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The osmium isotopic composition of organic-rich marine sediments
Earth and Planetary Science Letters, 110 (1992) 1-6
1
Elsevier Science Publishers B.V., Amsterdam [MK]
The osmium isotopic composition of organic-rich marine sediments G. R a v i z z a * a n d K.K. ~Furekian Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA
Received October 24, 1991; revision accepted February 3, 1992
ABSTRACT The osmium (Os) concentration and 187Os/186Os ratio of several recent, marine, organic-rich sediment samples from three widely separated sites have been measured. Os concentrations range from 0.095 to 0.212 ppb and 187Os/186Osratios range from 8.2 to 8.9. The calculated fraction of hydrogenousOs exceeds 78% in all samples. Thus, the 187Os/186Osratio of these samples reflects Os isotopic composition of seawater. The small range in measured 187Os/186Os ratio indicates that the Os isotopic composition at these sites is fairly homogeneous. The large magnitude of the Os burial flux at these sites indicates the Os burial in association with organic-richsediments is an important sink in the marine cycleof Os. These data also suggest that ancient organic-rich sediments may provide a record of past variations in the Os isotopic composition of seawater.
I. Introduction The osmium (Os) isotopic composition of seawater has not yet been measured. In lieu of direct measurement, we present Os isotopic data for several organic-rich, biogenic, marine sediment samples from three widely separated localities. Several features of these sediments make them particularly useful as recorders of the Os isotopic composition of seawater. Organic-rich marine sediments serve as natural preconcentrators of dissolved Os [1,2]. In addition, these sediments are relatively free of terrigenous material, thereby minimizing the influence of terrigenous Os on their isotopic composition. Finally, the sediments analyzed in this study all accumulated very rapidly, making the influence of cosmic Os negligible. Thus, as the Os in these sediments must be principally hydrogenous (e.g., derived from sea-
water), these data provide a measure of the 187Os/186Os ratio of the waters from which these sediments were deposited. The Os isotopic composition of manganese nodules [3-6] and abyssal red clays [7] have also been used to place constraints on the Os isotopic composition of seawater. However, these materials are strongly influenced by sources of Os other than seawater. The presence of Os from multiple sources complicates their application as recorders of the isotopic composition of Os in seawater. The organic-rich sediment data presented here provide a geochemically independent means of evaluating Fe and Mn oxide-bearing materials as recorders of the Os isotopic composition of seawater. These data also provide valuable insights into the marine geochemical cycle of Os and have important implications for Re-Os studies of ancient organic-rich sediments.
2. Samples * Presently at: WoodsHole OceanographicInstitution, Woods Hole, MA 02543, USA. Correspondence to: G. Ravizza, Department of Geology and Geophysics, Yale University, P.O. Box 6666, New Haven, CT 06511, USA.
Sediments from the Gulf of California (eastern North Pacific), Walvis Bay (eastern South Atlantic) and Jelly Fish Lake (western equatorial
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
2
G. RAVIZZA AND K.K. TUREKIAN
Pacific) have b e e n analyzed. T h e s e s e d i m e n t s are all c h a r a c t e r i z e d by high o r g a n i c m a t t e r c o n c e n tration, a n d rapid s e d i m e n t a c c u m u l a t i o n rate. W i t h i n the G u l f of California, samples from two b a s i n s - - C a r m e n Basin a n d San P e d r o M a r t i r B a s i n - - w e r e analyzed. I n t e n s e local upwelling in both the G u l f of California [8,9] a n d Walvis Bay [9,10] creates highly p r o d u c t i v e surface waters, which are, in t u r n , r e s p o n s i b l e for the rapid d e p o s i t i o n of organic-rich d i a t o m a c e o u s muds. Clastic material, s u p p l i e d by a d j a c e n t l a n d masses, does n o t m a s k the large b i o g e n i c c o m p o n e n t at these two sites. Jelly Fish Lake is a m e r o m i c t i c m a r i n e lake o n the uplifted c a r b o n a t e island of Eil Malk in the R e p u b l i c of Palau. Salinity-stratified d u e to int e n s e local rainfall, the lake c o n t a i n s n u t r i e n t - r i c h , sulfidic b o t t o m waters from a d e p t h of 15 m to the lake b o t t o m , 30 m d e e p [11]. T h e p r i n c i p a l
supply of n o n b i o g e n i c detritus to Jelly Fish Lake is aeolian.
