Marine geochemical cycles of the alkali elements and boron: The role of sediments

We have analysed the concentrations of Li, K, Rb, Cs, and B, and the isotop ic ratios of Li and B of a suite of pore fluids recovered from ODP Sites 10 37 (Leg 169; Escanaba Trough) and 1034 (Leg 169S; Saanich Inlet). In additi on, we have analysed dissolved K, Rb, and Cs concentrations for estuarine m ixing of the Ganges-Brahmaputra river system. Together, these data sets hav e been used to assess the role of sediments in the marine geochemical cycle s of the alkali elements and boron. Uptake onto clay minerals during estuarine mixing removes 20-30% of the riv erine input of dissolved Cs and Rb to the oceans. Prior to this study, the only other recognised sink of Rb and Cs was uptake during low-temperature a lteration of the oceanic crust. Even with this additional sink there is an excess of inputs over outputs in their modern oceanic mass balance. Pore fl uid data show that Li and Rb are transferred into marine sediments during e arly diagenesis. However, modeling of the Li isotope systematics of the por e fluids from Site 1037 shows that seawater Li taken up during marine sedim entation can be readily returned to solution in the presence of less hydrat ed cations, such as NH4+. This process also appears to result in high conce ntrations of pore fluid Cs (relative to local seawater) due to expulsion of adsorbed Cs from cation exchange sites. Flux calculations based on pore fluid data for a series of ODP sites indica te that early diagenesis of clay sediments removes around 8% of the modern riverine input of dissolved Li. Although NH4+-rich fluids do result in a fl ux of Cs to the oceans, on the global scale this input only augments the mo dern riverine Cs flux by similar to 3%. Nevertheless, this may have implica tions for the fate of radioactive Cs in the natural environment and waste r epositories. Copyright (C) 2000 Elsevier Science Ltd.