名古屋大学博物館報告
Bull. Nagoya Univ. Museum
No. 23, 1–12, 2007

Relative REE abundances of porewater in Pacific pelagic
sediment: estimation by equilibrium calculations basedon REE composition of Mn-oxide and apatite components
Masamichi TAKEBE1)* and Koshi YAMAMOTO2)
1) Department of Sciences, Matsue National College of Technology, 4-14, Nishi-ikuma-cho, 690-8518, JapanFax +81-852-36-5182 *: takebe@matsue-ct.jp
2) Graduate School of Environmental Studies, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan

Abstract
In order to reveal REE mobilization in sediment during early diagenesis, we estimate the REE composition of porewater in Pacific sediments deposited in the Equatorial area. The Pacific sediment was
divided chemically into leachate 1 (extracted by alkaline Na2S2O4), leachate 2 (extracted by 0.17 MHCl) and residue. The leachate 1, leachate 2 and residue consist mainly of Mn oxide, carbonate fluorapatite (CFA), and detrital materials, respectively. The Mn oxide and CFA component are considered
to provide information about REE composition of porewater. Assuming that Mn oxide and CFA in thesediment were in chemical equilibrium with the porewater in terms of REE, the REE composition (REE/
Gd ratio) of the porewater in the sediment was estimated from the REE abundance of the leachate 1 and 2.
The estimated REE pattern (REE/Gd ratio) of the porewater shows La depletion and Ce enrichment
relative to the deep-seawater. The difference of REE composition between deep-seawater and porewater suggests that REE in pelagic sediment is mobilized during early diagenesis.
Key words: REE composition, Porewater, Pelagic sediment, Equilibrium calculation

