The marine187Os/186Os record of the past 80 million years

EPSL ELSEVIER Earth and Planetary Science Letters 130 (1995) 155-167

The marine 1870s/ 1860s record of the past 80 million years

B. Peucker-Ehrenbrink a,b, G. Ravizza b, A.W. Hofmann a

a Max-Planck-lnstitut fir Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany

b Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, USA

Received 6 June 1994; accepted after revision 20 December 1994

Abstract

We report new “‘OS/ ls60s data and Re and OS concentrations in metalliferous sediments from the Pacific to construct a composite OS isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield 18’Os/ ‘s60s values of between 3 and 6.5 and 18’Re/ la60s values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the OS isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor( Following a rapid recovery to “‘OS/ 1860s of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in 18’Os/ lB60s to about 6 at 15 Ma. Variations in the isotopic composition of leachable OS from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous OS can occur during the hydrogen peroxide leach and demonstrate that OS data from pelagic clay leachates do not always reflect the OS isotopic composition of seawater.

New data for the late Cenozoic further substantiate the rapid increase in the ls70s/ “‘0s of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and OS isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and OS toward more radiogenic isotopic compositions. The positive correlation between *‘Sr/ ‘%r and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in 18’Os/ ls60s from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and OS record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic OS and Sr to the oceans and account for the periods of decoupling of the marine Sr and OS isotope records.

1. Introduction

The evolution of the osmium isotope ratio of seawater over the past 60 m.y. provides a means of deciphering the relative contributions of the various sources of osmium to seawater, namely

continental weathering, alteration of oceanic crust and, possibly, alteration of cosmic dust [l-3]. The prospect of combining the Sr isotopic evolution of seawater with the variations in the marine 1870s/ 18(jOs record is especially promising because the geochemically distinct systems record

0012-821X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00003-8

156 B. Peucker-Ehrenbrink et al. /Earth and Planetary Science Letters 130 (1995) 155-167

Table 1

Age, OS and Re concentrations, and “‘OS/ ls60s, ‘870s/1ssOs (20 in-run uncertainties are given for both ratios) and

“‘Re/ ls60s values for DSDP/ODP metalliferous sediment8 from the Pacific Ocean

DSDP/ODP type* age ‘870,/‘860,# 1*7Os/l**Os# Re OS 1*7Re/l860s

identifier

leg-site-core-section, cm (Ma) (pg/g) (pg/g)

5-39-2-6, 130-13’

