186Os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates

Earth and Planetary Science Letters, 118 (1993) 335-348

335

Elsevier Science Publishers B.V., Amsterdam [MK]

Variations of the 1870S/1860S ratio of seawater over the past 28 million years as inferred from metalliferous carbonates G. Ravizza Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received February 8, 1993; revision accepted June 6, 1993

ABSTRACT The Os concentration and isotopic composition of metalliferous carbonates deposited on the East Pacific Rise over the past 28 Ma are reported with complimentary Sr isotope data. Variations in the Os isotopic composition of these samples are interpreted as a record of past changes in the Os isotopic composition of seawater. These results are consistent with isotopic analyses of leachable Os in pelagic clays which have also been interpreted as a record of the 187Os/tS6Os ratio of seawater through time [1]. The metalliferous carbonate record clearly shows that seawater Os and Sr isotope systems are partially decoupled from one another over the past 28 Ma. Accelerated weathering of ancient organic-rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the 187Os//186Os ratio of seawater over the past 15 Ma. This rapid increase suggests that the seawater Os record can potentially be used as a stratigraphic tool in some Neogene marine deposits.

1. Introduction

Recently, Pegram et al. [1] reported the results of isotopic analyses of leachable Os from a North Pacific pelagic clay sequence, LL44-GPC-3, spanning nearly 60 Ma. Measured 187Os/186Os ratios varied from 8.2 to 2.8, with older sediments typically having lower ratios. These variations were interpreted as a record of temporal changes in the 187Os//186Osratio of seawater. This interpretation was based on two inferences: (1) that the Os isotopic composition of seawater is spatially homogeneous and (2) that Os liberated by leaching is indeed hydrogenous (i.e., derived from seawater). Although the Os isotopic composition of present-day seawater has not yet been measured, analyses of recent organic-rich sediments, rich in hydrogenous Os, from widely distributed localities suggest that the Os isotopic composition of seawater is nearly homogeneous with a 187Os/ ~86Os ratio of approximately 8.6 [2]. Pegram et al. [1] argued their leaching procedure selectively liberated a 'seawater' component from GPC-3

sediments because leachching of recent pelagic clays yields 187Os/186Os ratios similar to recent organic-rich sediments. Metalliferous sediments deposited near midocean ridges contain predominantly hydrogenous Os which is incorporated into the sediment in association with settling hydrothermal precipitates [3,4]. Therefore the Os isotopic composition of ancient metalliferous sediments may provide an alternative record of past variations in the Os isotopic composition of seawater. The present investigation of a sequence of metalliferous carbonates deposited near the East Pacific Rise over the past 28 Ma was undertaken to test empirically the interpretation of the G P C - 3 0 s data. The 28 Ma time span represented by the sediments investigated here overlaps the latter part of the GPC-3 record and allows direct comparison of the two datasets. Metalliferous sediments offer several advantages over pelagic clays as recorders of the Os isotopic composition of seawater. First, metalliferous sediments contain predominately hydroge-

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

336

nous Os and can therefore be subject to bulk analysis. In contrast, hydrogenous Os constitutes a relatively small fraction of the Os in pelagic clays and selective leaching methods must be employed in any effort to isolate the hydrogenous Os component. Second, the presence of calcareous micro- and nannofossils in metalliferous carbonates provides better independent age control than can be obtained in pelagic clays. Finally, the abundance of carbonate material also allows isotopic analysis of both Os and Sr in the same sample powders. Direct comparison of these two records of paleo-ocean chemistry cannot be made in pelagic clays because these sediments are devoid of calcium carbonate. Temporal changes in the Sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration (see [5] and [6] for recent summaries). Temporal changes in the Os isotopic composition of seawater should be influenced by similar processes. In both the Rb-Sr and Re-Os systems, the continental crust is characterized by large time-integrated p a r e n t - d a u g h t e r ratios, compared to the deep earth, imparting relatively radiogenic Sr and Os signatures to continental material. Thus, in a gross sense the seawater records of these two isotopic systems are expected to be similar. Pegram et al. [1] noted a crude correlation between these two parameters. However, because the geochemical affinities of Re and Os, both third series transition metals, differ radically from Rb and Sr, some degree of decoupling is also anticipated. The results reported here provide unambiguous evidence that the seawater Sr and Os isotope records do not covary in a simple linear fashion and are therefore at least partially decoupled from one another. 2. Samples Twelve bulk sediment samples from several cores drilled during DSDP Leg 92 were analyzed. Compositionally these sediments are essentially a two-component mixture of biogenic calcium carbonate and hydrothermal Fe and Mn oxides [7,8]. As discussed below, this attribute is desirable because the influence of nonhydrogenous Os is

