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Chemical Geology 220

Variations in seawater osmium isotope composition since the last

glacial maximum: A case study from the Japan Sea

Tarun K. Dalaia,*, Katsuhiko Suzukib, Masao Minagawac, Yoshiyuki Nozakia

aOcean Research Institute, Marine Inorganic Chemistry, University of Tokyo, Nakano-ku 164-8639, JapanbInstitute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

cGraduate School of Environmental and Earth Science, Hokkaido University, Sapporo 060-0810, Japan

Received 18 November 2003; accepted 14 April 2005

Abstract

Concentrations of Re and Os, and the isotopic composition of Os have been measured in the Japan Sea sediments to assess

the response of the Japan Sea to glacial–interglacial climate change and associated weathering fluxes. The osmium concentra-

tions in the sediment samples analyzed vary from 59 to 371 pg/g, and 187Os/188Os from 0.935 to 1.042. Only 187Os/188Os of

sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of Re

and Os, and with z80% hydrogenous Os are explained in terms of seawater composition. Lower 187Os/188Os were observed for

sediments deposited during the last glacial maximum (LGM) when planktonic foraminifera from the Japan Sea recorded lighter

oxygen isotopic composition. Decrease in dissolved Os fluxes from continents and/or change in the composition of the

dissolved load to the Japan Sea are suggested as the driving mechanisms for the observed lower LGM 187Os/188Os. The results

of this study, coupled with lower 187Os/188Os during the last glacial observed at other sites from ocean basins with different

lithology and contrasting sediment accumulation rates, suggest that this trend is characteristic of the global oceans.

Data from this study show that the Japan Sea recorded higher 187Os/188Os during the current interglacial coinciding with

excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values. This is explained in terms of

preferential release of 187Os during deglacial weathering and/or higher continental Os flux driven by warm and wet climate.

This study demonstrates that Os isotopic composition of reducing margin sediments has immense potential to track variations in

the seawater composition. In addition, 187Os/188Os of reducing sediments may be used to draw inferences about local

paleoceanographic processes in semi-enclosed basins such as the Japan Sea.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Japan Sea; 187Os/188Os; Paleoclimate; Margin sediment; Glacial–interglacial climate change

0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2005.04.012

* Corresponding author. Present address: Department of Geology

and Geophysics, SOEST, University of Hawaii, Manoa, 1680 East

West Road, Honolulu, HI 96822, USA.

E-mail address: dalai@hawaii.edu (T.K. Dalai).

1. Introduction

It is believed that climate variability influences

global weathering fluxes (Walker et al., 1981;

(2005) 303–314

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314304

Raymo and Ruddiman, 1992). Studies on past climat-

ic events and their impacts on weathering fluxes

require suitable proxies contained in marine sediments

and provide a means to test the climate-weathering

feedback. One of such proxies recently used by geo-

chemists is the marine Os isotopic record (Pegram et

al., 1992; Oxburgh, 1998; Cohen et al., 1999;

Peucker-Ehrenbrink and Ravizza, 2000; Ravizza et

al., 2001; Frank, 2002; Ravizza and Peucker-Ehren-

brink, 2003). The advantage of Os isotopes over other

isotopic systematics, particularly in the study of con-

tinental weathering, is that the mean isotopic compo-

sition of land derived dissolved Os (187Os/188Os=1.54, Levasseur et al., 1999) and upper conti-

nental crust (187Os/188Os=1.4, Peucker-Ehrenbrink

and Jahn, 2001) are very different compared to mantle

and cosmic sources (187Os/188Os=~0.13, Luck and

Allegre, 1983; Walker and Morgan, 1989; Meisel et

al., 2001). In addition, isotopic composition of the

dissolved Os flux is not buffered by carbonate weath-

ering, thus making the task of interpreting the marine

Os isotopic record easier. Furthermore, the marine

residence time of Os is such that variation in ocean

composition over several thousand years can be cap-

tured by marine 187Os/188Os records. Thus, one of the

approaches to assess the impact of glacial–interglacial

climate change on weathering fluxes is to study ma-

rine 187Os/188Os records. Although studies aimed at

reconstructing the seawater 187Os/188Os during the

glacial–interglacial climate cycles are few; they doc-

ument the potential of marine Os isotopic records to

ascertain impacts of these climate cycles on seawater

composition (Oxburgh, 1998, 2001; Peucker-Ehren-

brink and Ravizza, 2000; Williams and Turekian,

2004).

