LEG 113

Weddel Sea

During Leg 113, ODP investigated the climatic, glacial, and oceanographic history and biotic evolution of the Weddel Sea region in a wide range of sedimentary environments, providing a data base for evaluation of the environmental history of Antarctica and the adjacent ocean from the Early Cretaceous through Pleistocene. Nine sites were drilled during Leg 113 (Site 689 to Site 697) .

High organic content of the sediments from the Lower and Middle Cretaceous indicates deposition under oxygen-free or oxygen-deficient conditions, possibly during times of fluctuating surface- water salinity. By the late Campanian, open-ocean sediments were being deposited on Maud Rise. Diverse planktonic assemblages suggest that the surface water near Antarctica was relatively warm from the Late Cretaceous to the Eocene. East Antarctica was then a rich source of clay, dominated by smectite and kaolinite during the Paleocene, indicative of a warm climate, increasing humidity, and an unglaciated continent. Eocene clays are also dominated by smectite, signifying a continuing warm climate and a predominance of chemical over physical weathering processes. On Maud Rise, the sediments reveal a progressive transition from calcareous to siliceous microfossils, beginning at the Eocene/Oligocene boundary and becoming almost entirely siliceous by the late Miocene, suggesting that the water was probably highly unsaturated in CaCO3 and cool, even in shallow waters. Illite became important in the lower Oligocene sediments, whereas smectite began to decrease as a result of strong cooling and/or an increase in aridity in east Antarctica at that time. Ice-rafted debris indicates the presence of some ice on east Antarctica during the early Oligocene, the earliest of these fragments found at any of the Weddel Sea sites.

The west Antarctic ice sheet underwent major expansion during the middle Miocene, as indicated by an increase in ice-rafted debris in the lower/upper Miocene and the replacement of smectite by both illite and chlorite, and is interpreted to suggest physical weathering resulting from climatic cooling and that climatic cooling and glacial conditions developed much later on west Antarctica (late Miocene) than on east Antarctica (early Oligocene). A dramatic increase in sedimentation rate approximating the Miocene/Pliocene boundary results from an increased terrigenous and biosiliceous input and indicates an intensification of cooling and establishment of true polar-type glaciation and that the west Antarctic ice sheet may have become stable at this time.