Osmium in seawater: vertical profiles of concentration and isotopic composition in the eastern Pacific Ocean

Os concentrations (6.6 to 9.8 pg/l) and isotopic compositions (mean Os-187/ Os-188 = 1.067 +/- 0.011; 2 sigma error) were measured by a new method in s eawater samples collected from the eastern Pacific Ocean. In contrast to re cent work in the Indian Ocean, measured Os concentrations display a 30% dep letion coincident with both the core of the oxygen minimum zone, and a loca l minimum in light transmission. Whilst we cannot preclude the possibility that an oxidation-resistant form of Os in seawater prevented isotopic equil ibration between the enriched isotope spike and naturally occurring Os, whi ch would bias measured concentrations to low values, we interpret these dat a as evidence of non-conservative behaviour of Os in seawater in this zone, The data may be explained by two possible processes: the horizontal advect ion of a water mass of low Os concentration from the continental slope and/ or the in-situ adsorption of Os onto sinking particles. To what extent each process is contributing to the Os minimum is unclear from the data. The Os isotopic composition of these Eastern Pacific samples are within 2% of tho se recently reported from the Indian Ocean, which indicates that the isotop ic composition of seawater Os in these two ocean basins is indistinguishabl e at this level of precision. Neutrally buoyant hydrothermal plume samples collected in close proximity to active vents at 9 degrees 46'N on the East Pacific Rise have Os concentrations and isotopic compositions similar to am bient deep water. This shows that the flux of Os from high temperature hydr othermal vents is not large compared to the inventory of Os in ambient seaw ater. Comparisons between filtered and unfiltered samples show that the maj ority of seawater Os in these samples is in the dissolved phase. (C) 1999 E lsevier Science B.V, All rights reserved.