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 bee n deduced from a coarse fraction and clay mineral analysis of ODP Site 689 (Leg 113) sediments. In the middle Eocene, smectite-rich sediment s were a consequence of warm climate with alternating wet and dry seas ons on the Antarctic margins. Productivity in the Maud Rise area of th e Southern Ocean was low: no opal was deposited, preservation of carbo nate is very good. Carbonate preservation shows cyclic changes. Clinop tilolite 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 d etrital kaolinite indicates a change in oceanic circulation. In the la te 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 le ss smectite than warmer ones. The early Oligocene was a period of very high productivity. Highest opal accumulation rates correspond to stro ngest carbonate dissolution. Cyclic changes in productivity-with a mea n cycle duration of 0.4-0.5 Myr-were associated with cyclic changes in clay mineral composition. Highest productivity intervals are generall y those with highest smectite content, because of strong erosion of se diments of the Antarctic margins. Sediments from less productive warme r periods most of the time contain more illite. In the late Oligocene, the intensity of physical weathering on Antarctica was stronger, as d ocumented by increasing fluxes of illite, muscovite, amphiboles and ta lc. Ice expanded on the East-Antarctic continent. Lower productivity t han in the early Oligocene is indicated by lower accumulation rates of opal and weaker carbonate dissolution. Cyclic variations in productiv ity correlate to cyclic variations in illite and smectite contents as in the early Oligocene.