G. R A V I Z Z A

reduced to a minimum. The stratigraphy of the Leg 92 sediments is summarized in Fig. 1. The ages of the sediment samples analyzed in this study range from 1.5 to 28 Ma. These ages are derived from nannofossil biostratigraphy [9], based on the zonations of Bukry [10] and the time scale of Berggren et al. [11]. Where possible, sample depths were chosen close to nannofossil zone boundaries so that uncertainties associated with interpolating between biostratigraphic horizons could be minimized. When interpolation between nannofossil zones was necessary, the linear sedimentation rates reported by Rea and Leinen [12] were used to calculate ages. An effort was made to analyze material from the basal sections of the Leg 92 holes which accumulated relatively close to the ridge crest. These sediments tend to accumulate more rapidly than sediments deposited further from the ridge crest and contain larger amounts of biogenic and hydrothermal material and lesser amounts of detrital material. 3. Methods The Os isotopic analyses of the Leg 92 bulk sediment samples were performed by negative thermal ionization mass spectrometry on the NIMA-B instrument at W H O I [13]. A nickel sulfide fire assay method was employed to preconcentrate Os [14] and both distillation of OsO 4 [15] and ion exchange of OsBr6 2 [16] were required to purify the Os prior to mass spectrometric analysis. Os concentrations were determined by isotope dilution using a spike solution enriched in 19°Os added to the sample prior to preconcentration of Os by fire assay. Os was measured as a molecular species, OsO 3. Measured ion beam intensities were normalized to mass 236 and atom ratios were calculated from oxide ratios using an iterative procedure which simultaneously corrected for instrumental mass fractionation, normalizing atom ratios to 1920s/1880s = 3.0828 [17]. The reported 1870S/1860S ratios were calculated from measured 235/236 ratios. Based on counting statistics, the precision associated with 187Os/ 1860S ratios ranged from 0.08 to 0.45% (2o-), although replicate analyses of samples yield 1870s/1860s ratios which agree at the +0.5% level [3]. Concentrations of Os in similar sedi-

337

V A R I A T I O N S OF T H E 187Os/186Os R A T I O OF S E A W A T E R O V E R T H E PAST 28 m.y.

Depth Below H o l e Seafloor (m) 597 0

Age of Nannofossil ZoneBoundary (Ma) 0

Depth Below Seafloor (m) 0

1.5 !
15.1 . ~ . ~ 15.5

19.0

CN13-15

0

1.0 CN11-12 1.9 2.8 3.7

Interpolated Sample Ages (Ma)

CN2

Hole Age of Nannofossil 599 Zone Boundary (Ma)

CN9a-IO

Interpolated Sample Ages (Ma)

18.9

CNlc 19.2 19.7 - ~ w ,

22.2

22.4

8.1

34.8

CNla+b

27.8 29.1 " ~

40.8 24.8

<8.6

25.0

Depth Below Seafloor (m) 0

CP19b

Hole Age of Nannofossil 600c Zone Boundary (Ma) 0

CN13-15 4.4 4.8 6.2 ~

50.1

54.7

1.0 4.7 6.4 9.0 12.4 16.0 17.0 17.5 - ~

1.9

CN12 I 3.5 CN11 3.7

Interpolated Sample Ages (Ma) 3.7

28.0

=

Depth Below Seafloor (m) 0

~ 8.1

CN8b

CNlOa+b

<28.6

Hole Age of Nannofossil 598 Zone Boundary (Ma) ~CN13-1~ CN10-12

0

19.0

<4.6

1.9 5.4

CN9b CN9a CN8a+b CNTa+b CN6

6.7 8.1 9.2 12.0 12.5

Interpolated Sample Ages (Ma)

Depth Below Seafloor (m) 0

Hole Age of Nannofossil 601 Zone Boundary (Ma) 0

CN13-15

12.6

Interpolated Sample Ages (Ma) -~ 1.5

6.2 CN5a+b 8.0 30.1

29.7

14.2

14.3

8.9 10.0-~'

1.9 CN12 C~11

3.5

3.7

11.0 " ~ P

3.6 3.8

CN3-4 38.2

15.3

CNlOc+d

J<4.6 52.4 20.4 <17.2 Fig. 1. Diagrams illustrating the stratigraphy of Leg 92 D S D P drillholes. Nannofossil zones given in the columns are from Knuttle [9]. Ages of nannofossil zone boundaries are from Berggern et al. [11]. Samples analyzed in this study are marked with dots. Sample ages given to the right of the columns are based on linear interpolation between nannofossil zone boundaries. For samples from basal nannofossil zones it is a s s u m e d that the base of the sediment pile coincides with the underlying nannofossil zone boundary. In these cases the reported sample ages should be considered as m a x i m u m age estimates. Sample ages are also given in Table 1.

