Residence time of osmium in the oceans

Abstract

1] Abstract: Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations (35 ±50 kyr) appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater (3± 4 kyr). It is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree. As the inventory of osmium in the oceans is relatively well constrained, these disparate residence time estimates imply wildly different osmium input fluxes to the oceans. An osmium residence time of 8 ± 10 kyr is proposed by evaluating the uncertainties and limitations of both methods, and it is argued that osmium inputs to the ocean are currently underestimated by a factor of $3. This reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified. The very short residence time of 3 ±4 kyr inferred from the postglacial change in seawater composition (assuming a single step change in input flux) is rejected as it implies unreasonably high osmium input fluxes to the oceans. It is concluded that a postglacial spike in osmium flux, associated with a meltwater event, must have driven part of the change in seawater composition. However, it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187 Os/ 188 Os.

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

“…Input of unradiogenic riverine Os to the ocean from Papua New Guinea lowered the Os isotope composition of ambient seawater in the nearby Coral Sea (Martin et al, 2000). The seawater residence time of Os was probably shorter than the present-day value of $10 4 years (Oxburgh, 1998(Oxburgh, , 2001Levasseur et al, 1999) in less oxygenated Middle Proterozoic oceans. Thus, the Velkerri Formation I Os may reflect only the local water column and cannot be taken to represent a homogenous seawater composition for global seawater or even the Roper Superbasin at 1.4 Ga.…”

The transport of Osmium and Strontium isotopes through a tropical estuary

“…The relaxation time can be determined from the recovery rate of 187 Os/ 188 Os ratio following an excursion. Excursions to lower 187 Os/ 188 Os ratios associated with glacial maxima have been observed in the rapidly accumulating Fe-Mn sediments on the flanks of the East Pacific Rise (EPR) and from the anoxic Cariaco Basin (Oxburgh, 1998(Oxburgh, , 2001. They are also present in a core from the equatorial Indian Ocean (Marcantonio et al, 1999) and in the sub-oxic to anoxic sediments in the Santa Barbara Basin (Williams and Turekian, 2004).…”

“…The estimated relaxation time for the EPR sediments is 8-12 kyr (Oxburgh, 1998) and for the Cariaco Basin sediments it is 3-4 kyr (Oxburgh, 2001). Since the s SW Os thus estimated is much less than that determined from mass balance considerations, Oxburgh (2001) has argued that input fluxes of Os into the oceans have been underestimated and some sources have not been identified. Indeed, it is true that other sources of Os to the oceans such as aeolian (Williams and Turekian, 2002) or groundwater (Oxburgh, 2002;Peucker-Ehrenbrink, 2002) may be important but have not been quantified so far.…”

“…They are also present in a core from the equatorial Indian Ocean (Marcantonio et al, 1999) and in the sub-oxic to anoxic sediments in the Santa Barbara Basin (Williams and Turekian, 2004). The estimated relaxation time for the EPR sediments is 8-12 kyr (Oxburgh, 1998) and for the Cariaco Basin sediments it is 3-4 kyr (Oxburgh, 2001). Since the s SW Os thus estimated is much less than that determined from mass balance considerations, Oxburgh (2001) has argued that input fluxes of Os into the oceans have been underestimated and some sources have not been identified.…”

Dissolved osmium in Bengal plain groundwater: Implications for the marine Os budget

“…The analysis of the Os content and isotopic composition of river waters and seawater Levasseur et al, 1998Levasseur et al, , 1999 allows an Os residence time of $ 35-50 kyr to be calculated from mass balance. However, rapid variations in 187 Os/ 188 Os linked to glacial-interglacial variations, recorded by hydrogenous sediments of different oceans (Oxburgh, 1998;Williams and Turekian, 2004;Dalai et al, 2005;Oxburgh et al, 2007), indicate a much lower Os marine residence time of $8 Kyr (Oxburgh, 2001). This disagreement underscores the current gaps in our understanding of the Os marine budget.…”