Timing and causes of mid-Holocene mammoth extinction on St. Paul Island, Alaska

Relict woolly mammoth (Mammuthus primigenius) populations survived on several small Beringian islands for thousands of years after mainland populations went extinct. Here we present multiproxy paleoenvironmental records to investigate the timing, causes, and consequences of mammoth disappearance from St. Paul Island, Alaska. Five independent indicators of extinction show that mammoths survived on St. Paul until 5,600 +/- 100 y ago. Vegetation composition remained stable during the extinction window, and there is no evidence of human presence on the island before 1787 CE, suggesting that these factors were not extinction drivers. Instead, the extinction coincided with declining freshwater resources and drier climates between 7,850 and 5,600 y ago, as inferred from sedimentary magnetic susceptibility, oxygen isotopes, and diatom and cladoceran assemblages in a sediment core from a freshwater lake on the island, and stable nitrogen isotopes from mammoth remains. Contrary to other extinction models for the St. Paul mammoth population, this evidence indicates that this mammoth population died out because of the synergistic effects of shrinking island area and freshwater scarcity caused by rising sea levels and regional climate change. Degradation of water quality by intensified mammoth activity around the lake likely exacerbated the situation. The St. Paul mammoth demise is now one of the best-dated prehistoric extinctions, highlighting freshwater limitation as an overlooked extinction driver and underscoring the vulnerability of small island populations to environmental change, even in the absence of human influence.We thank all of the inhabitants of St. Paul Island and in particular Jason Bourdukofski, Bill Briggs, Aquilina Lestenkof, Gary Stanley, Mac Mandregan, Phillip Zavadil, Brenda Jones, Connie Newman, and the National Oceanic and Atmospheric Administration for their hospitality, assistance with site access, and field logistics. Simeon Swetzof, Laura Lestenkof, Nick Kozloff, Rob Owens, and Logan Tetov kindly provided specimens of woolly mammoth fossils from personal collections for radiocarbon dating. Voucher specimens have been preserved in the Earth and Mineral Sciences Museum collections at the Pennsylvania State University. The Limnological Research Center provided facilities for core splitting, imaging, description, and sampling. Elle Palkopoulou and Love Dalen provided prepublication access to the Wrangel Island woolly mammoth genome. We thank Nick Holschuh for assistance with figure drafting, Nancy Bigelow and Greg Smith for assistance with fieldwork, and two anonymous reviewers for their careful review and comments. This work was supported by National Science Foundation Grants PLR-1204233, PLR-1203772, and PLR-1203990