338

G. RAVIZZA

TABLE 1 Os and Sr isotopic data for DSDP Leg 92 metalliferous carbonates Sample 597 6-3 90-95

Age (Ma) 28 a

87Sr/86Sr

187Os/186Os

0.70815

6.156 ± 0.009

[Os] (ppt) 42.3

597 4-4 78-83

25

0.70827

6.198 ± 0.011

64.5

597 3-4 96-100

22.4

0.70834

6.248 ± 0.011

49.7

597 3-1 85-90

18.9

0.70842

6.122 ± 0.013

37.6

598 5-4 14-21

15.3 a

0.70892

6.188 ± 0.006

163.9

598 4-4 14-19

14.3

0.70891

6.400 ± 0.011

83.7

598 3-2 15-20

12.6

0.70890

6.592 ± 0.008

85.5

599 4-6 "19-24

8.1

0.70901

6.933 ± 0.015

58.2

601 2-2 21-24

3.8

0.70915

7.381 ± 0.034

71.9

600 1-5 16-19

3.7

0.70912

7.285 ± 0.006

76.3

601 2-1 15-20

3.6

--

7.503 ± 0.010

73.1

601 1-5 19-24

1.5

0.70911

7.675 ± 0.008

54.3

a These ages are based on extrapolation from overlying nannofossil zone boundaries and should be regarded as m a x i m u m age estimates. See Fig. 1 for more detailed stratigraphic information.

7

0

-

A

6-

A 4

"

3

"

A A

A []

A

2

.... 0

A

Leg 92 Metalliferous Carbonates LL44-GPC3 N. Pac. Pelagic Clay 0aegram et al. 1992)

I .... 10

I ....

I ....

20

30

Age

A

I .... 40

I 50

.

.

.

.

60

(Ma)

Fig. 2. Temporal variations in the 187Os/lS6Os ratio of seawater inferred from bulk sediment analyses of metalliferous carbonates from D S D P Leg 92 and analyses of leachable Os in pelagic clay from L L 4 4 - G P C - 3 [I]. The agreement of these two records over the past 28 Ma indicates that the major features of both records reflect changes in the Os isotopic composition of seawater rather than the influence of local diagenesis. T h e single-sided error bars associated with the Leg 92 data show the range of uncertainty introduced by the possible influence of cosmic Os supplied to the sediment by interplanetary dust particles. See text for further discussion.

339

V A R I A T I O N S O F T H E 187Os/186Os R A T I O O F S E A W A T E R O V E R T H E PAST 28 m.y.

ported sediment concentrations is approximately +5%.

ment samples determined by fire assay/isotope dilution are typically reproducible at the + 5 % level [3]. The Sr isotopic composition of the acid-soluble fraction of bulk sediment samples was determined in order to assess the role of diagenesis. Bulk sediment samples were leached in cold 2.5N HCI for 20 min. The soluble Sr was purified by ion exchange column chromatography [18]. Sr isotopic analyses were performed on a VG354 multicollector mass spectrometer at WHOI. The average and standard deviation of NBS 987 runs for the 6 months prior to these analyses are 0.710288 + 0.000026 (l~r). Major element concentrations of Leg 92 sediments were determined on powder splits by I C PAES at WHOI. Samples of approximately 30 mg were subjected to a two-step acid dissolution employing 8N nitric acid and 6.2N hydrochloric acid, followed by treatment with a mixture of nitric and hydrofluoric acids. The resulting solutions were diluted to 30 ml total volume and analyzed with matrix-matched standards to determine element concentrations. Quality control runs indicate that the precision associated with re-

4. Results and discussion

Table 1 gives the Sr and Os isotopic data for Leg 92 samples and the Os concentrations. The trend of temporal variations in the 187Os/186Os ratios of Leg 92 samples is shown in Fig. 2. Ancillary chemical data for Leg 92 samples are given in Table 2. There are two fundamental issues to consider in this discussion. First, it is necessary to evaluate how well the 187Os/186Os ratio variations observed in l_~g 92 sediments record past variations in the 187Os/186Osratio of seawater. Second, the Os isotopic data must be placed in a paleoceanographic context and related to other records of ancient ocean chemistry, bearing in mind the potential limitations of the dataset. In order to evaluate the quality of the Os record, sources of nonhydrogenous Os are assessed and the role of diagenesis is discussed. After making a case that the Leg 92 data provide a record of variations in seawater Os isotopic composition through time, the broader implications of this record are considered.