3. Analytical methods T h e m e t h o d s used in this study are described m o r e completely elsewhere [12]. A b r i e f s u m m a r y of the p r o c e d u r e s is given below. Os was p r e c o n c e n t r a t e d from the s e d i m e n t using a nickel sulfide fire assay. T h e nickel sulfide b e a d f o r m e d d u r i n g the fire assay was dissolved in 12 N HC1. T h e Os r e m a i n e d in a n insoluble sulfide r e s i d u e which was s e p a r a t e d from the solution by filtration. T h e chemical proc e d u r e following the filtration was d e v e l o p e d a n d is described by Luck [13]. T h e insoluble sulfide r e s i d u e was s u b s e q u e n t l y dissolved in 4 N H 2 S O 4 with Cr(VI), a n d Os was distilled from this solution as OSO4. Os was f u r t h e r purified, using a n ion exchange resin, prior to d e p o s i t i o n o n t o a Si
TABLE 1 Re-Os and bulk composition data from sediment analyses. All errors are twice the standard error of the mean based on counting statistics Sample
lS7Os/lS6Os
[Os]
CaCO 3 a
SiO2
(ppb)
(%)
(biogenic) (%)
Organic matter a (%)
Al b (%)
25 c
12.5
5.05
San Pedro Martir Basin Average 20-35 cm
8.92+0.15 8.65 + 0.09 8.79 + 0.19
0.151 +0.004 0.147 + 0.003 0.149 + 0.004
5
Carmen Basin 10-20 cm Carmen Basin 30-40 cm Carmen Basin 60-70 cm Carmen Basin 80-95 cm
8.77 + 0.11
0.212 + 0.004
1.9
9.4 c
11.7
5.96
8.59 + 0.09
0.152 + 0.003
2.3
14.0 d
12.0
6.38
8.75 + 0.09
0.166 + 0.003
2.7
14.4 d
11.7
6.41
8.53 + 0.12
0.170 + 0.004
3.1
14.4 d
12.3
6.33
Walvis Bay 10-36 cm Average
8.15 + 0.17 8.38 5:0.09 8.27 + 0.17
0.095 + 0.002 unspiked
2.5
40 c
24.9
2.69
Jelly Fish Lake 0-30 cm Average
8.71 +0.15 8.55 + 0.10 8.63 + 0.15
0.170+0.004 0.173 :t:0.003 •0.172 + 0.004
20 e
20 e
60 e
0.85
a Total and inorganic carbon concentrations determined by Leco carbon analyzer. Organic carbon calculated by difference [16]. The molecular formula of organic matter was approximated as CH20. b AI concentrations determined by ICPES [17] at the Woods Hole Oceanographic Institution ICP facility. ¢ Data from DeMaster [9]. d Data from DeMaster [22]. e Data from Burnett et al. [11].
Os ISOTOPIC COMPOSITION OF O R G A N I C - R I C H MARINE SEDIMENTS TABLE 2 Hydrogenous Os burial in organic-rich sediments Locality
Mass Flux (g/cm2/yr)
Os (ppb)
Detritus a (%)
Hydrogenous Os b (% of Total Os)
Hydrogenous Os Flux c (pg/cm2/yr)
San Pedro Martir Basin C a r m e n Basin Walvis Bay Jelly Fish Lake
0.1 d 0.05 e 0.11 d 0.07 f
0.15 0.175 0.095 0.17
57 72 33 0
81 79 83 100
12.2 6.9 8.7 11.9
a Detritus % = 1 0 0 - CaCO3% - Biogenic SIO2% - Organic Matter%. Data from Table 1. b Hydrogenous O s % = 1 0 0 - ( D e t r i t u s % ) × 0 . 0 5 / [ O s ] . The constant 0.05 ppb corresponds to the average Os concentration of crustal material [18]. c Hydrogenous Os Flux = (Mass F l u x ) x [Os] x (%Hydrogenous Os/100). d Data from D e M a s t e r [9]. e Data from D e M a s t e r [22]. f Data from Burnett et al. [11].
chip for isotopic analysis. Blank Os was present at detectable levels in the fusion reagents but was always less than 3% of the total analyte. All isotopic measurements were made using the M I T - H a r v a r d - B r o w n regional ion microprobe facility at MIT. Measured Os isotopic ratios were corrected for hydride interferences and mass discrimination on a scan by scan basis. Mass discrimination was assumed to be a linear function of the mass difference, and corrections were made by normalizing all measured 188Os//192Os ratios to the 188Os//192Osratio of 0.32439 [14]. Samples were spiked prior to the fire assay with Os solutions enriched in 190Os and measured 19°Os//192Os ratios were used to calculate Os concentrations. 187Os//186Os ratios were calculated from measured 187Os//192Osratios using the 186Os//192Os ratio of 0.03904 [15]. The 189Os// 192Os ratios measured were always within the error of the Nier ratio of 0.39680. Osmium hydride formation was monitored at masses 193 and 191. Hydride corrections based on data from either of these masses are indistinguishable. The consistent replication of the measured 189Os// 192Os ratio and the internal consistency of the hydride correction are indicative of an interference-free Os spectrum. Total carbon and inorganic carbon were measured in splits of the sample powders using a Leco Carbon Analyzer. Organic carbon concentrations were calculated by difference [16]. Replicate determinations of total and inorganic carbon concentrations were always within 3% of the re-
ported carbon concentrations. Al concentrations were determined by inductively coupled plasma emission spectroscopy [17], following fusion with a lithium metaborate flux and sample dissolution. The uncertainty associated with the AI measurements is estimated to be + 2 % , based on repeated analyses of standard solutions. 4. Results
The data from this study are summarized in Table 1. Os concentrations of the sediment samples range from 0.095 to 0.212 ppb. All of these concentrations exceed the estimated crustal abundance of Os of 0.05 ppb [18]. The spread of measured Os isotopic ratios is from 8.15 to 8.92; a small range of variation considering replicate analyses can differ by as much as 0.2. This range is also restricted in comparison to the range of Os isotopic compositions measured in manganese nodule leachates (5-11 [3,6]). Biogenic material," such as CaCO3, organic carbon and biogenic silica, comprises a large fraction of the total sediment. AI concentrations are all lower than the average Al concentration of shales [19]. These low AI concentrations are indicative of the limited influence of detrital material in these sampies. 5. Discussion
The supply of cosmic Os to these sediments is negligible due to their very rapid accumulation
4
