8.5 Seawater osmium

A comparison with Sr isotope systematics suggests that the large differences in osmium isotope composition between crustal and mantle reservoirs should generate large seawater Os isotope variations through time. Seawater itself contains very little osmium, but chemical sediments act to pre-concentrate osmium by scavenging it from seawater, thus reaching quite high abundances. Ravizza and Turekian (1992) showed that this ‘hydrogenous’ osmium component could be extracted from the substrate by leaching. Hence, they demonstrated that modern seawater has radiogenic ¹⁸⁷Os/¹⁸⁶Os ratios around 8.5 (¹⁸⁷Os/¹⁸⁸Os = 1.0). In contrast, residues from leaching have significantly lower Os isotope ratios due to the presence of a micro-meteorite (cosmic dust) component which is constantly raining down upon the Earth.

It is most convenient to review this subject under the same categories that were used for seawater Sr (section 3.6). Therefore, we first examine the evidence used to reconstruct a seawater osmium curve, before considering the competing fluxes which cause changes in seawater osmium through time.

8.5.1 Seawater Os isotope evolution

Pegram et al. (1992) analysed leached carbonaceous sediments of various ages in the first study of seawater osmium isotope evolution through the Cenozoic. Osmium was extracted by acid hydrogen peroxide leaching of pelagic black shales from a large piston core recovered from the North Pacific. Pegram et al. interpreted the measured osmium isotope ratios as primary signatures of the sediments, reflecting seawater osmium, rather than the product of secondary mixing between re-mobilised terrestrial and meteoritic osmium. Given this assumption, the data implied a sharp increase in seawater osmium isotope composition during the Tertiary period, mimicking the seawater Sr profile for this period (section 3.6.1).

These preliminary findings were confirmed by Ravizza (1993), who used metalliferous sediments deposited near mid ocean ridges as recorders of seawater osmium. Because these sediments have greater rates of deposition than the pelagic clays used previously, the fraction of hydrogenous (seawater derived) osmium dominates over the cosmic dust fraction. The results were in good agreement with leached pelagic clay data, and revealed a rapid increase in seawater Os isotope ratio over the last 15 Myr, but relatively constant ratios between 18 and 28 Myr ago (Fig. 8.27). Sr isotope ratios from the same samples were in good agreement with published data, but when compared with osmium date they showed significant decoupling between the two systems. This is not surprising, in view of the very different chemistries of the two elements.

Fig. 8.27. Plot of Os isotope ratio of sediment leachates against age, attributed to increasing seawater Os isotope ratio during the Tertiary. Data sources: ( " ) = Ravizza (1993) and Peucker-Ehrenbrink et al. (1995); ( ! ) = Pegram and Turekian (1999).

Peucker-Ehrenbrink et al. (1995) extended the detailed seawater osmium record to 80 Myr, using a combination of leached and bulk sediment analyses. However, they showed that for slowly deposited pelagic clays, a more gentle leaching procedure was necessary in order to remove hydrogenous osmium without releasing the cosmic dust fraction. Data from sediments with a relatively high deposition rate were in good agreement between different studies, but data from slowly deposited sediments were less reliable. Leached 80 Myr-old sediment showed for the first time a radiogenic osmium isotope signature below the K–T boundary (Fig. 8.27). In addition, gently leached samples on either side of the boundary constrained a sharp drop in osmium isotope ratio to the immediate vicinity of the boundary. To explain this dip at the K–T boundary, Peucker-Ehrenbrink et al. calculated that a meteorite impact could have released a pulse of dissolved osmium into the oceans equivalent in size to the global run-off flux over a 5 Myr duration (see below).

Data from the above studies were augmented by Pegram and Turekian (1999) in a larger study of the same sediment core used by Pegram et al. (1992). The bottom of the core was dated using cobalt accumulation rates (section 4.5.3), and the accuracy of Os measurements on bulk sediment was improved by correcting for the cosmogenic Os component in the sediment on the basis of ³He measurements. The result was a more detailed profile for the early Tertiary which was largely consistent with earlier work but also reached to even less radiogenic compositions at the K–T boundary. A similar dip was seen at the Eocene–Oligocene boundary (Fig. 8.27).

The first attempt to extend the seawater osmium curve back into the Jurassic was made by Cohen et al. (1999), based on the analysis of relatively very Os-rich organic-rich mud-rocks with total Os contents between 0.2 and 3 ppb. Samples were analysed by bulk dissolution techniques on the grounds that the meteoritic component was insignificant in these continental shelf sediments, which have deposition rates several orders of magnitude faster than pelagic sediments. Unlike the studies discussed above, these organic-rich mud-rocks also have high Re/Os ratios, so relatively large age corrections were necessary in order to recover initial Os isotope ratios indicative of seawater osmium. This was achieved by means of three Re–Os isochrons, with ages of intervening samples estimated by interpolation.