8-74-12-l. ‘29-130

8-74-12-3, 18-20

9-77B-53-‘,70-72

16-162-17-4, 139-140

20-196-2-5. I ‘2-l ‘4

34-319-12-3, ‘34-136

35323.l6-‘,68-70

91-596-l-1, 1’4-116

91-596-t-2.65-67

91-596-I-4.99-101

91-596-2-2,99-101

9 l-596-2-4, I28- ‘30

9’-596-2-5,42-44

91-596-3-l. ‘31-133

91-596-3-3, 102-104

9’-596-3-48-10

91.596-3.4,52-54

91-596-3-4, 122-124

91-596-3-5.97-99

91-596-3-6.48-50

91-596-3-7, ‘8-20

92-597-6-2, 106-107

129-80’ A-4R-2,45-47

b

b

b

b

1

b

b

1 b

b

1

b

I b

1

b

1

b

1

b

I

b

I b

I b

1

b

II

‘2

b

I r b

‘I

12

b

I

b

I

b

I

1

53

38

42.1

37.9

48.7

80

‘5.8

63.5

0.104

I .s%

6.34

‘7.07

24.35

28.22

51.01

63.15

65.08

66.56

69.12

74.61

78.63

83.7

26.3

57.1

4.0’9 f0.012 0.4837 f 0.0014

4.494f0.016 0.5408 f 0.00’9

4.752 f 0.099 0.57’9f0.0’19

4.848 * 0.0’5 0.5835 f 0.0018

4.835 f 0.01’ 0.5819 f0.00’3

4.642f0.0’6 0.5587 -r 0.00’9

5.379 + 0.018 0.6474 f 0.0022

6.182+10.0’9 0.7440 + 0.0023

6.082 + 0.026 0.7320 f 0.0031

3.154 +0.007 0.3796 f OX008

3.230 f 0.007 0.3887 f 0.@308

6.800+0.0’5 0.8’84?r0.00’8

7.886 f 0.02’ 0.9491 f 0.0025

7.363 + 0.02’ 0.886’ f 0.0025

8.120+0.0’0 0.9772f0.00’2

6.636 * 0.010 0.7986 + 0.0012

7.203 * 0.0’ 1 0.8669 f 0.00’3

7.036f0.0’3 0.8468 f 0.00’6

6.974 + 0.023 0.8393 f 0.0028

I .45 I f 0.003 0.1746 f 0.0004

4.9’5 *0.012 0.59’5 f0.00’4

3.136f0.014 0.3774 + 0.0017

4.272 f 0.0’7 0.5’4’ +0.0020

2.238 f 0.004 0.2693 f 0.0005

3.402 f 0.009 0.4094 + 0.001 I I.658 * 0.006 0.1995 + o.OcQ7

2.294 * 0.008 0.2761 ? 0.0010

1.402 f 0.003 0. I687 f O.MM4

2.27 1 f. 0.009 0.2733 + 0.00’ 1

1.592 f 0.004 0.19’6 f 0.0005

1.248 + 0.002 0.1502 f 0.0002

1.286 f 0.002 0.1548 f 0.0002

I.234 + 0.003 0.1485 f 0.0004

1.680 * 0.052 0.2022 + 0.0063

3.290f0.0’5 0.3959 f0.00’8

1.257 * 0.004 0.15’3 iO.CKJO5

2.426 +0.014 0.2920 f 0.00’7

3.263 + 0.024 0.3927 ? 0.0029

3.418 + 0.009 0.4’14 +0.0011

4.638 f 0.0’6 0.5582 f 0.00’9

3.12lf0.033 0.3756 f 0.0040

6.094 + 0.0’ 3 0.7334 k 0.0016

3.7 1 1 f 0.020 0.4466 r 0.0024

102

38

144

246

625

32’

42

75

69

48

4’6

74

75

64

22

38

I82

47

6’

237

‘3’

129

59

I50

75

554

2’3

‘29

75

10’4

52

230

66

2’5

51

218

28

476

30

50

‘492

253

682

236

2’

98

137

I9

76

14

8’

34

9

9.5

19.9

25.9

20.3

16.4

53.2

‘2.0

38.7

33.4

55.3

* b = Bulk sample; I = one-step peroxide leach; 11 = short dilute peroxide leach; 12 = normal peroxide leach on residue from step

II; r = residue after leaching. # “‘OS/ ls60s and ‘s70s/1880s values are uncorrected for analytical blank and are not age

corrected. Age corrections do not exceed 0.1 for the oldest samples analyzed and are thus insignificant for our interpretations. Analytical blanks are in all cases less than 5% of the measured OS concentration and do not affect our interpretations.

B. Peucker-Ehrenbrink et al. /Earth and Planetary Science Letters 130 (1995) 155-167 157

different but related global processes [l&I]. Ini- tial work on the seawater isotope record [l] emphasized the similarity of the Sr and OS records and noted that intensified weathering of continental crustal material with high 87Sr/ ‘%r and 18’os/ rg60s could account for the general trends toward more radiogenic seawater isotopic compositions during the Cenozoic. Subsequent work [2,4], with improved temporal resolution, revealed a clear plateau in the seawater OS record between 25 and 15 Ma. These studies emphasized the decoupling of the marine Sr and OS isotope records and tentatively invoked the rapid weathering of organic-rich sediments, noting a gross correlation between the OS and carbonate 613C record which is consistent with this interpretation.

The new data presented here further substantiate the late Cenozoic part of the seawater OS record. More importantly, these data refine our knowledge of the early part of the Cenozoic and place the first constraints on the OS isotopic composition of Cretaceous seawater, allowing a more complete comparison of the Sr and OS records. Existing data for the older part of the OS ‘seawater’ evolution curve (30-60 Ma) as derived from pelagic clay leachates show significant scatter [ll. This scatter has been interpreted by Richter and Turekian [5] as recording short-term fluctuations and thus could be an indication of a short residence time for osmium in seawater. If these fluctuations are real and OS truly has a short residence time in seawater (about lo4 yr) compared with Sr (2 x lo6 yr, [6]) the OS isotope

Fig. 1. Sample locations.


system may record short-term fluctuations in the rate of ocean floor alteration or continental weathering, which are damped in the Sr record because of the longer response time of Sr.