The goal of this study is to determine the varia-

tions in ocean chemistry driven by last glacial–inter-

glacial climate oscillations and associated weathering

fluxes by using high resolution 187Os/188Os record of

the Japan Sea sediments. The Japan Sea being a

marginal sea environment with high sediment accu-

mulation rates provides the opportunity to study the

records at high resolution. Its proximity to the land

and amplified paleoceanographic response to climate

change relative to the open ocean makes the Japan

Sea an ideal site to study the land–ocean interactions

(Tada et al., 1992, 1999). During the last glacial

maximum (LGM), the decrease of global sea level

by as much as 120 m (Fairbanks, 1989) virtually

isolated the Japan Sea from the open Pacific resulting

in anoxic bottom water condition and accumulation

of laminated sediments (Oba et al., 1991; Crusius et

al., 1999). Quaternary sedimentary records of the

Japan Sea have been extensively studied using vari-

ous proxies for paleoceanography and paleoclimate

(Crusius et al., 1999; Tada et al., 1992, 1999; Gorbar-

enko and Southon, 2000). In addition, data on Os

concentrations and 187Os/188Os are available for

loess, desert sands, arc volcanics and river water

and sediments (Levasseur et al., 1999; Peucker-

Ehrenbrink and Jahn, 2001; Alves et al., 2002; Hat-

tori et al., 2003), which are sources of dissolved and

detrital loads to the Japan Sea. Together these data

provide a valuable framework for interpreting the

results obtained in this study.

2. Geographic and oceanographic setting of the

Japan Sea

The Japan Sea is a semi-enclosed marginal basin

bordered by the Asian continent in the west and the

Japan Island Arc in the east (Fig. 1). It is connected to

the East China Sea in the southwest, to the Pacific

Ocean in the east and to the Okhotsk Sea in the north

through five shallow straits with sill depths b130 m.

In the modern circulation regime the Tsushima Warm

Current (TWC), a branch of Kuroshio Current, is the

only current flowing into the Japan Sea through the

Tsushima Strait. Deep water formed in the northern

part of the sea during winter ventilates the deeper part

and thus maintains oxygenated bottom water condi-

tions (Gamo et al., 1986). Since the amount and

properties of the water flowing in and out of the

Japan Sea have a major control on present-day ocean-

ographic condition, it is natural that glacio-eustatic sea

level changes during the Quaternary caused signifi-

cant paleoceanographic changes in the Japan Sea (Oba

et al., 1991; Tada et al., 1992, 1999). The proximity of

the Tsushima Strait to the mouths of two major rivers,

the Huanghe and the Changjiang, is suggestive of

riverine influence on the biogeochemistry of the

Japan Sea. Located in the downwind direction of the

desert area of central-east Asia, the Japan Sea receives

significant eolian dust (Kosa) contribution during

early spring from the Gobi and the Taklimakan

400N

300

1400E12001000800

200

Gobi

Taklimakan

Changjiang R

Huanghe R.

LoessSandsGravels

Japan Sea

Tsushima Strait

EastChinaSea

PC-5

PC-9

Fig. 1. Location map of the piston cores KT94-15 PC-5 (marked as PC-5) and KT94-15 PC-9 (marked as PC-9). Also shown is the distribution

of loess and major deserts in central and eastern Asia. Arrows represent present-day circulation in the Japan Sea, East China Sea and Yellow Sea

(after Tada et al., 1999).

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314 305

Deserts (Fig. 1, Tada et al., 2000 and references

therein).

0

20

40

60

80

0 5 10 15 20 25

Calendar Age (ky)

Dep

th (

cmb

sf)

Fig. 2. Age model for the sample depths studied. The age contro

points are based on the presence of TL-1 layer, top and bottom o

TL-2 layer.

3. Stratigraphy, materials and methods

Samples used in this study are from the piston core

KT94-15 PC-5 (hereafter referred as PC-5, Fig. 1),

which has been collected from the deep basin of the

Japan Sea (40801VN, 138812VE; water depth 2885 m).

This core is ~8.4 m long and covers the last 150 ky.

Possible loss of core-top materials has been inferred

by Itaki et al. (2004). In the Japan Sea, the Late

Quaternary sediments are characterized by alternating

dark and light layers which are synchronous and

correlatable in the whole basin (Tada et al., 1992,

1999). These layers have been suggested to be a result

of paleoceanographic and paleoclimatic changes and

possibly linked to Dansgaard–Oeschger (D–O) cli-

matic cycles observed in Greenland ice cores (Tada

et al., 1999). Most of the dark layers contain thin

laminations (TL) that are subsequently referred to as

TL layers (Tada et al., 1999). These TL layers of the

Japan Sea serve as excellent stratigraphic markers.

The depositional ages of TL layers, estimated based

on AMS 14C dating of monospecific foraminifers

(Oba et al., 1991, 1995) and tephra markers, have

been compiled by Tada et al. (1999). For the samples

analyzed in this study, age–depth relationship is based

on the age control points TL-1 layer (at 30 cmbsf),

and top and bottom of TL-2 layer (at 50 and 79 cmbsf,

respectively, Tada, 1998; Tada et al., 2000; Fig. 2).

