C 1.1 ± 0.3

GOM-S-8 MM Body 1995 JSL-2647 GC-185 C 0.6 ± 0.4

GOM-S-9 MM Shell 1995 JSL-2647 GC-185 C 0.5 ± 0.4

GOM-S-9 MM Body 1995 JSL-2647 GC-185 C 0.9 ± 0.4

J-2 MM Body 1991 JSL-3120 GC-272 C 1.5 ± 0.6

J-6 MM Shell 1991 JSL-3 120 GC-272 C 0.2 ± 0.4

J-6 MM Body 1991 JSL-3 120 GC-272 C 0.9 ± 0.4

L-5 MM Body 1991 JSL-3116 GC-185 C BDL

MAC-2 MM Shell 1990 Alvin-2209 Alaminos C 0.4 ± 0.4

Canyon

MAC-2 MM Body 1990 Alvin-2209 Alaminos C 0.1 ± 0.3

Canyon

709-1 MM Shell 1993 JSL-3560 MC-709 C 0.4 ± 0.4

709-1 MM Body 1993 JSL-3560 MC-709 C 0.4 ± 0.4

GOM-S-44 MM Shell 1997 JSL-2897 GB-338 B 1.6 ± 0.6

GOM-S-44 MM Body 1997 JSL-2897 GB-338 B 0.9 ± 0.4

GOM-S-46 MM Shell 1997 JSL-2897 GB-338 B 1.3 ± 0.3

GOM-S-46 MM Body 1997 JSL-2897 GB-338 B 0.8 ± 0.4

GOM-S-47 MM Shell 1997 JSL-2897 GB-338 B BDL

GOM-S-47 MM Body 1997 JSL-2897 GB-338 B 0.4± 0.8

GOM-S-48 MM Shell 1997 JSL-2897 GB-338 B 0.8 ± 0.4

GOM-S-48 MM Body 1997 JSL-2897 GB-338 B 1.3 ± 0.3

GOM-S-49 MM Shell 1997 JSL-2897 GB-338 B 0.4 ± 0.4

GOM-S-49 MM Body 1997 JSL-2897 GB-338 B 0.1 ± 0.3

GOM-S-50 MM Shell 1997 JSL-2897 GB-338 B 0.1 ± 0.3

GOM-S-50 MM Body 1997 JSL-2897 GB-338 B 0.4 ± 0.5

929-4 MM Shell 1993 JSL-3559 MC-929 B BDL

929-4 MM Body 1993 JSL-3559 MC-929 B 0.2 ± 0.4
CC- galatheid crab

SF- starfishes

PY- polychaetes

2 Year of collection

Submersible dive number; JSL = Johnson-Sea-Link

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

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

73

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

seep arid "B" denotes barite seep.

Sample

I.D.

Biota' Type Year2 Dive No.3 GUM

Block

Seep4 Ra-2265

dpm/g

GS-8 CC Skeleton 1992 JSL-3300 GC-l84 C 0.1 ± 0.3

GS-8 CC Body 1992 JSL-3300 GC-184 C 0.8 ± 0.4

97-GL1/Ci GC Skeleton 1997 JSL-2897 GB-338 B 1.0 ± ft4

97-GL1/CI GC Body 1997 JSL-2897 GB-338 B 6.0 ± 2.6

97-GL1IC2 GC Skeleton 1997 JSL-2897 GB-338 B 0.4 ± 0.3

97-GL1IC2 CC Body 1997 JSL-2897 GB-338 B 10.7±2.8

GS-1 SF Wholebody 1993 JSL-3300 GC-184 C 0.8± 0.4

97-SF2 SF Whole body 1997 JSL-2897 GB-338 B 1.7± 0.4

97-SF3 SF Whole body 1997 JSL-2897 GB-338 B 2.0± 0.5

97-SF1 SF Whole body 1997 JSL-2897 GB-338 B 3.9 ± 0.6

97-TW1 PY Skeleton 1997 JSL-2897 GB-338 B 0.5 ± 0.3

97.TW2 PY Skeleton 1997 JSL-2897 GB-338 B 1.0 ± 0.4
CD D cc cc cc cc 0 0 0 0 () N- CD C) CD C) - ? t '1 '? 1 ? 9 U) U) Cl) Cl) 0 Cl) U) C) N- o o 0 0 0 0 0 0 (9 (9 o (9 (9 0 0 0 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. 0 0 0 0 (0 cJ -, C/) 0 0 (9 0
3-5 3.0 -