The purpose of this study is threefold: The first is to evaluate the various approaches used to derive the so-called ‘seawater’ OS isotope value. Direct determination of the OS isotope composition and OS concentration has not been made for present-day seawater, so that the seawater value has to be inferred by other techniques which involve selective leaching to liberate the hydrogenous (seawater) OS component, or by bulk analyses on rapidly accumulating metalliferous carbonates. The second aim is to constrain the OS isotopic composition of seawater across the K-T boundary. The final aim of this study is to present a new composite seawater OS curve and to dis- cuss its significance. At present the early Ceno- zoic part of the seawater OS curve is based on data from a single core (LL4CGPC3) [ll. The data presented here are from a suite of samples distributed throughout the Pacific Ocean basin and allow us to test the robustness of the proposed trend. Homogeneity of present-day seawater with respect to OS is indicated [7,8], although not indisputably established [91, by data from recent oceanic sediments. The wide geographic coverage determines whether this apparent homogeneity extends back through the Cenozoic.

2. Samples

Twenty-five metalliferous sediments from several DSDP/ODP drillholes in the Pacific Ocean were analyzed (Table 1 and Fig. 1). The ages for most of the samples are derived from nannofossil biostratigraphy given in the initial reports of the DSDP/ODP and are based on the timescale of Harland et al. [lo], except for Site 596 where we use the ages of Zhou [ll] based on a constant Co flux model. Dating by carbonate fossils was im- possible for the Site 596 samples because sedimentation was below the carbonate compensation depth. The details and the limitations of this stratigraphy are discussed by Zhou [ill and Zhou and Kyte [12]. For some samples close to the

basement a combination of linear sedimentation rate and magnetic anomaly of the oceanic basement was used for age determination. The sedimentation rates and physical properties of the sediments are given in the initial reports of the DSDP/ODP.

All the samples were freeze-dried to avoid recrystallization of poorly crystalline Fe-Mn- oxyhydroxides to goethite during heating, and ground in a agate mortar prior to analysis. An aliquot of each sample was dried at 105°C to back-correct the Re and OS concentrations obtained on the freeze-dried samples. This procedure is necessary because the physical properties reported in the initial reports of the DSDP/ODP are based on samples dried at 105-110°C.

3. Analytical procedures

The analytical procedures for the bulk sediment analyses were the same as described by Ravizza [2] and Hauri and Hart [13]. Between 0.8 and 6 g of freeze-dried sample powder was used for the bulk analyses. Procedural blanks for 10 g bulk samples were in the range 1.9-2.6 pg/g; the is70s/ “‘0s of the blank varied between 8.7 and 9.3 (with one outlier at 5.7).

An acidic peroxide leaching technique was used to selectively leach the hydrogenous (seawater) OS component that is presumed to be bound on the Fe-Mn-oxyhydroxides. The technique was modified after Pegram et al. [l] and l-9 g of sample powder was used. Carbonate-rich samples were leached in a first step with a 10% acetic acid solution to selectively dissolve the carbonate phases. In the second step the samples were leached with a solution consisting of 2 ml of H,O, (30%) (0.64 ~01% cont. H,O,), 2 ml (2.13 ~01%) of cont. H,SO, (Seastar) and 90 ml of quartz-distilled H,O. This leaching solution is much weaker than that used by Pegram et al. [ll, who used an aqueous mixture of 5.8 ~01% cont. H,O, and 2.9 ~01% cont. H,SO,. Furthermore, we did not sonicate the samples prior to allowing them to reside overnight at room temperature. Some samples were leached in a stepwise procedure to check the influence of different leaching

B. Peucker-Ehrenbrink et al. /Earth and Planetary Science Letters 130 (1995) 155-167

solutions and leaching times on the analytical results. These samples (the Ll leach) were leached in a solution consisting of 1 ml of H,O, (30%), 1 ml of cont. H,SO, (Seastar) and 90 ml of sub-boiling distilled H,O. After being placed in an ultrasonic bath for 15 min the samples were centrifuged and the leachate was processed in the usual way. An OS spike solution enriched in r9’Os was added after the leaching procedure, and therefore the values given for the leachates are minimum OS concentrations. One procedural leaching blank was measured for a 5 g sample and yielded 0.3 pg OS/g sample. The separation and purification of osmium is described in detail by Pegram et al. El], Ravizza [2] and Hauri and Hart [13]. The osmium isotope ratio measure- ments were made on the NIMA-B thermal ion- ization mass spectrometer at the Woods Hole Oceanographic Institution following the procedure described by Hauri and Hart [13].

