We present a glacial record from the western Olympus Range, East Antarctica, that documents a permanent shift in the thermal regime of local glaciers, from wet- to cold-based regimes, more than 13.94 m.y. ago. This glacial record provides the first terrestrial evidence linking middle Miocene global climate cooling to a permanent reorganization of the Antarctic cryosphere and to subsequent growth of the polar East Antarctic Ice Sheet. The composite stratigraphic record constructed from field mapping and analyses of 281 soil excavations shows a classic wet-based till (Circe till, including an extensive melt-out facies), overlain by a weathered colluvial deposit (Electra colluvium), and then a series of stacked tills deposited from cold-based ice (Dido drift). Chronologic control comes from Ar-40/Ar-39 analyses of concentrated ash-fall deposits interbedded within glacial deposits. The shift from wet- to cold-based glaciation reflects a drop in mean annual temperature of 25-30 degrees C and is shown to precede one or more major episodes of ice-sheet expansion across the region, the youngest of which occurred between 13.62 and 12.44 Ma. One implication is that atmospheric cooling, following a relatively warm mid-Miocene climatic optimum ca. 17 to 15 Ma, may have led to, and thus triggered, maximum ice-sheet overriding.
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