Linear sedimentation rate was assumed between these

points to calculate the age of sample depths.

l

f

Table 1

Concentrations of Re and Os, 187Os/188Os and burial fluxes in the Japan Sea sediments

Depth (cmbsf) Age (cal. ky) Re (ng/g) Os (pg/g) 187Os/188Os F2ra Re burial fluxb (ng/cm2/ky) Os burial fluxb (pg/cm2/ky)

7.5 2.9 0.19 124 0.977 0.004 0.08 52

16.7 6.4 0.17 146 1.000 0.003 0.13 114

29.9 11.5 2.79 371 1.042 0.003 2.72 362

37.0 14.2 1.04 110 0.935 0.007 2.08 220

46.0 16.4 0.38 59 0.945 0.005 0.80 125

52.6 18.0 9.54 214 1.000 0.001 22.3 500

57.6 19.4 16.3 177 0.991 0.006 43.5 472

62.4 20.6 12.4 175 0.940 0.012 35.3 498

64.4 21.2 20.8 171 0.982 0.010 57.2 470

66.7 21.8 18.1 159 0.959 0.008 49.8 438

a Standard errors of the mean.b Os(Re) burial flux=linear sedimentation rate (cm/ky)�dry bulk density (g/cm3 )�Os(Re) concentration. Dry bulk density data from Tada

(1998) and Nakanishi (2003).

Table 2

Average concentrations of Re and Os, and 187Os/188Os in source

rocks of the Japan Sea and in Upper Continental Crust

Source rock Re

(ng/g)

Os

(pg/g)

187Os/188Os Reference

Chinese Loess 0.21 32 1.05F0.23 1

Taklimakan Desert

sand

0.18 11.2 1.29F0.09 2

Japan Island

Arc volcanics

0.47 1.9 0.24F0.13 3

Upper Continental

Crust

0.20 31 1.4F0.3 1

(1) Peucker-Ehrenbrink and Jahn (2001), (2) Hattori et al. (2003), (3)

calculated by averaging the data of Alves et al. (2002) after excluding

two samples with high 187 Os/188 Os and low Os concentrations

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314306

Osmium abundances and its isotopic composition

were measured in acid digests of 2–3 g of bulk sedi-

ments. Separation and purification of Os and measure-

ments of isotopic abundances followed the methods

outlined in Suzuki and Honda (2003). Samples were

spiked with solutions enriched in 190Os and 185Re,

and digested in reverse aqua regia in Carius Tubes

(Shirey and Walker, 1995). Osmium was extracted by

carbon tetrachloride solvent extraction (Cohen and

Waters, 1996; Pearson and Woodland, 2000) and

further purified by micro distillation (Birck et al.,

1997). Rhenium was extracted and purified from the

remaining solution by anion exchange procedure

using AG 1�8 resin (100–200 mesh). Isotopic abun-

dances were measured by negative thermal ionization

mass spectrometry (Finnigan MAT 262) of OsO3� and

ReO4� (Creaser et al., 1991; Volkening et al., 1991).

From the measured oxide ratios, atomic ratios were

calculated after correcting for oxide interferences,

instrumental mass fractionation and contributions

from the 190Os spike. Instrumental mass fractionation

for Os was corrected by normalizing the measured Os

isotope ratios to 192Os/188Os of 3.08271. Oxide cor-

rections were done using 17O/16O=0.00037 and18O/16O=0.002047 (Nier, 1950). Rhenium isotopic

ratios were determined by integrating data collected

after total ionization of all Re loaded on the filaments

(Suzuki et al., 2004). This method eliminates the

effect of instrumental mass fractionation and yields

isotopic ratios more precise than conventional mea-

surement techniques. Total procedural blank was ~7

pg for Re, and ~2 pg for Os with 187Os/188Os of

~0.298. Contribution of blank to measured Os con-

centrations and 187Os/188Os were b1.5% and b0.5%,

respectively. Precision of 187Os/188Os measurements,

based on analysis of an in-house standard over a

period of several months, was better than 0.4% (2

S.D.). Standard errors (2 S.D.) associated with the

measured 187Os/188Os are given in Table 1.

4. Results and discussion

Concentrations of Re and Os, and 187Os/188Os of

the Japan Sea sediment samples analyzed in this study

are given in Table 1. Rhenium concentrations vary

from 0.17 to 20.8 ng/g. Sediments deposited in the

intervals 2.9–6.4 ky and 14.2–16.4 ky have Re con-

centrations close to upper crustal values. Osmium

concentrations of samples analyzed are in the range

of 59 to 371 pg/g (Table 1). The highest Os concen-

.

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314 307

tration is more than ten times higher compared to the

upper continental crust (Table 2) whereas the lowest is

about two-fold higher. In comparison, average Os con-

centrations of sources of sediments to the Japan Sea,

i.e. Chinese Loess, Taklimakan Desert sand and the

Japan Island Arc volcanics are much lower, in the range

of ca. 2 to 30 pg/g (Table 2). Dark laminated layers

deposited during the LGM (TL-2) and at ca. 11 ky (TL-

1) have elevated Re and Os contents. 187Os/188Os of

samples analyzed ranges from 0.935 to 1.042 (Table 1).

