Browsing by Author "Storey, B"

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    Constraints on ice volume changes of the West Antarctic Ice Sheet and Ross
    (12th International Conference on Accelerator Mass Spectrometry (AMS-12), 2012-03-22) Fink, D; Storey, B; Joy, K; Shulmeister, J
    the Last Glacial Maximum (~20 ka), marine evidence indicates that the grounding line of the West Antarctic Ice Sheet (WAIS) advanced northwards into the Ross Ice Shelf (RIS), blocking drainage of outlet glaciers through the Transantarctic Mountains (TM) resulting in significant downstream thickening of glacier profiles. The Darwin and Hatherton Glaciers in the TMs provide geological and pedological records of WAIS fluctuations that are interpreted as evidence for a LGM ice volume at least ~800 m thicker than today. Cosmogenic 10Be and 26Al exposure ages at Lake Wellman and Dubris Valley from ice-sheet contact (850 masl) to mountain peak (600 masl) show a WAIS Pleistocene ice thickness some 800 to 400 meters thicker than today. However a cluster of mid-altitude moraine boulders, previously taken to demarcate the LGM advance, have exposure ages ranging from 30 to 40 ka. This suggests that while WAIS expansion during the early Pleistocene was large, its LGM ice volume was not as large as previously estimated and little different from what is observed today (at most 50 m above current ice surface). A second site further north, Diamond Hill, lies at the confluence of the Darwin Glacier and RIS. Two transects were sampled on Diamond Hill that cover an altitude range of 1100 meters. Preliminary 10Be cosmogenic dates show a similar trend to that seen further up glacier in Lake Wellman. For the case of Diamond Hill, the WAIS was approximately 900 meters thicker than the current Rose Ice Shelf configuration at ~1.5Ma and with only minor advances in the last 10ka. As with Lake Wellman no evidence of large scale LGM advances were found. These results raise serious questions about the implications of a reduced West Antarctic ice Sheet at the LGM, and how the Antarctic ice sheets respond to global warming. Copyright (c) 2011 AMS12
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    In-situ cosmogenic exposure dating in the Meirs and Garwood valleys, Denton Hills, Antarctica
    (12th International Conference on Accelerator Mass Spectrometry (AMS-12), 2011-03-23) Joy, K; Carson, N; Fink, D; Storey, B
    It has been hypothesised that during interglacials, thinning of the Ross Ice Shelf allowed a more open water environment with increased local precipitation. This resulted in adjacent glaciers within the Transantarctic Mountains to advance during moist warmer periods, apparently out of phase with colder arid dry periods. The geomorphology of the Denton Hills in the Royal Society Range, West Antarctica, is a result of Miocene fluvial incision reworked by subsequent warm and cold-based glacial advances throughout the Quaternary. Outlet glacials, which drain ice into the Shelf, should thus show maximum extent during interstadials. To understand the chronology of late Quaternary glaciations, 15 granitic boulders from terminal moraines in the Garwood and Miers Valleys were sampled for 10Be and 26Al cosmogenic dating. Obtaining reliable exposure ages of erratics within moraines that represent timing of deposition (i.e. glacial advances) is problematic in polar regions, where glacial activity is principally controlled by ice sheet dynamics. Recycling of previously exposed debris, uncertainty in provenance of glacially transported boulders and a lack of a post-depositional hydrologic process to remove previously exposed material from a valley system, leads to ambiguities in multiple exposure ages from a single coeval glacial landform. More importantly, cold-based ice advance can leave a landform unmodified resulting in young erratics deposited on bedrock that shows weathering and/or inconsistent age-altitude relationships. Primarily, inheritance becomes a difficulty in qualifying exposure ages from polar regions. Preliminary results based on average ages indicate that glaciers in the Denton Hills advanced to their maximum position between 30-35 ka, earlier than the Antarctic LGM (18-22 ka), then retreated leaving little evidence of late interglacial or Holocene advances. However, accounting for inheritance and taking the youngest 10Be ages, advance occurs at 20-22ka during the LGM. Hence support for the out-of-phase hypothesis depends largely on the exposure age model adopted. Copyright (c) 2011 AMS12.