2.5 -

.' 2.0 -

E

0.5-

Ra-226 Content

I

m m

N- c O c D - 1

e

- - 1' Cl) Cl) Cl) Cl) C (/D 0 0 0 0 0 0

CD (9 (9 0) 0 CD

LID C.) U

"1- CD CI , d - ct

o c.) o c)

C) , a) c'i (.9 (.7 Cl) Cl) o o a a

o w (9 0

75

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, respectively.
76

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

(control sites) average 0.8 ±0.7 dpm/g 226Ra (n7) and range from below

detection limit to 2.3 dpm/g. Among shell samples, GOM-S-2 from GC-234

exhibits the highest 226Ra value (2.3 dpmlg). 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 226Ra 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 3J2 and

graphed in Figure 3.28. Body tissues of galatheid crabs 97-GL1/Ci and 97-

GL1/C2 (B) from barite seeps yield higher 226Ra 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 226Ra. 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 226R a

activities is the accumulation of insoluble barite crystals in the UI 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 226Ra 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 a

activity levels (average 2.5 dpm/g) compared to a single starfish collected

from a carbonate seep (08 dpmlg). The barite-seep starfishes were second

highest of all heterotrophic faunal groups.

226Ra 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 226Ra 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 226R a
10.0 E 8.0 6.0 NN 4.0 2000 - C) CD CD CD CD CD- Cl) C-) CD CD CD N. r 0) 0) CDF- Cl) IN0-.4CDN- 0) Ra-226 Content CD CD 0 CD CD )C F- F- F- Cl) Cl) CD CO CD IN Cl) U- U-. Cl) Cl) F- F- CD . r- r. l- 0) 0) 0) 0) CDF-- CDLLCDCl-. 0) Figure 3.28.- Radium-226 activity concentrations of heterotrophic seep fauna. SK denotes hard skeletal component; ST marks soft tissue component; B and C marks barite and carbonate-bearing seeps, respectively. 
78

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

'MM- methanotrophic mussels Bathymodiolus spp. ; GC- galatheid crab; SF- starfish; PY- polychaetes 2 EEC- 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

Values reported in parts per million

4Values reported in parts per trillion with 2- random counting error

Sample ID

Barite Seeps

Soft Tissues

Biota' Type2 Ba (ppm)3 Ra-226 (ppt)4

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 BJ3C 5 0.57 ± 0.15

97-SF3 starfish body SF BBH 281 0.91 ± 0.21

97-SF2 starfish body SF BBH 1174 0.78± 0.20

97SFl starfish body SF BBH 3619 1.77 ± 0.25

97-GL1 crab 2 body GC BBH 472 4.82 ± 1.27

97-GLI crab 1 body GC BBH 4319 2.72 ± 1.15

Ex os k e I e ton

GOM-S-44 Shell MM BCE 496 0.72 ± 0.27

GOM-S-47 Shell MM BCE 96 0.12 ± 0.16

97-GL1 crabl Shell GC BITE 113 0.43 ± 0.17

97-GL1 crab 2 Shell GC BFIE 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

GOMS-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 0.34 ± 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 0.21 ± 0.19

GS-8 crab GC CHE 15 0.03 ± 0.14
BBC- chernotrophic 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

79

4

I

3 U 0

GB (end-member fluid) . 0 .

U

E

0.

0. m

0) 0

-J

MC (end-member fluid)

I

o

o BCE

1 CCB £ CHB

CCE CHE OEMF

2 3 4 5

Ra-226 (ppt)

Figure 3.29.- The Relation of Barium and 226Ra 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. Dprnlg 226Ra values may be obtained by multiplying parts per trillion by a factor of 2.22 dpm/ppt.
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 226Ra is used in this case to demonstrate

orders of magnitude differences between barium and radium. Parts per trillion

226Ra 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 226Ra 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 226Ra/Ba values from Mississippi Canyon and Garden Banks barite seeps which are also shown.
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 iii 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 bathyaj seawater (Table 3.1) suggest that these are primarily seawater trapped in the sediment. This contention is

further confirmed by the 87Sr/86Sr ratios (0.70916) which are identical to the

measured deepwater value (0.70917, Table 3.1).

