Reconstructing the history of the Antarctic ice sheet using internal reflecting horizons from radio-echo sounding
Understanding the contribution of the Antarctic Ice Sheet (AIS) to past and future sea-level rise has emerged as a scientific priority over the last four decades. Whilst our knowledge of ice-dynamical changes occurring as a result of current anthropogenic forcing has improved considerably since the start of the satellite era, significantly less is known about the evolution of the AIS during the pre-industrial Holocene (the last ~11.7 thousand years; ka). Quantifying these changes is crucial, however, as this time period corresponds to a time when the ice sheet was retreating from its maximal extent at the Last Glacial Maximum (LGM; ~20 ka) and environmental conditions were similar to today. Therefore, improving our understanding of this period may provide a long-term context to the decadal changes observed in recent times and how these may evolve in the future. Whilst point-based geochronological measurements of ice and sediment cores, or surface exposure dating, can be used to assess past ice-sheet changes over the AIS, it remains unclear how representative they are of a wider region. A complementary and spatially extensive resource across the ice sheet are Internal Reflecting Horizons (IRHs) as imaged by Radio-Echo Sounding (RES) techniques, which provide a cumulative record of accumulation, basal melt and ice dynamics that, if dated precisely at ice cores, can be used to inform numerical ice-sheet models projecting past and future changes on large spatial scales. The aim of this thesis is therefore to develop and extend age-depth models from IRHs across the AIS to assess the past stability of the ice sheet. In this thesis, an age-depth model of Pine Island Glacier spanning the LGM and Holocene periods is derived from spatially extensive IRHs. The connection between RES profiles and the WAIS Divide ice core enables the direct dating of the IRHs, and reveals that they match large peaks in sulphate concentrations which are unparalleled in the 68,000 year-old record, thus suggesting that the cause of these IRHs is from past explosive volcanic eruptions. By connecting this IRH stratigraphy with a previously developed age-depth model across the Institute and Möller Ice Streams (IMIS), I show that a precisely dated age-depth model now exists over 20% of the West Antarctic Ice Sheet (WAIS). One of these IRHs, precisely dated at ~4.7 ka, is then used as input into a one-dimensional ice-flow model to estimate past accumulation rates during the mid-Holocene over the catchments encompassing Pine Island Glacier, Thwaites Glacier, and IMIS, together representing 30% of the WAIS. The inferred mid-Holocene accumulation estimates are then compared with modern rates derived from climate models and observational measurements to show that accumulation rates were 18% greater during the last five millennia compared to the present over the Amundsen-Weddell-Ross Divide. These results match previous findings from isolated ice-core measurements and spatially targeted studies over the divide, and correspond to periods of grounding line retreat and readvance during the Holocene over the WAIS. Together, these show the potential for extracting further IRH information from other sectors of the AIS in order to build an age-depth model of the ice sheet. However, the underlying RES data necessary for this work were, until recently, relatively inaccessible to the wider scientific community, thus restricting the extraction and interpretation of age-depth models across the AIS. This motivated the release of ~300,000 line-km of RES profiles acquired by the British Antarctic Survey between 2004 and 2020. In addition to standardising and releasing these data, this thesis shows that large sections of continuous englacial layering exist widely across both East and West Antarctica, suggesting that, together with previously developed age-depth models of both regions and nearby ice-core stratigraphies, these newly released RES datasets will be critical in our aim to build an ice-sheet wide age-depth model of Antarctica, as motivated by the AntArchitecture Initiative. Together, the findings from this thesis reveal the spatially extensive nature of IRHs across West and East Antarctica and demonstrate how these can be used to infer past ice-sheet changes. This thesis also highlights the need to extract further age-depth models, particularly across East Antarctica, in order to provide important boundary conditions such as past accumulation rates and ice-elevation change which can be used by numerical ice-sheet models to help improve predictions of past and future ice-sheet change and ensuing sea-level rise contributions.