Clumped Isotope Thermometry in Plant-Derived Carbonates

The genus Celtis includes widespread trees that produce drupes with aragonite endocarps, or “hackberries.” These carbonate endocarps are preserved in the fossil record, often in cave deposits or packrat middens, and thus are targets for paleoclimate reconstructions. Stable oxygen isotopes in Celtis endocarps have been used as proxies for oxygen isotopic composition of past stream water and for paleothermometry. Here, we explore the suitability of hackberry carbonates for paleoclimate reconstructions based on carbonate clumped-isotope thermometry. We sampled modern hackberries grown at sites across North America (n = 37) for stable and clumped isotope analyses. Measured clumped-isotope temperatures are found to be within the range of measured local modern growing season surface temperatures and typically in dual clumped-isotope equilibrium. As such, we propose that hackberry clumped-isotope measurements can be used to reconstruct past Earth-surface air temperatures.