MAJOR MIDDLE MIOCENE GLOBAL CHANGE AND THE EXTINCTION OF TUNDRA COMMUNITIES: EVIDENCE FROM THE TRANSANTARCTIC MOUNTAINS

We present a glacial record from the western Olympus Range in the Transantarctic Mountains that documents a permanent shift in the thermal regime of local glaciers, from wet- to cold-based regimes between 14.11 and 13.85 million years ago (Ma). Drifts deposited before the shift include classic wet-based tills interbedded with glaciofluvial and glaciolacustrine deposits. In-situ fossils of algae, plants, freshwater mollusks, and insects indicate that the western Olympus Range supported sparse tundra vegetation during at least the waning stages of this wet-based glacial phase (see Ashworth et al., this volume). An overlying series of stacked, coarse-grained drifts, all lacking evidence for associated melt water, marks the shift to cold-based conditions. Chronologic control comes from ⁴⁰Ar/³⁹Ar analyses of six in-situ volcanic ash deposits: one interbedded within glaciolacustrine sediments and five interbedded within cold-based drifts. The shift from wet- to cold-based glaciation reflects a drop in mean annual temperature of 20 to 30 °C. A widespread erosion surface crosscuts wet-based drifts, as well as the oldest of the cold-based drifts. The style of dissection suggests erosion resulted from one or more major episodes of East Antarctic Ice Sheet expansion, occurring sometime between 13.62 and 12.44 Ma. Taken together, the western Olympus Range glacial record, fossils, and volcanic ash chronology imply 1) that atmospheric cooling preceded, and thus may have triggered, maximum overriding of the polar East Antarctic Ice Sheet, 2) that a temperature decrease combined with the cessation of most melt water production led to the extinction of the western Olympus Range tundra by 13.85 Ma, and 3) that alpine glaciers shifted from wet- to cold-based regimes sometime between 14.11 Ma and 13.85 Ma.