Rhenium was separated from 100 mg acid-dissolved sample aliquots by standard ion exchange techniques [131. A rhenium spike enriched in ‘*‘Re was used and the samples were analyzed by isotope dilution ICP-MS at the Lamont-Doherty Earth Observatory.

4. Results and discussion

4.1. The acidic hydrogen peroxide leaching technique

Two techniques have been used to reconstruct the seawater OS isotope record from the marine sediments, bulk sediment analysis and selective leaching of sediments. Only sediments which contain predominantly hydrogenous (seawater-derived) OS are suitable for bulk analysis. Therefore only a few sediment types, such as metalliferous carbonates [21 or metalliferous siliceous oozes, can be analyzed in this manner. Furthermore, high sedimentation rates are desirable in order to minimize the amount of cosmic material in the bulk sediment. If leaching techniques are em- ployed to selectively dissolve Fe-Mn-oxyhydroxides and release the associated hydrogenous OS the range of sediment types which can be used to

mbsf 19 19.5 20 20.5 21

-19 26.5 21

mbsf

Fig. 2. K-T boundary at DSDP Site 596 in the central South Pacific Ocean. Upper panel: Content of II (+I and shocked quartz (0). Middle panel: OS concentration (leach = o; bulk = 0) and OS/II ratios (black and white boxes). Lower panel: OS isotope ratios of leaching experiments (leach = O; bulk = 0). Weak leaches are marked 11, leaches on 11 residues are marked 12. mbsf = Meters below sea floor.

constrain the seawater OS evolution expands con- siderably.

Pegram et al. [ll used an aqueous mixture of 5.8 ~01% cont. H,O, and 2.9 ~01% cont. H,SO, with ultrasonic treatment for 30 min and left the samples overnight at room temperature to extract hydrogenous OS from metalliferous pelagic clays [14]. The results of our leaching experiments are shown in the middle and lowermost panels of Fig. 2. We used an aqueous mixture of 0.64 ~01% H,O, and 2.13 ~01% cont. H,SO, without soni- eating the samples before allowing them to react overnight at room temperature. Although our


leach is much weaker the OS isotope values measured in leaching solutions of slowly accumulating metalliferous pelagic clays from Site 596 are con- sistently less radiogenic than OS isotope values measured in rapidly accumulating bulk sediments of similar age. This discrepancy indicates that not only hydrogenous OS is leached during this procedure. A prime candidate for such a non-hydrogenous unradiogenic phase is cosmic material known to be susceptible to alteration to Fe oxyhydroxides [ 15,161. Slowly accumulating sediments are characterized by a larger fraction of cosmic dust particles which might be affected by the peroxide leach and thus distort the ‘hydrogenous’ OS isotope composition. This is illustrated by K-T samples from Site 596, which have high OS concentrations in both the bulk and the leachable fraction as well as 0s isotope compositions close to chondritic values. Therefore, K-T samples are used as a ‘worst-case’ test because of their high background of non-hydrogenous OS and the possibility that K-T impact debris may be more readily attacked by the leach than cosmic dust [17]. We leached two samples close to the K-T boundary at Site 596 in a two-step procedure in order to test this hypothesis (see Fig. 2 and Table 1): The first leach consists of an aqueous solution of 0.33 ~01% cont. H,O, and 1.09 ~01% cont. H,SO, for 30 min involving ultrasonic treatment for 15 min. In the second step the residue was leached with our standard mixture overnight at room temperature. The OS concentrations in the first leachate are lower by a factor of lo-15 than in the second leachate, while the i8’Os/ lg60s values of the first leachate are significantly more radiogenic (2.271 and 3.290) than the second leach (1.592 and 1.680 respectively). Pegram et al. 111 did not find such variations in their experiments on ‘non-K-T’ samples with different leaching solutions, although their mixtures are much stronger (2.7-30 ~01% H,O,) than our strongest leaching solution (0.64 ~01% H,O,), and K-T samples represent the ‘worst-case’, Our results illustrate the need for a more selective leaching procedure. Additional information on trace element concentrations in the leachates could be useful for de- tecting other soluble phases that might be attacked by the leach.

