BIOGENIC SILICA DISSOLUTION IN SEDIMENTS OF THE SOUTHERN-OCEAN .2. KINETICS

The dissolution kinetics of biogenic silica in surface sediments colle cted during the ANTARES I cruise were measured in stirred flow-through reactors. The rate data exhibit a distinctly non-linear dependence on the degree of undersaturation. Near equilibrium, the rates of silica dissolution and precipitation define a single linear trend, i.e. the k inetics are symmetric about the equilibrium point. When the dissolved silica concentration drops below a critical level, however, the dissol ution rate rises exponentially with increasing undersaturation. Hence, the data disagree with the linear rate law generally used to describe the dissolution kinetics of biogenic silica. It is hypothesized that the kinetic transition from the linear to the exponential regime repre sents the onset of localized dissolution centered on surface defects, e.g. small pores and crevices, or compositional defects. The effects o f temperature and pH confirm that the critical process controlling the overall dissolution kinetics is the hydrolysis of bridging Si-O-Si bo nds at the solid-solution interface. The rate measurements indicate th at the reactivity of biogenic silica decreases substantially with dept h in the sediment. The decrease in reactivity is explained by a progre ssive reduction of the defect density of the silica surfaces, through dissolution and reprecipitation of silica. It does not appear to resul t from the preferential dissolution of a more reactive fraction of bio genic debris deposited from the water column. Surface areas obtained b y the N-2-BET method or concentrations of extractable biogenic silica do not provide satisfactory proxies for the reactive surface area of s ilica in the sediments. However, a positive correlation was observed b etween the surface reactivity and the exchangeable Co2+ adsorption cap acity of biogenic silica. Specific kinetic effects on silica dissoluti on of the aluminum content of the silica surfaces or organic matter co atings were not observed. Both the non-linear dissolution kinetics and the aging of the silica surfaces help restrict the dissolution of dep osited biogenic silica to a narrow zone close to the water-sediment in terface. The results of the flow-through experiments highlight the imp ortance of in situ early diagenetic processes in controlling the behav ior and fate of deposited biogenic silica: no evidence was found suppo rting a significant effect of differences in solubility or reactivity inherited from the biomineralization process in the water column. (C) 1997 Elsevier Science Ltd.