TABLE 2 Ancillary chemical data for DSDP Leg 92 metalliferous carbonates

Sample

CaCO3%a

Fe %

Mn %

AI %

Os (ppt)

Os/Fe

Os/Al

xl0 9

xl0 8

597 6-3 90-95

78

5.1

1.8

0.74

42.3

0.8

0.6

597 4-4 78-83

82 b

5.0

1.6

0.26

64.5

1.3

2.5

597 3-4 96-100

95

2.1

0.46

0.21

49.7

2.4

2.4

597 3-1 85-90

94b

1.8

0.29

0.19

37.6

2.1

2.0

598 5-4 14-21

76

8.9

3.0

0.19

163.9

1.8

8.6

598 4-4 14-19

88

5.0

1.8

0.12

83.7

1.7

6.9

598 3-2 15-20

85

5.2

1.0

0.19

85.5

1.6

4.5

599 4-6 19-24

82

4.9

1.6

0.08

58.2

1.2

7.3

601 2-2 21-24

80

5.3

1.6

0.11

71.9

1.4

6.5

600 1-5 16-19

84

4.8

1.5

0.14

76.3

1.6

5.3

601 2-1 15-20

79

4.8

1.4

0.13

73.1

1.5

5.7

0.30

601 1-5 19-24

90

0.12

54.3

3.6

4.5

Average Continental Crust Mid-ocean Ridge Basalt

--

--

1.5

--

7.6 c

50 d

--

0.07

--

--

--

8.1 e

1f

--

0.001

Unless otherwise noted reported CaCO3% values were calculated from measured Ca concentrations assuming that CaCO 3 is the only Ca-bearing phase, b CaCO 3 values from nearby samples analyzed by Marchig and Erzinger [8]. e Data from [54]. d From [22]. e From [55]. f From [41].

340

Source of Os in Leg 92 sediments: On the basis of investigations of Os in recent metalliferous sediments [3,4] it is likely that the vast majority of Os in Leg 92 sediments is associated with hydrothermal Fe and Mn oxides. O s / F e ratios of Leg 92 samples (Table 2) are similar to O s / F e ratios of metalliferous sediments from the East Pacific Rise (1 to 10 × 10 -9 [3]) and the MidAtlantic Ridge (1-3 × 10 -9 [4]). Os concentrations of Leg 92 samples are positively correlated with Fe and Mn concentrations. However, it is likely that this correlation is the result carbonate dilution. Given that the thermodynamically stable species of Os in seawater should be an oxyanion [19], removal of Os from seawater in association with hydrothermal Fe and Mn oxides is consistent with our current knowledge of the marine chemistry of Os and other oxyanions [20,21]. In addition to hydrogenous Os carried by hydrothermal oxides, Os carried by cosmic dust particles and terrestrial detrital material may also influence the Os isotopic composition of Leg 92 sediments. As shown in the paragraphs below, neither of these potential sources of Os exerts a strong control on the 187Os/186Os ratio of Leg 92 sediments. Although Leg 92 Os concentrations, ranging from 38 to 164 ppt (Table 1), do not greatly exceed the average crustal abundance of Os ( = 50 ppt [22]), enrichments of Os of more than 100 x above detrital background are apparent when the dilution effects of biogenic calcium carbonate are eliminated. By comparing O s / A l mass ratios of Leg 92 samples (10 -7 to 10-8; see Table 2) to average crustal material (7 X 10-10) and mid-ocean ridge basalts ( < 10-10), it is clear that the influence of terrestrial detrital material is negligible. It is also important to note that Re concentrations of analogous modern sediments are much lower than average crustal material [23] and insitu production of 187Osin Leg 92 samples can be considered negligible. The Os isotopic composition of marine sediments can also be influenced by trace amounts of particulate extraterrestrial material [24] because Os concentrations of extraterrestrial material exceed the Os concentrations of terrestrial materials by several orders of magnitude. More rapid sediment accumulation rates elevate total Os burial fluxes which in turn diminish the influence of the background cosmic Os flux on the Os

