The Pleistocene Glacial History of the Lake Wellman Area, Darwin Mountains, Antarctica.

Abstract

The Darwin and Hatherton Glaciers form a major system that drains a significant portion of the East Antarctic Ice Sheet (EAIS) through the Transantarctic Mountains (TAM) into the Ross Sea. Flow lines along their length demonstrate that they connect back to Dome Cirque on the Polar Plateau. Very little is known about the way these outlet glaciers have drained the EAIS in the past. However, information on their previous behaviour in response to global climate change and EAIS activity is recorded in their geomorphology. An ice-free region adjacent to Lake Wellman contains a well preserved sequence of moraines that provides evidence of glacial ice fluctuation related to earlier climates. Consequently, this area has potential as an appropriate site for investigating past glacial movement and change. A study was therefore conducted in the Lake Wellman area in order to examine the drift material left behind as the Hatherton Glacier retreated. This was done to obtain information that would help explain the manner and timing of glacial recession. A geomorphology map was constructed using data obtained from a series of transects placed across the drift moraine material. The transects were located at different elevations ranging between 800 m and 1200 m a.s.l., and were at distances between 4 and 8 km from the present glacier edge near Lake Wellman. Field data were collected from clast material sampled at regular intervals along each transect. These records consisted of assessments and measurements of clast lithology type, average size, hardness measured with a Schmidt hammer, angularity or roundness, and degree of weathering. The field data demonstrated that for clasts of dolerite and sandstone, angularity decreased and roundness increased significantly with altitude. No such trend occurred with clasts of gabbro, granite or basalt. The field observations therefore indicated that clasts at higher elevations and greater distances from the present glacial ice had been freed from the receding ice earlier and hence exposed for longer periods to the effects of atmospheric weathering. This effect was more apparent on the less durable sandstone and dolerite lithologies. Rock samples were also collected in the field for subsequent Surface Exposure Dating (SED). These consisted of sandstone and granite rocks which contain a high level of quartz, and care was taken to ensure that the samples chosen had been transported by glacial ice. The rock samples were prepared in the laboratory at the University of Canterbury, Christchurch, New Zealand. The processed samples were sent for cosmogenic dating at the Australian Nuclear Sciences Technology Organisation (ANSTO), in Sydney, Australia, using 10Be and 26Al isotope analysis. Difficulties were experienced with the removal of aluminium from the quartz during the processing of some samples with excessive levels of this element, which interferes with the dating process, possibly leading to an underestimation of the true age. Also, for some samples there were significant differences in the ages determined by the two isotopes. Data were only accepted if there was agreement between the two values and there was no technical or physical reason to doubt the age determined. These results confirmed those from the clast weathering data and clearly demonstrated the past retreat of the Hatherton Glacier over a period of 2 Ma from an elevation of 1600 m a.s.l. down to its present altitude of ca. 800 m a.s.l. in the vicinity of Lake Wellman. In general, there was a trend of increasing age of exposure with greater elevation and distance from the present glacial edge. Between 800 and 1000 m a.s.l., rocks had been exposed for 1-60 ka. Between 1000 and 1200 m a.s.l., rocks were dated within the range 75-400 ka before present (B.P.) and between 1200 and 1600 m a.s.l., ages ranged up to a maximum around 2 Ma B.P. The dates obtained in this study are generally greater than those recorded in an earlier published study from the same area in which a different dating technique was used. In particular there is a discrepancy in the position of the ice during the Last Glacial Maximum (LGM; 18-20 ka B.P.). Instead, the results of this study support an alternative modelling analysis that indicates a thinner Hatherton Glacier during the LGM. This conclusion implies a more rapid recession of the Ross Ice Shelf as a result of rising sea levels during an increasingly warmer climate. Suggested future work to refine the outcomes from this study would include the collection of more samples for SED dating, particularly in the region of the greatest extent of ice during the LGM.