Ice-rafted feldspar grains in recent shelf sediments of West Antarctica: provenance pathways via lead isotope compositions

Knowledge of the past dynamics of the Antarctic ice sheets is essential for better understanding their present and future stability and, importantly, the resulting effects on global mean sea level rise in a warming world. The petrology and geochemistry of iceberg-rafted debris (IRD; >150 μm) produced by these ice sheets can provide vital information about past ice sheet extent, but the characteristics of geological sources are poorly constrained in ice-covered Antarctica. Lead isotope ratios (controlled by protolith age) in West Antarctic basement and derived sedimentary rocks (mostly <500 Ma) have previously been assumed to be largely homogenous, and therefore useful only for distinguishing West Antarctic IRD from older (0.5-3 Ga) East Antarctic IRD in Southern Ocean sediments. Laser ablation analysis of individual mineral grains, however, avoids the averaging effects of bulk mineral separate, sediment, or rock analyses and reveals the full variation of isotopic signatures in a sample of detrital grains. Here, we present a survey of lead isotope ratios determined in ~600 iceberg-rafted feldspar grains from 26 seafloor surface sediment samples from the West Antarctic continental shelf. Machine-learning clustering of the lead-isotope data reveals at least two major populations, including a previously unknown population of grains which are less radiogenic than the main cluster. These less radiogenic grains are only found in two areas: near the ice-shelf front of Thwaites Glacier and near the eastern edge of the Ross Ice Shelf. The Thwaites signal is not present at sites on the middle and outer continental shelf, suggesting an offshore dilution effect. Supervised clustering of the main group reveals additional subdivisions in Pb-isotope space that are geographically restricted to: (i) the wider Amundsen Sea, (ii) the Bellingshausen Sea, and (iii) Sulzberger Bay at the boundary between the Amundsen and Ross seas. These subdivisions will be further investigated using 87Rb-87Sr dating of a subset of the feldspar grains used for Pb-isotope analysis. Together, these new data provide a novel IRD provenance tool, allowing tracing of offshore IRD back to either Thwaites Glacier or the eastern Ross Ice Shelf source areas. Given the observed offshore dilution effect, detection of this signal in sediment cores from the outer shelf or deep-sea would indicate a significantly increased supply of detritus sourced from Thwaites Glacier or the Ross Ice Shelf, both important iceberg outlets of the West Antarctic Ice Sheet.