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. Plain Language Summary Hackberries are tree‐growing fruits that create carbonate mineral structures surrounding their seeds. Geochemical properties of these minerals can give us insight into growing conditions, particularly the growing season temperature of the hackberry plants. We tested the promise and limitations of clumped isotope as a paleothermometer for carbonate growth temperatures using modern hackberries collected across North America. We find recorded clumped‐isotope temperatures are consistent with mean growing season temperatures. We propose the use of hackberry carbonate clumped isotope measurements to reconstruct past environmental temperatures using ancient hackberry deposits found in caves (formed by humans) and middens (formed by animals such as packrats) throughout the past 10,000+ years. Key Points Hackberry plants create an aragonitic mineral endocarp within their fruit which reflects environmental conditions Clumped‐isotope analyses reconstruct temperatures comparable to mean growing season temperatures in modern hackberry endocarps Clumped isotopes in hackberries provide a potential paleothermometer for Quaternary‐aged cave deposits and packrat middens