The potential for carbonate clumped isotope thermometry to independently constrain both the formation temperature of carbonate minerals and fluid oxygen isotope composition allows insight into long‐standing questions in the Earth sciences, but remaining discrepancies between calibration schemes hamper interpretation of temperature measurements. To address discrepancies between calibrations, we designed and analyzed a sample suite (41 total samples) with broad applicability across the geosciences, with an exceptionally wide range of formation temperatures, precipitation methods, and mineralogies. We see no statistically significant offset between sample types, although the comparison of calcite and dolomite remains inconclusive. When data are reduced identically, the regression defined by this study is nearly identical to that defined by four previous calibration studies that used carbonate‐based standardization; we combine these data to present a composite carbonate‐standardized regression equation. Agreement across a wide range of temperature and sample types demonstrates a unified, broadly applicable clumped isotope thermometer calibration.

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ₄₇) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ⁴⁷ and Δ₄₇ values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ₄₇ (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale.

The clumped isotopic composition of carbonate‐derived CO₂ (denoted Δ₄₇) is a function of carbonate formation temperature and in natural samples can act as a recorder of paleoclimate, burial, or diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R¹³_VPDB, R¹⁷_VSMOW, R¹⁸_VSMOW, and λ) impact calculated Δ₄₇ values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed interlaboratory differences in the measured T‐Δ₄₇ relationship. Using the updated parameters, we reprocess 14 published calibration data sets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite data set (n = 1,253 sample replicates), we investigate the possibility for a “universal” clumped isotope calibration. We find that applying updated parameters improves the T‐Δ₄₇ relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all interlaboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect interlaboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ₄₇ (Δ*_25‐X), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of interlaboratory disagreement in the T‐Δ₄₇ relationship, but many more remain to be investigated.

Clumped‐isotope measurements in CO₂ and carbonates (Δ₄₇) present a number of technical challenges and require correcting for various sources of analytical nonlinearity. For now, we lack a formal description of the analytical errors associated with these correction steps, which are not accounted for in most data processing methods currently in use. Here we formulate a quantitative description of Δ₄₇ error propagation, fully taking into account standardization errors and their properties. We find that standardization errors are highly sensitive to the isotopic compositions (δ₄₇, Δ₄₇) of unknown samples relative to the standards used for analytical corrections, and in many cases constitute a non‐negligible source of uncertainty, causing true measurements errors to exceed traditionally reported error estimates by a factor of 1.5 (typically) to 3.5 (in extreme cases). Using Monte Carlo simulations based on the full InterCarb data set, we find that this model yields accurate error estimates in spite of small non‐Gaussian effects which remain entirely negligible in practice. We also describe various standardization strategies, along with the assumptions they rely on, in the context of this model, and propose a new, “pooled” standardization approach designed to yield more robust/accurate corrections. Among other uses, the mathematical framework described here may be helpful to improve standardization protocols (e.g., anchor/unknown ratios) and inform future efforts to define community reference materials. What is more, these models imply that the inter‐laboratory scatter (N = 5,329) observed in the InterCarb exercise (Bernasconi, Daëron, et al., 2021) can be entirely explained as the effects of current standardization procedures. Based on these findings, we recommend that future studies systematically report full analytical uncertainties taking standardization errors into account. In line with this recommendation, we provide user‐friendly online resources and an open‐source Python library designed to facilitate the use of these error models.

