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

Abstract

Os concentrations (6.6 to 9.8 pg/l) and isotopic compositions (mean 187Os/ 188Os = 1.067±0.011; 2 σ error) were measured by a new method in seawater samples collected from the eastern Pacific Ocean. In contrast to recent work in the Indian Ocean, measured Os concentrations display a 30% depletion coincident with both the core of the oxygen minimum zone, and a local minimum in light transmission. Whilst we cannot preclude the possibility that an oxidation-resistant form of Os in seawater prevented isotopic equilibration between the enriched isotope spike and naturally occurring Os, which would bias measured concentrations to low values, we interpret these data as evidence of non-conservative behaviour of Os in seawater in this zone. The data may be explained by two possible processes: the horizontal advection 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 those recently reported from the Indian Ocean, which indicates that the isotopic composition of seawater Os in these two ocean basins is indistinguishable at this level of precision. Neutrally buoyant hydrothermal plume samples collected in close proximity to active vents at 9°46′N on the East Pacific Rise have Os concentrations and isotopic compositions similar to ambient deep water. This shows that the flux of Os from high temperature hydrothermal vents is not large compared to the inventory of Os in ambient seawater. Comparisons between filtered and unfiltered samples show that the majority of seawater Os in these samples is in the dissolved phase.