The Holocene history of George VI ice shelf, Antarctic Peninsula from clast-provenance analysis of epishelf lake sediments
The Antarctic Peninsula has experienced a dramatic increase in temperature and the loss of ca. 14,000 km2 of ice-shelf area in recent years. During this time George VI Ice Shelf (GVIIS) has remained relatively intact, but it is now reaching its theoretical limit of viability. Epishelf lakes, formed when ice shelves dam the mouths of marine embayments, accumulate sediments that can be used to constrain past ice-shelf behaviour. They are stratified water bodies with an upper layer of fresh meltwater overlying a marine layer of water. Multi-proxy analysis of a sediment core from Moutonnée Lake, an epishelf lake dammed by GVIIS on the east coast of Alexander Island, has recently shown that it retreated to at least the Ablation Point area in the early Holocene, ca. 9600–7500 years BP, demonstrating its vulnerability to periods of atmospheric and oceanic warmth. This study tests this interpretation of ice-shelf collapse through detailed analyses of granulometric, geochemical and Sr and Nd isotope provenance data for > 8 mm clasts from the same cores. Clast data from Moutonnée Lake were compared with geological reference data from two further lakes on Alexander Island (Ablation Lake and Citadel Bastion Lake) and an extensive archive of rocks and isotope-geochemical provenance data from the Antarctic Peninsula region. Underpinning this provenance analysis is the contrast between the plutonic/igneous outcrops in Palmer Land on the western side and the predominantly sedimentary strata of Alexander Island on the eastern side of George VI Sound, and the different patterns in their deposition that would be expected at Moutonnée Lake during periods of ice-shelf presence and absence. Results show that changes in clast distribution and provenance reflect the early Holocene retreat and reformation of George VI Ice Shelf at Moutonnée Lake. The period of ice-shelf retreat was marked by the onset of marine conditions in the basin followed by a rapidly deposited zone of clasts whose provenance, distribution, varied lithology and larger than average clast size are indicative of a period of lake ice disintegration and ice-rafted debris deposition between ca. 9600–8450 years BP. Igneous clasts in the ice-rafted debris have a close affinity with the plutonic provinces of western Palmer Land suggesting transport to the site by icebergs or recycling through Alexander Island strata/moraines. When epishelf lake conditions returned after ca. 7500 years BP, the clasts were smaller, the number of lithologies more limited and the assemblage dominated by olive-green vitric tuff and pale-green rhyolitic clasts that isotope data link to Early Cretaceous volcanic activity in Palmer Land. Their dominance in the upper part of the core most likely relates to increased erosion of Palmer Land ash/tuffs from volcanic outcrops on the floor of Moutonnée Valley and implies a downturn in climate after ca. 7500 years BP and the reformation of George VI Ice Shelf.
Authors: Roberts, Stephen, Hodgson, Dominic, Bentley, M.J., Smith, James, Millar, Ian, Olive, V., Sugden, David E.