rates. Using Esser and Turekian's estimate of the flux of cosmic Os to marine sediments (5.7 ng/cm2/Ma [7]), the cosmic Os flux is more than 1000 times smaller than the total Os burial flux at these sites. In the absence of any significant cosmic Os component, all the Os in these sediments must be attributed to either a hydrogenous or terrigenous origin. The portion of non-hydrogenous Os in these samples can be estimated using the fraction of detrital material in each sample and assuming that the Os concentration of the detrital material in each sample is 0.05 ppb, the same as the average concentration of Os in crustal material [18]. Although the Os concentrations of terrigenous material at these sites may deviate from the average crustal value, the consistency of the data presented below argue against the influence of detrital material anomalously enriched in Os. Based on our estimates, terrigenous Os accounts for between 0%, at Jelly Fish Lake, and 21%, in the Gulf of California, of the total Os in these samples (Table 2). The remaining Os in these sediments is hydrogenous in origin. Thepresence of small amounts of non-hydrogenous Os in these sediments may cause the 187Os/186Os ratio of the sediment to deviate from that of the overlying water. However, it is improbable that hydrogenous and terrigenous Os of widely differing isotopic compositions should mix fortuitously in the appropriate proportions at three different sites to yield such similar bulk sediment Os isotopic compositions. Instead, we interpret these data as evidence that seawater at these three localities is fairly uniform in its 187Os//186Os ratio. Although the data do not preclude the possibility of subtle variations in the 187Os//186Os ratio of dissolved Os at these sites, the variations observed in 187Os//186Os ratios may arise from small contributions of nonhydrogenous Os (187Os//186Os = 10 for sialic rocks [18] and 187Os//186Os = 1 for young mafic rocks [20]). Manganese nodules were used as recorders of the Os isotopic composition of seawater in early studies [3-6]. Nodule samples were subjected to an oxalic acid leach, intended to dissolve selectively poorly crystalline authigenic Fe and Mn oxides and oxy-hydroxides, which incorporated hydrogenous Os during their formation. Significant variations in the 187Os//186Os ratios of man-
G. R A V I Z Z A A N D K.K. T U R E K I A N
ganese nodule leachates, ranging from 5.2 to 10.9, were observed. If the leaching procedure solubilizes only Os which has been recently scavenged from seawater, the nodule data provide a direct record of regional variations in the Os isotopic composition of seawater [3,5,6]. Alternatively, Palmer et al. [4] argued that residence time of Os in seawater should be long, relative to the mixing time of the oceans based on the expected speciation of Os in seawater. Under this assumption, variations in 187Os//186Os ratios of the nodules are attributed to the mixing of hydrogenous Os, of a relatively homogeneous isotopic composition, with oxalic acid leachable, meteoritic, or possibly ultramafic, Os with an 187Os//186Os ratio of 1. A third explanation of Os isotopic variations in nodule leaches involves labilization of hydrogenous Os precipitated from ancient oceans. If temporal variations in the 187Os//186Os ratio of seawater have occurred, nodule leaches might be able to access an admixture of hydrogenous Os of variable isotopic composition, thereby giving rise to the range of Os isotopic compositions observed. In the absence of additional data, each of these interpretations of the manganese nodule data is viable. In spite of the differences between manganese nodules and anoxic sediments as recorders of the Os isotopic composition of seawater, the results obtained from the two independent sources are largely consistent with one another. Palmer et al. [4] reported a model-based estimate of the 187Os//186Os ratio of seawater of between 8.5 and 11, based on manganese nodule data. The organic-rich sediment data presented here indicate that the 187Os//186Os ratio of dissolved Os at the study sites falls in the range 8.6 + 0.3. In addition, an oxalic acid leach of a recent pelagic clay [7] recovered from the abyssal North Pacific yielded an 187Os/186Os ratio of 8.40 + 0.12. Thus, the interpreted manganese nodule data, the pelagic clay leach and biogenic sediment data are all consistent with a fairly homogeneous lS7Os/ 186Os ratio of seawater, approximately 8.6. Isolated basins such as the Black Sea may have 187Os//186Os ratios which are influenced by local continental supply and, therefore, deviate from this open ocean value [2]. The burial flux of hydrogenous Os can be calculated from the sediment accumulation rate
Os ISOTOPIC COMPOSITION OF O R G A N I C - R I C H MARINE SEDIMENTS
and t h e estimate of the fraction of hydrogenous Os (Table 2). Hydrogenous Os burial fluxes at these three sites range from 6 to 12 pg/cm2/yr, roughly three orders of magnitude larger than those measured in pelagic clays from the North Pacific [7]. Thus, environments of organic-rich sediment deposition, in spite of their limited areal extent, represent an important sink for Os in the marine environment. Over the course of geologic time the residence time of Os in seawater should be sensitive to the areal extent of organic-rich sediment deposition and the efficiency of Os transfer to these sediments. The in situ decay of 187Re to lSTOs within ancient organic-rich rocks provides a chronometer which allows determination of the time of their deposition [12,21]. In order to utilize this chronometer successfully the 187Os/186Osratio of all samples must be homogeneous at the time of deposition. The data presented here provide empirical evidence that modern organic-rich sediments are indeed isotopically homogeneous at the time of their deposition. In addition to the chronometric information that Re-Os studies of ancient black shales can provide, the initial 187Os/186Os ratio of these rocks can also be determined. Just as modern organic-rich sediments have been employed in this study to infer the Os isotopic composition of the modern ocean, the initial 187Os/186Os ratio of ancient organic-rich marine sediments may provide a record of past variations in the Os isotopic composition of seawater. Thus Re-Os studies of marine black shales from the geologic record may provide paleoceanographic, as well chronometric information.
