SILICA CYCLE IN SURFACE SEDIMENTS OF THE SOUTH ATLANTIC

Production of biogenic silica and dissolution processes in the water c olumn and surface sediment are important aspects for the investigation and reconstruction of present and past productivity of the ocean. Alt hough the geological record of biogenic silica is often used as a prox y for paleoceanographic processes in the Southern Ocean, little is kno wn about the present regional distribution of biogenic silica flux and accumulation and their relation to primary production in surface wate rs. Based on more than 130 sediment and pore water samples, the region al differences of the biogenic silica flux to the sea floor of the sou thern South Atlantic were investigated. In contrast to biogenic silica content, the dissolved Si-flux through the sediment/water interface, caused by intense dissolution of BSi in surface sediments, reflects bi ogenic production in surface waters. This was inferred by observed inc reases of Si-fluxes in regions of recurrent polynya formation or in th e vicinity of Marginal Ice Zones as at the Weddelt-Scotia Sea boundary . In the Scotia Sea, where no benthic fluxes were reported before, we found a considerable burial of biogenic silica and biogenic silica flu xes to the sea floor of similar to 800-1300 mmol m(-2) a(-1). This is a significantly higher flux than derived for the known opal accumulati on area in the SE Atlantic, further to the east in the Antarctic Circu mpolar Current, where a flux of similar to 600-767 mmol m(-2) a(-1) wa s observed. This shows that the Scotia Sea is not a gap within the Cir cumpolar Antarctic Opal Belt as previously assumed. The geochemical bu dget for different sub-regions of the South Atlantic was considered by a Geographic Information System. In contrast to most previous attempt s, this ensures the accurate consideration of the spatial distribution of sampling sites, a crucial aspect for the accuracy of geochemical b udgets. For the South Atlantic we calculated the flux of biogenic sili ca to the sea floor as similar to 5.1 x 10(12) mol a(-1). Only similar to 0.84 x 10(12) mol a(-1) is buried in these sediments, which is con siderably less than previous estimates. (C) 1998 Elsevier Science Ltd. All rights reserved.