Pegram et al. [l] reported high-frequency OS isotope variations in the lower part of the LI_44- GPC3 sediment core. Unfortunately, bulk analyses of only a few samples from LL44-GPC3 were performed. Consequently it is unclear whether samples with very unradiogenic leachable OS carry high concentrations of unradiogenic OS in the bulk sediment, as would be expected in sediments containing significant amounts of cosmic material. The high-frequency variations might be due to analytical problems with the selectivity of the leaching technique, and we suspect that inferences based on these short-term fluctuations in the ‘hydrogenous’ OS isotope composition (i.e., the short residence time of OS in seawater of the order of lo4 yr> might not be supported by the data.

4.2. K-T boundary at DSDP Site 596

The shape of the seawater OS curve across the K-T boundary is of great interest because OS, like Ir, is enriched in these sedimentary sections throughout the world [18]. In addition, the lg70s/ lg60s values of K-T boundary clays are extremely unradiogenic compared to normal marine sediments [19,20], requiring that the OS and Ir within the boundary sections are derived either from the meteoritic material or from the Earth’s mantle [19]. Given the wide geographic extent and large magnitude of the PGE enrichment at the K-T boundary, it is likely that the PGE content of seawater was significantly perturbed during the K-T event 1171 (and see 1211 and references therein). We hypothesize that, in con- junction with the K-T event, the 18’Os/ lg60s of seawater was shifted to nearly meteoritic values.

In order to test the hypothesis that a K-T impactor has affected the OS isotope composition of seawater, we compare the mass of extraterrestrial OS derived from such an impactor with a crude estimate of the annual riverine OS flux. Assuming an annual riverine suspended matter flux of 1 x 1016 g [22] and a mean OS concentration for continent-derived material of 0.05 rig/g [23], the global annual riverine flux of OS is of the order of 5 x lo4 g, if we assume that 10% of the riverine OS is soluble (cf. U = 16%, MO = 30%


(table 4 in [24])). Our estimate of 10% dissolved riverine OS is highly speculative, but the resulting riverine OS flux is similar to the ‘labile OS fraction’ in river sediments used by Pegram et al. [25] to estimate the global riverine OS input to the ocean of about 7 x lo4 g. The extraterrestrial mass of OS can be estimated using a mean Ir concentration in the K-T impact horizon of 63 ng/cm* [26] and a chondritic Os/Ir value. The resulting mass of OS delivered by the impactor is of the order of 3 x 1011 g, which equals about 5 m.y. of riverine input to the ocean. This order of magnitude estimate demonstrates the enormous mass of extraterrestrial OS and supports the hypothesis that the K-T impactor could have had a significant effect on the OS isotopic composition of seawater.

Based on our analyses of the isotopic composition of total and leachable OS across the K-T boundary at Site 596, we have made an initial attempt to constrain the OS isotopic evolution of seawater during this time. The K-T boundary at DSDP Site 596 in the central South Pacific is characterized by one of the most prominent Ir anomalies in the world (320 ng Ir cm-‘, [27]), the occurrence of shocked quartz [27], and high-temperature relict minerals such as magnesioferrite spinels [28]. Although the Ir anomaly is smeared out over an interval of at least 1 m, the presence of shocked quartz over much of this interval indicates mixing (bioturbation) rather than diffusion as the main process. Diffusion is most likely a subordinate process in these oxic metalliferous pelagic clays. Our results on the concentration

K-T - 9 1.1

- 1.0 8

- koch

7

0 (high sed. rate) 0.9

- 0.8

6

I .* I ‘I I . I I ,‘I I i 0 lo 20 30 40 50 60 70 80 90

age W-0 Fig. 3. Rapidly accumulating bulk sediment and hydrogenous ‘s70s/1860s of marine sediments over the past 80 m.y. and the proposed OS isotope seawater evolution (gray field). The gray field was chosen to match the data obtained on rapidly accumulating metalliferous carbonates (large circles and dots and closed triangles), the best measure for the OS isotope composition of seawater. They tend to give minimum seawater values because we did not correct for small contributions by extraterrestrial OS. The data obtained by leaching slowly accumulating pelagic clays (small open symbols) tend to underestimate the OS seawater isotope composition due to the possibility of attacking extraterrestrial particles. The width of the gray band is a rough measure of the reliability of the inferred seawater OS evolution. Shapes of the symbols correspond to those used in Fig. 1. Open symbols = leachates; closed symbols = bulk analyses. Weak leaches are marked II. The small open symbols are leachates from DSDP Site 596 sediments. Uncertainties of age estimates for DSDP Site 596 sediments are discussed in detail by Zhou [ll] and Zhou and Kyte [12]. Age uncertainties for all other sediments are usually smaller than the symbols, except for sample 20-196-2-5,112-114 (80 + 3 Ma).


and isotope composition of OS in four samples across this K-T boundary are listed in Table 1 and shown in Figs. 2 and 3, together with literature data on the Ir concentration and the abun- dance of shocked quartz [12,27]. The OS concentration in the bulk sediment follows roughly the Ir concentration and reaches a maximum of 1.5 rig/g at the inferred K-T boundary at about 20.15 m below sea floor (mbsf).