1. Introduction
Whereas REEs are generally believed to behave conservatively in sediment, several studies reported
that REEs are mobilized during diagenesis. For example, Murray et al. (1991) discussed the mobility oflight REE in Japan Sea sediments. Milodowski and Zalasiewicz (1991) reported that REEs in Paleozoic
sedimentary rocks are concentrated in authigenic monazite, xenotime and apatite, which are generated
during early diagenesis. Hence, the behavior of REE during diagenesis is essential to interpret the REE
composition of sediments. To evaluate REE behavior during early diagenesis, we need more knowledge
about the REE chemistry of porewater. However, there have been very few studies about the REE chemistry of porewater because of sampling and analytical difficulty due to low concentrations of REE inporewater. (Sholkovitz et al., 1989; 1992; Haley and Klinkhammer, 2003).
Takebe (2005) conducted sequencial chemical leaching experiment for Pacific sediments (Fig. 1).
Fig. 2 shows the REE compositions of L1 (leachate by alkaline Na2S2O4), L2 (leachate by 0.17 MHCl) and R3 (residue) by the sequential leaching experiments in Takebe (2005)
The L1 leachate originates mainly from Mn oxide and its Mn content accounts for 65~85 % of thebulk Mn content. The shapes of the REE patterns and sizes of the Ce anomalies of the L1 leachate aresimilar to those of diagenetic Mn nodules (Fig. 3a: Calvert et al., 1987; Ohta et al. 1999). Takebe (2005)
concluded that the REEs of the L1s were associated with Mn oxide deposited during early diagenesis.
The L2 leachates are considered to be derived mainly from biogenic CFA. The L2 leachates contain
60~90 % of the bulk pohosphorus. It is known that biogenic CFA scarcely contains REEs in a living
body, but concentrates REEs after sedimentation (Toyoda and Tokonami, 1990). The REE patterns of——the L2 leachates normalized to deep-seawater REE abundances (SA5-5149m; Zhang and Nozaki, 1996)
show large positive Ce anomalies (Fig. 3b). Considering that REE incorporation into CFA from solution
may not be related to redox reaction of Ce and that REEs have similar chemical properties to each other, the REE distribution coefficient between the solution and CFA may not show a Ce anomaly. Deepseawater is not, therefore, appropriate as a solution that interacted with the CFA in terms of REE. An
alternative candidate for the solution is porewater. Toyoda and Tokonami (1990) suggested that a large
contribution of porewater likely contributes to the high REE concentrations of fish teeth in pelagic sediments. Taking into account the positive Ce anomalies shown in Fig. 3b, the porewater should show a
Ce enrichment relative to the deep-seawater. In fact, there are many reports showing that porewater areenriched in Ce relative to seawater (Sholkovitz et al., 1989; 1992; Haley and Klinkhammer, 2003).
The PAAS normalized REE patterns of the R3 residues show negative Ce anomalies and positive
Eu anomalies. R3 residues are the component not extracted by the reducing agent and acid. Thus thenegative Ce anomalies in the R3 residue probably indicate the contribution of authigenic clay deposited from sea or porewater, and the Eu anomalies suggest the influence of basaltic detrital materials fromoceanic island or andesitic eolian dust from American continent (Takebe, 2005; Nakai et al., 1993).
In this study, we calculate the REE composition of porewater in Pacific sediment, based on the analyzed REE compositions of Mn oxides and CFA examined in the sediments by Takebe (2005).
2. Samples and methods
Examined samples (NB7 and 59P159) were collected from the Western and Equatorial Pacific during NH 90-1 and GH 80-1 cruises, respectively, by the Geological Survey of Japan (NB7: 14ºN 138ºE,
11 cm below sea floor (cmbsf), 59P159: 1ºN 171ºW, 15 cmbsf: Fig. 1). These samples were from theseabed deeper than 5000 m and rarely contain Ca-carbonate. The sampling sites of NB7 and 59P159
are in areas with abundant biogenic productivity (Leinen et al, 1986). Takebe (2005) reported thatthese samples show high phosphorous concentrations reflecting enrichment of biogenic CFA.
Each sample was divided into three fractions by a sequential leaching method, following the similar
procedure to that of Toyoda and Masuda (1991). The analytical method in this study is summarized below, and the details were reported in Takebe and Yamamoto (2003) and Takebe (2005). A sample wasleached with Na-dithionite solution under alkaline conditions maintained by Na2CO3 and the resulting
leachate was named leachate 1 (L1). The
180º
150ºW
120ºE
150ºE
residue was subsequently leached with
0.17 M HCl, and the leachate was named
leachate 2 (L2). The residue was digested
by HF-HClO4, fused finally with Na2CO3,
and named R3.
40ºN
The REE concentrations of bulk, L1
leachate, L2 leachate and R3 residue were
Hawaii islands
measured by Inductively Coupled Plasma
Mass Spectrometry (ICP-MS; HP4500). 20ºN
There was no correction made for the
NB7
total blank of REEs during the procedure
because of its negligible contribution (< 10º
59P159%). REE concentrations were recalculated on an ignition loss-free basis. This recalculation does not affect the discussion 20ºSin this study, because all the elements in
Fig. 1. Location map of analyzed samples.
——the sample are corrected by the same factor. The recalculated concentrations are used for the following
discussions. Raw data were reported in Takebe (2005). The difference between the REE concentrationof the bulk analysis and the sum of the REE concentrations in L1 leachate, L2 leachate and R3 residue
was less than 5 %.
To depict REE patterns, the REE composition of Post-Archean Australian average shale (PAAS:
Nance and Taylor, 1976; Taylor and McLennan, 1985) was used for normalization.
3. Results and Calculation
REE abundances in each fraction are preNB7
sented in Table 1 and Fig. 2. On the basis of the
REE abundance of the L1 leachates (Mn oxide
1
Leachate 2
components), we estimate the REE/Gd ratioof the porewater except for the Ce/Gd ratio,
Residueassuming that chemical equilibrium was established between the Mn oxide and porewater.
Ce/Gd ratio of porewater is hard to estimatedLeachate 1
from Ce content in Mn oxide component, be0.1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
cause Ce3+/(Ce3++Ce4+) ratio in the component
is unclear. Subsequently, the distribution coefficient of REE (KdREE/KdGd) between the CFA
59P159and porewater is estimated by the REE/Gd
1
ratio of the L2 leachates (CFA component) andporewater. After that, the Ce/Gd ratio of theporewater is evaluated from the pattern of the
distribution coefficient. Reason for focusing
REE ratios (REE/Gd ratios) is that we can omit
the estimation of uncertain activity of H2O and
0.1
activity coefficients of REE in porewater.
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 2. PAAS-normalized REE patterns of the leachates
and residue.

