Dissolution of biogenic silica from land to ocean: Role of salinity and pH

Abstract

The dissolution rates of diatom frustules, phytoliths, two diatomaceous lake sediments, a siliceous ooze from the Southern Ocean, a diatomite deposit, and a synthetic amorphous silica were measured in flow‐through reactors supplied with either seawater or freshwater. Although the rates varied by more than one order of magnitude among the different siliceous materials, for any given solid the rate was systematically higher in seawater than freshwater, on average by a factor of five. Flow‐through reactor experiments with the diatom frustules and synthetic silica using mixtures of freshwater and seawater indicated that most of the rate increase occurred for seawater fractions between 0 and 50%. The observed rate enhancement is attributed to the higher pH of seawater and the catalytic effect of seawater cations on the hydrolysis of siloxane bonds at the silica surface. Because of their abundance in seawater, Na + and Mg 2+ are mainly responsible for the salinity‐induced rate increase. The large difference in dissolution kinetics between freshwater and seawater helps explain the very efficient recycling of biogenic silica in marine environments compared with freshwater lakes. Enhanced dissolution at the land‐ocean transition of biogenic silica produced by terrestrial plants and freshwater diatoms may represent a significant, but largely overlooked, source of nutrient silicon for estuarine and nearshore marine ecosystems.

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