G. RAVIZZA isotopic composition of the bulk sediment. Using the sediment mass accumulation rates reported by Rea and Leinen [12] and a cosmic Os flux estimate derived from investigation of nearby Bauer Basin sediments [3], the estimated fraction of cosmic Os in the bulk sediment samples ranges from 2 to 9%. The 187Os//186Osratio of extraterrestrial material is approximately 1. Therefore, addition of a small component of cosmic Os to Leg 92 sediments will shift bulk sediment 187Os/ 186Os ratios to values lower than the pure hydrogenous component. The single-sided errors bars on the Leg 92 data shown in Fig. 2 represent the magnitude of cosmic Os corrections assuming a constant background flux of cosmic Os over the past 28 Ma. The magnitude of these corrections is small compared to the range of temporal variations in 187Os//186Osratios. Sadler [25] has shown that average sediment accumulation rates decrease with increasing time span, and has attributed this phenomenon to the discontinuous nature of sediment deposition. Depth intervals corresponding to periods of rapid sediment accumulation represent a disproportionately large fraction of any sedimentary sequence, while hiatuses in sedimentation and periods of very slow accumulation necessarily constitute a much smaller fraction of the total length of a sedimentary sequence. By using sediment accumulation rates averaged over many meters of core as an approximation of the actual accumulation rate of samples which only span a few centimeters, it is likely that true sample accumulation rates of the Leg 92 samples have been systematically underestimated. Thus these calculations probably overestimate the influence of cosmic Os on Leg 92 sediments. Diagenesis in Leg 92 sediments: Potential problems associated with diagenesis are a fundamental concern in any effort to extract paleoceanographic information from the sedimentary record. As hydrogenous Os is expected to be associated with hydrothermal Fe and Mn oxides, evaluating potential reductive remobilization of these elements during diagenesis is of particular importance. The chemical composition of the sediments and pore waters indicate that diagenetic redistribution within the Leg 92 cores should be extremely limited. X R D studies indicate that amorphous iron oxides persist throughout the

VARIATIONS

OF

THE

187Os/IS6Os

RATIO

OF

SEAWATER

OVER

THE

PAST

28

341

m.y.

shows that three samples deviate slightly from the seawater Sr curve and that the slope of the four samples from Hole 597 is less steep than the seawater Sr curve. These deviations may reflect limited diagenetic Sr redistribution. However, small errors in age assignments, or liberation of small quantities of detrital Sr during acid leaching, could produce similar effects. The general concordance of Leg 92 samples with the seawater Sr curve has been established previously [29]. In general the Sr data do not give any indication of pervasive diagenetic Sr redistribution. Comparison of the Leg 92 record to the GPC-3 record also provides a means of assessing potential diagenetic problems. If the Os isotopic composition of seawater is spatially invariant, then, in the absence of significant diagenetic perturbations, independent sedimentary records should yield equivalent age-187Os/I86Os relationships. 1870s//1860s ratios of Leg 92 sediments are plotted as a function of time with Os isotopic data from Pegram et al's. [1] study of a North Pacific pelagic clay sequence in Fig. 2. Over the past 28 Ma, the two datasets are in good agreement. This

cores and have undergone only local recrystallization to goethite [26]. Organic carbon concentrations reported by Meyers et al. [27] are extremely low, ranging from 0.02 to 0.12%, thereby limiting the reductive capacity of the sediment. Most importantly, pore water nitrate concentrations persist at levels above bottom-water nitrate concentrations throughout the length of all of the cores [28]. The persistence of nitrate in sediment pore waters precludes the possibility of extensive reductive redistribution of Mn and Fe during organic matter decomposition. Sr isotopic data from the acid-soluble fraction of Leg 92 samples provide an additional means of assessing the possible role of diagenesis. It is important to note that sediment samples were subjected to a dilute hydrochloric acid leach to ensure total carbonate dissolution and thereby provide an indication of potential diagenetic Sr redistribution. If advection of hydrothermal fluids through the sediment pile persisted long after sediment deposition, the Sr isotopic composition of the acid-soluble fraction might be expected to be shifted off the seawater Sr curve. Figure 3

0.7095

0.7090

BN

c.~ 0.7085

[]

590B ('Richter&Depaolo 1988)

A-~ •

t~

car2 0.7080

0.7075

0.7070

L

,

[]

575 (Richter& DePaolo 1988)

O

607 (HodeUet al. 1990)

O

588 (Hodell et al. 1990)

A

MultipleDSDPCores (Hess et al. 1986)

•

Leg 92 (ThisStudy)

,

,

I I

5

L

,

i

,

] I

10

,

,

A

~,, ~-,%

^.

AA ~ A

,

,

I I

15

i

~

I

,

i I

20

i

L

,

J

] I

25

,

~

,

,

] I

30

i

I

I

I

]

I

35

Age (Ma) Fig. 3. Composite record of the Sr isotopic composition of seawater over the past 30 Ma. Leg 92 data are plotted as circles. All other data are from published studies [31,56,58]. All plotted 8 7 S r / 8 6 Sr ratios are normalized to a value for NBS 987 of 0.710220. Each of these studies uses the time scale of Berggren et al. [11] to make absolute age assignments.

