Insights into the behavior of osmium in the oceans

A thorough understanding of Os behavior in the oceans is critical for successful interpretation of the existing marine record and application of Os as a valuable, versatile, and sensitive environmental tracer. However, the existing literature on seawater Os distribution and behavior is limited and contradictory. This is a consequence of a) differences in the analytical techniques (Chen & Sharma, 2009 Anal. Chem.) and b) the common practice of storing acidified seawater in acid washed polyethylene bottles, which could contribute non radiogenic Os and/or adsorb Os from samples (Sharma et al., 2012 L&O: Methods). Here we sought to establish the concentration and isotope composition profiles of osmium in filtered and unfiltered sample-pairs from the US Geotraces missions in the North Atlantic Ocean and to draw from those profiles insights into the factors influencing marine Os concentration and isotope composition. Initial data were obtained from Geotraces Station No 12 (17.3983^o N, 24.4998^o W), which receives dust inputs from Sahara and is located in the highly productive Mauritanian Upwelling Zone. Although the water mass structure of the study area is quite complex, the presence of an NACW/SACW mixed upper layer, AAIW intermediate layer, and NADW deep layer was established using salinity-potential temperature relationship. These properties in the deepest sample (d =3500 m) also suggest mixing with AABW, which has not been present at such a shallow depth. The Os concentration (=[Os]) in the unfiltered samples increases from 9.3 pg/kg at the surface to 10.6 pg/kg at 800 m and then decreases to 9.6 pg/kg at 3500 m. There is a one-to-one correspondence between the [Os] from unfiltered and filtered samples with the [Os] of the unfiltered samples being higher than those of the filtered samples. The Os concentration profiles a) do not show any deficit in the oxygen minimum zone and b) correlate with nitrate concentration, illustrating a nutrient-like distribution (i.e., uptake from surface waters and regeneration at depth). Since low concentration of Os precludes a biological role it is likely that Os is absorbed/adsorbed onto sinking particulate matter. The ¹⁸⁷Os/¹⁸⁸Os ratio of unfiltered samples is 0.96 at the surface and ranges from 0.98 to1.048 at depth. In comparison, the filtered samples, in general, appear to have a relatively uniform Os isotope composition (average = 1.035 ± 1.6%, 2σ ). The isotope data suggest the following: a) some Os at the surface is likely contributed by a relatively unradiogenic mineral dust from Sahara; b) although the ¹⁸⁷Os/¹⁸⁸Os ratios of NADW at 2000 m from BATS and Stn 12 are identical (~1.045), the deepest sample from Stn 12 (depth =3500 m) has a ¹⁸⁷Os/¹⁸⁸Os ratio of 1.02 and is thus significantly less radiogenic than NADW. This sample suggests a distinctly lower Os isotope composition for the AABW; more analyses are underway to secure this observation. The estimated mean residence time of osmium in the oceans is 37 ± 14 kyr, which is much longer than ocean mixing time. The observed isotope disequilibrium between the surface and deep waters thus suggests a rather sluggish kinetics of isotope homogenization.