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 plays a critical role in a diverse array of environmental and climatic systems, including terrestrial carbon storage (Billings, 1987) and subglacial ice sheet processes (Cutler et al., 2000). 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 speleothem samples were collected from well-distributed areas throughout 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 257-2.2 ka. Over this time period, these data demonstrate only a single 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 also demonstrate that prior to the MIS 2 hiatus, continuous calcite growth occurred for ~220 ka at Cave of the Mounds. We conclude that over the last three LIS advances (MIS 2, 6, 8), the only significant duration of continuous permafrost in southwestern Wisconsin occurred relatively late during the last glacial period, which has major implications since this is the first dataset constraining permafrost extent in this region prior to the LGM.