Volcanic record of the West Antarctic Ice Sheet
There is intense scientific interest in the history, dynamics and future of the West Antarctic Ice Sheet because it is the world's only remaining marine ice sheet and is considered by many to be inherently unstable and prone to catastrophic collapse and melting (e.g. Hollin, 1962; Hughes, 1973; Mercer, 1978; MacAyeal, 1992; Binschadler, 1995). If the WAIS were to collapse, global sea level would rise by 6 meters. The stability of the WAIS is a function of some balance between external sea level and climate and internal controls of marine ice-sheet and ice-stream dynamics (Alley and Whillans, 1991; MacAyeal, 1992; Bentley, 1997). Ultimately, accurate predictions of future ice-sheet stability and behavior will require numerical models, based on an understanding of the physical dynamics of the present ice sheet and tested by geological constraints on past ice-sheet configurations.
One approach to providing geological constraints on the age of the West Antarctic Ice Sheet is through direct dating and geochemical correlation of englacial tephra layers within the ice sheet. The stratigraphy and morphology of many West Antarctic englacial tephra layers suggests that the layers were deposited on snow at the time of the volcanic eruption, and were incorporated into the ice with little reworking or mixing, indicating that the age of the tephra layer is truly representative of the age of the surrounding ice.
Several West Antarctic englacial tephra site exist. Mt. Waesche is located at between 1900-2000 meters elevation in a blue ice field on the south side of the volcano, and displays a complex, deformed stratigraphy. A second important site, Mt. Moulton, is located at 2800 meters elevation of the summit ridge of the extinct chain or The tephra layers at Mt. Waesche are dominantly either coarse and basanitic, probably of local derivation, or fine and trachytic, mainly derived from West Antarctic volcanoes Mt. Berlin or Mt. Takahe. Several new, locally-derived englacial tephra layers were found at Mt. Takahe during the 1998/99 field season. Although displaying boudin-shaped features, the thicker tephra layers at Mt. Waesche appear relatively undeformed, whereas some of the thinner, intercalated layers are strongly isoclinally folded. Three tephra layers at Mt. Waesche have been geochemically correlated with layers found at Mt. Moulton. The correlative layers have ages of between 15 and 27 ka, 27±2 ka, and between 106 and 119 ka, consistent with apparent stratigraphic order. A single tephra layer at Mt. Waesche has been directly dated using 40Ar/39Ar, and yields an apparent age of 117±7 ka. Although there are tephra layers that seem stratigraphically below the 117±7 ka layer, they appear to represent an overturned repetition of some part of the upper section. Hence, there is no evidence for ice significantly older
More information about the Mt. Moulton site is available on a separate page.
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