HIGHLY SIDEROPHILE ELEMENTS AND OS ISOTOPE CONSTRAINTS ON THE YOUNGER DRYAS IMPACT HYPOTHESIS: A CASE STUDY FROM HALL’S CAVE, TEXAS

There are many proposed mechanisms, most linked to meltwater cooling, for the cause of the Younger Dryas (YD) Event at ca. 12.9 ± 0.1 Ka that extended over North America and Europe. Among proposed theories, the YD Impact Hypothesis has been extensively debated for which single or multiple extraterrestrial (ET)impact body(ies) hit and exploded over the Earth surface at the onset of the YD period. Discovery of a suite of impact markers, for example, carbon spherules, magnetic grains and spherules, nanodiamonds, and Ir, that peak at or near the YD Boundary at numerous YD sites is consistent with the impact hypothesis. One major drawback of the hypothesis is its lack of marker reproducibility and chronological control for many of their YD sites. Although elevated Ir concentrations have been measured in samples from multiple YD sites, not all sites have increased Ir values and the strata containing the increased concentrations are not contemporaneous. To examine the YD impact hypothesis, we sampled a 60 cm thickness of cave sediments dating 9.4 to 16.1 Ka CAL BP from Hall’s Cave, Texas. We measured highly siderophile element (HSE; Os, Ir, Ru, Pt, Pd, Re) concentrations and Os isotopes. 31 of the samples, including 4 of 5 from the YD boundary layer, have chondrite-normalized (CI) HSE concentration patterns similar to those of upper continental crust (UCC) and ¹⁸⁷Os/¹⁸⁸Os ratios of 1.2 to 2.5, which indicate no significant ET HSE input. 7 samples, including 1 of 5 samples from the YD boundary layer, and 6 from layers above and below the boundary have contradictory terrestrial versus ET results. Their ¹⁸⁷Os/¹⁸⁸Os ratios are 0.12-0.42, which are ET values, while HSE values show UCC-like HSE patterns. Also, Os abundances are up to 2 orders of magnitude higher than UCC. These results exclude the possibility of a cosmic contribution, which would have resulted in flatter CI-normalized HSE patterns with as little as 0.5 % mixture in the target material. Furthermore, the 7 layers with low ¹⁸⁷Os/¹⁸⁸Os ratios would each require a separate impact event at times ranging from 11.7 to 16.1 Ka RC, and at least 3000 years before the YD’s onset. This is difficult to reconcile with a hypothesis that a single impact or airbust event caused the YD cooling episode. An alternative explanation based on Hall’s Cave data is episodic, distant volcanism occurring over this time period.