6. Summary and conclusions Measured 187Os/186Os ratios of recent, marine, organic-rich sediments range from 8.2 to 8.9. As the Os in these sediments must be derived principally from the waters from which these sediments were deposited, these data provide a measure of the 1870s/186Osratio of seawater. The small range in the Os isotopic composition of organic-rich sediment samples indicates that the 187Os/186Os ratio of seawater at these sites is relatively homogeneous. These conclusions are largely consistent with previous interpretations of Os isotopic data from pelagic clays and man-
5
ganese nodules. The large magnitude of the Os burial flux at the study sites suggests that burial of Os in association with organic-rich sediments is a major sink for dissolved Os in the oceans. If direct measurement of the Os isotopic composition of seawater confirms these inferences, the initial Os isotopic composition of ancient organic-rich sediments will provide a record of the Os isotopic composition of the ancient oceans.
Acknowledgements William C. Burnett and David J. DeMaster provided sample material for this study. B.K. Esser, C.E. Martin and S. Krishnaswami contributed to methods development and provided valuable discussions. We are also grateful to K. Burrhus for maintaining the ion microprobe and to S. Hart and N. Shimizu for advising GR on use of the ion probe. Comments from Martin Palmer on an earlier version of this work (as a thesis chapter) improved this manuscript significantly. A. Zindler and an anonymous reviewer provided helpful comments. This was supported by NSF grant OCE-8414735 to KKT.
References 1 M. Koide, E.D. Goldberg, S. Niemeyer, D. Gerlach, V. Hodge, K.K. Bertine and A. Padova, Osmium in marine sediments, Geochim. Cosmochim. Acta 55, 1641-1648, 1991. 2 G. Ravizza, K.K. Turekian and B.J. Hay, The geochemistry of Re and Os in recent sediments from the Black Sea, Geochim. Cosmochim. Acta 55, 3741-3752, 1991. 3 M.R. Palmer and K.K. Turekian, 187Os//186Os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium, Nature 319, 216220, 1986. 4 M.R. Palmer, K.K. Falkner, K.K. Turekian and S.E. Calvert, Sources of osmium isotopes in manganese nodules, Geochim. Cosmochim. Acta 52, 1197-1202, 1988. 5 J.-M. Luck and K.K. Turekian, Osmium-187/Osmium-186 in manganese nodules and the Cretaceous-Tertiary boundary, Science 222, 613-615, 1983. 6 K.K. Turekian and J.-M. Luck, Estimation of continental 187Os/186Os values by using 187Os/186Os and 143Nd/la4Nd ratios in marine manganese nodules, Proc. Natl. Acad. Sci. USA 81, 8032-8034, 1984. 7 B.K. Esser and K.K. Turekian, Accretion rate of extraterrestrial particles determined from osmium isotope systematics of Pacific pelagic clay and manganese nodules, Geochim. Cosmochim. Acta 52, 1383-1388, 1988. 8 S.E. Calvert, Factors affecting distribution of laminated
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G. RAV1ZZAAND K.K. TUREKIAN
9
10
11
12 13 14 15
diatomaceous muds in the Gulf of California, in: Marine Geology of the Gulf of California, T.H. Van Andel, ed., American Association of Petroleum Geologists, Tulsa, Okla., 1964. D.J. DeMaster, The supply and accumulation of silica in the marine environment, Geochim. Cosmochim. Acta 45, 1715-1732, 1981. J.M. Bremner, Biogenic sediments on the southwest African (Namibian) continental margin, in: Coastal Upwelling and its Sedimentary Record, B (NATO Conf. Ser. IV: 10b), J. Thiede and E. Suess, eds., pp. 73-104, Plenum, 1983. W.C. Burnett, W.M. Landing, W.B. Lyons and W. Orem, Jellyfish Lake, Palau: A model anoxic environment for geochemical studies, Eos 783, 777-779, 1989. G. Ravizza, Rhenium-Osmium isotope geochemistry of marine sediments, Ph.D. Thesis, Yale Univ., 1991. J.-M. Luck, G6ochimie du rh6nium-osmium: m6thode et applications, Ph.D. Thesis, Univ. Paris VII, 1982. A.O. Nier, The isotopic constitution of osmium, Phys. Rev. 52, 885, 1937. J.-M. Luck and C.J. Allegre, 187Re-187Os systematics in meteorites and cosmochemical consequences, Nature 302, 130-132, 1983.