Growth corrections for 187Os from decay of 185Re are

not required because even the largest correction of

0.02% in the measured 187Os/188Os is much less than

the analytical precision. Sediments of the TL-2 layer

deposited during LGM have lower 187Os/188Os where-

as the maximum 187Os/188Os was recorded in the lam-

ination layer TL-1 (Fig. 3). Low values of 187Os/188Os

observed in LGM sediments of the Japan Sea are

similar to those recorded in the Santa Barbara Basin

(Williams and Turekian, 2004), and are either similar or

lower than those observed in East Pacific Rise sedi-

ments (Oxburgh, 1998; Fig. 3). The LGM 187Os/188Os

minimum recorded in TL-2 and the 187Os/188Os max-

imum of TL-1 are associated with oxygen isotopic

excursions based on analysis of planktonic foraminif-

era (d18Opf) from the Japan Sea (Crusius et al., 1999;

Fig. 3. Age-variation of 187Os/188Os of the Japan Sea sediments (filled ci

the direction in which the data points would have plotted in absence of d

Results from the East Pacific Rise (open squares, Oxburgh, 1998) and th

for the last 30 ky are plotted for comparison. d18O (per mil relative to

umbilicata and N. pachyderma sampled from the core PC-9 (Fig. 1) for t

al., 1999; electronically archived by World Data Center-A for Paleocl

contibutions_by_author/crusius1999).

Fig. 3). This observation, together with the knowledge

that the dark laminations are a result of climatic and

paleoceanographic changes (Tada et al., 1992, 1999),

points to a general inference that the observed Os

isotopic variations in the TL layers of the Japan Sea

are also linked to these processes. In the following, we

assess if 187Os/188Os record of the Japan Sea sediments

over past 22 ky reflect change in seawater composition,

and determine mechanisms responsible for the ob-

served 187Os/188Os variations.

4.1. The LGM 187Os/188Os minimum

The Japan Sea sediments record lower 187Os/188Os

(0.94–0.99) during the LGM with a minimum at ~20

ky (Fig. 3). This excursion occurs in the TL-2 layer

and is associated with higher dust influx from the

Asian continent and d18O minimum observed in

planktonic foraminifera of the Japan Sea (Tada et

al., 1992, 1999; Crusius et al., 1999; Tada et al.,

2000). During the LGM, the sea level drop by 120

m had isolated the Japan Sea from the open Pacific

resulting in reduced deep water ventilation and strat-

ification (Oba et al., 1991). This caused widespread

bottom water anoxia in the Japan Sea as evident from

studies on abundance and assemblage of benthic fo-

rcles). TL-1 and TL-2 layers are shaded grey. The arrows indicate

etrital Os contributions from arc-volcanics (see text for discussion).

e Santa Barbara Basin (open circles, Williams and Turekian, 2004)

Vienna Peedee belemnites, VPDB) of planktonic foraminifera G.

he same interval are also plotted (dashed line, data from Crusius et

imatology, Boulder, Colorado at http://www.ngdc.noaa.gov/paleo/

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314308

raminifera, C/S and Re/Mo in sediments (Crusius et

al., 1999, Tada et al., 1999). Anoxic conditions

resulted in deposition of thick dark laminations layer

(TL-2). Prevailing anoxic conditions in the Japan Sea

must have favored enhanced scavenging of Os from

water as evident from high Os concentrations in the

LGM sediments. Rhenium concentrations of the TL-2

layer obtained in this study (12.4�20.8 ng/g) are

similar to those reported by Crusius et al. (1999)

and are about 50 times higher than the average

upper crustal value (~0.2�0.4 ng/g, Esser and Ture-

kian, 1993; Peucker-Ehrenbrink and Jahn, 2001). Os-

mium concentrations of TL-2 layer (159–214 pg/g)

are at least five-fold higher than the average crustal

value (~30 pg/g, Table 2). The osmium concentrations

show strong positive correlation with Corg (r2=0.91,

Fig. 4). Average Os/Corg weight ratio in sediments of

the Japan Sea as obtained from regression of data

(Fig. 4) is ~8 ng Os/g Corg which is higher than

those compiled for reducing margin sediments (3 ng

Os/g Corg, Ravizza, 1995) and in anoxic sediments of

the Cariaco Basin (5 ng Os/g Corg, Oxburgh, 2001).

Furthermore, Crusius et al. (1999) based on analysis

of organic matter of the LGM sediments from core

PC-9 (Fig. 1) inferred that they are mainly of marine

origin, thus strengthening the idea that Os in these

sediments are a result of scavenging by marine organ-

ic matter under anoxic bottom water condition. It is

widely recognized that dust transport from the Chi-

nese Loess was intensified during the LGM and con-

stituted bulk of the terrestrial contributions to the

0

100

200

300

400

0 2 3 4Corg (wt. %)

Os

(pg

/g)

1

Fig. 4. Co-variation plot of organic carbon (Corg) vs. Os concentra-

tion in Japan Sea sediments. A strong positive correlation (r2=0.91)

is suggestive of significant hydrogenous Os fraction (see text for

Discussion). Corg data from Tada (1998) and Nakanishi (2003).