3.1. REE/Gd Ratio of Porewater, except for Ce/Gd Ratio
Assuming that the changes of temperature and pressure cause negligible effects on the REE partitioning between Mn oxide and solution, we carry out the following calculation under the simplifying
conditions of 25ºC and 1 atm. Incorporation of REE into diagenetic Mn oxide from solution is expressed by the following reaction (Kawabe et al., 1999):
Ln(OH)3·nH2O + 3H+ = Ln3+ + (3+n)H2O,(1)

where Ln stands for REE. The equilibrium constant of Eq. (1) can be written as
KLn(OH)3·nH2O = (aLn3+ · aH2O3+n) / aLn(OH)3·nH2O · aH+3,(2)

where “a” is the activity for respective species. When the activities of Ln3+ and Ln(OH)3·nH2O are expressed in terms of the molar concentration of Ln3+ in solution, the molar fraction of Ln(OH)3·nH2O in
Mn oxide and the respective activity coefficients, Eq. (2) can be re-written as the following equation,——Table 1. REE concentratons (ppm) of bulk sample, leachates and residue.
NB7
Leachate 1 Leachate 2

59P159Residue

total

bulk

Leachate 1 Leachate 2

Residue

total

bulk

40.2

38.8

3.86

31.0

6.69

41.5

41.0

La

5.06

22.2

13.0

Ce

14.5

27.5

20.9

62.9

60.5

13.0

27.7

11.2

51.9

52.9

Pr

1.54

6.13

2.70

10.4

9.93

1.14

9.14

1.55

11.8

11.8

Nd

7.10

27.2

10.6

44.9

43.0

5.31

40.9

6.22

52.4

52.3

Sm

1.63

6.34

2.02

9.99

9.57

1.22

9.76

1.19

12.2

12.1

Eu

0.413

1.58

0.536

2.53

2.41

0.308

2.48

0.292

3.08

3.05

Gd

1.78

6.93

1.77

10.5

9.96

1.40

11.1

1.02

13.5

13.5

Tb

0.257

1.06

0.262

1.57

1.50

0.201

1.74

0.154

2.10

2.03

Dy

1.51

6.48

1.65

9.65

9.21

1.23

10.9

0.977

13.1

12.9

Ho

0.301

1.31

0.341

1.95

1.87

0.259

2.24

0.203

2.70

2.64

Er

0.833

3.69

1.01

5.54

5.33

0.743

6.36

0.596

7.70

7.49

Tm

0.118

0.514

0.153

0.784

0.750

0.104

0.899

0.0891

1.09

1.04

Yb

0.706

3.26

1.03

4.99

4.82

0.671

5.82

0.596

7.09

6.95

Lu

0.112

0.488

0.152

0.752

0.732

0.106

0.884

0.08701.08

1.03“total” is sum of leachates and residue.

KLn(OH)3·nH2O = ([Ln3+] · γ Ln3+ · aH2O3+n) / (XLn(OH)3 · nH2O · λ Ln(OH)3·nH2O · aH+3),(2’)

where [Ln3+] and XLn(OH)3·nH2O are the molar concentration of Ln3+ in porewater and the molar fractionof Ln(OH)3·nH2O in the Fe-Mn oxide, respectively. γ and λ denote activity coefficients in porewaterand Fe-Mn oxide, respectively. Although values of γ Ln3+ and λ Ln(OH)3·nH2O are not strictly constantacross the REE series, they may be presumed to be approximately constant. Normalization of Eq. (2’)
by that for Gd leads to the following form,
KLn(OH)3·nH2O / KGd(OH)3·nH2O = ([Ln3+] / [Gd3+]) / (XLn(OH)3·nH2O / XGd(OH)3·nH2O).