The marked SO4 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 09, 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 tllLe 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 1 8 enrichments in type I pore fluids of up to 1°/oo relative

to a seawater source also require an explanation (Fig. 3.5). Two processes

relevant to seeps could shift positively the ö1 80 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
82

water fraction of the hydrate would contribute to the elevated ö1 80 and

simultaneous freshening (i.e., lower salt content) of the pore fluids. For

example, a 10/00 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 o1 8 of 100/oo (assuming multiple thawing-freezing in a closed system with a

starting gas hydrate & 8 value of 2.60/00, Davidson et al., 1983) with seawaterderived fluid having a typical bathyal o180 value of 0.3°/oo (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 l8O 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 SO4 is enriched in 1 8

(&80 = 9.7°/oo SMOW, Faure, 1986) relative to seawater (H20), the microbial

breakdown of the S-0 bonds affords isotope exchange between the sulfateoxygen and water-oxygen and may cause an I 80-enrichment in the pore

fluids. However, it can be estimated that the sulfate reduction effect on the

ö18O of pore fluids is small (at most 0.02°Ioo) compared with gas hydrate

sublimation effects because the amount of oxygen in pore fluid SO4 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 &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 ö1 80 values in type II pore fluids with the 1 80 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)
where A is the fractional contribution

pf subscripts are the 87Sr/86Sr ratios

(0.70917) and pore fluids, respectively

ratio of 0.70797 measured in the pore

contribution from salt dissolution.

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 II pore fluids (0.34x103 to 1,l9x103) relative to seawater

(I .52x103), coupled with their near 1.0 NaJC1 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 SO4 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 (KC1) 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 87Sr/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 87Sr/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

87Sr/86Sr ratio in type II pore fluids can be estimated from a simple mass balance equation:

A (87Sr/86Sr)salt + (1-A) (87Sr/86Sr)sw (87Sr/865r)pf (4.1)

of Sr from salt dissolution; salt, sw and

of the dissolved salt (0.7070), seawater

Using the least radiogenic 87Sr/86S r

fluids, the equation above yields a 55% Sr

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

87Sr/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
84

z

y = - 375.33 + ILO7x R2 = 0.99

1000 2000 3000 4000

Cl (mMIL)

Figure 4,1. Na-Cl relationship in pore fluids type IL The almost 1:J slope

indicates that Cl and Na are derived primarily from dissolution of halite in the

salt domes.
3000

! 2000

1000

1000 2000 3000 4000

Cl (mMJL)

Cl (mMIL)

I

100

80

60

40

20

0

7000

6000-

5000

'000:

'; 3000

2000

1000

0

0

1000 2000 3000 4000

0 (mMJL)

.

1000 2000 3000 4000

Cl (mMIL)

Figure 4.2, Plots of Cl against Na, Ca, Sr and Ba in pore fluids type III. The

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.

85

1000 2000 3000 4000
86

0.7092

0.7090

- 0.7088

0.7086

0.7084

0

y = 0,70854 + 544*10-5 X

R2=0.99

y 0.70842 + 6.45*105 x

R2 = 0.99

6

1/Sr

8 10 12

Figure 4.3. Plot of 87Sr/86Sr against 1/Sr in pore fluids type IlL The empty

circle refers to seawater, solid triangles and solid circles represent the pore

fluids from Garden Banks and Mississippi Canyon, respectively.

2 4
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 III pore fluids it is essential to

derive the chemical composition of the saline end member. The two regression

lines in the plot of 87Sr/86Sr 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/86Sr 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 (l.52xl0-3 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 a!, 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 226Ra 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 a!, 1996; see below), and the similar negative relationship between pore fluids SO4 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.

Mesozoic and Cenozoic-sourced formation waters are distinguished on

the basis of their 1 80 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 ö180 values (i.e.,

>60/00 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 a!, 1994).

