THE MAMMOTH SITE AT HOT SPRINGS, SOUTH DAKOTA: USE OF OSL DATING TO CALIBRATE THE SINKHOLE TIME MACHINE

The Mammoth Site of Hot Springs, SD is an active paleontological excavation site and contains the highest concentration of mammoth remains in the world. The current mammoth count is 61, including 58 North American Columbian and 3 woolly mammoths; the first time both species have been found together. The sinkhole formed in the late Pleistocene when a ceiling overlying a cavern in the Minnelusa limestone collapsed opening a 20 m deep 37 m X 46 m wide sinkhole. The collapse caused a vertical breccia pipe to form along with ground collapse of the relatively soft Spearfish Shale. The breccia pipe provided a chimney-like opening for warm artesian water to percolate upward creating a steeply-sided pond. Apparently enticed by the warm water and pond vegetation, the mammoths entered the pond to eat, drink or bathe. On occasion the mammoths were trapped as they were unable to find a foothold to scale the steep banks during periods of low water. Once trapped the mammoths ultimately died of starvation, exhaustion, or were drowned. The watering hole, thought to be active for about 350-700 years, slowly infilled with silt. Eventually the sinkhole filled, and the artesian spring diverted to the lower elevation of Fall River, as the river cut deeper in the valley floor. Over thousands of years, the “hardened mud plug” inside the dried-up pond has remained stable. The surrounding sediment was subsequently eroded, leaving the sinkhole as a high point on the landscape.

The only geochronology on the sinkhole was obtained by radiocarbon ages on mammoth bone apatite, although apatite ages are now known to be highly unreliable. These results indicate that the sinkhole was used by mammoths about 26 ka ago. Samples for optically stimulated luminescence (OSL) were taken recently to verify these initial age estimates. Six samples were obtained throughout the excavated sinkhole and preliminary ages indicate that the sinkhole is older than 26 ka. Luminescence characteristics of the grains in the sediment will be discussed and shown along with the dates. Whether or not the sinkhole remained on the landscape longer than 700 years remains unanswered as of this writing.