Response of the Ice Sheets to Glacial Climate and Late Quaternary Ice Sheet Reconstructions

The climate around Antarctica during ''full glacial'' periods such as the LGM is likely to have been highly conducive to the presence of ice-sheets. Therefore, the 'minimum' reconstruction for the LGM Antarctic Ice Sheets is similar to the present geometry. However, due to the sea-level reduction of around 120m (e.g., Bard et al., 1990; Shackleton, 2000) and ocean temperature changes that occurred during the LGM, the ice-sheets in Antarctica are known to have grown out towards the continental shelf edge in several places. The ice shelves bordering the Antarctic Ice Sheet thickened, grounded and became part of the parent ice mass. A 'maximum'

reconstruction would therefore be an Antarctic Ice Sheet which expanded to reach the continental shelf break right around the continent.

There is some ice core evidence from around Antarctica, including Taylor Dome and Law Dome, which suggests a much lower rate of accumulation at the LGM. The Taylor Dome core also shows that LGM storm paths came from a different direction to the modern day. This suggests a reorganisation of the climate system at least at this part of the ice sheet (Morse et al., 1998). While reductions in both sea-level and air temperature at the LGM would have been conducive to ice-sheet expansion, decreasing accumulation may well have had a restraining effect on icesheet growth. To resolve how the ice sheet responded to such complicated changes in forcing requires the application of numerical models, which we will return to later.

Ice-sheet behaviour in the late Quaternary is coupled with the cryosphere, ocean and atmosphere. For example, if sea ice extent increased due to cooling of the air temperature over the Southern Ocean (Armand, 2000), then the moisture supply to the ice sheet may have decreased and so ice-sheet growth may have been impeded. This introduces the possibility that independent mountain glaciers in Antarctica may have responded differently to LGM conditions than areas connected to an ice-sheet. In other words, ice-sheet advance may have been related to sea level fall while, at the same time, glacier decay may be related to a significant reduction in rates of precipitation.

Since the CLIMAP (1976) reconstruction of Ice Age Earth, there have been numerous reconstructions of LGM ice extent in Antarctica based on evidence from sediments and geomorphology, isostatic rebound and ice-flow modelling. These reconstructions provide estimates ranging from 0.5-2 to 38 m of sea level equivalent (e.g., Budd and Smith, 1982; Nakada and Lambeck, 1988; Colhoun et al., 1992). While the techniques used vary, many of the early estimates placed the grounding line at the edge of the continental shelf, and have relatively steep ice profiles. As such these are at the upper end of estimates of sea-level lowering. More recent work has advanced our understanding of the dynamics of ice flow (in particular the longitudinal profile of ice streams and their controls), produced sophisticated 3D thermomechanical ice-sheet models and provided reliable geomorphic evidence that indicates relatively limited ice expansion in some areas. These advances in understanding have rendered the upper estimates implausible, such that recent reconstructions indicate an increase of Antarctic ice volume at the LGM with respect to today of only 5.9-19.2 m of sea level equivalent (Bentley, 1999; Huybrechts, 2002).

0 0

Post a comment