Weddell sea palaeoceanography: Preliminary results of ODP Leg 113
ODP Leg 113 drilled 22 holes at 9 sites in the Weddell Sea, to investigate the Cenozoic development of the modern cold, circum-Antarctic water mass structure and the continental glaciation. The sites sampled 4 contrasting environments: open-ocean pelagic sedimentation on Maud Rise (Sites 689 and 690), the East Antarctic continental margin (691–693), the deep Weddell basin (694) and the southeastern margin of the South Orkney microcontinent (695–697, a depth transect). Considerable work remains to be done on the cores, but preliminary results reported here are of great interest. The Maud Rise sites together provide a near-continuous section from the Late Cretaceous to the present day, which will form a southernmost anchor for Atlantic biostratigraphy, biogeography and isotopic studies. The sequence is calcareous through the Eocene, mixed calcareous and siliceous into the late Miocene and almost entirely siliceous thereafter. Apparently continuous K/T boundary sections were cored at each site. The Paleocene section at Site 690 contains eolian sediment from East Antarctica which suggests a warm, semi-arid continental climate at that time. At the East Antarctic margin, Lower Cretaceous organic-rich sediments below a 60-Ma hiatus indicate restricted circulation. Overlying Oligocene to Recent, mixed hemipelagic terrigenous and biosiliceous sediments suggest a pronounced middle Miocene increase in intensity of East Antarctic glaciation, with increased ice-rafting and hemipelagic deposition, and canyon-cutting on the margin. A rapid influx of terrigenous turbidites to the Weddell abyssal plain at Site 694 in the earliest Pliocene is considered to mark the growth of a West Antarctic ice sheet, subsequently stable. West Antarctic climatic history is also revealed at Sites 695–697: Nothofagus pollen and fern spores from the Eocene of Site 696 suggest a climate similar to present-day New Zealand. Overlying sediments indicate a cooler climate, with an almost completely siliceous biota, even at palaeodepths of 650 m or less, from the middle Miocene. Pliocene biosiliceous and ice-rafted terrigenous deposition was extremely rapid (reaching 200 m/Ma at times) at the South Orkney sites. After 2.4 Ma however, at all Leg 113 sites but 697, the deposition of biosiliceous and ice-rafted sediment decreased markedly, probably reflecting greater sea ice cover and grounding of the East Antarctic ice sheet to the shelf edge even during interglacial periods. An interesting exception to the siliceous domination of biogenic sedimentation through the Neogene is the occurrence at all Leg 113 sites above 3400 m of a calcareous horizon, containing Neogloboquadrina pachyderma and a restricted benthic foraminiferal assemblage, within the Quaternary.
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