Bringing the South Dakota southern Black Hills Mammoth Site formation and fossil discovery to light: a chronological reassignment using Luminescence Dating

The Mammoth Springs (MS) fossil site at Hot Springs, South Dakota, provides a unique opportunity to reconstruct paleoenvironmental conditions. We used luminescence dating on fine sand sized potassium feldspars to establish a chronological framework for the site. In addition, we dated a late Pleistocene paludal proxy site using luminescence and incorporated those results with previous radiocarbon dating. Radiocarbon dating of the MS sinkhole organics suggested that the ages of the sediments that hosted the fossils was ~22-26 ka, while luminescence dating on feldspar grains suggested substantially older ages of ~130-255 ka. Analysis of the equivalent dose dispersion of the luminescence samples showed that the sediments were mostly well bleached prior to deposition. This analysis suggests that eolian and shallow fluvial processes were the dominant transport for sediments into the sinkhole, and that luminescence provides a reliable, more rigorous chronology of the MS fossil site. The luminescence ages (n = 6), show that the fossil-bearing sediments span all of Marine Isotope Stage (MIS) 6 and MIS 7 with the sinkhole drying, cementing, and completely infilling at the start of the MIS 5 at the end of the global penultimate glacial maximum.