Geochemical fingerprints of glacially eroded bedrock from West Antarctica: Detrital thermochronology, radiogenic isotope systematics and trace element geochemistry in Late Holocene glacial-marine sediments
2018
Geochemical provenance studies of glacial-marine sediments provide a powerful approach to describe subglacial
geology, sediment transport pathways, and past ice sheet dynamics. The marine-based West Antarctic Ice Sheet
(WAIS) is considered highly vulnerable to ocean warming and sea level rise that is likely to cause its rapid and
irreversible retreat. Studies of its past response to climate change are hence essential for projecting its future
behaviour. The application of radiogenic and trace element provenance studies for past ice sheet reconstructions
requires surveying the geographic variability of geochemical compositions of glaciomarine sediments. In this
study, we characterize the provenance of the detrital fraction of 67 Late Holocene marine sediment samples
collected off the Pacific margin of West Antarctica (60°W to 160°W), including 40Ar/39Ar ages of individual
hornblende and biotite grains (> 150 μm), as well as Sr and Nd isotope and trace element composition of the
fine-grained (< 63 μm) sediment fraction. Overall, this approach allows differentiating West Antarctica into five
source regions: the Antarctic Peninsula, Bellingshausen Sea, Amundsen Sea, Wrigley Gulf-Hobbs Coast and
Sulzberger Bay. Minor geochemical variability is found within each individual sector due to local variability in
onland geology. 40Ar/39Ar ages of iceberg-rafted hornblende and biotite grains record primarily Carboniferous to
Lates Quaternary ages (~0 to 380 Ma), with a notable age peak of ~100 Ma, associated with plutonic intrusions
or deformation events during the mid-Cretaceous. Permian-Jurassic 40Ar/39Ar ages are widespread in the
Amundsen Sea sector, marking episodes of large-volume magmatism along the long-lived continental margin.
Metasedimentary rocks and Late Cenozoic alkali basalts in West Antarctica cannot be detected using detrital
hornblende and biotite 40Ar/39Ar ages due to the absence or small grain-size (i.e.<150 μm) of these minerals in
such rocks. These sources can however be readily recognized by their fine-grained geochemical composition. In
addition, geographic trends in the provenance from proximal to distal sites provide insights into major sediment
transport pathways. While the transport of fine-grained detritus follows bathymetric cross-shelf troughs, the
distribution of iceberg-rafted grains shows influence by transport in the Antarctic Coastal Current. Our study
provides the first systematic geochemical characterisation of sediment provenance off West Antarctica, and
highlights the importance of combining multiple provenance approaches in different size fractions of glacialmarine
sediments, and paves the way to investigate past WAIS dynamics.
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