Osmium in seawater : analysis and geochemistry

A method for the direct analysis of seawater osmium has been developed and is described in this thesis. Low and constant blanks are obtained (0.12 pg osmium with 187Os/186Os=2.6) and spike-sample equilibration is achieved using this method.A 191Os radiotracer was used to develop the method. The storage of samples and the distillation and microdistillation used for the isolation of the osmium were examined. It was demonstrated that seawater should be stored in either glass or Teflon bottles and acidified to pH2 using hydrochloric acid.Seawater was collected from the eastern Pacific Ocean and osmium concentrations (6.6 to 9.8 pg/L) and isotopic ratios (187Os/186Os = 8.7 to 9.2) were measured using the method. The vertical profile shows that osmium concentration is not constant through the water column in this ocean region. A minimum at around 500 m, coinciding with a minimum of both dissolved oxygen and light transmission, can be explained by two processes: the horizontal advection of a water mass low in osmium from the continental slope and the in-situ adsorption of osmium onto sinking particles. Whilst the concentration and isotopic composition values are similar to those measured in the Indian Ocean, the data from the Pacific Ocean shows reduced concentrations under anoxic conditions and presents an exception to the conservative behaviour of osmium. The seawater samples that were collected from close to the hydrothermal vents at 9°N of the East Pacific Rise, were not concentrated in osmium compared to deep seawater. Comparisons between filtered and unfiltered samples show that the majority of seawater osmium is in the dissolved phase.The results suggest that osmium exists in several oxidation states and chemical speciations in seawater, and that anoxic conditions may reduce the osmium to a particle-reactive species. A new calculation of the osmium oceanic residence time (34,800 years) indicates that osmium should be uniformly distributed throughout the oceans.