Japan Sea sediments (Tada et al., 2000; Irino and

Tada, 2002). The study of Tada et al. (2000) on the

core PC-5 showed that dust flux from the Asian Loess

was the major component of terrestrial load to the Japan

Sea during the LGM whereas contributions from riv-

erine detritus were relatively more important during the

interglacials. The observation that the Chinese Loess

and Taklimakan Desert sands and the upper continental

crust (UCC) have much lower Os concentrations and

higher 187Os/188Os (11–32 Pg/g Os with 187Os/188Os of

1.05–1.29; Table 2) compared to the LGM sediments of

the Japan Sea (z160 pg/g Os with 187Os/188Os of

0.94�1.00; Table 1) argues against significant influ-

ence of detrital Os contributions to the LGM sediments.

Thus, it is inferred that the majority of Os in the LGM

sediments is hydrogenous (z80%) and hence the LGM187Os/188Os excursion reflects change in Os isotopic

composition of the Japan Sea water. This inference is

consistent with conclusions of Oxburgh (1997) that all

osmium in the sediments from the Cariaco Basin are

hydrogenous. However, results from the Santa Barbara

Basin indicated that 187Os/188Os of bulk sediments

reflect signatures of variable contributions from detrital

sources. Low 187Os/188Os in the Japan Sea during the

LGM can arise from one or a combination of the

following: (i) enhanced inputs from mantle/hydrother-

mal sources (ii) decrease in total dissolved Os flux from

continents and (iii) decrease in 187Os/188Os of the dis-

solved Os flux to the Japan Sea.

There are no available studies that suggest enhanced

hydrothermal contribution to the Japan Sea during the

LGM. Enhanced hydrothermal Os contribution should

also have been accompanied by higher hydrothermal

metal inputs. In the core PC-5, Fe concentrations of the

LGM sediments are not distinctly different than those

of modern and interglacial sediments. Furthermore,

concentrations of Fe do not show significant correla-

tion with Os concentrations and 187Os/188Os. The in-

fluence of Os contributions from variations in cosmic

dust flux is likely to be negligible given that accumu-

lation rate of Os in marine sediments (4 pg/cm2/ky,

Esser and Turekian, 1988) is lower than burial fluxes of

Os in the LGM sediments of the Japan Sea (z440 pg/

cm2/ky, Table 1) bymore than two orders of magnitude.

Given the knowledge that the rate of chemical weath-

ering decreases in cold and arid climate with less

precipitation and surface runoff, it is reasonable to

invoke that decrease in total continental dissolved Os

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314 309

flux during the LGM have contributed to the LGM187Os/188Os minimum observed in the Japan Sea.

Lower LGM values of 187Os/188Os observed at East

Pacific Rise was also attributed to reduced dissolved Os

flux driven by glacial climate and prevailing aridity

(Oxburgh, 1998).

During the LGM, the Japan Sea was isolated from

the open ocean. The drop of sea level alone is thought

to have reduced influx of TWC through the Tsushima

Straight by more than a factor of 20 (Tada et al.,

1999). The decrease in flow from the Pacific resulted

in the increase in fresh water influence on the water

balance as evident from the lighter d18O values of

planktonic foraminifera, a unique feature of the Japan

Sea (Fig. 3). One of the major pathways of the mate-

rial transport from the Asian continent to the Japan

Sea is the riverine input and lateral transport via the

TWC (Otosaka et al., 2004). Although enhanced eo-

lian transport of the Chinese Loess was the major

source of terrestrial detritus to the Japan Sea during

the LGM, the transport of the dissolved load from the

Changjiang River and the Huanghe River was most

likely diminished due to a weaker TWC. Thus, the

dissolved fluxes from the Japan Island Arc are likely

to have played a more important role in regulating the

LGM Japan Sea chemistry. This suggests that during

the LGM, the dissolved load to the Japan Sea was

characterized by relatively lower 187Os/188Os. This

inference draws support from the suggestion that

weathering of arc volcanics and associated sedimen-

tary rocks is an important source of unradiogenic Os

to the modern sea water as evident from measured187Os/188Os in Coral Sea water, Fly and Sepik river

waters and sediments within the Gulf of Papua (Mar-

tin et al., 2000) that are lower compared to the open

ocean water and rivers draining the basins composed

of old continental crust (Levasseur et al., 1999). Thus,

composition of net dissolved flux to the Japan Sea

modified by decrease of freshwater flux from the

rivers draining the Asian continent was at least partly

responsible for the observed lower 187Os/188Os in the

Japan Sea during the LGM.

The mean 187Os/188Os (0.97F0.02, n =5) recorded

in the LGM sediments from the Japan Sea is identical

to glacial 187Os/188Os as recorded in the Santa Bar-

bara Basin (mean=0.97F0.04, Williams and Ture-

kian, 2004) and is similar to 187Os/188Os measured in

the glacial sediments of the EPR (Oxburgh, 1998), the

Cariaco Basin (Oxburgh, 1997) and the California

Basin (Peucker-Ehrenbrink and Ravizza, 2000).

