Case Studies in Evolution of the Antarctic Ice Sheet: From Inception to Instability

Anthropogenic climate change has accelerated the melting of the Antarctic Ice Sheet (AIS), stimulating enquiry into past AIS behaviour to more accurately predict future changes. As the onshore sediment record is mostly either concealed by ice or lost to erosion, past AIS behaviour must be reconstructed from the offshore archive. Antarctic paleo-ice sheet models typically predict development of substantial ice-sheet embayments during the Mid-Miocene Climate Optimum, a warm period between ~17-15 Ma ago. Here, we test these models using ice-rafted debris (IRD) recovered from the central Weddell Sea (ODP Site 113-694) and Prydz Bay (ODP Site 188-1165). These IRD are entrained by the ice-sheet expanding into the embayments during the cooling associated with the subsequent Mid-Miocene Climate Transition and ultimately delivered to the deep marine sink by iceberg armadas during higher-frequency instability events. Due to the near-absence of heavy minerals in these volumetrically small samples, we utilise the novel in-situ K-feldspar 87Rb/87Sr detrital geochronometer, as well as more conventional single-grain K-feldspar and plagioclase Pb-isotope and K-feldspar Ar-Ar provenance techniques. Results identify palaeo-ice-sheet instability in the Recovery and Aurora sub-glacial basins, consistent with embayment formation. Despite the onset of Antarctic glaciation (~34 Ma) having led to a global shift in climate conditions, the nucleation of the Antarctic Ice Sheet remains a poorly understood aspect of palaeo-climate change. Palaeo-ice sheet modelling suggests that the Gamburtsev Subglacial Mountains (GSM) acted as a nucleation site for early Antarctic mountain glaciers. We analyse sediments recovered from ODP Site 1166A, a proximal shelf site in Prydz Bay, to characterize GSM-sourced sediment drained by the Lambert ice stream. We observe apatite, titanite, and rutile U-Pb ages consistent with a regional Pan-African metamorphic overprint, and apatite and titanite trace element abundances suggesting predominantly metamorphic parent rocks. Our findings support passive tectonic models for GSM development whereby ancient crust was topographically rejuvenated by rifting during the Permo-Triassic and Cretaceous, leading to the uplift of the GSM as a central block between two rift branches.