(2”)

The left side of Eq. (2”) can be estimated from the distribution coefficients (KdLn) between Fe-Mn
oxyhydroxide precipitate and NaCl solution (Kawabe et al., 1999). The Kd value is defined by the following equation.
KdLn = XLn / [Ln]total,(3)

where [Ln]total and XLn are the REE concentration in solution and molar fraction of REE in Fe-Mn
oxyhydroxide, respectively. Because almost all dissolved REE is present as REE3+ under their experimental conditions (Kawabe et al.,1999), Eq. (3) can be re-written as follows:
KdLn = XLn / [Ln3+].

(4)

Normalized by the equation for Gd, Eq. (4) is expressed as follows:
KdLn / KdGd = (XLn / XGd) / ([Ln3+] / [Gd3+]).

(4’)

Considering XLn = XLn(OH)3·nH2O, we obtain the following equation from Eqs. (2”) and (4’).

——XLn(OH)3·nH2O / XGd(OH)3·nH2O = KdGd / KdLn.

(5)

Therefore, the left side of Eq. (2”), i.e. the ratio of the equilibrium constants of Eq. (1), can be calculated from the KdLn values of Kawabe et al. (1999). The KdLn/KdGd ratios other than the KdCe/KdGd
ratio are independent of the mole fractions of Fe and Mn in Fe-Mn oxyhydroxide, whereas KdCe/KdGd
ratio is dependent on these mole fractions (c.f. Kawabe et al., 1999). In order to simplify the problem,
Ce is omitted in the following calculation. The calculated KdLn/KdGd ratios are presented in Table 2.
The fraction of REE3+ and REE(III)-complexes in porewater was calculated (Fig. 4). In the calculation, we assume that pH is 7.84, salinity is 35 ‰, and the concentrations of total-CO2, Na, Ca, Mg, Cl,
and SO42– are 3.10 × 10–3 mol/kg, 0.468 mol/Kg, 10.0 × 10–3 mol/kg, 50.2 × 10–3 mol/kg, 19.3 ‰ and23.3 × 10–3 mol/kg, respectively. For these data, we refer to literatures published concerning porewaterin Pacific sediments without Ca-carbonate (Presley and Culp, 1973, Gieskes and Johnson, 1980). Cal––
culated concentrations of CO32 and HCO3 are 4.50 × 10–5 mol/kg and 1.87 × 10–3 mol/kg, respectively.
The stoichiometric constants of dissociation of carbonic acid in seawater (salinity = 35) at 5ºC are calculated according to the equation reported by Millero (1995). The stoichiometoric constants of complexation of metals other than REE are obtained from data at ionic strength = 0.7 and 25ºC by Millero
and Schreiber (1982). The activity coefficients of REE3+-complexes are calculated under the conditions of ionic strength = 0.7 and 25ºC, following Millero and Schreiber (1982) and Millero (1992). The
activity coefficients of the other ions in seawater (salinity = 35) at 5ºC are obtained from Millero andPierrot (2002). Stability constants of REE-complexes are refer to Mironov et al. (1982), Millero (1992),
and Ohta and Kawabe (2000b). An adjustment between concentration values in mol/kg and mol/l was
not made because it would have caused insignificant differences in the results. As shown in Fig. 4, the(a)hydrogenous Mn nodule
diagenetic Mn nodule

10

1

Table 2. Equilibrium constants (K Ln)
relative to KGd between Fe-Mn
oxyhydroxide and solution,and their 90% confidence
limits.
KLn / KGd
La

NB7 (L1)
59P159 (L1)10.1
La Ce Pr Nd(b) 10Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

8

NB7 (L2)
59P159 (L2)

10

10

7

6

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3. (a) PAAS-normalized REE pattern of hydrogenous
and diagenetic Mn nodules and leachate 1. Plots of
Mn nodules are average of analytical data of respective Mn nodules. The data are quoted from Ohta etal. (1999). (b) REE patterns of Leachate 2 normalized to deep-seawater (SA5-5149m; Zhang and Nozaki, 1996).
——

3.66

±

0.12

Pr

1.13

±

0.04

Nd

0.957

±

0.029

Sm

0.616

±

0.014

Eu

0.665

±

0.015

Gd

1.00

Tb

0.810

±

0.016

Dy

0.787

±

0.018

Ho

0.871

±

0.015

Er

0.810

±

0.021

Tm

0.640

±

0.024

Yb

0.506

±

0.019

Lu

0.556

±

0.021

These are calculated from data of
Kawabe et al. (1999)

following approximation is possible.
–[Ln]total = [Ln(CO3)2 ] + [LnCO3+].

