OPTICALLY STIMULATED LUMINESCENCE DATING OF SEDIMENT IN NATURAL TRAP CAVE, WYOMING

Paleontological excavation materials, such as vertebrate remains, microfossils (e.g., pollen, diatoms) and sediment can also be of critical importance for paleoclimatic reconstructions. Natural Trap Cave, Wyoming, has been the focus of intermittent excavations from the 1970's to now. Early work established depositional ages for the various sedimentary units, using radiocarbon ages from fossil material, correlative stratigraphy, and environmental markers (directly or indirectly) such as pollen assemblage changes. Radiocarbon ages analyzed in the 1970s and 1980s place the fossils between 23-12 ka, but some of the lower fossiliferous sediment units contain volcanic ashes and may be as old as 130-110 ka. Expeditions between 2014-2018 focused on understanding the stratigraphy, ages of the sediment and of preserved volcanic ash. We report on the first optically stimulated luminescence (OSL) ages from the cave sediments. This new data, combined with other excavational information, provides new insights into the depositional and paleo-environmental records during oxygen isotope stages 2 through 6 (and beyond).

Obtaining the OSL samples was challenging. Access to the cave is via a 15-foot-wide hole in the ground and the opening is almost impossible to see until you're next to it; it is also the reason there is such a rich record of Quaternary history at its bottom. Over tens of thousands of years, several thousand insufficiently wary animals plummeted more than 80 feet to death. These animal remains are entombed by sediments entrained into the cave through the same opening through either dust or overland flow, but also with autogenic sediments such as limestone derived from the cave walls and ceiling. Hence, depositional processes in the cave are not immediately obvious, requiring multiple methods to ensure stratigraphically successive sampling. Sampling locations are dispersed but representative of the sedimentologic units within the cave. Understanding continuity and duration of the deposition in the cave is better revealed with OSL. Eight OSL ages, with an additional five more, collected in 2018, will be discussed in the context of the stratigraphy and other known age data; the majority of the OSL ages however, are firmly in the latest Pleistocene at no older than 30 ka.