LATE PLEISTOCENE PERMAFROST CONDITIONS ALONG THE SOUTHERN MARGIN OF THE LAURENTIDE ICE SHEET (Invited Presentation)

Permafrost covered extensive areas of North America during past glacial periods of the late Pleistocene, yet determining the precise timing of permafrost emplacement beyond the Last Glacial Maximum (LGM) has proved challenging. This has primarily stemmed from the limitation of the geochronological methods traditionally used to date geomorphic relicts of permafrost as well as the preservation quality of those relicts beyond the LGM. Speleothems (cave carbonates), in combination with the U-series geochronometer, have provided precisely dated records of past climate change and have also proven useful for determining past permafrost conditions at high-latitudes and -altitudes because their growth requires liquid water. Here we present a new record of speleothem growth from Cave of the Mounds in southwest Wisconsin based on 141 U-series ages that span the last 250 ka (Batchelor et al., 2019, Geophysical Research Letters). This record demonstrates that local permafrost conditions were sustained during MIS 2 but intermittent across MIS 6. During MIS 2, subzero annually-averaged air temperatures persisted in southwest Wisconsin forming perennially frozen ground from 33 – 14 ka that inhibited speleothem growth. In contrast, we find recurring speleothem growth throughout MIS 6, including the coldest interval of the glacial phase. A compilation of published speleothem U-series records across central North America indicates that this pattern persists regionally. Furthermore, this result is consistent with a network of sea surface temperature reconstructions from mid- and tropical latitudes that suggest global temperatures were approximately 0.6 ± 0.4 °C warmer during MIS 6 than MIS 2.