Cenozoic climate and sea level history from glacimarine strata off the Victoria Land coast, Cape Roberts Project, Antarctica

摘要

This paper reviews the record of past climate and sea level from 34 to 17 Ma provided by con-tinuous core through 1500 m of shallow marine strata off the Victoria Land coast of Antarctica. Thesite was selected because it is close to the edge of the East Antarctic Ice Sheet. Previous drillingand seismic surveys had suggested a thick seaward-dipping sequence of Oligocene and older ageclose to the coast. However, the floor of the basin, lower Devonian sandstone, was encounteredbeneath uppermost Eocene conglomerate. The strata were deposited in a rift basin just seawardof the Transantarctic Mountains c. 20 m.y. after uplift began. Sediment was delivered by rivers andglaciers both from and behind the mountains from the East Antarctic interior. Sediment accumula-tion was rapid and the record more complete for the period 34 to 31 m.y., but then slowed as basinsubsidence declined, leaving major time gaps, but still a representative record of the entire timespan. Basin filling kept pace with subsidence. The sedimentary facies - conglomerate, sandstoneand mudstone with marine fossils throughout - are typical of the coastal margin of a subsidingsedimentary basin, with the addition of diamictite beds in the upper 900 m recording marine-terminating glaciers that extended periodically beyond the coast. Deposition was characterised byrepetitive vertical facies successions of conglomerate and fine sandstone in the lower part of thesection and by cyclic facies successions of diamictite, sandstone and mudstone from a few to over60 m thick in the upper part. These are thought to reflect glacio-eustatic changes in sea level ona wave-dominated coast in concert with advance and retreat of piedmont ice onto the continentalshelf, with diamicton and sand (nearshore) grading upwards to mud (shelf) and then to sand (innershelf toshoreline). Tephra dating of two cycles at 23.98 and 24.22 Ma allows their correlation with40,000 years cycles in the deep-sea isotope record, and ascribed to eustatic sea-level changes of30-60 m. This suggests that the cyclicity of the Cape Roberts section reflects the influence ofthe earth's varying orbital parameters on climate and sea level, with over 50 sedimentary cyclespreserved out of the possible 200-400 cycles during this time period. The Cape Roberts sectionalso records a dramatic increase in glacial influence at around 33 Ma, with the proportion of glacialfacies ranging from 10 to 30% of the sedimentary section through to 17 Ma. The section alsorecords a progressive shift from chemical to physical weathering (decline in CIA index and increasein % illite-chlorite) from 33 to 25 Ma, and a decline in marine palynomorphs characteristic of freshmelt water mingling over the same interval. The terrestrial pollen record records interglacialclimate and indicates this to be cool temperate for the entire period from 34 to 17 Ma thoughslightly cooler from c. 25 Ma.