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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: [email protected] (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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