Such similarity in 187Os/188Os recorded in five sepa-

rate sites from two ocean basins, with different lithol-

ogies and contrasting sediment accumulation rates,

establishes that marine 187Os/188Os during the last

glacial were lower. It is inferred that this global effect

is likely due to decrease in flux and/or 187Os/188Os of

the continental osmium delivered to the oceans.

Lower 187Os/188Os recorded in the glacial sediments

of the Japan Sea, during a period of maximum dust

flux from the Asian continent, suggest that variations

in the dust flux may not be significant in regulating187Os/188Os of the global oceans.

4.2. The current interglacial: was there a 187Os/188Os

maximum?

The TL-1 layer deposited at ~11 ky in the Japan

Sea has been suggested to have formed at elevated sea

level and higher freshwater influx to the sea (Tada et

al., 1999). Higher stands of sea level and larger influx

of sea water to the Japan Sea during this period is

reflected in the heavier d18O of planktonic foraminif-

era (Fig. 3; Crusius et al., 1999; Gorbarenko and

Southon, 2000). High organic carbon content of TL-

1 layer is a net result of enhanced surface productivity

as evident from higher biogenic silica contents (Tada

et al., 1992) and suboxic condition of bottom water

(Crusius et al., 1999; Tada et al., 1999). One sample

from this lamination was analyzed in this study. Ele-

vated Re and Os contents of this sample (2.8 ng/g Re

and 371 pg/g Os, Table 1) attests to the reducing

depositional condition. This information, coupled

with high concentrations of Corg, Os, and Re suggests

that a dominant fraction of Os in this sample is

hydrogenous. Furthermore, mineralogical studies of

the Japan Sea sediments show that riverine detritus,

particularly from Japan Island Arc, assumed impor-

tance over eolian contribution following the termina-

tion of the LGM (Tada et al., 1992). Significant Os

contribution from detritus of arc-volcanics composi-

tion would have lowered down 187Os/188Os of the

Japan Sea water which is contrary to what is observed.

The 187Os/188Os value of 1.04 observed for the

Japan Sea at 11 ky is within the range of those reported

for the interglacial sediments in the Santa Barbara

Basin (1.00F0.08, Williams and Turekian, 2004)

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314310

and is similar to those reported for the interglacial

records from the Cariaco Basin (Oxburgh, 2001) and

the California Basin (Peucker-Ehrenbrink and Ravizza,

2000). From the age vs. 187Os/188Os plot for the Japan

Sea sediments (Fig. 3), it is tempting to infer that the

Japan Sea experienced 187Os/188Os maximum at 11 ky.

A careful assessment needs to be done before such an

interpretation can be accepted. Although two Holocene

samples (with age 2.9 and 6.4 ky) analyzed in this study

have higher Os concentrations compared to upper

crustal Os abundance, they were deposited in oxic

conditions. Existing studies indicate that oxygenated

condition prevailed in the Japan Sea beginning at about

8 ky BP (Oba et al., 1991; Tada et al., 1999). This,

coupled with low Re abundances in these samples,

suggests that the apparent 187Os/188Os maximum at

11 ky (Fig. 3) can be an artifact of detrital Os contribu-

tions to the Holocene sediments. We perform a simple

mass balance calculation for the sample with age 2.9 ky

to test if the measured Os concentration (124 pg/g) and187Os/188Os (0.977) can be a result of contributions

from seawater and detritus of arc-volcanics composi-

tion. Using 187Os/188Os value of 1.04 (as observed at 11

ky) for seawater and 0.24F0.13 for the arc volcanics

(Table 2), it is estimated that ca. (7–9)% of Os in

sediments (i.e. 8–12 pg Os per gram of sediment) has

to be from arc-volcanics detritus. Similarly, for the

sample with age 6.4 ky, contributions from materials

of arc-volcanics composition should account for 6–9

pg Os per gram of sediment.

In addition to arc-volcanics, materials from the

Asian Loess are also transported to the Japan Sea

both via eolian transport and regional circulation sys-

tems. Considering that incorporating the Asian Loess

in the above calculation will result in a higher fraction

of Os contributions from arc-volcanics detritus, the

estimations of the required Os contributions from arc

volcanics are minimum values. Sediment traps studies

show that fraction of arc-volcanics detritus in the

lithogenic flux to the Japan Sea at 1 km depth varies

from 15% in the Yamato Basin to 87% in the eastern

Japan Basin (Otosaka et al., 2004). Considering that

the location of the core PC-5 is between the Yamato

Basin and the eastern Japan Basin, the contribution of

arc-volcanics detritus to the lithogenic flux at this site

is expected between 15% and 87%. This is consistent

with the estimation of Tada et al. (2000) that Kosa

fraction (derived from the Asian Loess) in sediments

of the core PC-5 during the current interglacial is at

least 30%, the rest being derived from the arc-volca-

nics materials. If the mean Os concentration of the

Japan Island arc volcanics (1.9F2.2 pg/g, Table 2) is

assumed to be representative of the sediments in the

Japan Sea derived from the arc volcanics, it can be

seen that that even the minimum requirement of Os

contribution from the arc volcanics is difficult to meet.

