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Rationale: Clumped isotope geochemistry examines the pairing or clumping of heavy isotopes in molecules and provides information about the thermodynamic and kinetic controls on their formation. The first clumped isotope measurements of carbonate minerals were first published 15 years ago, and since then, interlaboratory offsets have been observed, and laboratory and community practices for measurement, data analysis, and instrumentation have evolved. Here we briefly review historical and recent developments for measurements, share Tripati Lab practices for four different instrument configurations, test a recently published proposal for carbonate‐based standardization on multiple instruments using multi‐year data sets, and report values for 21 different carbonate standards that allow for recalculations of previously published data sets. Methods: We examine data from 4628 standard measurements on Thermo MAT 253 and Nu Perspective IS mass spectrometers, using a common acid bath (90°C) and small‐sample (70°C) individual reaction vessels. Each configuration was investigated by treating some standards as anchors (working standards) and the remainder as unknowns (consistency standards). Results: We show that different acid digestion systems and mass spectrometer models yield indistinguishable results when instrument drift is well characterized. For linearity correction, mixed gas‐and‐carbonate standardization or carbonate‐only standardization yields similar results. No difference is observed in the use of three or eight working standards for the construction of transfer functions. Conclusions: We show that all configurations yield similar results if instrument drift is robustly characterized and validate a recent proposal for carbonate‐based standardization using large multiyear data sets. Δ47 values are reported for 21 carbonate standards on both the absolute reference frame (ARF; also refered to as the Carbon Dioxide Equilibrated Scale or CDES) and the new InterCarb‐Carbon Dioxide Equilibrium Scale (I‐CDES) reference frame, facilitating intercomparison of data from a diversity of labs and instrument configurations and restandardization of a broad range of sample sets between 2006, when the first carbonate measurements were published, and the present. [ABSTRACT FROM AUTHOR]

The global geochemical cycles of calcium and strontium in seawater link the chemical composition of the oceans to the global carbon cycle and Earth's climate through the precipitation, diagenetic alteration, and sedimentary recycling of marine carbonate minerals. Here we present a record of calcium isotopic composition and [Sr] of seawater over the last 100 million years using measurements from modern and fossil shark teeth. Although there is significant variability in modern elasmobranch tooth enamel associated with physiology and diet, our record suggests a first-order increase in the average calcium isotopic composition of seawater by 0.5‰ and a decline in Sr/Ca ratios of ∼40% over the last 100 million years. These observations are in agreement with trends seen in other archives. We propose that the observed changes in both the calcium isotopic composition and Sr concentration of seawater can be explained by changes in the partitioning of the global carbonate sink associated with the development of a deep-sea carbonate reservoir in the mid-Mesozoic. • Fossil elasmobranch tooth enamel can be used for paleoseawater reconstructions. • We observe an enrichment in seawater δ 44 / 40 Ca and decrease in Sr/Ca since 100 Ma. • These changes may be explained by a decrease in the size of the neritic carbonate sink. [ABSTRACT FROM AUTHOR]