A HIGH-PRECISION U-TH CHRONOLOGY OF CALCITE DEPOSITION AT CAVE OF THE MOUNDS, WISCONSIN, AND ITS IMPLICATIONS FOR CLIMATE AND PERMAFROST IN THE LATE PLEISTOCENE

Permafrost forms when mean annual air temperatures are consistently below freezing, and it plays a critical role in a diverse array of environmental and climatic systems, including terrestrial carbon storage and subglacial ice sheet processes. Our understanding of late Pleistocene permafrost extent in the upper Midwest is temporally restricted to the last 25 ka and primarily derived from radiocarbon dates. Fortunately, expansive areas of the upper midcontinent of North America are underlain by cave carbonates (speleothems), which can be precisely dated using the ²³⁰Th-²³⁴U chronometer to 600 ka (Edwards et al., 1987). Since speleothem growth requires water, and therefore temperatures above freezing, these deposits can be used as a tool to constrain the extent of past permafrost and to better reconstruct pre-Last Glacial Maximum (LGM) climate near the Laurentide Ice Sheet (LIS) margin.

Seventeen different speleothems were sampled from Cave of the Mounds, a cave located in the unglaciated Driftless Area of southwestern Wisconsin. This cave represents an ideal study site because it is located <20 km from the maximum extent of the LIS margin during the LGM. A total of 132 U-series dates were analyzed and span several glacial-interglacial cycles, ranging from 257-2 ka. Over this time period, these data distinguish a notable period of no calcite growth (hiatus) from 33 to 14 ka that overlaps Marine Isotope Stage (MIS) 2. The end of this hiatus (14 ka) corresponds with previously published geomorphological observations that constrain the onset of permafrost retreat in this area (Mickelson et al., 1983; Attig et al., 1989; Clayton et al, 2001). Our results demonstrate that between 233 and 33 ka, there is no significant hiatus in speleothem growth at Cave of the Mounds. We conclude that over the last two LIS advances (MIS 2, 6), the only prolonged duration of continuous permafrost in southwestern Wisconsin occurred relatively late during the last glacial period, which has major implications for constraining permafrost extent and climate conditions at these latitudes prior to the LGM.