Continuing Holocene retreat in West Antarctica

There are several methods available that allow us to determine the past configuration of ice sheets; these are primarily marine geophysical and geomorphological techniques, but one relatively new technique is substantially improving our understanding of the history of West Antarctica and is worthy of special mention. Using isotopic analysis, it is now possible to determine how long rocks have been exposed to cosmic radiation. When this technique is applied to glacial erratics deposited on the slopes of nunataks

160W 180 160E

160W 180 160E

Figure 42.7 The retreat of the grounding line in the Ross Sea embayment since the LGM. The figure shows that the retreat was substantially later than the post-LGM rise in global sea level, and is perhaps still ongoing. (Redrawn from Conway et al. (1999), courtesy H. Conway, University of Washington.)

Figure 42.7 The retreat of the grounding line in the Ross Sea embayment since the LGM. The figure shows that the retreat was substantially later than the post-LGM rise in global sea level, and is perhaps still ongoing. (Redrawn from Conway et al. (1999), courtesy H. Conway, University of Washington.)

during the last deglaciation, we obtain dates for when those erratics were last covered by a significant thickness of ice, i.e. the timing of deglaciation at that point.

This method of cosmogenic dating has been applied in several areas of Antarctica (Brook et al., 1993; Ackert et al., 1999; Tschudi et al., 2003), but one study provides a detailed insight into the continuing response of the ice sheet to changes after the LGM. Stone et al. (2003) used rock-exposure dating to provide evidence of more than 700 m of thinning of the ice sheet in Marie Byrd Land over the past 10kyr. This provides direct evidence of prolonged retreat of the ice sheet in this area in response to changes after the LGM. Indeed, their measurements show no evidence that the Holocene thinning of the ice sheet has slowed in the past few millennia, and it is entirely possible that Holocene deglaciation of Antarctica may not yet be complete, but continuing at a similar rate to the last few millennia.

A similar type of signal has also been noted in the retreat of the grounding line across the Ross Sea onto the Siple Coast (Fig. 42.7, reproduced from Conway et al., 1999). Conway et al. used a variety of glaciological and geological sources to map the retreat of the grounding line in the Ross Sea embayment since the LGM. Although there is uncertainty over the precision of each of the dates, they found reason to believe that the retreat since the LGM was linear, and is still ongoing.

Although Stone et al. (2003) measured ice thickness and Conway et al. (1999) measured ice-sheet extent, both studies came

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Figure 42.8 Major glaciers in West Antarctica.

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to the same conclusion, that the post-LGM response of the ice sheet in West Antarctica was not tied to periods of rapid change in sea level, but has been prolonged and may be ongoing. The mean rate of ice-sheet thinning noted by Stone et al, ca. 7cmyr-1 is of sufficient magnitude that eventually it could become measurable using satellite altimetry, but the likelihood is that it will take many decades and huge improvements in our understanding of snowfall variability before we can be confident in assigning some portion of modern elevation change to the long-term trend, without further evidence from cosmogenic dating.

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