The Holocene history of George VI Ice Shelf, Antarctic Peninsula

Abstract

Meteorological records have shown that the Antarctic Peninsula region has warmed at a rate of 3.7 ± 1.6 С during the last century. One of the most publicised aspects of this warming has been the retreat and disintegration of several of the regions ice shelves. It is unknown at present however, whether advance and retreat of these ice shelves has occurred repeatedly in response to natural Holocene climate change, or whether their recent collapse is the result of the recent rapid regional warming that has been linked to anthropogenic forcings. George VI Ice Shelf is the largest ice shelf on the western side of the Antarctic Peninsula and its northern margin marks the southern most latitudinal limit of recent ice shelf retreat. At Ablation Point, on the east coast of Alexander Island, the ice shelf impounds two epishelf lakes: Moutonnée and Ablation. These lakes are tidal, stratified water bodies with a lower marine layer that extends under the ice shelf and an upper freshwater layer whose maximum thickness is determined by the draught of the ice shelf. The first aim of this thesis was to document the physical limnology and sedimentary environment of Moutonnée and Ablation Lakes. Collectively, this information has led to the development of a conceptual model for detecting ice shelf collapse in epishelf lakes and has provided important baseline data for any future changes in the stability of the ice shelf. The second aim was to reconstruct the long-term (Holocene) history of George VI Ice Shelf through detailed analysis of lake sediments from Moutonnée and Ablation Lakes. In addition, this thesis provides a detailed review of the dynamics of George VI Ice Shelf, which provides an important context to any future changes in the configuration of the ice shelf. Studies of sediment cores extracted from Moutonnée and Ablation Lakes have included; micropaleontology (diatoms/ foraminifera), stable isotope (δ(^18)0, ô(^13)C), geochemistry (Corg, Norg, C/N ratios) and physical (grain-size/magnetic susceptibility) analyses. Together with the ideas developed in the conceptual model, these data provide robust evidence for one period of past ice shelf absence during the Holocene. The timing of this period has been constrained by 10 AMS(^14)C dates performed on mono-specific foraminifera samples. These dates suggest that George VI Ice Shelf was absent between ca. 8962 cal. (calibrated) yr B P. and ca. 7945 cal. yr в.р. This early Holocene absence immediately followed a period of maximum Holocene warmth that is recorded in some Antarctic ice cores and coincides with an influx of warmer ocean water onto the western Antarctic Peninsula shelf at ca. 9000 cal. yr в.р. The absence of the ice shelf during this time interval suggests that early Holocene ocean-atmosphere variability in the Antarctic Peninsula was greater than that measured in recent decades. The lake sediment record from Moutonnée Lake also provides evidence to suggest that Holocene (ca. 8000 to 0 cal yr в.р.) climatic change on the Antarctic Peninsula may have been coupled to atmospheric and/or oceanic changes recorded in the tropical western Pacific Ocean.