Foraminiferal isotopes are widely used to study past oceans, with different species recording conditions at different depths. Their δ¹⁸O values record both seawater oxygen‐18 and temperature according to species‐specific fractionation factors, while their Δ₄₇ signatures likely depend only on temperature. We describe an open‐source framework to collect/combine data relevant to foraminiferal isotopes, by constraining species‐specific oxygen‐18 fractionation factors (¹⁸α) based on culture experiments, stratified plankton tows or core‐top sediments; compiling stratified plankton tow constraints on living depths for planktic species; extracting seawater temperature, δ¹⁸O, and chemistry from existing databases for any latitude, longitude, and depth‐range; inferring calcification temperatures based on the above data. We find that although ¹⁸α differs between species, its temperature sensitivity remains indistinguishable from inorganic calcite. Based on > 2,600 observations we show that, although most planktic δ¹⁸O values are consistent with seawater temperature and δ¹⁸O over their expected living depths, a sizable minority (12%–24%) have heavier‐than‐predicted δ¹⁸O, best explained by calcification in deeper waters. We use this framework to revisit three recent Δ₄₇ calibration studies of planktic/benthic foraminifera, confirming that planktic Δ₄₇ varies systematically with oxygen‐18‐derived temperature estimates, even for samples whose δ¹⁸O disagrees with assumed climatological conditions, and demonstrating excellent agreement between planktic foraminifera and modern, largely inorganic Δ₄₇ calibrations. Benthic foraminifera remain ambiguous: modern benthic Δ₄₇ values appear offset from planktic ones, yet applying equilibrium Δ₄₇ calibration to the Cenozoic benthic foraminifer record of Meckler et al. (2022, https://doi.org/10.1126/science.abk0604) largely reconciles it with δ¹⁸O‐derived temperatures, with discrete Δ₄₇/δ¹⁸O discrepancies persisting in the Late Paleocene/Eocene/Plio‐Pleistocene.

Most of our knowledge of past seawater temperature history is based on δ¹⁸O values of calcium carbonate fossil shells. However, the determination of past temperatures using this proxy requires the knowledge of past seawater δ¹⁸O values, which is generally poorly constrained. Other paleothermometers using carbonate archives, such as $\mathrm{Mg}/\mathrm{Ca}$ ratios and clumped isotopes (Δ₄₇), have been developed to allow for paleotemperatures to be estimated independently and to allow past ocean δ¹⁸O values to be calculated using various groups of calcifying organisms. Articulated brachiopod shells are some of the most commonly used archives in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and for decades, their low Mg–calcite mineralogy has been considered relatively resistant to diagenetic alteration. Here, we investigate the potential of brachiopod shells as recorders of seawater temperatures and seawater δ¹⁸O values using new brachiopod shell geochemical data by testing multiple well-established or suggested paleothermometers applied to carbonate archives. Modern articulated brachiopod shells covering a wide range of temperatures (−1.9 to 25.5 ^∘C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU) were analysed for their stable isotope compositions (δ¹³C, δ¹⁸O and Δ₄₇) and their elemental ratios ($\mathrm{Mg}/\mathrm{Ca}$ , $\mathrm{Sr}/\mathrm{Ca}$ , $\mathrm{Na}/\mathrm{Ca}$ and $\mathrm{Li}/\mathrm{Ca}$ ). Our data allowed us to propose a revised oxygen isotope fractionation equation between modern-brachiopod shell calcite and seawater: $$\begin{array}{}\text{(1)}& T=-\mathrm{5.0}(\pm \mathrm{0.2})({\mathit{\delta }}^{\mathrm{18}}{\mathrm{O}}_{\mathrm{c}}-{\mathit{\delta }}^{\mathrm{18}}{\mathrm{O}}_{\mathrm{sw}})+\mathrm{19.4}(\pm \mathrm{0.4}),\end{array}$$ where δ¹⁸O_c is in ‰ VPDB, δ¹⁸O_sw is in ‰ VSMOW, and T is in ^∘C. Our results strongly support the use of clumped isotopes as an alternative temperature proxy but confirm significant offsets relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Brachiopod shell $\mathrm{Mg}/\mathrm{Ca}$ ratios show no relationship with seawater temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Despite significant correlations with brachiopod living temperature, brachiopod shell $\mathrm{Sr}/\mathrm{Ca}$ , $\mathrm{Na}/\mathrm{Ca}$ and $\mathrm{Li}/\mathrm{Ca}$ values indicate the influence of environmental and biological factors unrelated to temperature, which undermines their potential as alternative temperature proxies. Kinetic effects (growth rates) could explain most of the deviation of brachiopod shell calcite from expected isotopic equilibrium with seawater and part of the distribution of $\mathrm{Sr}/\mathrm{Ca}$ , $\mathrm{Na}/\mathrm{Ca}$ and $\mathrm{Li}/\mathrm{Ca}$ ratios.