The Os/Ir values, calculated with our OS data and interpolated Ir data from the literature [12], become lower near the K-T boundary, a common feature in K-T sections worId wide [20,29] except at Stevns Klint [30]. The non-chondritic ratios require that OS and Ir were fractionated from one another and thus could not have been pas- sively deposited as particles of meteoritic material. This implies that these elements passed through the dissolved state, either following va- porization or by dissolution of impact ejecta. These less-than-chondritic Os/Ir values could reflect condensation or fractionation of the volatile 0~0, under oxidizing conditions in association with impact-related processes, as discussed by Evans et al. [31]. These low ratios may also reflect less efficient OS burial in oxic pelagic clays relative to Ir, as indicated by Os/Ir values generally lower than unity in such sediments compared with ratios larger than unity often found in organic-rich shales [32,33]. Preferential diffusion of K-T osmium out of the sediment is less likely because of the oxic nature of these sediments.

The maximum OS concentration in the leachate at the K-T boundary might result from attacking non-hydrogenous phases such as altered cosmic particles, as discussed in the previous section. The i8’Os/ rs60s of the bulk sediment shows a minimum of 1.248, very close to the chondritic value of about 1.1 and one of the lowest values ever measured in marine K-T sections. The data for the leachates, which are supposed to reflect hydrogenous (seawater) OS isotope compositions, are somewhat problematic at the K-T boundary because of the possibility of attacking non-hydrogenous phases during the leaching procedure. Given the unusually high OS concentrations and unradiogenic isotopic compositions associated with K-T sections it is likely that the leach data

are biased toward “‘OS/ ‘s60s values which are lower than the actual l ‘OS/ ls60s of seawater. We have attempted to correct for this bias by assuming that the true hydrogenous OS concentration in the K-T sample is 25.8 pg OS/g, iden- tical to the valies obtained by our weak leaches on neighboring samples, and that all non-hydrogenous OS in the K-T sample has a chondritic ‘*‘OS/ iE60s value. Based on these assumptions, mass balance requires that the ‘*‘OS/ 1860s value of the hydrogenous component is 2.93, which we regard as a conservative estimate of the maximum “‘OS/ ls60s value of K-T seawater. This value strongly depends on the hydrogenous OS concentration. If, for instance, the hydrogenous OS concentration is 20 pg/g instead of 25.8 pg/g, the OS isotope ratio of the hydrogenous fraction would be 3.45 instead of 2.93. The 18’Os/ lg60s value of about 1.6 in the Stevns Klint K-T boundary clay [19] could be a more appropriate estimate for the K-T seawater than the value of 1.29 measured in the leached fraction of our K-T sample from DSDP Site 596 provided that the OS in the Stevns Klint boundary clay is mainly hydrogenous in origin.

The gradual shift toward unradiogenic OS isotopic compositions prior to the K-T boundary and the slow recovery after the impact as indicated by the Site 596 data are more likely related to bioturbation of these slowly accumulating sediments, as indicated by the occurrence of shocked quartz over at least 1 m. Bioturbation depths of more than 1 m have been reported in other marine sediments [34,35]. Therefore, on the basis of the Ir and shocked quartz data we assume that the sharp primary signal was smeared out over a l-l.5 m thick interval, equivalent to 3-5 m.y. of pelagic sedimentation. The apparently slow recovery after the K-T boundary is somewhat problematic. If the proposed short residence time of OS (about lo4 yr> turns out to be correct, the noble-metal spike should be flushed through the marine system within less than 1 m.y. The gradual increase during the early Paleogene could be related to weathering of impact material which was stored for some time on the continents and entered, with a delay, the marine environment. Another important factor is high oceanic crust


production rates during the Maastrichtian and Paleocene [36], which led to increased input of unradiogenic OS to the ocean.

