Osmium isotope geochemistry of a tropical estuary

Fundamental aspects of the estuarine geochemistry of Os were investigated by analysis of the Os isotope composition and concentration variations in filtered water samples from a salinity transect taken in the estuary of the tropical Fly River of New Guinea. Os-187/Os-181 of the waters in the salinity transect increase with salinity from 0.6165 in the river endmember (0.1 parts per thousand salinity) to a maximum of 0.913 in the most saline water (33 parts per thousand). These values are distinctly lower than the Os-187/Os-188 of 1.06 reported for open ocean waters from the Indian and Pacific Oceans (Sharma et al., 1997; Levasseur et al., 1998; Woodhouse et al., 1999). The Os concentrations in the waters range from a minimum of 4.59 pg Os/g in the river to a maximum of 5.66 pg Os/kg at 12 parts per thousand salinity. These concentrations are all significantly lower than the open ocean range of 6.6 to 10.86 pg Os/kg (Levasseur et al., 1998; Woodhouse et al., 1999). The isotopic results indicate that the rivers draining the New Guinea landmass provide unradiogenic dissolved Os. to the oceans and led to a lowering of the Os-187/Os-188 of the local seawater. The Os concentration-salinity relationship indicates that the mixing process is not conservative and that removal of Os from the dissolved state has taken place. The proportion of Os lost from solution increases with salinity, resulting in removal of a large proportion of seawater Os in the estuary. High concentrations of Os in marine sediments within the Gulf of Papua were previously interpreted to be due to incorporation of seawater-derived Os into organic matter (Martin et al., 2000, Earth Planet. Sci. Lett. 183, 261-274) and are the likely complement to the processes observed in the dissolved state. Enhanced removal of seawater Os in estuaries could help to resolve the differences in estimates of Os residence time made on the basis of oceanic deposits versus those based on measured river water concentrations. Copyright (C) 2001 Elsevier Science Ltd.