Carbonate Formation Induced by Evaporation: Controls on Isotopic Composition and Implications for Quantitative Paleoclimate Reconstruction

Isotopic compositions of terrestrial carbonates (e.g., pedogenic carbonates, lacustrine carbonates, and speleothems) reflect the physicochemical conditions of their formation environment and are among the best tools for constraining past continental environmental changes. Particularly, the recent development and application of novel isotope techniques, such as triple oxygen isotope (17O) and clumped isotope analyses (47), to terrestrial carbonates have opened exciting new opportunities for continental paleo-environment and paleo-hydroclimate reconstruction. However, extraction of quantitative paleoclimate information from these carbonate archives can be challenging, partly because they often form under dynamic environments involving intensive evaporation and/or CO2 degassing and accordingly the conventional assumption of isotope equilibrium between these carbonates and their formation water might not apply. Here I present an isotope model of carbonate formation under evaporative conditions, and systematically evaluate the robustness of these carbonates in recording the temperature and isotopic composition of their formation water. This model, building on our recent simulation of isotope effects associated with speleothem formation [1], explicitly tracks the evolution of the abundances of all species and isotopologues containing 12C, 13C, 16O, 17O and 18O (including clumped isotopologues) during evaporative carbonate formation. The model reproduces the characteristic covariation of 18O and 13C observed in many lacustrine and pedogenic carbonates, with the slope of the correlation varying mainly as a function of evaporation intensity. Similar correlations among carbonate 18O, 17O and 47 are also predicted. Under highly evaporative conditions, isotopic compositions of the carbonate precipitates are predicted to deviate significantly from the expected equilibrium with the water, complicating the interpretation of the related isotope signals. The effects of other environmental factors on the isotopic composition of evaporative carbonates and their implications for multi-proxy reconstruction of continental paleoclimate will be discussed at the meeting. [1] Guo and Zhou (2019), Geochim. Cosmochim. Acta, 267, 196-226, doi:10.1016/j.gca.2019.07.028