4.3. ‘870s /1860s seawater evolution

We propose an OS isotope seawater evolution curve over the past 80 Ma, as shown by the gray field in Fig. 3, based on an evaluation of all the available data. Rapidly accumulating metalliferous sediments and carbonate-rich metalliferous samples are likely to be the best providers of the past variations in seawater 1870s/ rg60s, and, un- til leaching methods are developed which selectively liberate exclusively seawater-derived OS, are better suited to yielding this information than pelagic clay leachates. For this reason all the seawater OS data obtained on pelagic clay leachates must be interpreted with great care, because of the likelihood that additional non-hydrogenous phases are attacked during leaching. The correlation with the marine Sr isotope record

IO

9

8

7

6

5

4

3

2

1

[37,381 is shown in Fig. 4, together with a variety of processes which cause specific changes in the combined Sr-0s isotope record with time. The combined Sr-0s isotope record shows several important features, most notably periods of corre- lated increase in both isotope ratios (40-25 Ma and 15-O Ma) as well as periods of decoupling between Sr and OS (70-40 Ma and 25-15 Ma).

The inset in Fig. 4 is a schematic ‘process diagram’ which represents the various driving forces in the Sr-0s isotope space of seawater. High weathering rates of the continental crust (+ CC) will increase both “Sr/ 86Sr and 1870s/ lg60s, whereas low weathering rates (--CC) lead to decreases in these two ratios, given a constant mantle-derived input. Exposure and weathering of organic-rich shales on continental shelves caused by a drop in sea level can only become an important driving force of changes in the marine OS record if the time span between deposition and weathering is long enough to allow development of a radiogenic OS isotopic com-

OMa

impact

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Fig. 4. The combined Sr-0s isotope record of seawater since 80 Ma. The shade and width of the gray band is a rough measure of the reliability of the inferred seawater OS evolution. Note that the seawater OS isotope ratio at the K-T boundary is especially uncertain (?). The Sr data are taken from [37,38]. The rates of change in the two isotope systems are given by the relative spacing of the 1 Ma time marks for the last 10 Ma and for the 5 Ma time marks thereafter. Abbreviations used in the process diagram: CC = continental crust; OC = oceanic crust; ultramafics = abyssal ultramafic rocks; + indicates enhanced weathering; - indicates decrease in weathering. See text for details.

164 B. Peucker-Ehrenbrink et al. /Earth and Pianetaly Science Letters 130 (1995) 155-167

position. In order to increase the 18’Os/ ig60s from 5 (average Cenozoic seawater) to 15 (average continental runoff) about 130 Ma is required, given a mean ‘s7Re/ i%Os value of about 5000 for organic-rich shales [39,40]. It is therefore much more likely that sea-level fluctuations and weathering of young organic-rich sediments buffer the seawater 0s isotopic composition, in a way similar to that for carbonates in the marine Sr record. Input from old black shales and large cosmogenic impacts affects OS but not Sr isotope ratios. Dis- solution of cosmic dust is unlikely to be an important driving force of changes in seawater OS isotope composition because the cosmic dust flux has been more or less constant over the past 80 Ma, while the seawater OS isotopic composition has varied significantly [41,42]. The inner circle of the process diagram shows processes most likely to be subordinate driving forces during the past 80 Ma, whereas processes listed in the outer circle are likely to be of major importance for the seawater record.

One of the reasons for the recent heightened interest in the marine Sr isotope record is the notion that this record might serve as a proxy indicator of the past intensity of chemical weathering on the continents [43,44] and hence the drawdown of CO, via silicate weathering [451 and nutrient reIease to the oceans [46]. The process diagram in Fig. 4 shows that the marine OS record, like the marine Sr record, should shift toward more radiogenic values during times of more intense continental weathering. During the past 15 Ma the positive correlation between Sr and OS is consistent with the general inference of intensified continental weathering. The positive correlation between 87Sr/ 86Sr and U concentration [47], the large flux of dissolved U, and the covariation between U and Re concentrations [48] found in rivers and groundwaters of the Ganges-Brahmaputra (G-B) drainage system corroborates the speculation that the coupled increase in the marine Sr and OS isotope ratio during the past 15 Ma is mainly caused by weathering of the Himalayas. A similar positive correlation between Sr and OS appears to exist from 40 to 25 Ma, although reliable OS data for this time interval are scant. However, from 65 to 40 Ma

and from 25 to 15 Ma the marine Sr and OS isotope records do not covary. Between 25 and 15 Ma Sr rises rapidly toward more radiogenic values while OS remains essentially invariant. From 65 to 40 Ma the situation is reversed, with the OS record shifting toward higher 18’Os/ lg60s values, while the “Sr/ “%r of seawater is nearly constant, exhibiting a shallow minimum between 50 and 40 Ma. These periods of decoupling between Sr and OS are important because they raise ques- tions about the validity of basing inferences of past chemical weathering intensity solely on the Sr isotope record.

