Antarctic glacial history inferred from cosmogenic-nuclide measurements in rocks

Abstract

This dissertation describes three research projects on the glacial history of Antarctica using measurements of cosmogenic-nuclides in glacial deposits and bedrock surfaces.

The first chapter investigates the deglaciation chronology of the Ross Sea following the last ice age. Abrupt thinning of glaciers in the southern Transantarctic Mountains occurred ~9-8 kyr B.P. This coincided with deglaciation of the Scott Coast, ~800 km to the north. At the end of this period the grounding line was located near Shackleton Glacier, indicating that most of the central and western Ross Sea deglaciated in less than 2 kyr. The rapidity of this event appears to have been influenced by unstable grounding-line retreat into deep marine basins and, potentially, enhanced melting at the marine margin. Because the majority of the deglaciation occurred during the early Holocene, the Ross Sea sector could not have significantly contributed or responded to rapid sea-level rise during Meltwater Pulse 1A.

The second chapter discusses sites in West Antarctica for subglacial drilling to test for past ice-sheet collapse. It has been hypothesized that marine-based portions of the WAIS deglaciated during past warm interglacial periods. Measurements of cosmogenic nuclides in subglacial bedrock surfaces therefore have the potential to establish whether and when this occurred. However, because most of the bedrock revealed by ice-sheet collapse would remain below sea level, shielded from the cosmic-ray flux, drill sites for subglacial sampling must be located in areas where thinning of the residual ice sheet would expose presently subglacial bedrock surfaces. In this chapter I discuss the criteria and considerations for choosing drill sites where subglacial samples will provide maximum information about WAIS extent during past interglacial periods. I evaluate candidate sites in West Antarctica and find that sites located adjacent to the large marine basins of West Antarctica will be most diagnostic of past ice-sheet collapse. There are important considerations for drill-site selection on the kilometer scale that can only be assessed by field reconnaissance. As a case study of these considerations, I describe reconnaissance at sites in West Antarctica, focusing on the Pirrit Hills, where in the summer of 2016-2017, an 8 m bedrock core was retrieved from below 150 m of ice.

The third chapter investigates the glacial history of three isolated groups of nunataks in West Antarctica. The objectives of this chapter are to examine (i) the development and preservation of alpine landscapes, and (ii) past variations in ice thickness on timescales ranging from thousands to millions of years. Alpine landscapes were carved during the mid-Miocene and have since remained exceptionally well preserved. A trimline at the Pirrit Hills is an extension of a prominent trimline that occurs throughout the Ellsworth Mountains to the north. At the divide, ice levels have rarely, if ever, been higher than present, but they appear to have been lower for prolonged periods in the past. Midway between the divide and the grounding line, ice levels have repeatedly been ~300-400 m higher than present, as occurred during the last ice age. Ice levels here also appear to have been lower than present during past interglacial periods.