This will lead to the suggestion that 187Os/188Os of

seawater in the Japan Sea water was less than 1.04

during the last 3–6 ky and that the Japan Sea indeed

witnessed a transient 187Os/188Os maximum at 11 ky.

This inference, however, is critically dependent on

the assumption that suspended materials derived

from weathering of arc volcanics have Os concentra-

tions same as the reported mean value of their source

rocks. The geochemical behavior of Os suggests that

it is particle reactive and is likely to be scavenged by

Fe–Mn oxyhydroxides. Significant leachable Os con-

centrations in the sediments of the Ganga river

(Pegram et al., 1994) and the Sepik river (Martin

et al., 2000) is suggestive of scavenging of Os most

likely by Fe–Mn oxyhydroxides. The sediments of

the Fly and Sepik rivers, draining island arc volca-

nics and associated sedimentaries, have Os concen-

trations in the range of 82–102 pg/g (Martin et al.,

2000). Thus, it is likely that suspended sediments

carried by rivers draining the Japan Island Arc vol-

canics have higher Os concentrations compared to

their source rocks. In addition, Alves et al. (2002)

reported Os concentration for the Japan Island Arc

volcanics as high as 7.8 pg/g which suggests that

estimates for the required Os contributions to the

Japan Sea sediments from the arc volcanics is not

entirely impossible. Together, these data and infor-

mation suggest that 187Os/188Os of the Japan Sea

water during the current interglacial has been likely

~1.04 as recorded in the TL-1 layer and lower187Os/188Os values recorded in the Holocene sedi-

ments are most likely a result from contributions

from arc-volcanics detritus.

The possible mechanisms responsible for higher

interglacial 187Os/188Os can be preferential release of187Os during deglacial weathering (Peucker-Ehren-

brink and Blum, 1998) in the Asian continent and

its efficient delivery to the Japan Sea due to higher

influx through the Tsushima Strait, or higher conti-

nental dissolved Os flux resulting from enhanced

0

150

300

450

0 15 30 45 60

Re burial flux (ng/cm2/ky)

Os

bu

rial

flu

x (p

g/c

m2 /

ky)

Fig. 5. Plot of Os burial flux vs. Re burial flux in Japan Sea

sediments. At Re concentrations higher than the crustal value

increase in Re burial flux is associated with higher Os burial flux

This trend, however, breaks down at very high Re concentration

z10 ng/g.

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314 311

weathering in wet and humid climate or a combination

of both.

4.3. Abundances of Re and Os, and 187Os/188Os of

margin sediments: role of depositional condition

The Japan Sea sediments deposited during the last

deglacial (14–16 ky) have very low 187Os/188Os

(0.935�0.945, Table 1). Available information on

geochemical parameters such as C/S and Re/Mo sug-

gest that sediments of this interval deposited under

oxic condition (Crusius et al., 1999; Tada et al., 1999).

The results on Re and Os concentrations, obtained in

this study, seem to suggest that these sediments have

significant detrital Os. Rhenium concentrations of

these sediments (b1 ng/g) show no appreciable en-

richment over crustal level. Osmium concentrations

are the lowest among all samples analyzed in this

study (59�110 pg/g). Lower 187Os/188Os of these

sediments is in line with the knowledge that detritus

of arc-volcanics composition were more important

than eolian inputs following the termination of the

LGM (Tada et al., 1992). In the previous section, it

was argued that contributions from arc-volcanics de-

tritus in the Holocene sediments can be significant and

may bias the interpretation of 187Os/188Os in terms of

seawater composition. Together, all this information

show that the utility of bulk 187Os/188Os of oxic

margin sediments as records of seawater composition

is limited; and underscore the importance of data on

proxies of bottom water oxygen level, such as C/S and

Re/Mo, and on Os contents of sediments which pro-

vide at least qualitative information on detrital contri-

bution to Os budget of sediments.

That reducing bottom water condition enhances

depositional flux of Os is best illustrated in the plot

of burial flux of Re vs. burial flux of Os (Fig. 5). A

similar trend also exist between concentrations of Re

and Os burial flux. The plot shows that at Re con-

centrations higher than the upper crustal level, in-

crease in Re burial flux is associated with higher Os

burial flux (Table 1, Fig. 5) thus strengthening the idea

that Os is readily scavenged in reducing conditions. It

is noteworthy that at very high Re concentrations

(z10 ng/g) observed for LGM sediments deposited

in anoxic to euxinic conditions (Crusius et al., 1999;

Tada et al., 1992, 1999), the increasing trend between

burial fluxes of Re and Os breaks down. While the

,

.

