West Antarctic ice-rafted debris: Sources, signals and signatures of past ice sheet dynamics

Abstract

The West Antarctic Ice Sheet (WAIS) is a critical component of the Earth’s climate system, and its stability (or instability) is a major source of uncertainty in projections of global sea level rise in the coming decades to centuries. Investigating past WAIS behaviour provides essential information on processes that control its response to warming and its potential for future instability. Iceberg-rafted debris (IRD) in marine sediments serves as a valuable archive of WAIS history, and tracing IRD back to its subglacial source can provide vital insights into the past extent and dynamics of the ice sheet. However, knowledge of the subglacial geology of West Antarctica is limited because of extensive ice cover, which makes source attribution challenging.

In this thesis, I develop an inverse provenance approach to address this problem (Chapter 2), using clustering analysis of lead (Pb) isotopic measurements of individual iceberg-rafted potassium feldspar grains from surface sediment samples collected from the continental shelf on the Pacific margin of West Antarctica. Coupled in situ rubidium–strontium (Rb–Sr) geochronology of the same grains confirms the validity of the detected clusters and provides age constraints which aid in identifying West Antarctic source terranes.

I then apply the Pb isotopic provenance method to late Pliocene–Pleistocene samples from marine sediment core PC493 on the Amundsen Sea continental slope, producing a record of IRD provenance and accumulation spanning the past 3.1 million years, which provides insights on ice-sheet dynamics in this important sector of the WAIS (Chapter 4). A new age model for this core (Chapter 3) provides the chronostratigraphic context for down-core analysis of PC493, constructed by high-resolution cyclostratigraphic tuning using barium (Ba) chemostratigraphy, constrained by calcareous nannofossil and diatom biostratigraphy and magnetostratigraphy.

This research demonstrates that Pb isotopic composition, along with Rb-Sr geochronology, is a powerful tool for investigating the past response of the vulnerable WAIS to climate change.