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).
88

Table 4.1. Chemical compositions of the two possible saline end member

fluids of pore fluids type III.

Note: Concentrations are in mMIL, salinity is in permil, ö13C is in

perrnil (PDB) and 1x0 is in perrnil (SMOW).

Components Saline end member fluids at:

Garden Banks Mississippi Canyon

salinity 139 155

Cl 2792 3520

Br 2.6 3.2

SO4 0.5 0.5

Na 2485 3338

K 11 14.8

Ca 84.4 76

Mg 43.2 59.5

Sr 3.2 1,7

Ba 6.1 0.6

Si 1 0.6

DIC 2.4 2.4

ö'C -3.8 0.8

0.70854 0.70843

18O (1120) 1.7 0.1
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 ö1 80 of

formation waters and depth in the sediment (Land and Macpherson, 1989), the

ö180 of pore fluids are expected to be heavier than measured (Table 4.1) if they

are derived from Mesozoic Strata. The ö1 8 values for the possible end-member

fluids are 1.7 and 0.J°/oo (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 sourcedl 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/865r 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 Krislinaswamj, 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 234TJ 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 are in
90

1000

100

10:

1E

.1

U0

U

1 10 100 1000 10000

SO4 (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.
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 alpharecoil 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 228Ra analyses of pore fluids in

deep-sea sediments, the few available data tend to support the model outlined

above. For example, determination of 226Ra 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 3xlO3dpmIL for 228Ra).

The 226Ra and 228Ra activities of the pore fluids analyzed in this study,

ranging from 27.2 to 36.6xl02 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, 228R a

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 SO4 data (Fig. 4.4).
92

106

io

io

io

102

+

-t

102

Salinity (°Ioo)

o formation waters

+ discharged produced waters

A pore fluids

io

Figure 4.5. 226Rasa1inity 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 226R a

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.
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)t (22SRa/226Ra)0et (4.2)

where (228Ra/226Ra)t 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; (228Ra226Ra)0

is 228Ra/228Ra activity ratio of the fluid at the time it was trapped in the

sediment; and 2 is the decay constant of 228Ra (7. 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.).
94

The initial 228Ra/226Ra activity ratio of the barite chimneys could be used

as a measure of (228Ra226Ra)0 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 Rarich 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
I

25

20

15

10

5

0

GB-Ch

(n 26)

GB-Cr

(n = 6)

MC-Ch

(n = 8)

MC-Cr (n3)

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 228ThJ228Ra method on pure barites and hardened chimneys.

The ages of crusts are obtained from 210Pb/226Ra method on pure barites.

GBCh: 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
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, 199]; 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 thiat 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 228Raf226Ra in the Barites

The 228Ra isotope decays much faster than 226Ra anid 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)t (228Ra/226Ra)0et (4.3)

where (228Ra/226Ra)t is the measured ratio in the barite, t is the age as

determined by the 228Th/228Ra dating method, (228Ra/226Ra)0 is the initial

ratio, and X is the decay constant of 228Ra (A228Ra = 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 t'he 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 ±0.08, 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
15

10

5

E

1.0

y = 2531 + 9.32x R"2 0.99

1.5 2.0

Age (yrs)

2.5 3.0

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
25

2

20

15

10

5

E

0

0 1

y = 21.30 - 8.93x RA2 = 0.95

Age (yrs)

Figure 4.8. Vertical age profile for chimney #8, Garden Banks block 382. Growth rate determined on the basis of slope is 893 cm/yr.

2 3
E

15

10

5

E

3 4

y = 29.84 - 4.38x RA2 0.94

5

Age (yr)

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.

6 7

99
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

228RaI226Ra 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/226R a

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/226R a

than under Mississippi Canyon area implying different Th/U ratios iii 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 (226R a

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 22SRa/22ôRa is that the

presence of 228Ra in the barites indicates that the maximum migration time

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/226 Ra could also be used to date barites if the initial ratio is known.

