The Holocene history of George VI Ice Shelf, Antarctic Peninsula from clast-provenance analysis of epishelf lake sediments

Abstract

The Antarctic Peninsula has experienced a dramatic increase in temperature and the loss of c. 14,000 km 2 of ice-shelf area in recent years. During this time George VI Ice Shelf (GVIIS) has remained relatively intact, but 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 melt-water 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, c. 9600-7500 yr 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 c. 9600-8450 yr 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 c. 7500 yr 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 c. 7500 yr BP and the reformation of George VI Ice Shelf.

Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years

“…Firstly, it has two ice fronts that are almost 500 km apart and experience quite different climatic conditions. Secondly, it is an ice shelf that is constrained within a narrow channel (25 km wide in the north, widening to over 70 km in the south) and loses most of its mass to melting (both surface melt-pools and basal melting), rather than changes in ice-dynamics brought about by a changing calving rate (Roberts et al, 2005).This suggests that this ice shelf may be more sensitive to changes in ocean conditions, and less sensitive than its neighbours to atmospheric change (Doake and Vaughan, 1991b).…”

Mechanisms of Holocene palaeoenvironmental change in the Antarctic Peninsula region

“…Near-coastal lakes or lagoons can also be used as isolation basins to determine relative sealevel change (Bentley et al, 2005a). Epishelf lakes can provide proxy records of ice shelf presence and absence (Bentley et al, 2005b;Smith et al, 2006Smith et al, , 2007Roberts et al, 2008).…”

“…For example, immediately post-dating the early Holocene climate optimum there was a retreat of George VI Ice Shelf on the west side of the AP, with the onset of collapse at 9595 cal yr BP and complete or partial re-formation by 7945 cal yr BP (Bentley et al, 2005b;Smith et al, 2007, Roberts et al 2008. This coincides with evidence of the southward intrusion of warmer, more subpolar waters at 9000-6700 cal yr BP in the Palmer Deep (Leventer et al, 2002).…”

Glacial discharge along the west Antarctic Peninsula during the Holocene

“…1a) during a time of widespread warmth along the WAP exemplified by ice sheet retreat and thinning 2 , high SSTs 3 and thin sea ice cover with warm winters 4 in Palmer Deep, the collapse of the George VI Ice Shelf (Fig. S1) at 9.6 ka 16 and increased influence of glacial melt and precipitation in Maxwell Bay 13 . This warmth was concomitant with the last influence of orbitally-forced deglaciation 5,15 and ENSO-sensitive proxy records from the Peru margin and Ecuador 5,17 that indicate a persistent warm, mean La Niña-like state (relative to mean El Niño-like conditions).…”