Distribution of dissolved iron in sediment pore waters at submillimetre resolution

Abstract

MUCH effort has been directed at measuring concentration gradients at the sediment/water interface of aquatic systems, where the biogeochemical cycling of natural and pollutant species is particularly active¹. Precise measurements of oxygen gradients using microelectrodes² and estimates from independently determined fluxes³ suggest that concentration gradients in this region often extend only to depths of ˜1 mm, much less than the resolution (˜1 centimetre) of conventional techniques^4–7. We have developed a new method for measuring pore-water composition in which diffusive equilibrium is established rapidly (within minutes) in a thin film of gel inserted in the sediment. On removal, the dissolved components are fixed, allowing chemical measurements to be made at high spatial resolution (<1 mm) on a stable solid phase. Using MeV-proton-induced X-ray emission (PIXE) to analyse the dried gel, we have measured iron concentrations in lacustrine pore waters at submillimetre resolution, revealing steep concentration gradients and sub-surface maxima consistent with a hypothesis of localized, reductive dissolution of fresh material.