However, because the initial 228Ra/226Ra of the barites vary, 228Ra/226Ra can not be used to calculate the exact age. A plot of the ages versus the
1.2

1.1

1.0

.- 0.9

0.6

0.5

101

0 1 2 3 4 5 6 7

Age (yrs)

Figure 4,10. Initial 228Raf226Ra versus age of ba:rite chimneys.
102

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

high 228Ra/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#ll- F/i, 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 226R a 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
10

8

6

4

2

I I A Garden Banks

Mississippi Canyon

A

103

0.0 0.2 0.4 0.6 0.8 1.0

228Ra/226Ra

Figure 4.11. 228Raf'-26Ra AR versus 228Th/228Ra ages (yrs) of chimneys.

These relationships can be used to quantitatively estimate the ages of samples.
104

HY-B

M-B

GOM

GB-Ch

GOM

GB-Cr

GOM GOM

MC-Ch MC-Cr

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: Bariterich 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).
105

are in excess of the amount which can be supported by the coexisting 234

(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 deepseated 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 (RaJBa)solution (4.4)

where D is the homogeneous distribution coefficient of Ra and Ba;

(Ra/Ba)crystaj and (Ra/Ba)soiutjon 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.
Table 4.2, 22Ra1Ba ratios in barites and fluids.

Note: Pore fluids 2559-2-32 and 3566-1-21 are representatives of end member fluids inMississippi Canyon and Garden Banks,

respectively (see text). Their Ra activities are from Table 3.3.

Sampling

area Barite deposits End member fluids

Samp!e name Ra

(dpm/g BaSO)

2Ra/Ba

(dpm/mg Ba)

Sample name 2Ra

(dpmlL)

Ba

(mgfL)

Ra/Ba

(dpm/mg Ba)

Ch#2-a 1452 2.469 3566-1-21 2492 919 2.7 12

Ch#2-b 1184 2.014

Ch#9-Tfa 1279 2.175

Ch#9-M/a 1246 2.119

Ch#9-B/a 1339 2.277

Ch#9-T/b 1229 2.090

Ch#9-M/b 1346 2.289

Ch#9-BIb 1174 1.997

Garden Banks Ch#10-T/a 1125 1.913

Chfl0-M/a 1332 2.265

Ch#1 0-B/a 1250 2.126

Block 382 Ch#10-T/b 1345 2.287

Ch#!0-M/b 1211 2.060

Ch10-B/b 1302 2.214

93-7-7/1 1352 2.299

93-8-5/3/1 1545 2.628

93-8-8/3/u 1502 2.554

93-8-7/2/u 1203 2.046

93-8-7/2/1 1725 2.934

93-8-7/3 1271 2.162

Average 2.246±0.245

Ch#!!-FIl 3945 6.709 2559-2-32 739 80 9.238

Mississippi Canyon Chl l-F/2 3697 6.288

93-1-3/I 3797 6.458

Block 929 93-4-4/1/u 2701 4.594

93-4-4/1/1 3940 6.701

Average 6A50±0.888
4.3.4. The Source of Sulfate in the Earites

Sulfur (334S) and oxygen (& 80) 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.

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 334S and 3180

compositions of modern seawater sulfate are constant within narrow limits

and are represented by a mean 334S value of 20.30/00 CDT and a mean 3180

value of 9.7°/oo SMOW, respectively (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 i.tm), 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.

Barites formed in anoxic environments display heavier 334S and öl8O

values than those of the coeval seawater sulfate. The enrichments of 34S and

1 80 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 334S and 31 8 values than those of the coeval seawater

sulfate. This process has been used to explain the heavier 334S and 3180 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).

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

highly negative 834S values, barites formed by this process often exhibit light

sulfur isotope compositions. The 3180 of the barites formed by this process

varies depending on the 3180 values of dissolved oxygen (02) and the water

(H20) from which the barites precipitated. This is because three of the four

sulfate oxygens are derived from dissolved oxygen (02) and the forth one from

water (H20) during the oxidation of sulfides (Lloyd, 1968). Smaller barites (<5

tim) in modern pelagic sediments usually show lighter 34 values than

modern seawater sulfate and were explained by this sulfide oxidation process

(Church, 1970).

