Carbonate clumped isotope (Δ<sub>47</sub>) thermometry andits application in paleoelevation reconstruction

Abstract

Paleoaltimetry provides crucial constraints on tectonic processes and precise paleotemperature measurement is an important component in paleoelevation reconstruction. Carbonate clumped isotope thermometry is based on homogeneous isotope equilibrium and is used to provide direct estimation of carbonate formation temperature independent of δ 18O of the parent water from which the carbonate formed. The error regarding clumped isotope temperature estimation can be reduced to ±2°C. The characteristics of temperature-dependence, and the small uncertainty using carbonate clumped isotope thermometry, greatly improve accuracy and precision in paleoelevation reconstruction. In this review, we introduce the basic theory, laboratory methodology and standardization processes underpinning carbonate clumped isotope thermometry. Theoretical calculations indicate a linear relationship between parameter Δ47 and 1/ T 2, which establishes the foundation from which carbonate clumped isotopes provide reliable paleotemperatures. The laboratory treatment of carbonate includes two systems: the modified Kiel IV device for small weight samples (0.15−0.2 mg) installed within microvolume nitrogen traps and packed microvolume columns; and a heavy sample device that is installed with McCrea/common bath acid digestion devices, nitrogen traps and packed columns. These systems are designed to remove potential interference from water and hydrocarbons in the CO2. The measurement of purified CO2 is finished on gas source isotope mass ratio spectrometry (IMRS), especially on MAT 253 and 253 Plus machines. The standardization of the Δ47 to an absolute reference frame (ARF) accounts for the non-linearity and scaling effect of the IMRS, and the ARF also enables direct comparison of data from different laboratories. The practical procedure for the standardization includes interpolating the relationship between the measured δ 47 and Δ47 of the equilibrium gases, and conversion of the measured Δ47 to theoretical values to derive an empirical transfer function. Due to the great potential for solving Earth science problems, empirical calibration of the carbonate clumped isotope thermometry has been explored over a wide range of temperature in different types of carbonates, but is still plagued by discrepancies between different calibrations, probably due to variability in acid preparation, CO2 purification and data reduction methods. We summarized all the published synthetic calcite data to ARF with the same acid fractionation factor and yield a composite empirical calibration that could resolve the discrepancies between different calibrations. However, bio-genetic carbonates still fail to yield a universal empirical calibration, even when using the same methodology. The cause of this difference is probably due to different environmental, species discrimination and vital effects. The application of carbonate clumped isotope thermometry to paleoaltimetry is based on the theory that the δ 18O of precipitation (meteoric water) decreases proportionally as elevation increases. Knowing the formation temperature and δ 18O of carbonate, we can trace back to the precise value of δ 18O in precipitation, and then, using the theoretical model, or empirical relationship, between the δ 18O of precipitation and elevation, reconstruct paleoelevation. Furthermore, clumped isotope thermometry can be used to test for diagenetic effects on pedogenic samples to ensure they retain original sedimentary isotopic compositions. We summarized previous applications of carbonate clumped isotope thermometry to accurately and precisely reconstruct the paleoelevation of the Andes mountains, Western North America and the Tibetan Plateau to provide insights into the uplift history and geodynamic process of those regions. Despite the great potential for carbonate clumped isotopes in paleoelevation reconstruction, there are still challenges regarding equilibrium mechanisms, the discrepancies between different calibrations and the broader application in different carbonates. To resolve these challenges, more theoretical research is needed, as well as improving the resolution of IMRS.