16 M.D. Krom and R.A. Berner, A rapid method for the determination of organic and carbonate carbon in geological samples, J. Sediment. Petrol. 53, 660-663, 1983. 17 M.A. Floyd, V.A. Fassel and A.P. D'Silva. Computer controlled scanning monochromator for the determination of fifty elements in geochemical and environmental samples by inductively coupled plasma emission spectrometry. Anal. Chem. 52, 2168-2173, 1980. 18 B.K. Esser, Osmium isotope geochemistry of terrigenous and marine sediments. Ph.D. Thesis, Yale Univ., 1991. 19 K.H. Wedephol, Environmental influences on the chemical composition of shales and clays, in: Physics and Chemistry of the Earth,Ahrens, Press, Runcorn and Urey, eds., Permagon, Oxford, 1970. 20 C.E. Martin, Os isotopic characteristics of mantle derived rocks, Geochim. Cosmochim. Acta 55, 1421-1434, 1991. 21 G. Ravizza and K.K. Turekian, Application of the 187RelS7Os system to black shale geochronometry. Geochim. Cosmochim Acta 53, 3257-3262, 1989. 22 D.J. DeMaster, The marine budgets of silica and 32Si. Ph.D. Thesis Yale University 1979.
1
Elsevier Science Publishers B.V., Amsterdam [MK]
The osmium isotopic composition of organic-rich marine sediments G. R a v i z z a * a n d K.K. ~Furekian Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA
Received October 24, 1991; revision accepted February 3, 1992
ABSTRACT The osmium (Os) concentration and 187Os/186Os ratio of several recent, marine, organic-rich sediment samples from three widely separated sites have been measured. Os concentrations range from 0.095 to 0.212 ppb and 187Os/186Osratios range from 8.2 to 8.9. The calculated fraction of hydrogenousOs exceeds 78% in all samples. Thus, the 187Os/186Osratio of these samples reflects Os isotopic composition of seawater. The small range in measured 187Os/186Os ratio indicates that the Os isotopic composition at these sites is fairly homogeneous. The large magnitude of the Os burial flux at these sites indicates the Os burial in association with organic-richsediments is an important sink in the marine cycleof Os. These data also suggest that ancient organic-rich sediments may provide a record of past variations in the Os isotopic composition of seawater.
I. Introduction The osmium (Os) isotopic composition of seawater has not yet been measured. In lieu of direct measurement, we present Os isotopic data for several organic-rich, biogenic, marine sediment samples from three widely separated localities. Several features of these sediments make them particularly useful as recorders of the Os isotopic composition of seawater. Organic-rich marine sediments serve as natural preconcentrators of dissolved Os [1,2]. In addition, these sediments are relatively free of terrigenous material, thereby minimizing the influence of terrigenous Os on their isotopic composition. Finally, the sediments analyzed in this study all accumulated very rapidly, making the influence of cosmic Os negligible. Thus, as the Os in these sediments must be principally hydrogenous (e.g., derived from sea-
water), these data provide a measure of the 187Os/186Os ratio of the waters from which these sediments were deposited. The Os isotopic composition of manganese nodules [3-6] and abyssal red clays [7] have also been used to place constraints on the Os isotopic composition of seawater. However, these materials are strongly influenced by sources of Os other than seawater. The presence of Os from multiple sources complicates their application as recorders of the isotopic composition of Os in seawater. The organic-rich sediment data presented here provide a geochemically independent means of evaluating Fe and Mn oxide-bearing materials as recorders of the Os isotopic composition of seawater. These data also provide valuable insights into the marine geochemical cycle of Os and have important implications for Re-Os studies of ancient organic-rich sediments.
2. Samples * Presently at: WoodsHole OceanographicInstitution, Woods Hole, MA 02543, USA. Correspondence to: G. Ravizza, Department of Geology and Geophysics, Yale University, P.O. Box 6666, New Haven, CT 06511, USA.
Sediments from the Gulf of California (eastern North Pacific), Walvis Bay (eastern South Atlantic) and Jelly Fish Lake (western equatorial
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
2
G. RAVIZZA AND K.K. TUREKIAN
Pacific) have b e e n analyzed. T h e s e s e d i m e n t s are all c h a r a c t e r i z e d by high o r g a n i c m a t t e r c o n c e n tration, a n d rapid s e d i m e n t a c c u m u l a t i o n rate. W i t h i n the G u l f of California, samples from two b a s i n s - - C a r m e n Basin a n d San P e d r o M a r t i r B a s i n - - w e r e analyzed. I n t e n s e local upwelling in both the G u l f of California [8,9] a n d Walvis Bay [9,10] creates highly p r o d u c t i v e surface waters, which are, in t u r n , r e s p o n s i b l e for the rapid d e p o s i t i o n of organic-rich d i a t o m a c e o u s muds. Clastic material, s u p p l i e d by a d j a c e n t l a n d masses, does n o t m a s k the large b i o g e n i c c o m p o n e n t at these two sites. Jelly Fish Lake is a m e r o m i c t i c m a r i n e lake o n the uplifted c a r b o n a t e island of Eil Malk in the R e p u b l i c of Palau. Salinity-stratified d u e to int e n s e local rainfall, the lake c o n t a i n s n u t r i e n t - r i c h , sulfidic b o t t o m waters from a d e p t h of 15 m to the lake b o t t o m , 30 m d e e p [11]. T h e p r i n c i p a l
supply of n o n b i o g e n i c detritus to Jelly Fish Lake is aeolian.