Barites formed by mixing of bariumrich 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 334S of barites are controlled by the mixing of the following sources: (i) seawater

sulfate (634S 20.3°/oo CDT); (ii) sulfides and sulfate leached from basalt (634S

of about 00/00 CDT), and (iii) residual seawater sulfate after thermal chemical

sulfate reduction occurred at high temperatures

107

(334S > 20.3°/oo CDT).
108

Therefore, the Ô34S 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; J3ertine arid Keene, 1987; Kusakabe

et al., 1990). The ö180 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 GUM barites, it is important to note that both 34 and ö'80

of the chimneys, listed in Table 3.5, are isotopically similar to modern 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 34S

and 1 80 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 ö34S values. Sulfate reduction at high

temperature could cause enrichments of 34S and 1 8 in barites because

sulfates containing 60 and 32 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°/oo 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 34S arid l 80 of the barites

because our direct observations indicate relatively cold (S.60C) and practically

invariable bottom-water temperatures.

The observed enrichments of 34 and 180 in the GUM 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 13C of calcite in

the crusts ranges from -21 to 360/oo PDI3 (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 13C

inherited from the fossil fuel- derived carbon. Thus, the light 3'3C values of calcite strongly indicate the occurrence of sulfate reduction during the

formation of the barite crusts.

The enrichments in 34S and 1 80 relative to the seawaiter in samples from

inner 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

1 80 relative to the seawater in chimneys are much smaller than those in the
109

crusts (Fig. 3.11). For example, the 34S of barite crusts are as high as 62.30/oo

(CDT, Table 3.5), which indicates that 95% of seawater sulfate has been reduced

based on the values of ö34S and the concentration of sulfate in pore fluids. In

contrast, the highest ö34S value in chimneys is 21.6°/oo (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/86Sr

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 87Sr/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 irom 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 (87Sr/86Sr)sw + (1-A) (87Sr/865r)f (87Sr/86Sr)br (4.5)
110

0

U

_ U

: .

IL iii, U

1

.- .. o. ..U.

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 GUM bariles 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).

.U

GOM

o other settings

10 20 30 40
where A is the fractional contribution of Sr from seawater; Sw, f and br

subscripts are the 87Sr/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 87Sr/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 pooi of pore water sulfate consumed by sulfatereducing 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 34S and 18 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, HCO3 with light 13C and impoverished in SO4 with heavier 834S and

1 8 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 ö13C-calcite and heavy ö34S and 18O 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 EI3C of carbonates in

the crusts (-21 to -36°/oo 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

111
112

rj

Venting fluids

(Ba2, Sr2,

Ra2,Ca2)

.. SO42Ti

.. . sO42.

Gaseous

hydrocarbons

(CH4)

Conduit

pore fluid sulfate with . seawater signature Ci-st

C114 + SO42 = H2S + CO3 +11,0

£Lw Ca2 + CO32 = CaCO3

B:+}04=BaSO4 2+ 2-

after reduction

S

Venting fluids

(Ba2, Sr2,

Ra2, Ca2)

Figure 4.14. Depositional model of GOM barites. Note that chimney arid crust are

not shown to scale.
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 (VpARaj)/t (4.6)

where F is the average flux of radium in dpm/yr; V is the volume of the

barite deposit in cm3; p is the density of the barite deposit in gm/cm3; t is the duration of growth (Ra-rich fluid emission) in years; and Rai is the average

total initial radium activity in dpm/g scavenged by the barite. The Ra 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:

ARai = 226Ra + (4.7)

where 226Ra and 228Ra 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.

can be obtained from the following equation:

228Ra = 226Ra(228Ra/226Ra)i (4.8)

where (228Ra/226Ra) is the initial 228Ra/226Ra 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 [l/3mh(r12+r22 + rlr2)pARa(i)]/t (4.9)

Where r1 and r 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

cm (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 l/3cG(r12+r22 + rlr2)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
114

Figure 4,15. Dominant morphological truncated conical-shaped barite

chimneys used in the modeling of radium fluxes (see text).
Table 4.3. Fluxes of radium-rich fluids calculated from barite chimneys.