,

lack of co-variation between burial fluxes of Os and

Re at high Re concentrations is suggestive of limited

supply of Os to the Japan Sea during LGM, this trend

may also have been caused by lower flux of organic

matter during peak glacial conditions (Tada et al.,

1992). Alternatively, burial of Re and Os in extreme

reducing conditions may have been driven by separate

removal mechanisms.

4.4. The origin of the LGM salinity minimum in the

Japan Sea: constraints from Os isotopic composition

Earlier studies have demonstrated that Japan Sea

sediments deposited during last glacial period contain

a dark and thick layer which is pyrite bearing and has

parallel laminations. A decrease in d18O of planktonic

foraminifera (by 3x) in the uppermost part of this

dark layer suggests lowering of surface water salinity

during this period (Oba et al., 1991). Such a reduction

in salinity is thought to be responsible for density

stratification, reduction of vertical mixing and the

resultant euxinic condition in the basin (Crusius et

al., 1999; Tada et al., 1999). The cause for the salinity

minimum, however, has been a topic of debate. En-

hanced inflow of the Huanghe (Yellow) River water

has been suggested as the cause of LGM salinity

minimum by Oba et al. (1991). An alternative expla-

nation is the reduction of sea level and isolation of

Japan Sea from the open ocean during LGM and

consequent increase in relative influence of freshwater

from rivers (Koizumi, 1984; Tada et al., 1992). The

T.K. Dalai et al. / Chemical Geology 220 (2005) 303–314312

knowledge that Chinese Loess and the Taklimakan

Desert sand have higher average 187Os/188Os (1.05–

1.29, Table 2) and that rivers draining them, such as

the Huanghe and the Changjiang (Fig. 1), have rela-

tively higher dissolved Os concentrations and187Os/188Os (13.9–42.1 pg/L and 1.32–1.95 respec-

tively, Levasseur et al., 1999), suggests that larger

inflow of the Huanghe River during LGM would

have elevated 187Os/188Os of the Japan Sea, contrary

to what is observed. As argued previously, one of the

mechanisms responsible for low LGM 187Os/188Os

values observed in the Japan Sea was the dominance

of dissolved Os flux from the rivers draining the arc

volcanics with relatively lower 187Os/188Os. Lower187Os/188Os values during the LGM also suggest

that inputs from rivers draining the Chinese Loess to

the Japan Sea were most likely diminished due to

restricted flow through the Tsushima Strait. Thus, the

suggestion that an overall increase in the relative sig-

nificance of freshwater at sea level low stand resulted

in the LGM salinity minimum in the Japan Sea seems

more viable. The above discussions highlight the pos-

sible potential of 187Os/188Os as a tracer to draw

inferences on local paleoceanographic processes in

semi-enclosed basins like the Japan Sea. However, it

should be recognized that such an application is region

specific and requires knowledge on Os isotopic com-

position of various sources that contribute dissolved

and detrital fluxes to the basin under study.

5. Summary and conclusions

Results of this study provide supporting evidences

for lower seawater 187Os/188Os during the last glacial

maximum and higher 187Os/188Os during the current

interglacial. Lower LGM 187Os/188Os values are like-

ly a result of decrease in flux and/or 187Os/188Os of

the dissolved continental Os flux to oceans. Variations

in the dust flux seem to exert insignificant influence in

regulating marine 187Os/188Os during glacial–intergla-

cial climate change. Higher 187Os/188Os values in the

Japan Sea during the current interglacial, coinciding

with excursion to heavier d18Opf, is most likely

caused by enhanced dissolved Os fluxes from con-

tinents driven by warm and wet climate and/or pref-

erential release of 187Os from older crust during

deglacial weathering.

Combined results on Re and Os abundances and

their burial fluxes suggest they are enriched and corre-

lated in sediments deposited under suboxic condition.

However, in anoxic to euxinic sediments deposited

during the LGM, higher abundances and burial fluxes

of Re relative to Os seem to be governed by either

limited supply of Os to the Japan Sea and/or separate

removal mechanisms. This study brings to light the

potential of 187Os/188Os of reducing margin sediments

in reconstructing the variations in seawater composi-

tion driven by climate shifts and associated changes in

fluxes and composition of dissolved load to the oceans.

The depositional conditions exert significant control on

whether or not a dominant fraction of Os in margin

sediments is hydrogenous and thus have implications

to interpretation of 187Os/188Os of sediments in terms of

variations in seawater composition.

Acknowledgements

We thank Kyoko Abe for her assistance in sample

analysis. This paper benefited from constructive com-

ments of two anonymous reviewers and suggestions

of L.M. Walter. Useful suggestions from Greg

Ravizza are gratefully acknowledged. The first author

acknowledges a post doctoral fellowship from the

Japan Society for the Promotion of Science (JSPS).

This study was partially supported by JSPS through

Grant-in-Aid for scientific research to KS (Grant

#14703002). [LW]

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