PLEISTOCENE WATER-TABLE FLUCTUATIONS AND SHIFTS IN AQUIFER CONTRIBUTIONS IN WIND CAVE NATIONAL PARK, BLACK HILLS, SOUTH DAKOTA, USA

Wind Cave records a long history of karst formation including Pleistocene calcite deposits that are mostly restricted to the lower levels of the cave. Deposits form coatings on open passage and precipitated under phreatic conditions, although textures and mineralogy also indicate periods of vadose deposition. Coating thickness ranges from a few mm to 1–2 cm and generally decreases with height above the modern water table. These deposits offer an opportunity to understand the history of Pleistocene water table fluctuations as well as changing water sources in the Madison aquifer.

We collected speleothems along an 85 m vertical transect from the top of Boxwork Chimney to Calcite Lake (CL), which represents the modern regional water table. Calcite rafts and outermost surfaces of laminated crusts represent the latest phreatic mineral precipitation at a given site. Basal layers were deposited on altered substrates and represent initial flooding with calcite-saturated water. Most ²³⁰Th/U ages range from about 280–70 ka and vary systematically with height above CL. The resulting paleohydrograph suggests a long period of non-deposition prior to 300 ka followed by a rapid rise of groundwater to its maximum level of ~80 m above CL at ~220 ka. By 70 ka, water levels dropped to ~20 m (long-term average rate of ~0.4 m/ka). Growth rates for 14 subsamples from a single 13-mm-thick laminated crust at ~42 m above CL imply that growth was more rapid during glacial or stadial periods between 276–261 ka, 233–222 ka, and 192–171 ka, with no deposition between 170 and the outermost layer dated at 126 ka. Young phreatic deposits are scarce, although a few rafts within ~17 m of the water table have ages <12 ka.

Isotopes of C, O, Sr, and U were used to evaluate how groundwater sources varied over time. Deposits <12 ka have compositions consistent with deposition from groundwater similar to modern CL that reflects local recharge and relatively short flowpaths. Older deposits precipitated from groundwater with substantially lighter δ¹⁸O and higher ²³⁴U/²³⁸U AR values implying recharge during colder climates and longer/deeper flowpaths. Elevated δ⁸⁷Sr in some samples indicates a greater component of water from the Precambrian core of the Black Hills. Changes in groundwater recharge or aquifer transmissivity are likely drivers of the observed variations.