Collaborative Research: Refining a 500-kyr Climate Record From the Mt. Moulton Blue Ice Field in West Antarctica

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 than 117 ka at Mt. Waesche.