3. Analytical methods T h e m e t h o d s used in this study are described m o r e completely elsewhere [12]. A b r i e f s u m m a r y of the p r o c e d u r e s is given below. Os was p r e c o n c e n t r a t e d from the s e d i m e n t using a nickel sulfide fire assay. T h e nickel sulfide b e a d f o r m e d d u r i n g the fire assay was dissolved in 12 N HC1. T h e Os r e m a i n e d in a n insoluble sulfide r e s i d u e which was s e p a r a t e d from the solution by filtration. T h e chemical proc e d u r e following the filtration was d e v e l o p e d a n d is described by Luck [13]. T h e insoluble sulfide r e s i d u e was s u b s e q u e n t l y dissolved in 4 N H 2 S O 4 with Cr(VI), a n d Os was distilled from this solution as OSO4. Os was f u r t h e r purified, using a n ion exchange resin, prior to d e p o s i t i o n o n t o a Si
TABLE 1 Re-Os and bulk composition data from sediment analyses. All errors are twice the standard error of the mean based on counting statistics Sample
lS7Os/lS6Os
[Os]
CaCO 3 a
SiO2
(ppb)
(%)
(biogenic) (%)
Organic matter a (%)
Al b (%)
25 c
12.5
5.05
San Pedro Martir Basin Average 20-35 cm
8.92+0.15 8.65 + 0.09 8.79 + 0.19
0.151 +0.004 0.147 + 0.003 0.149 + 0.004
5
Carmen Basin 10-20 cm Carmen Basin 30-40 cm Carmen Basin 60-70 cm Carmen Basin 80-95 cm
8.77 + 0.11
0.212 + 0.004
1.9
9.4 c
11.7
5.96
8.59 + 0.09
0.152 + 0.003
2.3
14.0 d
12.0
6.38
8.75 + 0.09
0.166 + 0.003
2.7
14.4 d
11.7
6.41
8.53 + 0.12
0.170 + 0.004
3.1
14.4 d
12.3
6.33
Walvis Bay 10-36 cm Average
8.15 + 0.17 8.38 5:0.09 8.27 + 0.17
0.095 + 0.002 unspiked
2.5
40 c
24.9
2.69
Jelly Fish Lake 0-30 cm Average
8.71 +0.15 8.55 + 0.10 8.63 + 0.15
0.170+0.004 0.173 :t:0.003 •0.172 + 0.004
20 e
20 e
60 e
0.85
a Total and inorganic carbon concentrations determined by Leco carbon analyzer. Organic carbon calculated by difference [16]. The molecular formula of organic matter was approximated as CH20. b AI concentrations determined by ICPES [17] at the Woods Hole Oceanographic Institution ICP facility. ¢ Data from DeMaster [9]. d Data from DeMaster [22]. e Data from Burnett et al. [11].
Os ISOTOPIC COMPOSITION OF O R G A N I C - R I C H MARINE SEDIMENTS TABLE 2 Hydrogenous Os burial in organic-rich sediments Locality
Mass Flux (g/cm2/yr)
Os (ppb)
Detritus a (%)
Hydrogenous Os b (% of Total Os)
Hydrogenous Os Flux c (pg/cm2/yr)
San Pedro Martir Basin C a r m e n Basin Walvis Bay Jelly Fish Lake
0.1 d 0.05 e 0.11 d 0.07 f
0.15 0.175 0.095 0.17
57 72 33 0
81 79 83 100
12.2 6.9 8.7 11.9
a Detritus % = 1 0 0 - CaCO3% - Biogenic SIO2% - Organic Matter%. Data from Table 1. b Hydrogenous O s % = 1 0 0 - ( D e t r i t u s % ) × 0 . 0 5 / [ O s ] . The constant 0.05 ppb corresponds to the average Os concentration of crustal material [18]. c Hydrogenous Os Flux = (Mass F l u x ) x [Os] x (%Hydrogenous Os/100). d Data from D e M a s t e r [9]. e Data from D e M a s t e r [22]. f Data from Burnett et al. [11].
