Biogenic silica dissolution in sediments of the Southern Ocean. I. Solubility

A stirred flow-through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the Indian sector of the Southern Ocean (ANTARES I cruise). The results show that the apparent silica solubility in the cores may decrease, increase or remain constant with depth. The silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material. By combining the solubility data with measured dissolved silica profiles, it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility. In sediments that experience a significant detrital input, the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica. The principal oceanographic control on pore water silica build-up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material. Solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels.