Processes controlling solubility of biogenic silica and pore water build-up of silicic acid in marine sediments

Dissolution experiments in batch and flow-through reactors were combined wi th data on sediment composition and pore water silicic acid profiles to ide ntify processes controlling the solubility of biogenic silica and the build -up of silicic acid in marine sediments. The variability of experimentally determined biogenic silica solubilities is due, in part, to variations in s pecific surface area and Al content of biosiliceous materials. Preferential dissolution of delicate: skeletal structures and frustules with high surfa ce areas leads to a progressive decrease of the specific surface area. This may cause a reduction of the solubility of deposited biosiliceous debris b y 10-15%, relative to fresh planktonic assemblages. Dissolution of lithogen ic (detrital) minerals in sediments releases dissolved aluminum to the pore waters. This aluminum becomes structurally incorporated into deposited bio genic silica, further decreasing its solubility. Compared to Al-free biogen ic silica, the solubility of diatom frustules is lowered by as much as 25% when one out of every 70 Si atoms is substituted by an Al(III) ion. The build-up of silicic acid in pore waters of sediments with variable prop ortions of detrital matter and biogenic silica was simulated in batch exper iments using kaolinite and basalt as model detrital constituents. The stead y-state silicic acid concentrations measured in the experiments decreased w ith increasing detrital-to-opal ratios of the mixtures. This trend is simil ar to the observed inverse relationship between asymptotic pore water silic ic acid concentrations and detrital-to-opal ratios in Southern Ocean sedime nts. Flow-through reactor experiments further showed that in detrital-rich sediments, precipitation of authigenic alumine-silicates may prevent the po re waters from reaching equilibrium with the dissolving biogenic silica. Th is agrees with data from Southern Ocean sediments where, at sites containin g more than 30 wt.% detrital material, the pore waters remain undersaturate d with respect to the experimentally determined in situ solubility of bioge nic silica. The results of the study show that interactions between deposited biogenic silica and detrital material cause large variations in the asymptotic silic ic acid concentration of marine sediments. The production of AI(III) by the dissolution of detrital minerals affects the build-up of silicic acid by r educing the apparent silica solubility and dissolution kinetics of biosilic eous materials, and by inducing precipitation of authigenic alumino-silicat e minerals. (C) 2001 Elsevier Science B.V. All rights reserved.