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Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δ_iis a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δ_ivalues measured by mass spectrometry (Δ₄₇, Δ₄₈) or laser spectroscopy (Δ₆₃₈) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δ_ivalues that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals. ▪ Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments. ▪ Mature analytical methods enable carbonate clumped Δ₄₇, Δ₄₈, and Δ₆₃₈measurements to address diverse questions in geological and biological sciences. ▪ These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.

 

The δ18O of carbonate minerals that formed at Earth’s surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly lower than the δ18Ogw of –4.4‰ to –9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations. 

Faunal analog reconstructions suggest that Last Interglacial (MIS 5e) sea surface temperatures were cooler around Bermuda and in the Caribbean than modern climate. Here we describe new and revised clumped isotope measurements ofCittarium picafossil shells supporting previous findings of cooler than modern temperatures in Bermuda during the Last Interglacial. We resolve temperature and δ¹⁸O_wdifferences between two closely located and apparently coeval sites described in Winkelstern et al. (2017),https://doi.org/10.1002/2016pa003014through reprocessing raw isotopic data with the updated Brand/IUPAC parameters. New subannual‐resolution clumped isotope data reveal large variations in δ¹⁸O_wout of phase with seasonal temperature changes (i.e., lower δ¹⁸O_wvalues in winter). Supported by modern δ¹⁸O_wmeasurements identifying similar processes occurring today, we suggest past variations in coastal δ¹⁸O_wwere driven by seasonally variable freshwater discharge from a subterranean aquifer beneath the island. Taken together, our results emphasize the importance of δ¹⁸O_win controlling carbonate δ¹⁸O, and suggest that typical assumptions of constant δ¹⁸O_wshould be made cautiously in nearshore settings and can contribute to less accurate reconstructions of paleotemperature.

 

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.