THE SOUTHERN OCEAN SILICA CYCLE: KNOWNS AND UNKNOWNS

On the one hand, the Southern Ocean surface waters receive considerable amounts of silicic acid (dissolved silica - DSi) from the rest of the world's oceans through the upwelling of the Circumpolar Deep Water, fed by contributions of deep waters of the Atlantic, Indian and Pacific oceans. On the other hand, the Southern Ocean exports a considerable flux of the silicic acid that is not used by diatoms in surface waters through the northward pathways of the Subantarctic Mode Water, of the Antarctic Intermediate Water, and of the Antarctic Bottom Water; thus the Southern Ocean is a considerable source of DSi for other ocean basins. In a preliminary article Tréguer (2014) addressed the question of whether the Southern Ocean is a net importer or exporter of DSi. To further address this question, new estimates of the external silica inputs to the Southern Ocean (subglacial discharge, ice shelf melt and icebergs, aeolian and hydrothermal inputs), the long-term burial rate of opal in sediments, and the accumulation of silica by siliceous sponges are presented.