HR: 0800h
AN: PP51D-1351    [Abstracts]
TI: Radiolarian and Sedimentologic Evidence for Late Eocene Origin of Southern Ocean Environments
AU: * Lazarus, D B
EM: david.lazarus@rz.hu-berlin.de
AF: Museum fuer Naturkunde, Invalidenstrasse 43, Berlin, 10115 Germany
AU: Hollis, C J
EM: c.hollis@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, 69 Gracefield Rd, PO Box 30-368, Lower Hutt, 6315 New Zealand
AU: Apel, M
EM: marcus.apel@museum-hu.berlin.de
AF: Museum fuer Naturkunde, Invalidenstrasse 43, Berlin, 10115 Germany
AB: Benthic foram $\delta$$^{18}$O and Mg/Ca records indicate that the deep ocean cooled by 5-$10\deg$C between the early Eocene and earliest Oligocene, although the timing, magnitude and dominant style of change (continuous vs. steps) is still debated. The largest change occurs at the Eocene/Oligocene boundary (33.5 Ma), associated with the first major Antarctic glaciation. Current debate concerns whether this glaciation was a threshold event related to long-term decline in CO$_{2}$ or was caused by opening of the Tasmanian Gateway, with thermal isolation of Antarctica and development of circumpolar circulation. The Southern Ocean, characterised by circumpolar current flows and bounded by the Polar Front, has contained since the mid Paleogene a distinct Antarctic radiolarian biota consisting of endemic forms and bipolar high-latitude taxa adapted to seasonal high productivity environments. It is also characterised by common biogenic silica in pelagic sediments. Past changes in the abundance and geographic distribution of these faunas and sediments provide qualitative proxies for the origin and development of this environment. We examine trends in radiolarian evolution, biogeography and biosiliceous facies development from the mid-Eocene to early Oligocene. We calculate diversity, speciation and extinction rates; track the origin and dispersal patterns of Antarctic radiolarian biota (percent Antarctic species at each site); and map sedimentary facies distribution changes through a series of time slices from the late Middle Eocene to the middle Early Oligocene. Data comes from numerous DSDP and ODP sites drawn from the Pacific, Indian and Atlantic sectors of the Southern Ocean. Results show maxima in rates of evolutionary change, increases in Antarctic species, and appearance of biosiliceous components in sediments in the late Middle to middle Late Eocene interval. There is little evidence for major change in, or expansion of the Antarctic radiolarian fauna; or increase of biosiliceous sediment at the time of the opening of the Tasmanian Gateway and Eocene-Oligocene glaciation. We conclude that the Antarctic environment largely developed prior to the Eocene-Oligocene boundary.
DE: 9310 Antarctica
DE: 9604 Cenozoic
DE: 4855 Plankton
DE: 4863 Sedimentation
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting