Osmium isotope geochemistry of a tropical estuary

Fundamental aspects of the estuarine geochemistry of Os were investigated b y analysis of the Os isotope composition and concentration variations in fi ltered water samples from a salinity transect taken in the estuary of the t ropical Fly River of New Guinea. Os-187/Os-181 of the waters in the salinit y transect increase with salinity from 0.6165 in the river endmember (0.1 p arts 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 O ceans (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 salinit y. These concentrations are all significantly lower than the open ocean ran ge 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 landm ass 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 r elationship indicates that the mixing process is not conservative and that removal of Os from the dissolved state has taken place. The proportion of O s lost from solution increases with salinity, resulting in removal of a lar ge proportion of seawater Os in the estuary. High concentrations of Os in m arine 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 comple ment 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 ba sed on measured river water concentrations. Copyright (C) 2001 Elsevier Sci ence Ltd.