chip for isotopic analysis. Blank Os was present at detectable levels in the fusion reagents but was always less than 3% of the total analyte. All isotopic measurements were made using the M I T - H a r v a r d - B r o w n regional ion microprobe facility at MIT. Measured Os isotopic ratios were corrected for hydride interferences and mass discrimination on a scan by scan basis. Mass discrimination was assumed to be a linear function of the mass difference, and corrections were made by normalizing all measured 188Os//192Os ratios to the 188Os//192Osratio of 0.32439 [14]. Samples were spiked prior to the fire assay with Os solutions enriched in 190Os and measured 19°Os//192Os ratios were used to calculate Os concentrations. 187Os//186Os ratios were calculated from measured 187Os//192Osratios using the 186Os//192Os ratio of 0.03904 [15]. The 189Os// 192Os ratios measured were always within the error of the Nier ratio of 0.39680. Osmium hydride formation was monitored at masses 193 and 191. Hydride corrections based on data from either of these masses are indistinguishable. The consistent replication of the measured 189Os// 192Os ratio and the internal consistency of the hydride correction are indicative of an interference-free Os spectrum. Total carbon and inorganic carbon were measured in splits of the sample powders using a Leco Carbon Analyzer. Organic carbon concentrations were calculated by difference [16]. Replicate determinations of total and inorganic carbon concentrations were always within 3% of the re-
ported carbon concentrations. Al concentrations were determined by inductively coupled plasma emission spectroscopy [17], following fusion with a lithium metaborate flux and sample dissolution. The uncertainty associated with the AI measurements is estimated to be + 2 % , based on repeated analyses of standard solutions. 4. Results
The data from this study are summarized in Table 1. Os concentrations of the sediment samples range from 0.095 to 0.212 ppb. All of these concentrations exceed the estimated crustal abundance of Os of 0.05 ppb [18]. The spread of measured Os isotopic ratios is from 8.15 to 8.92; a small range of variation considering replicate analyses can differ by as much as 0.2. This range is also restricted in comparison to the range of Os isotopic compositions measured in manganese nodule leachates (5-11 [3,6]). Biogenic material," such as CaCO3, organic carbon and biogenic silica, comprises a large fraction of the total sediment. AI concentrations are all lower than the average Al concentration of shales [19]. These low AI concentrations are indicative of the limited influence of detrital material in these sampies. 5. Discussion
The supply of cosmic Os to these sediments is negligible due to their very rapid accumulation
4
rates. Using Esser and Turekian's estimate of the flux of cosmic Os to marine sediments (5.7 ng/cm2/Ma [7]), the cosmic Os flux is more than 1000 times smaller than the total Os burial flux at these sites. In the absence of any significant cosmic Os component, all the Os in these sediments must be attributed to either a hydrogenous or terrigenous origin. The portion of non-hydrogenous Os in these samples can be estimated using the fraction of detrital material in each sample and assuming that the Os concentration of the detrital material in each sample is 0.05 ppb, the same as the average concentration of Os in crustal material [18]. Although the Os concentrations of terrigenous material at these sites may deviate from the average crustal value, the consistency of the data presented below argue against the influence of detrital material anomalously enriched in Os. Based on our estimates, terrigenous Os accounts for between 0%, at Jelly Fish Lake, and 21%, in the Gulf of California, of the total Os in these samples (Table 2). The remaining Os in these sediments is hydrogenous in origin. Thepresence of small amounts of non-hydrogenous Os in these sediments may cause the 187Os/186Os ratio of the sediment to deviate from that of the overlying water. However, it is improbable that hydrogenous and terrigenous Os of widely differing isotopic compositions should mix fortuitously in the appropriate proportions at three different sites to yield such similar bulk sediment Os isotopic compositions. Instead, we interpret these data as evidence that seawater at these three localities is fairly uniform in its 187Os//186Os ratio. Although the data do not preclude the possibility of subtle variations in the 187Os//186Os ratio of dissolved Os at these sites, the variations observed in 187Os//186Os ratios may arise from small contributions of nonhydrogenous Os (187Os//186Os = 10 for sialic rocks [18] and 187Os//186Os = 1 for young mafic rocks [20]). Manganese nodules were used as recorders of the Os isotopic composition of seawater in early studies [3-6]. Nodule samples were subjected to an oxalic acid leach, intended to dissolve selectively poorly crystalline authigenic Fe and Mn oxides and oxy-hydroxides, which incorporated hydrogenous Os during their formation. Significant variations in the 187Os//186Os ratios of man-
G. R A V I Z Z A A N D K.K. T U R E K I A N
ganese nodule leachates, ranging from 5.2 to 10.9, were observed. If the leaching procedure solubilizes only Os which has been recently scavenged from seawater, the nodule data provide a direct record of regional variations in the Os isotopic composition of seawater [3,5,6]. Alternatively, Palmer et al. [4] argued that residence time of Os in seawater should be long, relative to the mixing time of the oceans based on the expected speciation of Os in seawater. Under this assumption, variations in 187Os//186Os ratios of the nodules are attributed to the mixing of hydrogenous Os, of a relatively homogeneous isotopic composition, with oxalic acid leachable, meteoritic, or possibly ultramafic, Os with an 187Os//186Os ratio of 1. A third explanation of Os isotopic variations in nodule leaches involves labilization of hydrogenous Os precipitated from ancient oceans. If temporal variations in the 187Os//186Os ratio of seawater have occurred, nodule leaches might be able to access an admixture of hydrogenous Os of variable isotopic composition, thereby giving rise to the range of Os isotopic compositions observed. In the absence of additional data, each of these interpretations of the manganese nodule data is viable. In spite of the differences between manganese nodules and anoxic sediments as recorders of the Os isotopic composition of seawater, the results obtained from the two independent sources are largely consistent with one another. Palmer et al. [4] reported a model-based estimate of the 187Os//186Os ratio of seawater of between 8.5 and 11, based on manganese nodule data. The organic-rich sediment data presented here indicate that the 187Os//186Os ratio of dissolved Os at the study sites falls in the range 8.6 + 0.3. In addition, an oxalic acid leach of a recent pelagic clay [7] recovered from the abyssal North Pacific yielded an 187Os/186Os ratio of 8.40 + 0.12. Thus, the interpreted manganese nodule data, the pelagic clay leach and biogenic sediment data are all consistent with a fairly homogeneous lS7Os/ 186Os ratio of seawater, approximately 8.6. Isolated basins such as the Black Sea may have 187Os//186Os ratios which are influenced by local continental supply and, therefore, deviate from this open ocean value [2]. The burial flux of hydrogenous Os can be calculated from the sediment accumulation rate
Os ISOTOPIC COMPOSITION OF O R G A N I C - R I C H MARINE SEDIMENTS
and t h e estimate of the fraction of hydrogenous Os (Table 2). Hydrogenous Os burial fluxes at these three sites range from 6 to 12 pg/cm2/yr, roughly three orders of magnitude larger than those measured in pelagic clays from the North Pacific [7]. Thus, environments of organic-rich sediment deposition, in spite of their limited areal extent, represent an important sink for Os in the marine environment. Over the course of geologic time the residence time of Os in seawater should be sensitive to the areal extent of organic-rich sediment deposition and the efficiency of Os transfer to these sediments. The in situ decay of 187Re to lSTOs within ancient organic-rich rocks provides a chronometer which allows determination of the time of their deposition [12,21]. In order to utilize this chronometer successfully the 187Os/186Osratio of all samples must be homogeneous at the time of deposition. The data presented here provide empirical evidence that modern organic-rich sediments are indeed isotopically homogeneous at the time of their deposition. In addition to the chronometric information that Re-Os studies of ancient black shales can provide, the initial 187Os/186Os ratio of these rocks can also be determined. Just as modern organic-rich sediments have been employed in this study to infer the Os isotopic composition of the modern ocean, the initial 187Os/186Os ratio of ancient organic-rich marine sediments may provide a record of past variations in the Os isotopic composition of seawater. Thus Re-Os studies of marine black shales from the geologic record may provide paleoceanographic, as well chronometric information.