342

G. RAVIZZA

agreement strongly suggests that over this time period the major features of both records are relatively free from the confounding effects of diagenesis. Similarity in the Os isotopic composition of 3.6-3.8 Ma samples from Leg 92 Holes 601 and 600 (Table 2) provides additional evidence that sediments from different cores yield compatible results. On the basis of the agreement of the data from Leg 92 sediments and L L 4 4 GPC-3 and the supporting chemical and isotopic data, the temporal variations in Os isotopic composition shown in Fig. 2 are interpreted as a record of variations in the Os isotopic composition of seawater through time. Implications of the Leg 92 Os Data: As noted by Pegram et al. [1], both the Os and Sr records show younger samples shifting toward more radiogenic isotopic signatures. In this sense the two isotopic systems are grossly correlated. However, the higher resolution Leg 92 data reveal striking differences in the evolution of the Os and Sr isotopic signatures over the past 28 Ma. The very rapid increase in the 87Sr/86Sr ratio of seawater, commencing at approximately 40 Ma and persist-

ing to 15 Ma (Fig. 3), has been attributed to enhanced input of radiogenic Sr (driven by the Himalayan orogeny) to the oceans via the Ganges Brahmaputra river system [5,6,30-32]. Over the period 25-15 Ma, the Sr isotopic composition of seawater continued to become more radiogenic very rapidly, while the Leg 92 Os record shows little variation in the Os isotopic composition of seawater. From 15 Ma to the present, the rate of increase of the 187Os//186Os ratio of seawater became more rapid, with the data suggesting a very rapid increase during the last 4 Ma (Fig. 2). In contrast the slope of the seawater Sr curve over the last 15 Ma is much less steep than for the preceding 10 Ma. Decoupling of the Os and Sr seawater records is clearly illustrated in a plot of 187Os//186OsVS. 87Sr//86Sr (Fig. 4). Another respect in which the seawater Os and Sr records differ is illustrated by comparing the range of seawater variation to continental and mantle end members (Fig. 5). Seawater Os isotopic variations span nearly 60% of the range between the continental and mantle end members, while seawater Sr isotopic variations span

8.0

1.% 7.5

3.8~ "7

©

7.0

14.%

6.5

2% 6.0

' 0.708

0.7082

0.7084

0.7086

0.7088

0.709

0.7092

87Sr/86Sr Fig. 4. Plot of 187Os//186Osratio of Leg 92 samples vs. seawater 87Sr/S6Sr ratio. The numbers adjacent to the plotted symbolsgive the sample age (Ma). The plotted 87Sr/86Sr ratios are derived from the sample age and the Sr seawater curve in Fig. 3. This plot illustrates clearly that over the past 28 Ma these two isotopic systems do not covary in a simple linear manner.

343

V A R I A T I O N S O F T H E 187Os/186Os R A T I O O F S E A W A T E R O V E R T H E PAST 28 m.y.

only slightly more than 12% of this range. In part this difference reflects the fact that the dissolved Sr isotopic composition of global mean river runoff (87Sr/86Sr= 0.712 [33]) is significantly lower than that of average continental crust, as inferred from riverine particulate matter (87Sr/ 86Sr ~ 0.716 [34]). This discrepancy is generally attributed to the buffering influence of marine carbonates on the exogenic Sr cycle [35] and the relatively slow erosion rate of continental shield areas [36]. The inferred 187Os/186Os ratio of present-day seawater ( = 8.6) places a lower bound on the Os isotopic composition of global river runoff. The fact that this value closely approaches the estimated 187Os//186Osratio of average crustal material (10.5; [22]) suggests that no Os reservoir analogous to Sr in marine carbonates acts to buffer the Os isotopic variations in seawater. Esser et al. [37] presented data suggesting that the isotopic composition of dissolved Os carried by some rivers may be more radiogenic than the watershed that they drain. Such a scenario requires preferential dissolution of Os from phases

12

A

LL44-GPC3 (Pegram et al. 1992)

[]

