Reactive surface area control of the dissolution kinetics of biogenic silica in deep-sea sediments

Flow-through dissolution experiments demonstrate that the specific reactive surface area of biogenic silica decreases significantly with depth in surficial sediments of the Southern Ocean. Specific surface areas obtained from nitrogen gas BET adsorption isotherms, however, do not exhibit the systematic changes of the specific reactive surface area inferred from the kinetic experiments. Furthermore, the decrease in surface reactivity of the siliceous skeleton fragments continues at depths in the sediments where there is no longer a net release of silicic acid to the pore waters. Thus, the specific reactive surface changes most likely reflect a progressive reduction of the surface density of reactive sites (aging) during burial, rather than the preferential dissolution of a more reactive fraction of siliceous debris deposited from the water column. The observed aging of the silica surfaces has important consequences for the benthic regeneration of nutrient silicic acid from deep-sea sediments.