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au=marchant

HR: 10:50h
AN: C42A-03
TI: Major Mid-Miocene Climate Change In The Transantarctic Mountains
AU: * Ashworth, A C
EM: allan.ashworth@ndsu.edu
AF: North Dakota State University, Department of Geosciences, Fargo, ND 58105, United States
AU: Lewis, A R
EM: adam.r.lewis.1@ndsu.edu
AF: North Dakota State University, Department of Geosciences, Fargo, ND 58105, United States
AU: Marchant, D R
EM: marchant@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215, United States
AB: Independent lines of evidence from paleoecology, glacial geology and marine isotopes indicate major climate change in the Dry Valley sector of the Transantarctic Mountains (TAM) at c.14 Ma. A fossil assemblage of pollen and spores, freshwater diatoms, ostracods, mosses, and insect remains has been recovered from lacustrine sediments preserved in a small morainic lake basin in the western Olympus Range. The diatom assemblage indicates that the lake existed for >10³yr and was ice-free during summers. Based on the moss and insect fossils the minimum mean summer temperature (MST- Dec-Feb) was 2°C but could have been as high as 5°C. Today at the site the MST is c. -15°C. The lake-marginal vegetation was a sparse tundra dominated by mosses and liverworts. Based on pollen, Nothofagus (southern beech) was part of the lowland regional vegetation and individual dwarfed shrubs may have grown on the slopes surrounding the lake basin. The age of the deposits is well-constrained by an ⁴⁰Ar/³⁹Ar age of 14.11 ± 0.11 Ma from an in situ volcanic ash within related lacustrine sediments. Based on an independent study of the glacial stratigraphy of the western Wright and McKelvey valleys, diamictites of a wet-based glacial regime had been replaced by those of cold-based regime by 13.85 ± 0.03 Ma. The drop in temperatures and the cessation of meltwater at c. 14 Ma would have caused the regional extinction of all plant and insect life with the exception of the hardiest of soil-dwelling organisms. Paleobotanical evidence indicates that Antarctica had likely been vegetated throughout the Cenozoic, with forests replaced by tundra during the early Oligocene. The mid-Miocene extinction marks the end of tundra in the interior of Antarctica and its replacement by the polar desert biota which exists today. Changes in δ¹⁸O and Mg/Ca ratios from different sectors of the Southern Ocean indicate sea surface temperature cooling and ice sheet growth between 13.8 - 14.2 Ma. The close correlation of events in these marine records with the glaciological and biological records from the Dry Valleys strongly suggests that they are part of a major hemispheric or global climatic event.
DE: 0726 Ice sheets
DE: 3030 Micropaleontology (0459, 4944)
DE: 4207 Arctic and Antarctic oceanography (9310, 9315)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4950 Paleoecology
SC: Cryosphere [C]
MN: 2007 Fall Meeting

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