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

115

Chimney Top radius Bottom radius Growth rate Density Ra Rn Flux Fluid flux

name (cm) (Cm) (em/yr) (gm/cm3) (dpmlgm) (dpm/yr) (L/yr)

Garden Banks block 382

Ch#6 2.0 4.0 9.3 2.6 2000 L4x106 270

Ch#8 2.0 3.5 8.9 2.6 2132 1.2x106 230

Mississippi Canyon block 929

Ch#11 2.5 7.0 4.4 2.6 3987 3,5x106 2920
116

1.2x 106 and l.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 (GB382) and Mississippi Canyon(MC-929), respectively.

Thus, leakage of 230 to 270 L/yr of fluids in GB382 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 derive 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 reporited by MacDonald et

al. (1993) we estimate that about 5% of the upper Louisiana slope area of 15,552

km2 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., 2.3x101° and

2.1x101° g/yr, respectively) and are of the same order of magnitude as the total

annual Ba discharge from the Mississippi River (4.lxlO1° g/yr). Calculations

of the total Ra fluxes (i.e., 226Ra + 228Ra) yield a similar picture, namely close

agreement between Garden Banks and Mississippi Canyon rates (1.2x1014 and

2.8x1014 dpm/L) which amount to fluxes having the same order of magnitude

as the Mississippi River (l.5x1014 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 i:he 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.
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 Gulf of Mexico are shown for comparison purposes.

'Garden Banks: GB-382 and GB-338; Mississippi Canyon: MC-929.

2 Seeps fluid fluxes data from Table 4.3; Mississippi River discharge is the 46 year mean of 507 km3/yr (Dinnel and Wiseman, 1986).

Ba concentration in seep pore fluids from Table 3.4; Mississippi River data from Buerkert (1997).

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

F= FU1d xC (orA): Annual discharges from seeps are estimated over the Louisiana upper slope area of 15,552 km2 assuming abarite-seep distribution of

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

Total radium flux (22Ra + 228Ra).

Source FflUd C0a 2211-Ra F5 71

22(1-Ra F22Ra 9t Ra (L/yr) (mg/L) (dpm/L) (dpm/L) (g/yr) (dpm/yr) (dpm/yr) (dprnlyr)

Garden Banks 250 1,173 3,656 2,453 2.3 x 1010 7.3 x i0' 4.9 x i0 1.2 x 10'

Mississippi Canyon 2,920 91 739 491 2.1 x iO 1.7 x i0' 1.1 x io 2.8 x

Mississippi River 507 x 1012 8 x 102 0.18 0.12 4.1 x 10u 9.1 x 10B 6.1 x l0' 1.5 x
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 5.5x105, 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 226R a 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 ±0.002 dpm/L) is

statistically indistinguishable from the inflowing Caribbean Sea waters (0.123 ±0.008 dprn/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 vvas 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 faultrelated 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 surround][ng the seeps in GB382 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 (arid 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 226R a
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 226Ra 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 softbody 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 UI 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 bariumradium-rich sites may cause rapid precipitation of BaSO4 when the venting

fluids come into contact with seawater sulfate. The accompanying soluble

226Ra 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 KSP for barite;

Hence, available 226Ra does not coprecipitate with BaSO4 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 226Ra 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 l012,

biouptake of Ba should be greatly enhanced over that of 226Ra 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
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) (L2) (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 bariumradium-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 226Ra results from marine animals worldwide has been completed by Iyengar and Narayana

(1990) for the International Atomic Energy Agency. 226Ra 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 226Ra activities to range

from 0.03 dpm/g to 0.3 dpm/g. These values overlap with some of the lowest

226Ra activities measured in seep mussels studied here (Table 3.1 1). 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, respectiEvely. 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, 226Ra concentrations in most cases constitute approximately

2/3 of the total radium. The adjusted 226Ra 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. P, 1990), one of three

deployed oysters exhibited an elevated 226Ra activity (6.9 dpm/g) which

exceeds by far all the mussel soft tissues measured in this study (Table 3.11).
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 226Ra levels (2.72 dpm/g). Again, these high values are likely to reflect barite crystals harbored in the GI

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 G.I. tract (International Commission on Radiological Protection, 1973), which accounts for its potential oral radio-toxicity (Raabe, 1994).