Leg 92 Data (This Work)

carrying large time-integrated R e / O s ratios, and thus high 187Os/186Osratios. As no direct measurements of either riverine Os fluxes or the isotopic composition of dissolved riverine Os have been made to date it is only possible to speculate about the reason(s) for the recent rapid increase in the 187Os/186Osratio of seawater over the past 15 Ma. This increase requires either a decrease in the flux of nonradiogenic Os to the oceans or an increase in the flux of radiogenic Os. Both extraterrestrial material [38,39] and ultramafic rocks [40,41] carry high concentrations of Os, with 187Os//lS6Os ratios close to 1. A diminishing flux of soluble Os from either of these sources beginning at 15 Ma could produce the observed increase in the 187Os/186Os ratio of seawater without perturbing seawater Sr systematics. Without further investigation these possibilities cannot be discounted. On the basis of the correlation between the Os and Sr seawater records, Pegram et al. [1] argued that an increase in the flux of radiogenic Os to the oceans due to enhanced continental weathering was the driving force for the increase in the

10 Present Day Seawater

8

© 6 A

A

©~

Cenozoic Seawater Minimum

©

Global Riverine Sr Input

Mantle "End Member"

0,

0.700

I'l'l",",",",

~',

0.705

' ', ' ', '

' J' I' i' l' l' I' i' i",",",",'

0.710

0.715

0.720

0.725

87Sr / S6S r Fig. 5. A schematic diagram illustrating the range of seawater Os and Sr isotopic variations during Cenozoic time in relation to mantle and continental end members. Note that seawater Os isotopic compositions span a m u c h larger fraction of the range between continental and mantle end m e m b e r s than seawater Sr isotopic compositions.

344

G. RAVIZZA

187Os/186Os ratio of seawater over the Cenozoic. In addition, these workers suggested that weathering of ancient organic-rich sediments on continents may constitute a significant and unusually radiogenic component of soluble riverine Os. This claim was based on the observation that this particular lithology carries high concentrations of both Re and Os and has R e / O s ratios 10-20 × larger than average crustal material [42]. The reduced character of black shales, and the associated occurrence of iron sulfides, renders this rock type vulnerable to rapid weathering under acidic conditions once exposed. Given the paucity of Os isotopic analyses of continental rocks and the absence of riverine Os data it is difficult to assess the volumetric significance of black shale weathering as a source of radiogenic Os to the oceans. However, the fact that Os burial fluxes in association with modern organic-rich sediments are 2-3 orders of magnitude larger than Os burial fluxes associated with more common marine sediments qualitatively

suggests that organic-rich sediment deposition represents a major sink in the marine Os budget [2]. Thus, with regard to marine sinks Os in marine black shales may be analogous to Sr in marine carbonates. The significant contrast between these two reservoirs is the radical difference in parent-daughter ratios. Marine carbonates have extremely low R b / S r ratios and therefore in-situ production of 87Sr is rarely significant. In contrast, roughly 50% of the present-day riverine Re flux is removed from the ocean in association with organic-rich sediments [23]. As a consequence, marine black shales have high Re concentrations and evolve lS7Os rapidly. If burial with organic-rich sediments is the major marine sink for Os it is reasonable to expect that black shale weathering plays an important role in supplying Os to the oceans. In addition, rather than acting to buffer changes in the ~87Os/lS6Os ratio of seawater black shale weathering may actually force large-amplitude changes in the Os isotopic composition of seawater.

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Age (Ma) Fig. 6. Carbon isotopic record of bulk sediment in DSDP Sites 525 and 528 [58] reported as permi! deviations from the PDB standard. Shackleton [44] argued that this shift toward lighter 313C values is a consequence of a net decrease in the size of the buried organic carbon reservoir. The coincidence of the shift in ~13C toward lighter values and the increase in seawater 187Os/lS6Os ratio at 15 Ma suggests that the rise in the 187Os/186Os ratio of seawater may be related to weathering of ancient organic matter.

V A R I A T I O N S O F T H E 187Os//186Os R A T I O OF S E A W A T E R O V E R T H E PAST 28 m.y.