As discussed in the previous section, the K-T event almost certainly drives the initial recovery to higher ‘870s/1860s in the earliest Cenozoic, but it seems improbable that this recovery is responsible for the gradual increase in the marine ‘870s/ 1860s during the subsequent 15 Ma. Therefore we interpret the present Sr-0s data as requiring an explanation which allows both constant 87Sr/ 86Sr to occur with variable 18’Os/ ls60s and constant 18’Os/ lB60s to occur with changing *‘Sr/ ‘%r. The former case can be accommodated by the processes indicated in the inset of Fig. 4-changes in black shale or ultramafic rock weathering rates. The latter case cannot be easily explained by the processes in Fig. 4. We suggest that the heterogeneous distribution of lithologies within individual drainage basins can account for both of the situations described above. For example, erosion of old gneisses, gran- ites and granodiorites in the Lesser Himalaya are known to contribute a major portion of the radio- genie Sr to the G-B river system 1491. However, given the very sialic character of these rocks they may contain only small quantities of Re and OS and may therefore not significantly impact upon the OS budget of the G-B river system. Extensive weathering of this type of terrain could explain the seawater trend of rapidly increasing 87Sr/ “%r and constant lg70s/ ‘*‘jOs between 25 and 15 Ma. The case of constant 87Sr/ *%r and increasing “‘OS/ 1860s could be readily attributed to extensive weathering of ancient sedimentary sequences containing marine carbonates and organic-rich shales. Chemical erosion of this type of terrain would reflect the radiogenic character of OS typi-

B. Peucker-Ehrenbrink et al. /Earth and Planetary Science Letters 130 (1995) 155-167 16.5

cal in black shales [7,39,40] and the seawater-like Sr signature characteristic of marine carbonates. The age of the drainage terrain may be a much more important factor with respect to OS, because Re/Os fractionation can be so much larger than Rb/Sr fractionation. If changes in the slope and sign of the marine Sr-0s isotope evolution trend (Fig. 4) do in fact reflect changes in the lithology and age of eroding material, use of these isotopic records as indicators of the overall intensity of chemical weathering may not be justi- fied.

Variations in the mantle-derived input of OS to the ocean ( + OC and - OC in Fig. 4) may be caused by variations in the spreading rate, phases of major plate reorganization and the accessibility of abyssal ultramafic rocks to seawater alteration [50]. Global variations in spreading rate have been extensively documented ([36] and references therein) and demonstrate deceleration of seafloor spreading since the Cretaceous. The increasing lg70s/ ig60s of seawater is consistent with this phenomenon. Unfortunately, no information about the intensity of the seawater alteration of abyssal ultramafic rocks or normal oceanic crust through time is available. As a result the relative importance of continental weathering and seafloor alteration in controlling the Cenozoic evolution of the seawater OS record cannot be quantitatively assessed. We therefore urgently need data on the OS concentration in marine hydrothermal fluids and rivers from different ge- ographical settings in order to decipher the processes which led to the observed marine OS isotope evolution.

Acknowledgements

The samples used in this study were supplied through the assistance of the Ocean Drilling and Deep Sea Drilling Programs. We are indebted to Stan Hart, who generously provided access to his clean laboratory for the OS separation and NIMA-B for the OS isotope ratio measurement. Mike Bacon also kindly allowed the use of his laboratory facilities for our work. We have bene- fited from discussions on various aspects of this

study with Erik Hauri, Jurek Blusztajn, Brad Esser, W.J. Pegram and Alex Rocholl. Thought- ful reviews were provided by D. Colodner, K. Falkner, R. Oxburgh and M.E. Raymo. This work was carried out during a visit by B.P.E. to the Woods Hole Oceanographic Institution and forms part of his Ph.D. thesis at Mainz University (D77). G.R. acknowledges financial support from NSF grants OCE 9301049 and EAR 9204409. [CL]

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The marine187Os/186Os record of the past 80 million years

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Transcript of The marine187Os/186Os record of the past 80 million years