6. Summary and conclusions Measured 187Os/186Os ratios of recent, marine, organic-rich sediments range from 8.2 to 8.9. As the Os in these sediments must be derived principally from the waters from which these sediments were deposited, these data provide a measure of the 1870s/186Osratio of seawater. The small range in the Os isotopic composition of organic-rich sediment samples indicates that the 187Os/186Os ratio of seawater at these sites is relatively homogeneous. These conclusions are largely consistent with previous interpretations of Os isotopic data from pelagic clays and man-
5
ganese nodules. The large magnitude of the Os burial flux at the study sites suggests that burial of Os in association with organic-rich sediments is a major sink for dissolved Os in the oceans. If direct measurement of the Os isotopic composition of seawater confirms these inferences, the initial Os isotopic composition of ancient organic-rich sediments will provide a record of the Os isotopic composition of the ancient oceans.
Acknowledgements William C. Burnett and David J. DeMaster provided sample material for this study. B.K. Esser, C.E. Martin and S. Krishnaswami contributed to methods development and provided valuable discussions. We are also grateful to K. Burrhus for maintaining the ion microprobe and to S. Hart and N. Shimizu for advising GR on use of the ion probe. Comments from Martin Palmer on an earlier version of this work (as a thesis chapter) improved this manuscript significantly. A. Zindler and an anonymous reviewer provided helpful comments. This was supported by NSF grant OCE-8414735 to KKT.
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diatomaceous muds in the Gulf of California, in: Marine Geology of the Gulf of California, T.H. Van Andel, ed., American Association of Petroleum Geologists, Tulsa, Okla., 1964. D.J. DeMaster, The supply and accumulation of silica in the marine environment, Geochim. Cosmochim. Acta 45, 1715-1732, 1981. J.M. Bremner, Biogenic sediments on the southwest African (Namibian) continental margin, in: Coastal Upwelling and its Sedimentary Record, B (NATO Conf. Ser. IV: 10b), J. Thiede and E. Suess, eds., pp. 73-104, Plenum, 1983. W.C. Burnett, W.M. Landing, W.B. Lyons and W. Orem, Jellyfish Lake, Palau: A model anoxic environment for geochemical studies, Eos 783, 777-779, 1989. G. Ravizza, Rhenium-Osmium isotope geochemistry of marine sediments, Ph.D. Thesis, Yale Univ., 1991. J.-M. Luck, G6ochimie du rh6nium-osmium: m6thode et applications, Ph.D. Thesis, Univ. Paris VII, 1982. A.O. Nier, The isotopic constitution of osmium, Phys. Rev. 52, 885, 1937. J.-M. Luck and C.J. Allegre, 187Re-187Os systematics in meteorites and cosmochemical consequences, Nature 302, 130-132, 1983.
16 M.D. Krom and R.A. Berner, A rapid method for the determination of organic and carbonate carbon in geological samples, J. Sediment. Petrol. 53, 660-663, 1983. 17 M.A. Floyd, V.A. Fassel and A.P. D'Silva. Computer controlled scanning monochromator for the determination of fifty elements in geochemical and environmental samples by inductively coupled plasma emission spectrometry. Anal. Chem. 52, 2168-2173, 1980. 18 B.K. Esser, Osmium isotope geochemistry of terrigenous and marine sediments. Ph.D. Thesis, Yale Univ., 1991. 19 K.H. Wedephol, Environmental influences on the chemical composition of shales and clays, in: Physics and Chemistry of the Earth,Ahrens, Press, Runcorn and Urey, eds., Permagon, Oxford, 1970. 20 C.E. Martin, Os isotopic characteristics of mantle derived rocks, Geochim. Cosmochim. Acta 55, 1421-1434, 1991. 21 G. Ravizza and K.K. Turekian, Application of the 187RelS7Os system to black shale geochronometry. Geochim. Cosmochim Acta 53, 3257-3262, 1989. 22 D.J. DeMaster, The marine budgets of silica and 32Si. Ph.D. Thesis Yale University 1979.