A cosmogenic glacial chronology of Lake Judd, southwest Tasmania and latitudinal shifts in the Roaring Forties

Fink, D; Wilcken, KM; Simon, KJ; Schulmeister, J; Thackray, GD

A cosmogenic glacial chronology of Lake Judd, southwest Tasmania and latitudinal shifts in the Roaring Forties

Date

2019-07-27

Authors

Publisher

International Union for Quaternary Research (INQUA)

Abstract

Limited attention has been paid to the glacial history of Tasmania (40-44 S). At the northern limits of today’s Westerly storm track (the Roaring Forties), it experiences precipitation changes due to shifts in the Southern Annular Mode and ITCZ. Cosmogenic dating in southwest Tasmania (Kiernan 2010) highlights the marginal nature of its glaciation where small precipitation or temperature changes cause large changes in mass balance. This makes southwestern Tasmania an excellent location to study Westerly flow through the Last Glacial Cycle. The prevailing idea is that during glacial times, Westerlies expand northward to lower Southern Ocean latitudes, delivering increased rainfall to Tasmania, whilst during interglacials, their migration polewards reduces precipitation. Somewhat counterintuitively, from a Tasmanian perspective of increased Westerly circulation and wind strength, maxima glaciation should coincide with peak global glacials. Although well preserved, extensive last glacial cycle moraine sequences are not common, a series of more than 20 terminal moraine ridges have been mapped over 3 km concentric with the southern margin of Lake Judd, in the SW wilderness corner of Tasmania. Morphologically, the Lake Judd sequence is believed to represent three phases; pre MIS-6, MIS 5-3 and MIS 2 (LGM), representing substantially different volumes of ice accumulation. Our first field campaign collected 16 dolerite samples (36Cl) from the three innermost concentric moraines adjacent to lake edge and 4 quartzite (10Be, 26Al) samples from the most distal moraines. Our aim is to test correlation of the long-term glacial history with records of westerly fluctuations from NZ speleothems (Whittaker et al., 2011) to determine whether the latitudinal variation in Westerly winds in the Australasian region are zonal during glacial times as they are demonstrated to be in interglacial intervals (Fletcher and Moreno, 2012). Preliminary exposure age calculations on the full set of 20 samples are in poor agreement with Kiernan’s age associations. The 10Be (and 26Al) ages on the distal moraine indicate a most probable MIS 4/5 affiliation (>50 ka), significantly younger than expected. In contrast, the 16 dolerite samples from the 3 innermost proximal moraines at lake edge gave a complex set of ages that ranged from MIS 3 to MIS 4 affiliation (45-25ka), somewhat older than predicted. To add further complexity, these 16 ages show no intra or inter moraine age trends with respect to their morphologic positions and ice flow direction. These results contrast with those from other Tasmanian glacial records and reveal the need for a concretive effort to apply cosmogenic exposure dating to the numerus glacial deposits already mapped throughout Tasmania.

Keywords

Glaciers, Tasmania, Australia, Lakes, Moraines, Geologic deposits, Turbulence, Ambient temperature, Ice, Atmospheric precipitations, Wind

Citation

Fink, D., Wilcken, K., Simon, K., Schulmeister, J., & Thackray, G. (2019). A cosmogenic glacial chronology of Lake Judd, southwest Tasmania and latitudinal shifts in the Roaring Forties. Poster presented to the 20th INQUA Congress, Dublin, Ireland, 25th - 31st July 2019. Retrieved from: https://virtual.oxfordabstracts.com/#/event/public/574/submission/1538