The micromorphology of iceberg-keel scoured diamictons from the Bellingshausen and Amundsen Seas: An approach to improving reconstructions of West Antarctic Ice Sheet extent.

In the past the West Antarctic Ice Sheet (WAIS) extended beyond its present-day limits, sometimes as far as the continental shelf edge during cold periods, such as the Last Glacial Maximum (LGM, ~19-23 ka). Sediment deposited at the base of grounded ice is known as subglacial diamicton (or ‘till’). In addition, diamictons can be formed in a range of other glacimarine depositional environments including sub-ice shelf or seasonally open marine settings, as iceberg rafted and scoured diamictons, or glacigenic debris flows. Whilst there has been some progress in characterising subglacial and iceberg-keel scoured diamictons at both macro- and micro-scales, historically it has been difficult to distinguish between different types of diamictons formed in very different settings. This is particularly true for areas where several glacial and glacimarine processes operate, and thus, overprint each other. However, distinguishing between the different types of diamictons is crucial if we are to reliably reconstruct the maximum extent of the WAIS in the past and the timing of its retreat. This information is urgently needed for ice sheet and climate models that are used to predict future WAIS changes and resulting global sea-level rise. The aim of this study is to macro- and microscopically examine, and determine the origin of, diamictons from the outer shelves of the Bellingshausen Sea (core GC371) and the Amundsen Sea (cores VC430, VC436), in West Antarctica. Although the three cores examined in this study were retrieved from sea floor areas affected by iceberg-keel scouring, their diamictons may also represent any or all of the other aforementioned diamicton-forming processes. Micromorphological analyses show that diamictons in all three cores have undergone stress resulting in pervasive deformation subsequent to deposition. Cores GC371 and VC430 contain diamictons with more abundant and better developed microstructures than core VC436, which suggests cores GC371 and VC430 have undergone more intense deformation than core VC436. Micromorphological structures and features at all three core sites demonstrate complicated and/or inverse down-core deformation patterns, which often do not complement a traditional strain profile, and are not consistent between core sites. This indicates potential overprinting of structures at several horizons after multiple deformation events. Future research should focus on attempting to identify and unravel separate deformation events in diamictons, and to further distinguish between diamictons formed in different Antarctic depositional settings.