ALBERT

Description: The majority of glaciers draining the Antarctic Peninsula Ice Sheet are thinning and retreating rapidly1. It is widely understood that these changes are driven by both a warming ocean and atmosphere. However, there are other mechanisms, including pinning points created by bathymetric highs and a reverse bed gradient, that are thought to have an important control on ice stream behaviour (Weertman, 1974; Jamieson et al., 2012). Our understanding of the interplay between these mechanisms and time-scales over which they are important is currently limited in time to the advent of satellite monitoring. By reconstructing the cause and style of ice stream retreat following the Last Glacial Maximum (LGM; 25-19 ka BP), it is possible to gain a greater insight into the mechanisms which drive glacier retreat (Ó Cofaigh et al., 2014). Sedimentary sequences deposited during the LGM and the subsequent deglaciation on polar continental shelves, provide an important archive of past changes (Ó Cofaigh et al., 2014). Previous studies have typically identified three sediment facies assemblages; sub-glacial, transitional and open marine (Ó Cofaigh et al., 2014; Domack et al., 1988; Smith et al., 2011). Transitional sediment facies are deposited at the grounding line and are often targeted for radiocarbon dating, as they represent the onset of glaciomarine sedimentation following the retreat of grounded ice (Domack et al., 1988; Smith et al., 2014; Heroy et al., 1996). Despite the development of depositional models to help explain the processes occurring at grounding lines (Powell et al., 1995 and 1996), there is still significant uncertainty about the temporal and spatial variations in grounding line sedimentation along and across a palaeo-ice stream trough. Here we use a multi-proxy approach (water content, shear strength, magnetic susceptibility, density, contents of biogenic opal, Total Organic Carbon and CaCO3, grain size distribution and X-radiographs) on marine sediment cores recovered from the Anvers-Hugo Palaeo-Ice Stream Trough (AHT), western Antarctic Peninsula shelf, to identify variability in transitional sediment facies deposited along and across the trough. We discuss possible controls on the variability in transitional sediment facies and how this is related to the rate and style of ice stream retreat. Our data reveal systematic variability in the types and volume of transitional sediments deposited during the last deglaciation of AHT. A detailed analysis of the transitional sediment facies shows that this variability reflects different phases of ice stream behaviour. Large volumes of ice proximal sediment facies recovered seawards of grounding zone wedges are indicative of episodes of grounding line still-stands. Re-advances of the grounding line, concurrent with a shallowing of the reverse bed gradient and a narrowing of the trough, appear to have occurred during the final stages of deglaciation. This is indicated by interlaminated ice-proximal and ice-distal sediment facies within inner shelf cores. Transitional sediment variability additionally captures the evolution of the ice stream during deglaciation, including the formation of a small ice shelf on the inner shelf. Keywords: Antarctic Peninsula, Last Glacial Maximum, ice stream, sediment cores References Cook, A. J., Holland, P. R., Meredith, M. P., Murray, T., Luckman, A. & Vaughan, D. G, 2016. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353, 283-286. Weertman, J, 1974. Stability of the Junction of an Ice Sheet and an Ice Shelf. Journal of Glaciology, 13, 3-11. Jamieson, S. S. R., Vieli, A., Livingstone, S. J., Cofaigh, C. O., Stokes, C., Hillenbrand, C.-D. & Dowdeswell, J. A, 2012. Ice-stream stability on a reverse bed slope. Nature Geoscience, 5, 799-802. Ó Cofaigh, C., Davies, B. J., Livingstone, S. J., Smith, J. A., Johnson, J. S., Hocking, E. P., Hodgson, D. A., Anderson, J. B., Bentley, M. J., Canals, M., Domack, E., Dowdeswell, J. A., Evans, J., Glasser, N. F., Hillenbrand, C.-D., Larter, R. D., Roberts, S. J. & Simms, A. R, 2014. Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum. Quaternary Science Reviews, 100, 87-110. Domack, E. W. & Harris, P. T, 1998. A new depositional model for ice shelves, based upon sediment cores from the Ross Sea and the Mac. Robertson shelf, Antarctica. Annals of Glaciology, 27, 281-284. Smith, J. A., Hillenbrand, C.-D., Kuhn, G., Larter, R. D., Graham, A. G. C., Ehrmann, W., Moreton, S. G. & Forwick, M, 2011. Deglacial history of the West Antarctic Ice Sheet in the western Amundsen Sea Embayment. Quaternary Science Reviews, 30, 488-505. Smith, J. A., Hillenbrand, C.-D., Kuhn, G., Klages, J. P., Graham, A. G. C., Larter, R. D., Ehrmann, W., Moreton, S. G., Wiers, S. & Frederichs, T, 2014. New constraints on the timing of West Antarctic Ice Sheet retreat in the eastern Amundsen Sea since the Last Glacial Maximum. Global and Planetary Change, 122, 224-237. Heroy, D. C. & Anderson, J. B, 1996. Radiocarbon constraints on Antarctic Peninsula Ice Sheet retreat following the Last Glacial Maximum (LGM). Quaternary Science Reviews, 26, 3286-3297. Powell, R. D., Dawber, M., McInnes, J. N. & Pyne, A. R, 1996. Observations of the Grounding-line Area at a Floating Glacier Terminus. Annals of Glaciology, 22, 217-223. 1Powell, R. D. & Domack, E, 1995. Modern Glacimarine Environments. In: Glacial Environments, Volume 1 (ed. J Menzies). Butterworth-Heinemann, 445-486.

Description: We will present new multibeam bathymetry data that make the Anvers-Hugo Trough west of the Antarctic Peninsula one of the most completely surveyed palaeo-ice stream pathways in Antarctica. We interpret landforms revealed by these data as indicating that subglacial water availability played an important role in facilitating ice stream flow in the trough during late Quaternary glacial periods. Specifically, we observe a set of northward-shoaling valleys that are eroded into the upstream edge of a sedimentary basin, extend northwards from a zone containing landforms typical of erosion by subglacial water flow, and coincide spatially with the onset of mega-scale glacial lineations. Water was likely supplied to the ice stream bed episodically as a result of outbursts from a subglacial lake previously hypothesized to have been located in the Palmer Deep basin on the inner continental shelf. In a palaeo-ice stream confluence area, close juxtaposition of mega-scale glacial lineations with landforms that are characteristic of slow, dry-based ice flow, suggests that water availability was also an important control on the lateral extent of these palaeo-ice streams. These interpretations are consistent with the hypothesis that subglacial lakes or areas of elevated geothermal heat flux play a critical role in the onset of many large ice streams. The interpretations also have implications for the dynamic behaviour of the Anvers-Hugo Trough palaeo-ice stream and, potentially, of several other Antarctic palaeo-ice streams. Keywords: multibeam bathymetry, ice stream, subglacial water, landform