TIMING OF THE LAST GLACIAL MAXIMUM IN FIORDLAND, NEW ZEALAND – BUT WAS IT THE LARGEST ADVANCE ?

Dated speleothems interdeposited with glacifluvial sediments in Aurora Cave beside Lake Te Anau in Fiordland, southwest New Zealand, indicate that a glacial advance at MIS-4 was far larger than those that succeeded it. We compliment this archive of late Quaternary chronology with paired 10Be and 26Al in-situ cosmogenic exposure ages from lateral moraine erratics deposited along the glacial valley trough of Lake Te Anau directly overlying the cave. Whereas the speleothem evidence identifies non-glacial periods, in-situ exposure ages register the timing of the close of the intervening glacial phases or stadials. Our new results conclusively supports the assertion, unlike that observed in the Northern Hemisphere, that over the past 100 ka of the Last Glaciation cycle, the largest glacial advance in New Zealand was during MIS-4.

Over the face of the Te Anau valley sides, the ebb and flow of past glaciers have crafted a sequence of lateral moraine terraces, that staircase back in time with increasing altitude from 203 m at lake level to 975 m. In all we dated 27 granitic erratics from 9 of the 11 terraces mapped. Correction factors to local production rates included valley slope, terrace geometry, vegetation cover and variations in paleogeomagentic dipole strength.

Coupling the uranium-series data and exposure ages at Te Anau, a coherent and interesting picture of the regional glacial chronology is emerging. Preliminary results, as a function of decreasing altitude, indicate glacial advances in Fiordland at 75- 65 ka BP, 28-25 ka BP, 21-19 ka BP and 17-14 ka BP. The data indicate that the maximum ice advance of the Last Glacial cycle was at 75-65 ka BP and, importantly, not at 20-18 ka LGM as observed in the Northern Hemisphere. Of interest, the youngest exposure dated terrace is the lowest altitude lateral moraine (20 meters above lake level) and is dated at 14-17 ka, occurring 2-3 ka earlier than the Younger Dryas readvance. Taken together this comprehensive chronology and ice extent questions the assumed synchronicity and amplitude of global glacial advances at the millennial age scale.