East Antarctic ice sheet stability recorded in a high-elevation ice-cored moraine

Abstract

Till in an extensive blue ice moraine in the central Transantarctic Mountains at Mt. Achernar shows relatively continuous deposition by East Antarctic derived ice throughout the last glacial cycle. The most recently exposed material along the active margin of the Law Glacier (Zone 1) has hummocky topography that transitions into to a relatively flat region (Zone 2), followed by a series of ∼2 m high continuous, parallel/sub-parallel ridges and troughs (Zones 3–5). The entire moraine is ice-cored. Past surface changes of <40 m are indicated by a lateral moraine at the base of Mt. Achernar and substantial topographic relief across Zone 3.Pebble lithology and detrital zircon geochronology were analyzed on samples along a 6.5 km transect across the moraine which formed from sub- and englacial debris. Beacon and Ferrar Supergroup rocks comprise most rock types on the moraine surface. Overall, pebbles in Zones 1, 4 and 5 are dominated by igneous rocks of the Ferrar dolerite, whereas Zones 2 and 3 have ∼40% more Beacon Supergroup sedimentary rocks. Zone 4 is characterized by distinctly colored lithologic bands, 5–20 m wide, that alternate between dominant Beacon and Ferrar rock types. When combined with surface exposure ages, we conclude that Zones 2 and 3 contain sediment accumulated through the last glacial maximum (LGM). In contrast to pebble data, the U-Pb zircon data from till across all zones show little variability and are consistent with a Beacon Supergroup source, as samples show significant populations from the Proterozoic, ∼550–600 Ma and ∼950–1270 Ma, as well as the late Archean ∼2700–2770 Ma. The Mackellar, Fairchild, and lower Buckley Formations are interpreted as dominant sources of the detrital zircons. The zircon data lack the spatio-temporal variability indicated by the pebble fraction because the local Ferrar dolerite is not zircon bearing, highlighting the broader importance of using multiple techniques when interpreting provenance changes over time.Rather than reflecting major changes in ice flow path over time, the provenance changes are interpreted to indicate relative stability of the East Antarctic ice sheet, as the Law Glacier tapped into and eroded successively lower stratigraphic units of the Beacon Supergroup. This has important implications for interpreting offshore provenance records.