The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO2-charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca2+ and OH- enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO3 at surface springs. This environment enables us to apply the recently developed Δ47-Δ48 dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO2 transformation in a region with the potential for geological CO2 sequestration. We analyzed CaCO3 recovered from various localities and identified significant kinetic fractionations associated with CO2 absorption in a majority of samples, characterized by enrichment in Δ47 values and depletion in Δ48 values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ­47: 0.163‰, ΔΔ­48: -0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 hours to achieve isotopic equilibria. The Δ48/Δ47 slope of samples from the Cedars (-3.223±0.519; 1 SE) is within the range of published theoretical values designed to constrain CO2 hydrolysis-related kinetic fractionation (-1.724 to -8.330). The Δ47/δ18O slope (-0.009±0.001) and Δ47/δ13C slope (0.009±0.001) are roughly consistent with literature values reported from a peridotite in Oman of -0.006±0.002 and -0.005±0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to study CO2-absorption processes in applied systems, including sites of interest for geological sequestration.

Lacustrine, riverine, and spring carbonates are archives of terrestrial climate change and are extensively used to study paleoenvironments. Clumped isotope thermometry has been applied to freshwater carbonates to reconstruct temperatures, however, limited work has been done to evaluate comparative relationships between clumped isotopes and temperature in different types of modern freshwater carbonates. Therefore, in this study, we assemble an extensive calibration dataset with 135 samples of modern lacustrine, fluvial, and spring carbonates from 96 sites and constrain the relationship between independent observations of water temperature and the clumped isotopic composition of carbonates (denoted by Δ47). We restandardize and synthesize published data and report 159 new measurements of 25 samples. We derive a composite freshwater calibration and also evaluate differences in the Δ47-temperature dependence for different types of materials to examine whether material-specific calibrations may be justified. When material type is considered, there is a convergence of slopes between biological carbonates (freshwater gastropods and bivalves), micrite, biologically-mediated carbonates (microbialites and tufas), travertines, and other recently published syntheses, but statistically significant differences in intercepts between some materials, possibly due to seasonal biases, kinetic isotope effects, and/or varying degrees of biological influence. Δ47-based reconstructions of water δ18O generally yield values within 2‰ of measured water δ18O when using a material-specific calibration. We explore the implications of applying these new calibrations in reconstructing temperature in three case studies.

Carbonate clumped isotope thermometry is a useful tool practised in studies of temperature history and fluid composition of surface and subsurface environments, with application to both inorganic and biological precipitates. Its measurements are based upon the propensity with which 13C and 18O isotopes, within a carbonate mineral, are bound to one another, in relation to a stochastic distribution. The quantity of these 13C-18O bonds (commonly referred to as “clumps”) is determined by gas source mass spectrometry on CO2 produced from acid digestion of carbonate minerals, and is controlled by physiochemical parameters of the solution at the time of mineral precipitation. If equilibrium is reached, then 13C-18O abundance, measured against the stochastic distribution and represented by the variable Δ47, can be used to measure the temperature of precipitation of the carbonate without the need to characterize the isotopic composition of coeval fluids. However, long-term reproducibility of these measurements is a critical factor contributing to uncertainties in all calibrations and applications. Here we discuss the impact of using different standardization procedures on the accuracy and precision of Δ47 measurements, as compared across three mass spectrometers with four different configurations within the Tripati Lab at UCLA. Specifically, we assess the long-term reproducibility of carbonate standard Δ47 values across mass spectrometers, using a correction scheme that incorporates either gas and carbonate standards of known composition, or both, and the impact of different approaches for characterizing instrument drift (i.e., averaging for an interval or using a moving window). We also recommend best practices to promote reproducibility.

Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of m/z 47 CO2 for geothermometry, but theoretical calculations and recent experiments indicate paired analysis of the m/z 47 (13C18O16O) and m/z 48 (12C18O18O) isotopologues (referred to as Δ47 and Δ48) can be used to study non-equilibrium isotope fractionations and refine temperature estimates. We utilize 5,465 Δ47 and 3,400 Δ48 replicate measurements of carbonate samples and standards, and 183 Δ47 and 195 Δ48 replicate measurements of gas standards from 2015-2021 from a multi-year and multi-instrument dataset to constrain Δ47 and Δ48 values for 27 samples and standards, including Devils Hole cave calcite, and study equilibrium Δ47-Δ48, Δ47-temperature, and Δ48-temperature relationships. We compare results to previously published findings and calculate equilibrium regressions based on inter-laboratory values. We report acid digestion fractionation factors, Δ*63-47 and Δ*64-48, and account for their dependence on the initial clumped isotope values of the mineral.