Effect of pressure on silica solubility of diatom frustules in the oceans: Results from long-term laboratory and field incubations

Abstract

The oceanic cycle of silicon (Si) has been studied extensively due to its close coupling to the oceanic carbon cycle and the biological CO2 pump. The oceanic Si cycle is dominated by the uptake of dissolved silicate (dSi) by planktonic organisms, predominantly diatoms, which use it to synthesize siliceous frustules. As oceanic waters are undersaturated with respect to biogenic silica (bSiO2) the frustules dissolve after death of the organisms, thereby regenerating dSi. Because the dissolution rate of bSiO2 depends on the degree of undersaturation, the thermodynamic solubility of bSiO2 is a key parameter controlling the recycling efficiency of nutrient Si in the water column and sediments. While an extensive body of data exists describing the dependence of bSiO2 solubility on temperature, the effect of pressure on the solubility of natural diatom frustules has never been measured directly. In this study, we conducted long-term (up to 22 months) laboratory and field equilibration experiments to determine the solubility of cleaned frustules of a cultured marine diatom (Thalassiosira punctigera) in seawater, for pressures between 1 and 700 bar, and temperatures between 2 and 21°C. According to our results, the solubility of the frustules decreases by about 10% when pressure increases from 1 to ~200 bar. From 200 bar on, the pressure dependence reverses, and at 700 bar the solubility is about 15% higher than at atmospheric pressure. Integrated over an average oceanic water depth of 4000 m, a drop in temperature of 15–20°C has a far more significant effect on the solubility of bSiO2 than a corresponding 400 bar increase in pressure.