The results from this study (Fig. 8.28) revealed very unradiogenic osmium near the Triassic– Jurassic boundary, with a subsequent very rapid rise, reminiscent of the K–T boundary. Despite the relatively proximal (near-shore) deposition site of the sediments, the osmium isotope signatures were taken to be indicative of seawater osmium, on the grounds that only the open ocean could supply the relatively very large amounts of Os necessary to support the deposition flux in these Os-rich sediments. However, Cohen et al. attributed the presumed variations in seawater osmium to varying terrestrial osmium fluxes (see below), rather than a meteoritic source. One case where a continental weathering model seems justified is the Os isotope peak in the early Jurassic, which was shown by Cohen et al. (2004) to be much sharper than previously realised, with a duration of only 1 Myr. The Os peak is correlated with the ‘Toarcian oceanic anoxic event’, a global warming event that caused a sharp increase in continental weathering. This could have released a spike of radiogenic osmium to the oceans.

Fig. 8.28. Plot of inferred Os isotope ratios of seawater through the Jurassic to Tertiary periods. After Cohen et al. (1999). Open symbols are data of Peucker-Ehrenbrik et al. (1995).

8.5.2 Os fluxes and residence times

In order to understand seawater osmium evolution it is important to quantify the fluxes which control its composition. A comparison between these fluxes and the oceanic osmium inventory will then allow a calculation of the residence time of osmium in seawater. The principal source of radiogenic osmium is river-borne run-off from old continental crust. On the other hand, possible sources of unradiogenic osmium are low temperature hydrothermal alteration of ultramafic rocks (Sharma et al., 2000) and the dissolution of cosmic dust. The magnitude of the unradiogenic component is unknown, but its composition is well constrained. Therefore, a major objective has been to constrain the size and composition of the river water budget.

In early work on this subject, Pegram et al. (1994) argued that the oxidising conditions of river water would cause osmium to be adsorbed on the ferro-manganese coatings of particulate sediment, rather than remaining in solution. They speculated that when this sediment reached the sea, reducing conditions would break down the ferro-manganese oxides, releasing osmium to seawater. By analysing leaches of river sediment, Pegram et al. found very variable ¹⁸⁷Os/¹⁸⁸Os ratios, ranging from 0.17 – 0.85 in rivers draining ultramafic rocks to values of 1.4 – 2.8 in more typical drainage basis. However, large rivers such as the Mississippi and Ganges had ratios close to 2.2, a best estimate for the global average. This was substantially more radiogenic than an estimate of 1.26 in average upper crust (Esser and Turekian, 1993).

The first direct measurements on dissolved river water osmium were made by Sharma and Wasserburg (1997). Because these concentrations are so low, it is convenient to quote them in picogram/kg (10^-15 g/g). Analysis of four major rivers suggested a concentration range from 2.8 to 8.5 pg/kg and a range of ¹⁸⁷Os/¹⁸⁸Os from 1.2 to 2.0. Hence Sharma and Wasserburg estimated the total riverine supply of dissolved osmium at 320 kg/yr. These results were refined by Levasseur et al. (1999), using data from 17 of the world’s largest rivers. The average dissolved osmium concentration was estimated as 7.9 pg/kg, with a ¹⁸⁷Os/¹⁸⁸Os ratio of 1.5, leading to an estimated global riverine flux of 295 kg/yr, in good agreement with Sharma and Wasserburg (1997). In addition, the flux of non-dissolved Os carried on particulate matter was estimated at less than 25% of the dissolved flux.

The first direct measurements of seawater osmium concentration were made by Sharma et al. (1997), yielding a best estimate of 3.6 pg/kg, with an isotope ratio of 1.04 (¹⁸⁷Os/¹⁸⁶Os = 8.7). Subsequent work by Levasseur et al. (1998) and Woodhouse et al. (1999) confirmed the mean isotope ratio of modern seawater as 1.06 (8.8), but suggested that the concentration determined by Sharma et al. was an under-estimate, due to a failure to achieve complete isotopic homogenisation between sample and spike osmium. Thus, Levasseur et al. determined a constant concentration of 10.9 pg/kg in a 5 km deep section from the southwest Indian ridge (Fig. 8.29). A similar experiment by Woodhouse et al. (1999), on a 3 km deep section from the eastern Pacific, gave identical isotope ratios but suggested some variations in osmium concentration. Below 2 km depth, these ranged from 8.5 to 9.5 pg/kg, but at 500 m depth the concentration dropped as low as 6.5 pg/kg.

Fig. 8.29. Profiles of (a) osmium isotope ratio, and (b) concentration, in sections through the SW Indian Ocean. After Levasseur et al. (1998).

A comparison between the isotopic composition of average river water and seawater allows the relative fluxes of riverine osmium and unradiogenic osmium (meteoritic and mantle derived) to be estimated. Estimates of the size of the riverine flux vary from a low of 50% (using the riverine ¹⁸⁷Os/¹⁸⁸Os ratio of 2.2 from Pegram et al. 1994) to a high of 81% (using the value of 1.5 from Sharma and Wasserburg, 1997). The intermediate riverine value of 1.8 from Levasseur et al. (1999) corresponds to a 70% contribution. Ignoring the redissolution of particulate osmium from rivers, this implies a global osmium flux to the oceans (riverine and unradiogenic) of 420 kg/yr.