(6)

Ohta and Kawabe (2000a) reported that a similar approximation is also valid under seawater conditions.
REE(III)-complexes are formed according to the following reactions.
––

Ln3+ + 2CO32 = Ln(CO3)2–
Ln3+ + CO32 = LnCO3+

(7)
(8)

Complex formation constants of Eqs. (7) and (8) are written as follows, respectively:
KLn(CO3)2– = aLn(CO3)2– / (aLn3+ ·aCO32–2).
KLnCO3+ = aLnCO3+ / (aLn3+ ·aCO32–).

(9)
(10)

Eqs. (9) and (10) can be re-written as the following equations, using molar concentrations and activity
coefficients for the respective species,–

KLn(CO3)2– = ([Ln(CO3)2 ] · γ Ln(CO3)2–) / ([Ln3+] · [CO32–] 2· γ Ln3+ · γ CO32–2).
KLnCO3+ = ([LnCO3+] · γ LnCO3+) / ([Ln3+] · [CO32–] · γ Ln3+ · γ CO32–).

(9’)
(10’)

Assuming γ Ln(CO3)2– = γ LnCO3+ (c.f. Cantrell and Byrne, 1987), we obtain the following equation from
Eqs. (9’) and (10’).
[Ln(CO3)2–] = (KLn(CO3)2– / KLnCO3+) · [LnCO3+] · [CO32–] · γ CO32–.

(11)

From Eqs. 6) and 11), [Ln]total is expressed as,[Ln]total = [LnCO3+] · {(KLn(CO3)2– / KLnCO3+) · [CO32–] · γ CO32– + 1}.

Fig. 4. Calculated concentration ratios of REE3+ and other REE3+-complexes to total dissolved
REE in porewater. Assumptions for this calculation are presented in text.
——

(12)

Normalized by the equation for Gd, Eq. 12) is expressed as,([Ln]total / [Gd]total) = ([LnCO3+] / [GdCO3+]) ·{(KLn(CO3)2– / KLnCO3+) · [CO32–] · γ CO32– + 1} / {(KGd(CO3)2– / KGdCO3+) · [CO32–] · γ CO32– + 1}.

(12’)

From the chemical equations of (1) and (8), incorporation of REE into diagenetic Mn oxide fromthe porewater is also expressed by the following reaction
Ln(OH)3·nH2O + 3H+ + CO32– = LnCO3+ + (3+n)H2O.

(13)

Equilibrium constant of chemical reaction (13), can be written as follow.
KLn(OH)3·nH2O-LnCO3+ = ([LnCO3+] · γ LnCO3+ · aH2O3+n) / (XLn(OH)3·nH2O · aH+3 · [CO32–] · γ CO32–).

(14)

The right hand equation of Eq. (14) can also be obtained from the equilibrium constants of Eqs. (2’)
and (10’).
KLn(OH)3·nH2O · KLnCO3+ = ([LnCO3+] · γ LnCO3+ · aH2O3+n) / (XLn(OH)3·nH2O · aH+3 · [CO32–] · γ CO32–).

(15)

Assuming that γ LnCO3+ is constant across the REE series, the following equation can be obtained fromEq (15) normalized by the equation for Gd.
([LnCO3+] · XGd(OH)3·nH2O) / ([GdCO3+] · XLn(OH)3·nH2O)= (KLn(OH)3·nH2O · KLnCO3+) / (KGd(OH)3·nH2O · KGdCO3+).

