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LATE PLEISTOCENE ECOSYSTEM EVOLUTION IN SOUTHEASTERN NORTH AMERICA: A TRANSDISCIPLINARY APPROACH USING FOSSIL AMERICAN ALLIGATOR ENAMEL ISOTOPES**

Abstract

Existing research points to utilizing ancient ecosystem functionality as a means of understanding contemporary biological responses to climate change and human population expansion. In this study, we use enamel isotope values (δ13C, δ18O) from modern and fossil American alligators (Alligator mississippiensis) from the Clark Quarry Site Complex (CQSC; Brunswick, GA) dating to ~60,000 years ago to better understand the aquatic ecosystems and climatic conditions of coastal landscapes in southeastern North America during the late Pleistocene. In particular, we use these data as a proxy for the position of the CQSC relative to the paleoshoreline during this period, which has been variably hypothesized in the existing literature. Serially sampled δ13C values from modern alligator enamel (N=13; Jekyll Island, GA and Aiken, SC), after correcting for contemporary atmospheric CO2 levels, averaged -10.8‰ (±6.2), while δ18O values averaged 1.0‰ (±1.7). δ13C values from serially sampled fossil alligator enamel (N=14; CQSC) averaged 8.3‰ (±1.4), while δ18O values averaged -2.9‰ (±2.2). These data indicate that CQSC alligator δ13C values are intermediate between modern alligator samples characteristic of strictly marine (Jekyll Island, GA) or strictly freshwater (Aiken, SC) environments, and CQSC alligator δ18O values are depleted relative to those from all modern aquatic systems sampled here (freshwater, brackish and marine). These data suggest that 1) the CQSC was part of a brackish system during the late Pleistocene, and 2) southeastern North America was dominated by cooler ambient temperatures during this period. This pattern in temperature is also supported by δ18O values from CQSC megaherbivores (Mammuthus columbi, Bison latifrons). These enamel data indicate that the CQSC ecosystem was markedly different from those characteristic of the region today. Our findings can be used as an ecological framework for testing hypotheses related to the extinction of species due to climatic changes and the expansion of human populations during the late Pleistocene.

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