Do Uncharred Plants Preserve Original Carbon and Nitrogen Isotope Compositions?

Abstract

The isotopic compositions of plants can provide significant insights into paleodiets, ancient agricultural activities, and past environments. Isotopic compositions of charred (aka carbonized) ancient plant remains are typically preferred over those of uncharred/uncarbonized plants, both because charred plants are more commonly preserved and because early research suggested they experience less post-depositional isotopic alteration. In this paper, we re-explore the question of whether uncharred plants experience large-magnitude post-depositional changes in carbon and nitrogen isotope compositions by analyzing Terminal Pleistocene–Early Holocene plant specimens from rockshelters in the Escalante River Basin (Colorado Plateau, southeastern Utah). Several lines of evidence, including C3-CAM differences, plant-part comparisons, and dietary estimates from ancient herbivore collagen, suggest that the original carbon isotope compositions of these plants have not been significantly altered. The preservation status of plant nitrogen isotope compositions is equivocal. The direction of temporal shifts in plant δ15N matches global trends and the magnitude of the shift may have been exacerbated by the extinction of megafauna in an arid environment. However, the Pleistocene plant δ15N values are higher than would be expected based on herbivore bone collagen δ15N. Nevertheless, in contrast to previous research, the ancient uncharred plants in this study did not have exceptionally high δ15N values (> + 25‰). Overall, our research suggests that uncharred plants could be useful substrates for isotopic paleodietary and/or paleoenvironmental studies.