(16)

Eq (16) can be transformed into following equation.
([LnCO3+] / [GdCO3+])= (KLn(OH)3·nH2O · KLnCO3+ · XGd(OH)3·nH2O) / (KGd(OH)3·nH2O · KGdCO3+ · XLn (OH)3·nH2O)(16’)

From Eqs. (12’) and (16’), we obtained the following equation.
([Ln]total / [Gd]total) = (KLn(OH)3·nH2O / KGd(OH)3·nH2O) · (XLn (OH)3·nH2O / XGd(OH)3·nH2O)· (KLn(CO3)2– · [CO32–] · γ CO32– + KLnCO3+) / (KGd(CO3)2– · [CO32–] · γ CO32– + KGdCO3+).

(17)

Using Eq. (17), we could calculate the Ln/Gd ratio of porewater from the KdLn/KdGd ratio (Table 2)
and the Ln/Gd ratio of the Mn oxide (L1).
The results of the calculations are presented in Table 3 and Fig. 5a (except for Ce/Gd ratio). The
calculated error caused by the analytical error of the Ln/Gd ratio is smaller than the size of points plotted in Fig 5a. There is no significant difference in the calculated Ln/Gd ratio of the porewater between
NB7 and 59P159. The calculated porewater shows La depletion relative to the deep-seawater. Except
for the La/Gd ratio, the calculated Ln/Gd ratio of the porewater is similar to the ratio of deep-seawater(SA5-5149 m: Zhang and Nozaki, 1996).
3.2. Distribution coefficient of REE between CFA and porewater
Using the calculated Ln/Gd ratio of the porewater, we can evaluate the Gd-normalized distribution——Table 3. Ln/Gd ratios of pore water and their
90 % confidence limits.
NB7
La

59P159

2.86

±

0.10

2.78

±

0.09

Pr

0.727

±

0.028

0.683 ±

0.026
0.09

Ce
Nd

3.09

±

0.09

2.94

Sm

0.698

±

0.016

0.666 ±

±

0.015

Eu

0.184

±

0.004

0.175 ±

0.004

Gd

1.00

Tb

0.165

±

0.003

Dy

1.19

±

Ho

0.291

±

1.00
0.164 ±

0.003

0.03

1.24

±

0.03

0.005

0.320 ±

0.006

Er

0.934

±

0.024

1.06

±

0.03

Tm

0.139

±

0.005

0.156 ±

0.006

Yb

0.841

±

0.032

1.02

±

0.04

Lu

0.148

±

0.005 0.177

±

0.007

Table 4. Distribution coefficient (KdLn) relative
to KdGd between CFA and porewater,
and their 90% confidence limits.
NB7
La

1.12

Ce

1.17

±

59P159
0.04

1.01

±

0.03

1.10

Pr

1.22

±

0.04

1.21

±

0.04

Nd

1.27

±

0.04

1.26

±

0.04

Sm

1.31

±

0.03

1.32

±

0.03

Eu

1.24

±

0.03

1.28

±

0.03

Gd

1.00

Fig. 5. (a) Comparison of REE/Gd ratios between calculated porewater and deep seawater. The Ce/Gd ratio
was calculated after the calculation of distribution
coefficient of REE between CFA and porewater.
(b) Distribution coefficient (KdREE/KdGd) of REE
between CFA and porewater. The KdREE/KdGd was
calculated from Eq. (18) assuming that the distribution coefficient shows no Ce anomaly.

1.00

Tb

0.925 ±

0.018

0.961 ±

0.019

Dy

0.786 ±

0.017

0.797 ±

0.018

Ho

0.647 ±

0.011

0.632 ±

0.011

Er

0.570 ±

0.014

0.542 ±

0.014

Tm

0.534 ±

0.019

0.521 ±

0.019

Yb

0.559 ±

0.020

0.517 ±

0.019

Lu

0.476 ±

0.017

0.450 ±

0.016

coefficient (KdLn/KdGd ratio) between the CFA (L2) and porewater. The estimated KdLn/KdGd ratios arepresented in Table 4 and Fig. 5b. Although the Ce/Gd ratio of the porewater is unclear, the KdCe/KdGd
ratio can be estimated from the following equation, assuming that the distribution coefficient shows no
Ce anomaly.
log(KdCe / KdGd) = (log(KdLa / KdGd) + log(KdPr / KdGd)) / 2.