Additionally, 226Ra's 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 (218Po and 214Po, 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 (3.84xl0 1 6

Joules) per atom of 226Ra and its three short lived alpha emitting daughters

(where the assumption is made that no 222Rn 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
122

Table 4.5. Radium-226 Activities (dpmlg) in Marine Molluscans and Crustaceans

aBDL: below detection limit

hNR: did not report data

cBarite Seeps

Organism Iyengar and Naraynna

(1990)

Alam Ct al.

(1999)

This Study Rabalais et al.

(1991)

MoUusca

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 NRh

Crustacea

Soft parts BDLa_ 0.18 NRh 6.04 10.7 NRb

Exoskeletons 0.032 - 0.204 NRb 0.42 - 0.95 NRb
12

1110

9

(4) 7 Crustacean Cxoskeletons

Mussel Shells (4)

6 2Ø Mussel Soft Parts (1)

5 Mussel Shells (2)

Mussel Soft Parts (2) 3 Crustacean Exoskeletons

(1)

2

j Crustacean Soft Parts (1)

Mussel Shells (1)

Oyster Soft Parts (3)

Mussel Soft Parts (4)

0

Crustacean Soft Parts (4)

2 4 6 8 10 12 Ra-226 (dpm/g)

Figure 4.16.-A comparison of 226Ra 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%.

Iyengar and Narayana (1990)

Alam et aT. (1999)

Rabalais et al. (1991)

J3arite Seeps, this study

123
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

(cOy) 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 LD50 is approximately 400-500 cGy. For

molluscs, the acute dose LD5O is approximately 2000 cGy (Bacq and Alexander,

1961).

Genetic effects (mutations caused in chromosomes) manifested by

radiation-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 238U and 232Th decay series, respectively (Fig. 23) 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 t'hrough 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 0.1. tract via fine barite
A. Chemical Flow of Radium into Fauna

V

Heterotrophs

-4

Heterotrophs

scavenging

B. Radiation Flow to Fauna

Ra-rich Fluid

Barite

Barite

226Ra-- 222Rn-4" 210Pb

(internal)

by ingestion

Consequences

Impaired cell function

Cellular death

Genetic damage

Methanotrophic

Mussels

125

Methanotroph ic

Mussels

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).

(external) (external)

Y cx
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

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

ray emissions up through the radionuclide 210Pb. 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 proxilinity 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 226Ra, 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 (un 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
127

gives total target dose due to the entire radioactive volumetric source. The mathematical concept is a special case of a Greens 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 226Ra 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 226Ra atom deposited. Actual calculations

based on measured 226Ra 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 226Ra 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 iGy and 12.8 Gy 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.Ol28mGy/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 222Rn in the surrounding bottom waters.

Laboratory tests of sampled barites reveal that bari'te can emanate (or lose) as

much as 3% of its radon gas in secular equilibrium with radium to the

environment. Internally deposited 222Rn and its alpha emitting decay products in biological tissues is potentially damagirg. Since no direct measurements of radon gas dissolved in water could be carried out during this
128

Table 4,6. 226Ra and 228Ra mean activities in barite deposits with 2-iy random counting error

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

Location Number

226Ra

Mean dpmlg

228Ra

Mean dpmlg

GardenBanks 28 2796±414 1872±564

Mississippi Canyon 4 7146 ± 3168 2568 ± 1644

Location 226Ra

Dose Rate :tGyTh

22Ra

Dose Rate j.tGylh

Total

Dose Rate :iGy/h

Garden Banks 3.4 (0.34 mradlh) 3.0 (0,30 mracl/h) 7.4 (0.74 mradlh)

Mississippi Canyon 8.7 (0.87 mradlh) 4.1 (0.41 mradlh) 12.8 (1.28 mradlh)
study, there is a need to know the extent to which radon is present in

chemosynthetic ecosystems before assessing the radon radiation hazards.

129
5. CONCLUSIONS AND RECOMMENDATIONS

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.

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.

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.

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.

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

210Pb/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.

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

LIyr, 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

131
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),

The uptake of Ba and Ra by the fauna inhabiting the baritebearing

seeps was found to be proportional to the Ba/Ra ratio in the pore fluids. Mussels harboring methanotrophic endosymbiorits 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.

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.
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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 Services (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.