HR: 13:55h
AN: PP53C-02    [Abstracts]
TI: The Case for a Late Oligocene-Early Miocene Ice Sheet in Marie Byrd Land
AU: * LeMasurier, W E
EM: wesley.lemasurier@colorado.edu
AF: Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309 United States
AU: Rocchi, S
EM: rocchi@dst.unipi.it
AF: Dipartimento di Scienze della Terra, University of Pisa, Pisa, I-56126 Italy
AB: A rough estimate of Neogene climate history can be inferred for coastal Marie Byrd Land (MBL) from the record contained in volcanic deposits, and from anomalies in the erosion characteristics of Cenozoic volcanic and plutonic features. As a prelude to the Neogene, a 34 Ma gabbroic pluton in eastern MBL was exhumed by removal of at least 4 km of overburden, apparently during a period of rapid uplift and erosion between 34 and 25 Ma. The depth of erosion represented by the exposure of this pluton contrasts sharply with the generally minimal erosion that seems to have characterized the MBL environment throughout much of the Neogene. Thus, the Neogene seems to have been ushered in by a pronounced decrease in erosion rate that has climatic implications. Late Oligocene hydrovolcanic deposits at the summit of Mt. Petras, on the crest of the MBL dome, seem to record the presence of glacial ice at the time they were erupted (25-29 Ma). However, uplift of the dome did not begin until the early Neogene, and the landscape 25 m.y. ago was probably one of very low relief, represented by the West Antarctic erosion surface. The inferred glacial ice was therefore most likely an ice sheet, rather than a mountain glacier. Although tentative, and contrary to interpretations of the deep sea proxy record, this suggestion of mid-Tertiary glaciation in West Antarctica is supported by two other lines of evidence. (1) The deep dissection of Mt. Petras (Oligocene and older rocks) by glacial cirques is anomalous when compared with nearby mid-Miocene (12-14 Ma) Mt. Hampton volcano, which is virtually untouched by erosion. Our analysis of this anomaly suggests that the Petras cirques were cut mainly before Mt. Hampton formed, i.e. between 14 Ma and 25 Ma, as the Petras fault block was rising. (2) Further support can be found in recently reported seismic-stratigraphic evidence for grounding of an expanded West Antarctic ice sheet in the Ross Sea around 15-17 Ma. Although the MBL data are, individually, quite fragmentary and inconclusive, the consistency among disparate sources is considerably more compelling than the story from any single source. Offshore drilling in the Amundsen Sea seems the most likely way to resolve the questions raised by these field observations.
UR: http://www.instaar.colorado.edu
DE: 9310 Antarctica
DE: 5416 Glaciation
DE: 5480 Volcanism (8450)
DE: 1600 GLOBAL CHANGE (New category)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting