Pliocene Paleoenvironment and Antarctic Ice Sheet Behavior: Evidence from Wright Valley

Investigations in Wright Valley, adjacent to the Transantarctic Mountains in East Antarctica, shed light on the question of whether high-latitude Pliocene climate was warm enough to cause widespread deglaciation of the East Antarctic craton with a concurrent Magellanic moorland-like environment. If Pliocene-age diatoms, presently in glaciogenic deposits high in the Transantarctic Mountains, had come from seaways on the East Antarctic craton, an expanding Late Pliocene ice sheet must have first eroded them from marine sediments and then deposited the diatoms at their present high-altitude locations. This hypothetical expanding glacier would have had to have come through Wright Valley. Glacial drift sediments from the central Wright Valley were mapped, sampled, analyzed, and 40Ar/39Ar wholerock dated. Our evidence indicates that an East Antarctic outlet glacier has not expanded through Wright Valley, and hence cannot have overridden the Dry Valleys sector of the Transantarctic Mountains, any time in the past 3.8 myr. Rather, there was only moderate Pliocene expansion of local cold-based alpine glaciers and continuous cold-desert conditions in Wright Valley. Persistence of a cold-desert paleoenvironment implies that the sector of the East Antarctic Ice Sheet adjacent to Wright Valley has remained relatively stable without melting ablation zones since at least 3.8 Ma, in Early Pliocene time. A further implication is that Antarctic Ice Sheet behavior in the Pliocene was much like that in the Quaternary, when the ice sheet consisted of a stable, terrestrial core in East Antarctica and a dynamic, marine-based appendage in West Antarctica. Introduction: The Problem Numerous paleoclimate records indicate that Pliopresent within Sirius Group glaciogenic deposits high in the Transantarctic Mountains, once lived cene climate was warmer than at present in many sectors of the planet between 3.0 and 4.8 Ma (Kenin interior marine basins that would have formed if the East Antarctic Ice Sheet collapsed (figure 1). nett and Hodell 1993; Cronin and Dowsett 1993). An unresolved issue is whether the climate of East The ice sheet is thought to have eroded the diatoms and emplaced them in Sirius sediments when it reAntarctica was sufficiently warm during this time interval to feature inland seas with surface temperadvanced over the basins and then across the Transantarctic Mountains in Late Pliocene or Early Pleisatures of 2–5°C (Harwood 1986), temperate ice caps, and Nothofagus (southern beech) forests in tocene time (Webb et al. 1984; Webb and Harwood 1991; Barrett et al. 1992). In addition, Nothofagus place of the present-day continent-wide polar ice sheet (Denton et al. 1993; Webb and Harwood 1991; and Dacrydium fossil remains occur essentially in situ in Sirius outcrops beside Beardmore Glacier Webb et al. 1984). This problem bears not only on the evolution of a polar East Antarctic Ice Sheet, (Askin and Markgraf 1986; Ashworth et al. 1996). Given the assumptions about the age of the Sirius but also on its sensitivity to greenhouse warming (Barrett 1991). deposits, they are thought to be representative of Late Pliocene vegetation in the Transantarctic The hypothesis of massive deglaciation is based on the assumption that Pliocene diatoms, now Mountains (Webb and Harwood 1993). Pliocene ice-sheet collapse and development of krummholz 1 Manuscript received September 5, 1996; accepted DecemNothofagus in Antarctica both require atmospheric ber 12, 1996. temperatures 20–25°C greater than now (Barrett 2 Also: Institute for Quaternary Studies, University of Maine, 1991; Huybrechts 1993; Sakai et al. 1981). Orono, Maine 04469. The hypothesis of Pliocene ice-sheet collapse 3 Geologisches Institut, Universität Bern, CH-3012 Bern, Switzerland. carries specific predictions. The combination of [The Journal of Geology, 1997, vol. 105, p. 285–294]  1997 by the University of Chicago. All rights reserved. 0022-1376/97/10503-0007$01.00