Assuming that the system is in a steady state condition, comparison of the seawater osmium inventory with the total input flux allows the oceanic residence time of osmium to be calculated. An average seawater concentration of 10 pg/kg, divided by the mass of the oceans (1.4 H 10²¹ kg) leads to an oceanic osmium inventory of 1.4 H 10⁷ kg. Given a total input flux of 420 kg/yr, this leads to a residence time (tau) of 33 kyr. This residence time is near the middle of many estimates made over the past ten years. It is much shorter than that of strontium (section 3.6.2), but also substantially longer than the residence time of non-conservative elements such as Nd, Th and Be. In fact it is similar to the time taken for river water to fill the oceans (37 kyr).

Despite the efforts described above to place constraints on global osmium fluxes, there are still major uncertainties. One of these concerns the fate of dissolved osmium in the estuarine and coastal zone. As noted above, Pegram et al. (1994) initially proposed that particulate riverine osmium would be released into solution in estuaries. On the other hand, a study of the Lena river in Siberia (Levasseur et al., 2000) showed that nearly 30% of dissolved riverine osmium was lost from solution in the estuary by adsorption onto suspended particles. If this process occurred on a world-wide scale, the seawater residence time of osmium would have to be increased by 30%. However, a study of the Fly and Sepik rivers in Papua New Guinea (Martin et al., 2001) suggested that both riverine and seawater osmium were removed by adsorption in the estuary and the near-shore zone. This was demonstrated in a 200 km long ‘salinity transect’ (Fig. 8.30).

When osmium isotope ratios are plotted against salinity (Fig. 8.30a) they provides evidence of fairly simple mixing between seawater and the unradiogenic osmium carried by the river (reflecting its source in young ultramafic rocks). However, the plot of osmium concentration against salinity (Fig. 8.30b) shows that removal of dissolved osmium has occurred, particularly at the seaward end of the transect. This implies that seawater osmium may be lost at a similar or greater rate than riverine osmium. The relative magnitude remains unknown, certainly at the global scale, but it suggests that 33 kyr may represent a maximum rather than a minimum value for seawater osmium residence.

Fig. 8.30. Salinity profiles from the estuary of the Fly River, Papua New Guinea. a) against isotope ratio; b) against Os concentration. After Martin et al. (2001).

The tentative consensus of seawater osmium residence times around 30 – 40 kyr was shattered by evidence for very rapid changes in seawater osmium isotope composition during the Quaternary period (Oxburgh, 1998). This evidence comes from two 200 kyr sediment cores from the East Pacific Rise which display osmium isotope variations in step with glacial cycles, as represented by oxygen isotope analysis (Fig. 8.31). The two cores yielded consistent results, suggesting that they might be recording real changes in the seawater osmium signature. However, the sharpness of the changes (particularly during the deglaciations 20 and 140 kyr ago) implied an extremely short seawater osmium residence time, possibly as low as 3000 yr. Given such a short residence time, the apparent sharp drops in the isotope signature during the last two glacial periods were attributed to a reduction in chemical weathering at these times. However, in order to reconcile these data with the box model estimate of seawater residence it is necessary to postulate a riverine flux at least three times greater than that estimated by Sharma and Wasserburg (1997) and Levasseur et al. (1999).

Fig. 8.31. Plot of osmium isotope ratio against age for two sediment cores ( ! , " ) from the flanks of the East Pacific Rise. These are compared with a record of oxygen isotope variations, representative of glacial cycles. After Oxburgh (1998).

Further evidence for very rapid changes in seawater osmium was presented by Oxburgh (2001) for the Cariaco Basin in the Caribbean (Fig. 8.32) This basin shows annual varves that have been calibrated against radiocarbon ages (section 14.1.5). This data set gave much better time resolution for seawater osmium evolution and appeared to show a sudden jump from a steady state during the last glacial maximum to a new steady state during the Holocene.

The data are compared in Fig. 8.32 with three of the data points from core V19-55 in the Pacific (Oxburgh, 1998). This comparison reveals some positive and some negative points (for an interpretation of these signals as real seawater osmium variations). On the positive side, the core top analyses from the Pacific and the Caribbean samples are consistent with osmium analysis of young Fe–Mn crusts from the Pacific and Atlantic oceans (Burton et al., 1999a), as shown in Fig. 8.32. However, on the negative side, the Cariaco basin records a very early rise in the osmium signature, largely in advance of the oxygen isotope profile, and while the Pacific core is still displaying a ‘glacial’ signature. Hence it is concluded that more work must be undertaken before these findings can be accepted as genuine reflections of seawater osmium isotope evolution.

Fig. 8.32 Plot of Os isotope ratio against age for sediment cores from the Cariaco Basin ( ! ) and the East Pacific Rise ( " ), compared with an oxygen isotope record from the Cariaco Basin. Recent Fe–Mn crusts from the Atlantic and Pacific Ocean are also shown ( Q ). Modified after Oxburgh (2001).

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