ALBERT

Beschreibung: Middle Eocene to late Oligocene sediments recovered at Ocean Drilling Program Sites 689 and 690 on Maud Rise in the southernmost Atlantic Ocean and at Sites 738 and 744 on Kerguelen Plateau in the southernmost Indian Ocean were analysed in order to study the depositional environment and the palaeoclimate. Special emphasis was placed on the reconstruction of the formation of a cryosphere on the Antarctic continent. The investigations include quantifications of carbonate and opal contents, grain size analyses and studies of clay mineral assemblages. The sedimentary sequence at all sites under investigation is highly pelagic, with nannofossil oozes and chalks dominant. The first indication of probable glacierization at sea level is in the form of isolated gravel and terrigenous sand grains, which indicate ice-rafting from middle Eocene time at c. 45.5 Ma. This is supported by enhanced concentrations of detrital chlorite and reworked kaolinite. Probably, some glaciers reached the sea, while most of the continent remained under the influence of a humid and warm to temperate climate. The growth of the inland ice resulted in enhanced physical weathering and in increased contents of detrital chlorite and kaolinite from about 40 Ma. A strengthening of the glacial conditions and the onset of continental East Antarctic glaciation is recorded in early Oligocene sediments with an age of 36 Ma. All major sediment parameters document this event. The clay mineralogy changed between c. 36.3 Ma and c. 35.5 Ma from smectite-dominated assemblages to illite- and chlorite-dominated assemblages, the latter being indicative of physical weathering under cooler climates. Large quantities of ice-rafted gravel and sand accumulated on the Kerguelen Plateau between 36.0 Ma and 35.8 Ma. At the same time, an increase in opal content occurred as well as a decrease in carbonate. The sediment parameters imply that the East Antarctic continent was more or less totally buried beneath the ice during Oligocene time. The ice sheet, however, probably did not have a polar but a temperate character, as indicated by the occurrence of Nothofagus and relatively warm surface water temperatures prohibiting the long-distance transport of debris by icebergs.

Beschreibung: Cretaceous to Quaternary sediments recovered at Leg 119 Sites 738 and 744 on the southern tip of the Kerguelen Plateau were studied in order to determine the depositional environment and the paleoceanography of the southern Indian Ocean and especially the long-term glacial history of East Antarctica. Emphasis is given to bulk-sediment composition, grain-size data, and clay mineralogy. The sediment sequence at the two sites is generally of a highly pelagic character, with nannofossil oozes, chalks, and limestones dominant from the Turanian to upper Miocene and diatom oozes dominant within the uppermost Miocene to Holocene interval. The first indication of glaciation at sea level is the occurrence of isolated gravel and terrigenous sand grains, which indicate ice rafting in the middle Eocene interval of 45.0-42.3 Ma. A major intensification of glaciation, probably the onset of continental East Antarctic glaciation, is recorded in sediments of early Oligocene age (36.0 Ma). All major sediment parameters document this event. The clay mineralogy changes from smectite-dominated assemblages, typical of moderately warm and humid climatic conditions in which chemical weathering processes are prevalent, to illite- and chlorite-dominated assemblages, indicative of cooler climates and physical weathering. Ice-rafted debris of both gravel and sand size occurs in large quantities in that interval and coincides with a change in the mode of carbonate deposition. Carbonate contents are relatively high and uniform (90%-95%) in strata younger than early Oligocene; in Oligocene to upper Miocene strata they fluctuate between 65% and 95%. Oligocene and Neogene hiatuses reflect an intensification of oceanic circulation and the increased erosional force of Circumpolar Deep Water. The long-term Cenozoic cooling trend was interrupted by a phase of early Miocene warming indicated by maximum Neogene smectite concentrations. Although ice-rafted debris is present only in minor amounts and mainly in the silt fraction of early Oligocene to late Miocene age, it shows that glaciers advanced to the East Antarctic shoreline throughout that time. Ice-rafting activity drastically increased in latest Miocene time, when carbonate deposition decreased and diatom ooze sedimentation started. This suggests a pronounced intensification of Antarctic glaciation combined with a northward migration of the Polar Front.