Glacial processes and sediment provenance in basal ice, subglacial and fluvial sediments from Greenland: insights from mineralogy, grain morphology, and isotopic analyses

The accelerated melting of the Greenland Ice Sheet is one of the consequences of current global warming. In addition to being affected by Arctic amplification, Greenland could contribute dramatically to future sea-level rise. However, our current knowledge of the evolution of the Greenland Ice Sheet (GrIS) during the warmest periods of the Quaternary, as well as of subglacial geology and geochemistry, remains limited, notably due to the scarcity of available basal ice and subglacial sediment samples. Within the framework of the ERC Green2Ice project, we present preliminary results from the analysis of basal ice and subglacial sediments from the Camp Century ice core (1966, northwestern Greenland, 1388 m depth beneath the ice sheet, frozen bed). For comparison, we also studied samples collected from different glacio-geological settings in the Kangerlussuaq region (western margin of Greenland): (i) a subglacial drilling sample (H1-1, 1250 m depth, temperate bed) and (ii) a sediment sample from the Kangerlussuaq River. Morphological, mineralogical, and isotopic analyses were conducted to characterize the geological and geochemical nature of the debris, their provenance, and the sequence of processes recorded, such as deglaciation phases and subglacial weathering. Six samples (four from the Camp Century basal sediment section, one from the Kangerlussuaq River, and one from the H1-1 drill) with grain sizes ranging from 125 µm to 2 mm were analysed using Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive Spectroscopy (EDS). Grain morphologies observed under SEM reflect different transport modes (glacial, fluvial, aeolian), allowing the identification of local phases of ice-sheet retreat and advance. EDS provides information on grain mineralogy, notably the presence of clay coatings, which are indicative of stable, ice-free environmental conditions. The clay fraction of the basal and subglacial ice from Camp Century, as well as that of H1-1 and the Kangerlussuaq River, was analysed by X-ray Diffraction, providing information on the different clay mineral species present, some of which indicate deglaciation conditions. Finally, the isotopic ratio 87Sr/86Sr and ɛNd of Camp Century samples and those from the Kangerlussuaq region constrain the provenance of the debris. The morphological and mineralogical analyses reveal (i) distinct geological source areas depending on location and (ii) a complex grain history combining sedimentary transport and weathering phases during ice free conditions. 87Sr/86Sr and ɛNd isotopic analyses from the silicates of the basal and subglacial ice samples will provide further constrains on the source materials, this constraint being notably key to assess the origin of the clay fraction in the silty ice of Camp Century, and in the intermediate ice rich unit with the basal sedimentary section.