Carbonate clumped isotope paleothermometry: a review of recent advances in CO2 gas evolution, purification, measurement and standardization techniques

Carbonate clumped isotope paleothermometry is based upon the principle that the formation temperature of carbonates is proportional to the relative abundance of 13C18O16O in CO2 produced through carbonate acid digestion. Furthermore, the relative abundance of 13C18O16O is independent of the δ18O value of parent water, providing a viable alternative to the classic oxygen isotope carbonate-water paleothermometer. The temperature resolution of this newly developed paleothermometer primarily relies on the analytical techniques required to determine the clumped isotope composition of a carbonate mineral. These analytical techniques involve: (1) CO2 gas evolution, in which carbonate is reacted with phosphoric acid, yielding CO2 with a clumped isotope composition proportional to the carbonate formation temperature; (2) CO2 purification, in which a rigorous purification of acid-liberated CO2 is achieved by removing contaminants; (3) clumped isotope measurement, in which a customized gas-source isotope ratio mass spectrometer quantifies the raw clumped isotope composition (Δ47) of the purified CO2 gas; and (4) inter-laboratory standardization, in which raw Δ47 values are normalized with respect to a community-accepted reference frame. This review provides an overview and comparison of the analytical techniques currently utilized in stable isotope laboratories for Δ47 measurements, specifically discussing carbonate pre-treatment techniques, phosphoric acid digestion systems, isotope and temperature effects during carbonate phosphoric acid digestion, CO2 purification processes, and the challenges associated with the measurement and standardization of the Δ47 value. Continued refinement of these analytical techniques will aid in reducing sample size, increasing sample throughput, and improving Δ47 external precision and associated paleotemperature resolution. Furthermore, these advances may assist in elucidating the source of discrepancies in slope between two groups of reported carbonate clumped isotope thermometer calibrations.