Biogenic silica recycling in surficial sediments across the Polar Front of the Southern Ocean (Indian Sector)

Abstract

The processes controlling preservation and recycling of particulate biogenic silica in superficial sediments must be understood before one uses biogenic silica as a proxy in paleooceanographic studies, and in order to compute oceanic mass balances for silica. In this respect, the Antarctic Ocean is certainly a key region due to its high productivity and export of biogenic silica. In order to quantify sedimentary fluxes and identify crucial processes that allow the preservation of biogenic silica, pore water and solid phase silica profiles were performed on sediment cores from the Southern Ocean (Indian Sector) during the ANTARES 1 cruise. In combination with solubility data reported by Van Cappellen and Qiu (1997a), a process model representing the early diagenesis of silica was developed. In this model, a dependence with depth of the kinetic constant was introduced to allow the preservation of biogenic silica in sediment porewater undersaturated with respect to that phase. Using this steady-state model, it is proposed that a proportionality of the reactivity of the biogenic silica with its settling flux is necessary to explain the observed profiles. It is then shown using this model that the preservation of biogenic silica is not a linear function of the deposited flux. Using a modified version of this model containing an explicit term of reprecipitation, we hypothesize that reprecipitation alone cannot counterbalance dissolution and that its effect is certainly related to a decrease in either surface solubility or kinetics of dissolution.