A Quaternary history of ice sheet dynamics in the Transantarctic Mountains.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The Antarctic Ice Sheets responded significantly to climatic conditions during the Last Glacial Maximum (LGM) and the subsequent warming that followed. Therefore, an understanding of how Antarctica reacted to past climates is necessary to predict the response of its ice sheets to current and future climate change.
This thesis presents new evidence about the timing and magnitude of East and West Antarctic ice sheet (EAIS & WAIS) changes during the Quaternary Period, from the Darwin Hatherton glacial system (DHGS, 79.5S, 158E). The DHGS drains the EAIS through the Transantarctic Mountains into the Ross Ice Shelf and glacial deposits have been used to constrain ice sheet thicknesses in this sector of the Ross Sea Embayment. At four sites along the length of the system, glacial deposits were mapped and 73 erratic and bedrock samples collected for ¹⁰Be and ²⁶Al surface exposure dating (SED). The exposure ages range from 0.01 to 2.2 Ma and generally show a trend of oldest ages at the highest elevations, thus suggesting an overall decrease of ice volume within the DHGS over the Quaternary. The older ages suggest that during the Plio-Pleistocene, DHGS ice was at least 800-1000 metres thicker than present, while in the mid to late-Holocene thickening was less than 50-80 metres. Four glacial advance and retreat events were described and mapped previously from the DHGS by Bockheim et al (1989). The Isca and Danum drifts, are ~1-2 and 0.6 Ma respectively. The Britannia-II Drift, previously assumed to mark the maximum extent of the Last Glacial Maximum advance is more complex, with clusters of ages at ~6.5, ~36 and ~125 ka. The youngest drift, the Hatherton is mid to late-Holocene (<4.5 ka) and suggests that the DHGS has been near its equilibrium position during this period.
Throughout the DHGS no unequivocal evidence of the LGM was observed and therefore poses questions about the past thickness of the Antarctic ice sheets during the LGM. Exposure ages from sites near the head of the Hatherton Glacier (Dubris Valley & Lake Wellman) suggest that at the LGM, the East Antarctic Ice Sheet may have been of similar size, or slightly smaller, than present. In stark contrast, at the confluence of the Darwin Glacier and the Ross Ice Shelf, a WAIS ~400-900 metres above the modern ice surface is tentatively suggested; A value in agreement with that proposed by modern glaciological models. Additionally, while the results from the Dubris and Bibra valleys show that the EAIS thins during glacial climates (i.e. the LGM), it also suggests thickening during interglacials. The Britannia-I and II drifts representing retreats at~6.5 and ~125 ka from glacial highstands.
A number of key findings related to the application of SED in Antarctic settings are also presented. The use of dual-nuclides (¹⁰Be & ²⁶Al) show that within the DHGS, the proportion of samples displaying a prior burial history increases with distance from the catchment. The spread of exposure ages observed in the dataset also show the complexity of the depositional processes occurring at cold-based glacial margins and therefore judicious sample selection is required to obtain exposure ages that are representative of the true deglaciation age.