CYCLING OF ORGANIC-CARBON AND BIOGENIC SILICA IN THE SOUTHERN-OCEAN -ESTIMATES OF WATER-COLUMN AND SEDIMENTARY FLUXES ON THE ROSS SEA CONTINENTAL-SHELF

We examined the cycling of organic carbon and biogenic silica in the w ater column and upper sediments of the Ross Sea, seeking to understand the processes leading to the formation of opal-rich, organic-poor sed iments over much of the Southern Ocean. Between January, 1990 and Dece mber, 1994 we conducted three cruises, performing tracer incubation st udies (C-14, N-15, Si-30, Si-32) to measure rates of primary productio n, nitrate-based ''new'' production, biogenic silica production and bi ogenic silica dissolution in the upper 50 m over most of the Ross Sea shelf in spring, mid summer and late summer. We deployed sediment trap s from January, 1990 to early March, 1992 to measure the mid-water (25 0 m) and near-bottom gravitational fluxes of particulate organic carbo n, nitrogen and biogenic silica year-round at three sites, and obtaine d sediment cores at 15 sites to assess the accumulation rates of organ ic carbon and biogenic silica in all known sediment regimes on the she lf. At 9 of those sites we also measured nutrient efflux from the sedi ments, enabling us to calculate benthic recycling fluxes of organic ma tter and opal. These data permit estimates of the annual production, n ear-surface recycling, vertical sinking flux, delivery to the seabed, benthic regeneration and long-term burial of both organic and siliceou s material, integrated over a 3.3 x 10(5) km(2) area that covers 75 - 80% of the Ross Sea shelf. The resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column, as well as i n the upper seabed. Selective preservation of silica within the upper 50 m is not indicated, and both organic matter and silica are transpor ted from 250 m to the sea floor with virtually 100% efficiency. The Si O2/C mass ratios for surface-layer production, 250-m sinking flux, del ivery to the seabed and long-term burial are approximately 0.85, 6.1, 6.2 and 27, respectively. This progressive enrichment in silica result s in long-term burial of 5.8% of the biogenic silica and 0.17% of the organic carbon produced by phytoplankton in the surface layer, a facto r of 30 greater preservation efficiency for silica than for carbon. Ne vertheless, the ratio of opal burial to opal production in the Ross Se a is only about twice the apparent global average of 3% and < 1/3 of t he estimated burial/production ratio for the Southern Ocean as a whole . It thus appears that both silica preservation and the decoupling bet ween the cycles of silica and carbon must be even more effective in th e waters overlying abyssal Southern Ocean sediments than they are over the Ross Sea shelf.