The Last Glacial Maximum

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We distinguish the LLGM from the LGM to emphasize the regional variability in the timing of ice-sheet and glacier fluctuations relative to the globally integrated sea-level record used to define the LGM.

There is evidence that some ice margins experienced small (tens of kilometers) fluctuations while at or near their LLGM position, such as the New England margin at Martha’s Vineyard or the margin of the Ohio-Erie lobe in Ohio (7). We define the termination of the LLGM on the basis of when the margin began a sustained retreat from its LLGM position.

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The CIS and IIS remained at their maxima until after 19 ka. The southern margin of the CIS (where dating constraints are best) retreated from its maximum extent between 16 and 17 ka, whereas the IIS was the last of the global ice sheets to start to retreat from its maximum extent at ~13.5 ka (Fig. 3B).

Several additional lines of evidence support a significant change in WAIS volume at this time. The isotopic (δD and δ¹⁸O) records from two ice cores bordering the Ross Sea (the Siple and Taylor Domes) show an abrupt warming at 14.5 ka (15, 16), which may reflect ice-surface lowering or some regional climate change induced by ice-surface lowering. If attributed solely to a change in ice-surface elevation, the 3° to 4°C warming at Siple Dome (16) would indicate 500 to 650 m of ice-surface lowering, assuming a free atmospheric lapse rate of 6°C per 1000 m. This magnitude of lowering is supported by ice-sheet modeling, which suggests thinning of Siple Dome ice by 350 m between 14 and 15 ka (17). Within the large dating uncertainties of dated marine samples, widespread retreat of the Antarctic Peninsula Ice Sheet margin on the continental shelf also occurred at this time (18). These lines of direct evidence for WAIS retreat at the time of MWP-1A are consistent with geophysical modeling of the noneustatic component of the sea-level rise during MWP-1A, which suggests a dominant source from Antarctica (19).

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Based on a larger data set, the mean age for regional deglaciation of mid-latitude Southern Hemisphere mountain glaciers (17.7 ka) is not significantly different from the previously reported mean age (17.3 ka), using the same scaling factor (21). Nevertheless, the age of regional deglaciation in the mid-latitude Southern Hemisphere is significantly younger than the regional deglaciation age established for LLGM glaciers at northern mid-latitudes (Fig. 3A), a finding counter to the conclusion that deglaciation in the two hemispheres was synchronous (21). The source of this disagreement appears to be that Schaefer et al. (21) identified “outer LGM” and “inner LGM” moraines in any given region, which in western North America are typically 20 to 21 ka and 16.5 to 17.5 ka, respectively (22). In these cases, and to be consistent with the concept of LLGM, we used the TCN ages of the “outer” moraines to mark the onset of regional deglaciation from the LLGM, whereas we viewed the “inner” moraines as recording a subsequent millennial-scale ice-margin fluctuation that is unrelated to the termination of the LLGM.

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To estimate this mass increase, we used the area of the LIS at 13 ka (7.35 × 10⁶ km²) multiplied by the mass balance increase of 0.17 m year^–1 over 6500 years, resulting in 24 m of sea-level equivalent. The LIS expanded to its LGM area of 11.63 × 10⁶ km² over 6500 years, resulting in an average annual rate of area increase of 611 km² year^–1. The assumption that the mass balance increase of 0.17 m year^–1 applied to this expanding area over 6500 years results in an additional 7 m of sea-level equivalent.

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