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.