Integrated Mineralogic, Magnetic, Geochemical, and Isotopic Tracers of Sediment Provenance for the Circum-Antarctic Margin (Invited)

An important component to understanding past dynamics of Antarctica’s ice sheets is determining which parts of the ice sheets have responded to climate change. If a fingerprint for different ice drainage basins can be determined, we can potentially match Southern Ocean ice rafted debris back to its most probable source, providing a means of identifying the source of iceberg-generating events through time. Obtaining the maximum information on the regional variability of sediment sources requires a spectrum of strategies. This work integrates a new study of iron oxide geochemistry and textures with magnetic properties, major and trace element and Nd isotope composition of bulk sediment, and Ar-Ar ages of detrital hornblende grains. We analyzed the geochemistry and mineral textures of iron oxide grains contained in glacial-marine diamict and dropstones from the East Antarctic Margin. Diamict and dropstones recovered in fjords, bays, and inner shelf basins represent sub-glacially derived and transported material from the adjacent ice sheet drainage basin. These materials can be used to investigate the bedrock below the Antarctic Ice Sheet in regions where outcrops are absent or inaccessible. Last glacial maximum diamict from the George V Coast, Prydz Bay, Svenner Channel, and the Mac. Robertson Coast were divided into size fractions and mineral fractions to construct a suite of provenance tracers. Iron oxides in the 45-500 µm size fraction were mounted and polished for reflected light microscopy and electron microprobe analysis. The George V Coast is dominated by magnetite, with minor ilmenite, and hematite. The Mn-rich silicate rhodonite was also present. The magnetite grains contain significant Mn substituting for Fe. Grains are generally smaller than 100 μm, euhedral and homogeneous. Iron oxides from Prydz Bay are 100-250 μm in diameter and show three main textures. We observed homogeneous grains, magnetite host grains with narrow exsolved ilmenite bodies, and grains with thick (several tens of microns) lamellae of magnetite and ilmenite, each of which hosts its own subset of exsolution blebs. Svenner Channel and Prydz Bay ilmenite are Fe-rich with V, Mg, and Mn impurities. Magnetite grains contain up to 2% Al. Svenner Channel iron oxides display mermekitic textures with intergrowths of magnetite, ilmenite and quartz. Mac. Robertson Land is dominated by Fe-poor ilmenite with a variety of alteration textures. Many of the ilmenites show twinning with chemical alteration along the twins and cracks within the grains. Mac. Robertson ilmenites contain substantial Nb impurities, the only site to have elevated levels of this element. Analysis of west Antarctic sites is in progress. The developing library of iron oxide chemical profiles and mineral textures can be combined with other provenance tracers to form a suite of tools for sourcing major iceberg generating events and reconstructing past ice sheet flow lines.