(18)

The pattern of the KdLn/KdGd ratio (Fig. 5b) shows a slight increase from La to Sm and a distinct
decrease from Sm (1.3) to Lu (0.45~0.48).

——3.3. Ce /Gd Ratio of Porewater
On the basis of the KdCe/KdGd ratio, we can estimate the Ce/Gd ratio of the porewater, assumingthat the contributions of components other than CFA to the Ce content of the L2 leachates are negligible. The REE/Gd ratio of the porewater is shown in Fig. 5(a). The size of the negative Ce anomaly of
the porewater is smaller than that of the deep-seawater (SA5-5149 m; Zhang and Nozaki, 1996).
4. Discussion
4.1. Ce anomaly
The calculated REE concentration in porewater shows a smaller negative Ce anomaly than those in
the deep-seawater. In fact, Sholkovitz (1989, 1992) and Haley and Klinkhammer (2003) observed thatporewater has smaller Ce anomaly relative to bottom seawater. The smaller Ce anomaly implies thepreferential Ce release from sediment to porewater (Haley and Klinkhammer, 2003). It is known thatsuspended particles are effective carriers of REE from seawater to sediment (Murphy and Dymond,
1984; Moffet, 1993; Sholkovitz et al., 1994). Suspended particles coated by Fe-Mn oxyhydroxide andorganic matter adsorb REE while sinking through the water column (Moffet, 1993; Sholkovitz et al.,1994). In particular, Ce is effectively adsorbed on suspended particles relative to the other REEs due tothe oxidation of Ce(III) to Ce(IV), which is less soluble in oxic seawater (Sholkovitz et al., 1994). As aresult, suspended particles show positive Ce anomalies (Sholkovitz et al., 1994). After sedimentation,
the particles derived from the seawater column may release Ce due to the degradation of organic matter and Fe-Mn oxyhydroxide during early diagenesis. This process is considered to cause the smaller
Ce anomaly of the porewater in the Pacific sediments than that of the deep-seawater.
4.2. La depletion
The La depletion estimated by us is actually observed by Haley and Klinkhammer (2003) in porewater of deep-sea sediments, suggesting that our estimation is valid. The calculated porewater shows
La depletion relative to the deep-seawater. The La depletion in porewater in pelagic sediment is alsoinferred from the fact that diagenetic Mn nodules usually show La depletion relative to hydrogenous
ones (Fig. 3a: Ohta et al., 1999). The suspended particles, that are responsible for the Ce enrichment in
the porewater, are also considered to affect the distributions of other REE in the porewater.
Ohta and Kawabe (2000b) reported that the pattern of the REE distribution coefficient between FeMn oxyhydroxide and seawater shows La depletion, which becomes distinct with decreasing total carbonate concentration. Equatorial Pacific sediments are generally enriched in organic carbon (Ohkouchi
et al., 1997). Porewater in equatorial sediments may have a higher total carbonate concentration thanseawater due to the degradation of organic matter in the sediment (c.f. Libes, 1992). Thus, the suspended particles with Fe-Mn oxyhydroxide coatings adsorb REE in seawater with lower total carbonate content and, then after sedimentation, the particles release REEs into porewater with higher total
carbonate content. As a result, the La migration from seawater to porewater via the suspended particles
may be small relative to the flux of the other light-REEs. These processes may cause the La depletionof the porewater in the Pacific sediment.
Another possible reason for the La depletion of the porewater is that authigenic materials in thesediment preferentially take up La from the porewater. However the Mn oxide (L1) and CFA (L2) inthe Pacific sediments do not show enough La enrichment to explain the La depletion of the porewater.
The R3 residues of NB7 and 59P159 are considered to contain authigenic materials, because the R3
residues show negative Ce anomalies (Fig. 2), which cannot be explained by the contributions of detrital materials (Takebe, 2005). Because R3 is not extractable by a reducing agent and 0.17M HCl, theauthigenic materials might be barite and/or clay minerals.
