A 20-MA-LONG RECORD OF ANTARCTIC CRYOSPHERE EVOLUTION FROM THE FRIIS HILLS, EAST ANTARCTICA

Marine isotope records suggest that ice sheets began to grow in Antarctica 40 Ma ago, equaling modern volume by 34 Ma ago. From 34 to 17 Ma, marine records register fluctuations from 50 to 125% of modern volume. Modeling suggests these glaciers grew outward from high-elevation terrain and recent subglacial mapping of alpine valleys provides evidence for this. Because of extensive ice cover, however, it is all but impossible to test estimates of early ice sheet growth and volume against unambiguous geologic evidence.

Here we present a 20-Ma-long glacial record from the Friis Hills, in the southern McMurdo Dry Valleys block of the Transantarctic Mountains (TAM). The record is preserved on the upper surface of an 800-m-tall inselberg; it includes stacked drifts as much as 28 m thick, some with fossiliferous interbeds, and crosscutting glacial erosion surfaces. The deposits and underlying surfaces register three phases of glaciation. (1) An initial phase of alpine glaciation in which glaciers flowed out of the TAM to the SW, toward the Antarctic interior. Tills from this phase were deposited 19.76 Ma ago based on dating of a tephra interbed. (2) A later phase of alpine glaciation in which glaciers flowed to the SE into a dendritic drainage network leading offshore. Fossiliferous lacustrine interbeds indicate regional climate was mild, even during glacier advance. (3) Lastly, a phase of ice sheet growth in which east-flowing ice overtopped the Friis Hills and cut deep troughs across divides of the TAM. Based on the regional glacial record this took place ~13 Ma ago. Tephra interbeds in drifts along trough margins indicate that outlet glaciers have been confined to these troughs since at least 10.76 Ma.

The Friis Hills record confirms modeling studies and recent subglacial mapping that suggest the Antarctic cryosphere began with alpine ice caps. Our dating shows, however, that ice in the Dry Valleys block of the TAM continued to drain toward the Antarctic interior until at least 19.76 Ma, which is 3 to 5 Ma after ice volumes are thought to have reached 125% of modern volume. Ice only became thick enough to breach topographic divides during a restricted period ~13 Ma ago, at which time the modern pattern of ice drainage became established. Since this period of ice sheet overriding, glaciers in the region have not altered large-scale topography.