Past variations in the 13C content of inorganic carbon dissolved in the oceans are consistent with the inference of accelerated weathering of buried organic matter during the past 15 Ma. Temporal variations in the carbon isotopic composition of marine carbonates are believed to record fluctuations in the relative sizes of the global reservoirs of organic and inorganic carbon [43] where shifts toward lighter ~13Cin marine carbonates indicate of a decrease in the size of the global organic carbon reservoir. The ~13Cvalues of bulk carbonate from the South Atlantic (DSDP Leg 74) display a marked negative shift of 2%0 which commences at 15 Ma and continues to the present (Fig. 6). This shift has been interpreted as evidence of a decrease in the size of the buried organic carbon reservoir due to a diminished rate of organic matter burial [44]. However, the observed shift may also reflect enhanced organic matter oxidation during continental weathering. The coincident shifts of the ~13Crecord to more negative values and the 187Os/186Os ratio of seawater to higher values are compatible with a scenario of enhanced weathering of marine organic-rich sediments over the past 15 Ma. If the covariation of these two isotopic records is driven by an increased rate of organic matter weathering, the Os isotopic data require that the organic matter be of significant age. Qualitative similarity between the Phanerozoic 87Sr/86Sr record and the ~348 and 613C trends of marine sulfates and carbonates have been noted by several workers [45-47]. However, this similarity may be fortuitous because these different isotope records depend on several processes which are unrelated or poorly coupled [48]. In the light of the difficulties inherent in inferring the forces driving changes in records of paleo-ocean chemistry [6] the apparent similarity of the Os and carbon isotope records is at best considered provocative. More detailed investigation of both the carbon [49] and the Os records, as well as comprehensive field study of the behavior of Os during weathering, are required. It is noteworthy that prior to 28 Ma the L L 4 4 - G P C - 3 record exhibits large-amplitude fluctuations in the 187Os/186Os ratio independent of any shift in the carbon isotope record. Even in the absence of a clear understanding of the factors controlling temporal variations in

345

the Os isotopic composition of seawater, it is possible to evaluate the potential utility of this system as a stratigraphic tool. This application of the seawater Os record would be analogous to applications of the seawater Sr curve to stratigraphic problems [50-53] in which the established relationship between age and isotopic composition is used to determine the age of sediments on the basis of their isotopic composition. The available Os data delimit the broad features of seawater Os isotopic variations over the past 27 Ma and allow a preliminary assessment of the possible temporal resolution attainable by this method. Between 27 and 15 Ma the seawater Os curve is nearly flat and therefore cannot yield useful age constraints. If the increase in the 187Os/186Os ratio of seawater over the past 15 Ma is approximated as a linear increase from 6.1 to 8.6, and the present level of reproducibility of Os isotopic analyses is used to define resolvable differences, some 62 different 187Os//186Osratios can be resolved. This corresponds to a temporal resolution of approximately 250 ka. As the precision and reproducibility of Os isotopic analyses approach the levels associated with other heavy isotope systems, temporal resolution could increase dramatically. The agreement between the Leg 92 data and the pelagic clay data from L I A 4 - G P C - 3 is particularly interesting because it suggests that the seawater Os isotope record may provide a means of correlating nonfossiliferous red clays with better established marine stratigraphies. Before the seawater Os record can be applied effectively to stratigraphic problems a composite record of high temporal resolution must be established. 5. Conclusions

(1) The Os isotopic composition of leachable Os from a North Pacific pelagic clay sequence and bulk sediment samples of metalliferous carbonates deposited on the East Pacific Rise displays similar patterns of temporal variation. This similarity indicates that the composite record of 1870s/1860s ratio variations reflect changes in the Os isotopic composition of seawater. (2) The metalliferous carbonate data reveal that the seawater Sr and Os records are decoupled from one another over the past 28 Ma. From

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28 to 15 Ma 875r/86Sr increased rapidly while the 187Os/186Os ratio of seawater remained nearly constant. At 15 Ma the 187Os/186Os ratio of seawater began to increase rapidly but the rate of increase in the 87Sr/86Sr ratio diminished. (3) Although there are not sufficient data to identify with certainty the cause of the increase in the 187Os/186Os ratio of seawater, enhanced weathering of ancient organic-rich sediments is a plausible mechanism. The preferential burial of Re and Os in association with modern organicrich sediments and the carbon isotopic record preserved in marine carbonates over the past 28 Ma are consistent with this hypothesis. (4) The wide range of seawater Os isotopic compositions and the apparent monotonic increase in 187Os/186Os ratios over the past 15 Ma suggest that the seawater Os isotope record has the potential to be used as a stratigraphic tool.

Acknowledgements I am indebted to Stan Hart, who generously provided access to his clean laboratory and NIMA-B for Os separations and isotopic analysis respectively. Mike Bacon also kindly allowed use of his laboratory facilities for this work. Jureck Blusztajn and Erik Hauri provided assistance with Os analyses on NIMA-B and help in the clean laboratory. Ken Burrhus provided valuable technical help with NIMA-B. Mark Kurz allowed access to his VG354 multicollector mass spectrometer for the Sr analyses. Dave Kammer provided much needed assistance with the VG354. Ed Brook and Delia Oppo provided helpful comments on an early draft of the manuscript. I have also benefited from helpful discussion with W.J. Pegram and B.K. Esser. Three anonymous reviewers provided constructive criticism which improved this paper. Samples for this work were provided by the International Ocean Drilling Program. This work was supported by NSF grants OCE-9115253 and EAR 9204409 and is WHOI contribution 8349

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