PALEOCEANOGRAPHIC AND PALEOCLIMATIC EVOLUTION IN THE WEDDELL SEA (ANTARCTICA) DURING THE MIDDLE EOCENE-LATE OLIGOCENE, FROM A COARSE SEDIMENT FRACTION AND CLAY MINERAL DATA (ODP SITE 689)

Paleoceanographic and paleoclimatic variations from the middle Eocene to the late Oligocene in the Antarctic area, Atlantic sector, have been deduced from a coarse fraction and clay mineral analysis of ODP Site 689 (Leg 113) sediments. In the middle Eocene, smectite-rich sediments were a consequence of warm climate with alternating wet and dry seasons on the Antarctic margins. Productivity in the Maud Rise area of the Southern Ocean was low: no opal was deposited, preservation of carbonate is very good. Carbonate preservation shows cyclic changes. Clinoptilolite is a common mineral. At the middle/late Eocene boundary (40.8 Ma), productivity suddenly increased enough to allow preservation of opal skeletons. Clinoptilolite disappeared where opal appeared. Illite appearance points to cooling of Antarctic climates. The presence of detrital kaolinite indicates a change in oceanic circulation. In the late Eocene, cyclic changes in productivity (cycle length 0.42 Ma) were associated with clay mineral compositional variations: sediments from colder, highly productive periods generally contain more illite and less smectite than warmer ones. The early Oligocene was a period of very high productivity. Highest opal accumulation rates correspond to strongest carbonate dissolution. Cyclic changes in productivity-with a mean cycle duration of 0.4-0.5 Myr-were associated with cyclic changes in clay mineral composition. Highest productivity intervals are generally those with highest smectite content, because of strong erosion of sediments of the Antarctic margins. Sediments from less productive warmer periods most of the time contain more illite. In the late Oligocene, the intensity of physical weathering on Antarctica was stronger, as documented by increasing fluxes of illite, muscovite, amphiboles and talc. Ice expanded on the East-Antarctic continent. Lower productivity than in the early Oligocene is indicated by lower accumulation rates of opal and weaker carbonate dissolution. Cyclic variations in productivity correlate to cyclic variations in illite and smectite contents as in the early Oligocene.