CLUMPED ISOTOPES OF HACKBERRY ENDOCARP CARBONATE RECORDS INFORMATION ON GROWING SEASON TEMPERATURE

The genus Celtis includes a range of widespread trees producing drupes with carbonate endocarps, or “hackberries,” composed of aragonite. Fossil hackberries have been found stored in packrat middens and human deposits throughout the Quaternary across multiple continents (e.g., North America, Europe and Asia). The carbonate endocarps are stable and can withstand diagenesis due to their mineral structure, and as such, are an excellent potential tool to examine ancient environments, particularly ones occupied by early humans. Previously, stable carbon and oxygen isotopes in Celtis endocarps have been used as a proxy for environmental water and, based on correlations between the ¹⁸O/¹⁶O ratio of water and temperature, as a potential paleothermometer. Here we present data exploring the suitability of hackberry aragonite for use a paleothermometer based on carbonate clumped-isotope analyses. For carbonate formed in internal isotopic equilibrium, the clumped isotopic compositions is solely a function of formation temperature. The carbonate endocarps within drupes generally form two months into the growing season after which single isotope values are relatively stable, thus, endocarps should reflect peak growing season air temperatures. We will present results from the measurement of 31 different modern endocarps collected from a latitude range of 30.8-47.6 °N. Temperatures generally range from 10-30°C and generally correspond to reasonable air temperatures for the expected growing season. We will discuss how this paleothermometer could be applied to study hackberries from cave excavations of caches deposited by humans and mammals during the Pleistocene-Holocene transition like those found in Alabama, South Dakota, Nevada, Spain, and more.