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Reconstruction of Sub-Ice Volcanoes and Ice Sheet Thicknesses from Geomorphological and Lithofacies Analysis and Volatile Compositions |
1 Department of Geological Sciences, Albion College, Albion MI 49224, USA twilch{at}albion.edu
2 Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
Palaeoenvironmental reconstructions and 40Ar/39Ar geochronology of volcanism at Mt. Murphy and the Crary Mountains in eastern Marie Byrd Land (MBL), West Antarctica, provide records of changing ice levels of the West Antarctic Ice Sheet (WAIS) since the late Miocene. Interpretations of eruptive and depositional environments are based on lithofacies studies and indicate whether the volcanoes erupted below, near or above the level of the ice sheet. Seventy-seven new 40Ar/39Ar dates offer a precise chronological frame-work for the ice volcanic history.
Late Miocene (9–8 Ma) basal volcanic sequences at Mt. Murphy and the Crary Mountains (Mt. Rees and Mt. Steere) exhibit fluctuations between ‘wet’ ice-contact lithofacies and ‘dry’ subaerial lithofacies. The ‘wet’ lithofacies include pillow lava and hyaloclastite breccia; the ‘dry’ lithofacies include massive and deuterically oxidized lava and associated welded breccia deposits. The sequences at Mt. Murphy include several erosion surfaces and tillites, which are inferred to represent fluctuations in the WAIS. At Mt. Rees and Mt. Steere, the alternating lithofacies form the constructional slopes of the volcano and are inferred to represent interactions with local slope ice that occurred above the level of the regional ice sheet.
The Miocene to Pleistocene volcanic history of the area provides a proxy record of ice-level changes in West Antarctica, with the following three major conclusions. First, the oldest evidence for a large-scale WAIS is from Late Miocene (c. 9 Ma) glaciovolcanic sequences at Mt. Murphy and several other sites in Marie Byrd Land. The combined Mt. Murphy and Crary Mountains records indicate that ice-level expansions of the WAIS were more extensive at coastal sites than at inland sites. Second, the present-day WAIS appears to be in a near maximum configuration that has existed at several times since 9 Ma but was rarely exceeded. Finally, a significant expansion of the WAIS above its present-day level occurred at 590 ± 15 ka, when ice levels were 550 m higher at the coastal volcano, Mt. Murphy.
