Kinetics of dissolution of Antarctic diatom frustules and the biogeochemical cycle of silicon in the Southern Ocean

Abstract

In order to simulate the fate of biogenic silica generated in the surface waters of the Southern Ocean, the dissolution of silica frustules was studied for seven natural assemblages of diatoms, collected during summer 1984 in the Indian sector, and two typical Antarctic diatoms (Nitzschia cylindrus and Chaetoceros deflandrei), following the procedure of Kamatani and Riley (1979). For mean summer conditions in the surface waters of the Southern Ocean (2<T°C<12; 7.5<pH<8) rate coefficients of dissolution range from 2.2 to 18.5x10-3d-1 for the natural assemblages. The silica frustules trapped by fecal pellets and by gelatinous aggregates, and rapidly transported through the cold waters of the Circumpolar Current, reach the sea bottom of either the continental shelves of the abysses without loosing much of the initial amount of silica (less than 10%). A model based on Stokes' law, modified to take in account of non ideal conditions and of the upwelling rate, is used in order to simulate the fate of silica of unaggregated particles settling down in the cold waters of the Antarctic Divergence. It supports the ideas that 1-the cycle of siliceous particles which radii are <2 μm (i.e., of a part of the nanoplankton) is completely achieved in the surface layer, 2-although the biogenic silica of large unaggregated particles (radii over 25 μm) may reach the seabottom (within one month to a few years) without complete dissolution, the main explanation for the accumulation of biogenic silica on Antarctic abysses remains transport by fecal pellets and gelatinous aggregates.