——4.3. Distribution coefficient of REE between CFA and porewater
There has been no satisfactory explanation as to how biogenic CFA concentrates REE from ambient solution, although several hypotheses have been proposed (Shemesh, 1990; Grandjean-Léuyer etal., 1993; Reynard et al., 1999). Koeppenkastrop and De Carlo (1992) performed a REE sorption experiment with synthetic hydroxyapatite under seawater conditions, and reported that the distribution
coefficient showed a slight increase from La to Nd and a distinct decrease from Nd to Lu. Taking into
account the similarity of the ionic radii between the middle-REE3+ and Ca2+, they considered that lattice substitution causes preferential sorption of the middle-REE onto hydroxyapatite relative to other
REEs. This mechanism may also influence the REE abundance of the biogenic CFA, because the
KdLn/KdGd ratio of the CFA in this study also shows the maximum value for a middle-REE (Sm). On
the other hand, the KdLa/KdLu ratio (5.1) obtained experimentaly by Koeppenkastrop and De Carlo(1992) is twice higher than the ratio (2.2~2.5) of our study. Moreover, the Kd values of Koeppenkastrop and De Carlo (1992) range from 1×105 to 1×106, while the Kd value for the CFA in the present
study is estimated to be up to ~109 because of the high La concentrations in the CFA (> 3000 ppm:
Takebe, 2005) and the low concentration in deep-seawater (5~6 ppt: Zhang and Nozaki, 1996). Theselarge differences may be caused by the fact that Koeppenkastrop and De Carlo (1992) and this study
dose not take complexation into consideration in the KdLn calculation. In addition, it is reported thatvariations in crystallinity, carbonate content and F order in the apatite lattice potentially influence the
REE abundances in fish apatite in sediment (Shemesh, 1990).
5. SUMMARY
In this study, we estimate the REE composition of porewater in Pacific sediment, based on the REE
compositions of Mn oxides and CFA component analyzed by Takebe (2005), assuming that Mn oxide
and CFA in the sediment were in chemical equilibrium with the porewater in terms of REE.
At first, we estimate the REE/Gd ratio of the porewater except for the Ce/Gd ratio, on the basisof the REE abundance of Mn oxide component. In the calculation, we assumed that REE are mainly
dissolved as Ln(CO3)2– and LnCO3+ in porewater, according to the equilibrium calculation. After theestimation of REE composition of porewater, the distribution coefficient of REE (KdREE/KdGd ratio)
between the CFA and porewater is calculated from the REE/Gd ratio of CFA component and porewater. Subsequently, the Ce/Gd ratio of the porewater is evaluated from the pattern of the distribution coefficient.
Our calculated REE composition of porewater shows La depletion and a small negative Ce anomaly
relative to deep-seawater (SA5-5149m: Zhang and Nozaki, 1996) when normalized to PAAS, whereas
the REE/Gd ratio except for La and Ce is similar to the ratio of the deep seawater. The La depletion is
actually observed in porewater (Haley and Klinkhammer, 2003). The difference of REE composition of
deep-seawater and porewater suggest that REE in pelagic sediment is mobilized during early diagenesis.
Acknowledgements
We especially wish to express my gratitude to K. Tanaka, Hiroshima University, who contributedwith suggestion and criticism. We also express my gratitude to R. Metcalfe, Quintessa Japan for theirpatience in revising the English version of this paper. The surface sediments used in this study werecollected during GH 80-1 Cruises of the Geological Survey of Japan, and NH 90-1 Cruises of theNorthwest Pacific Carbon Cycle Study (NOPACCS) assigned to the Kansai Environmental Engineering Center Co. Ltd. (KEEC) by the New Energy and Industrial Technology Development Organization(NEDO). A. Nishimura, Geological Survey of Japan kindly gave us these Pacific sediments.

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(Received: 18 Sept. 2007, Accepted: 15 Oct. 2007)— 12 —