During past glacial periods, extensive areas of North America were covered by permafrost. The timing and extent of these paleo-permafrost conditions, however, remains ambiguous. Here we present a 250,000-year record of speleothem growth from a midlatitude North American cave and report 141 U-Th ages with hiatuses in growth that reflect the development of temporally continuous permafrost. Combined with U-Th ages from other speleothem studies, we demonstrate that regional permafrost conditions occurred during both of the prior two glacial maxima but were markedly shorter in duration during the penultimate (Marine Isotope Stage 6, MIS 6) versus the last (MIS 2) glacial period. Notably, a network of sea surface temperatures indicates that mid- and low-latitude temperatures were 0.9 °C ± 0.2 °C warmer during the culmination of MIS 6 versus MIS 2. Our results illustrate the importance of developing regional paleo-permafrost records and highlight the sensitivity of permafrost conditions during glacial periods to relatively small differences in global-scale temperature.
Bibliographical noteFunding Information:
This work was supported by the University of Wisconsin-Madison (S. A. M.), the Center for Climatic Research (I. J. O.), the Geologic Society of America (C. J. B.), and the National Science Foundation (P2C2-1805629 to S. A. M. and I. J. O.). We thank J. Klimczak and A. Wescott for their permission to collect samples at Cave of the Mounds. Samples analyzed in this study are curated at the University of Wisconsin-Madison Geology Museum. All speleothem U-Th data and SST reconstructions compiled for this study are entirely contained within the paper and references and are available at the National Center for Environmental Information.
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