Volcanic rocks, glaciogenic sediments, and recycled marine microfossils at Mount Murphy provide evidence for large-scale fluctuations in the mass of the West Antarctic ice sheet during Neogene time. Mount Murphy is a large shield volcano with an atypical structure. Its basal unit consists of alternating subaquatic and subaerial rock types through a stratigraphic interval of at least 300 m, suggesting that ice level changed several times during shield building. Much larger changes are suggested by an outcrop of glacial lake sediment near the volcano summit, 1,300 m above present ice level. It contains an assemblage of recycled marine microfossils derived from marine basins in the interior of West Antarctica. They suggest multiple intervals of near-complete deglaciation in West Antarctica between about 24 and 3.5 Ma. The microfossils were probably deposited by meltwater from the ice sheet during a much higher stand of ice, in late Pliocene time.

It has long been suspected that the West Antarctic ice sheet is unstable and vulnerable to greenhouse warming, because it is grounded more than 1,000 m below sea level. Our results provide field evidence consistent with that proposal. More than the West Antarctic ice sheet may be involved, however. The nature of the evidence and timing of events at Mount Murphy are similar to the record in East Antarctica, suggesting continent-wide synchroneity of the major glacial and interglacial intervals. This conflicts with interpretations of marine data, Which suggest a shorter history for the West Antarctic ice sheet and a more stable history for the entire ice sheet.

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