Silica in the sea — its forms and dissolution rate

Abstract

Reactive, particulate, and total silica were investigated in seawater samples collected in the Oyashio and Kuroshio areas of the western North Pacific and Funka Bay near Hokkaido Island. The concentrations of particulate silica and suspended inorganic matter in the Japan Sea were also determined. A vertical diffusion model was first applied to the profiles of particulate silica to estimate the rate of dissolution of particulate silica in the intermediate water of the Japan Sea. The dissolution rate, which corresponds to the rate of increase of reactive silica, was estimated to be 0·04–0·4 μg-atoms Si 1 −1 γr −1, providing the vertical eddy diffusivity is 1–10 cm 2 s −1 and the settling velocity of particulate matter is (1·16–11·6) × 10 −4 cm s −1. Reactive silica is the dominant species of silica in seawater. No colloidal silica is detectable in samples taken either near shore or in open seas, in any of the whole water column from the surface to near the deep-sea bed. Particulate silica concentrations varied from 0·1 to 2·0 μM Si in the open seas and from 0·5 to 11 μM Si in Funka Bay. Higher values were observed in the surface and subsurface waters in the Oyashio area and the bottom waters in the coastal region.