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Carbonates are common paleoclimate archives in a variety of environments with land carbonates recording both temperatures and hydrological conditions. Whereas δ18O has long been used for that purpose, triple oxygen isotopes (given as 17Oexcess) has only recently been analyzed in CaCO3. Carbonate 17Oexcess is expected to reflect the temperature, in which the carbonate formed, and the triple oxygen isotopic composition of its parent water. The reconstruction of past water 17Oexcess values from carbonate records requires characterization of the 17O fractionation between water and CaCO3. Here, we use freshwater mollusk shells grown in well constrained spring environments, at a temperature range of 15–28 °C, to derive the fractionation factor 17α between CO2 extracted from CaCO3 and the carbonate habitat water. The spring water temperature and isotopic composition are constant year-round, avoiding uncertainty associated with seasonality. The derived 18α and 17α between CO2 extracted from mollusk aragonite and spring water are used to calculate the fractionation slope θ (=ln17α/ln18α), resulting in 0.5231 ± 0.0003, with no discernible temperature dependence. Using this value, we evaluated the fractionation slope of the acid digestion of CaCO3 as 0.5170 ± 0.0004. The minor difference between the fractionation slope between aragonite and parent water and the reference slope of 0.528, implies that 17Oexcess in carbonates is only weakly temperature dependent. On the other hand, carbonate 17Oexcess values directly record variability in 17Oexcess of the parent water, making it a useful paleo-hydrological proxy.

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., Key Points The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurementsMeasurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishableWe propose new consensus values for a set of seven carbonate reference materials and updated guidelines to report clumped isotope measurements

All major groups of Mesozoic dinosaurs exhibited metabolically mediated thermoregulation based on clumped isotopes in eggshells., Studying the origin of avian thermoregulation is complicated by a lack of reliable methods for measuring body temperatures in extinct dinosaurs. Evidence from bone histology and stableisotopes often relies on uncertain assumptions about the relationship between growth rate and body temperature, or the isotopic composition (δ18O) of body water. Clumped isotope (Δ47) paleothermometry, based on binding of 13C to 18O, provides a more robust tool, but has yet to be applied across a broad phylogenetic range of dinosaurs while accounting for paleoenvironmental conditions. Applying this method to well-preserved fossil eggshells demonstrates that the three major clades of dinosaurs, Ornithischia, Sauropodomorpha, and Theropoda, were characterized by warm body temperatures. Dwarf titanosaurs may have exhibited similar body temperatures to larger sauropods, although this conclusion isprovisional, given current uncertainties in taxonomic assignment of dwarf titanosaur eggshell. Our results nevertheless reveal that metabolically controlled thermoregulation was the ancestral condition for Dinosauria.

A fundamental issue in the interpretation of speleothem calcite δ18Occ records is the correct partitioning of the effects of temperature and water δ18O variations. This study explores the paleo-environmental evolution of the Eastern Mediterranean Sea (EMS) region using Soreq Cave speleothems in the last 160 ka, a period covering glacial MIS6 to the present, through the independent determinations of temperatures via carbonate clumped isotopes (Δ47) and fluid inclusion δ18Ow, δDw and d-excess values of cave water entrapped in the speleothems. The observed general agreement between the trends in water δ18Ow composition directly measured from fluid inclusions and calculated from Δ47 temperatures and δ18Occ support the robustness of the data. Δ47 temperatures, derived using a revised thermometer calibration that includes recent modifications in 17O correction, vary from high values of 20–22 °C during interglacial periods to low values of 11–12 °C in cold glacial and stadial periods. Temperatures of 19–22 °C are observed during the time of deposition in the EMS of sapropel S5 in the last interglacial MIS5e period and sapropel S1 in the early Holocene, immediately following Terminations II and I, respectively. The Soreq Cave Δ47 temperatures generally agree with previously estimated EM sea surface temperatures, in support of the absence of a significant land-sea temperature gradient. Cave water δ18Ow values show large variations (from ∼-3 to −7.5‰), with high glacial and low interglacial values. These variations are largely explained by changes in Mediterranean Sea water composition due to glacial ice melting and freshwater input during sapropel periods, combined with the effect of rainfall amounts during interglacial periods. An analysis of relative timing shows that temperature rise precedes hydrological changes in the initial stages of Termination events. Fluid inclusion d-excess values show clear variations between values similar to the Global Meteoric Water Line during cold periods and values similar to the local Eastern Mediterranean Meteoric Water Line (EMMWL) in warm periods.

The carbonate clumped isotope (Δ47) thermometer was applied to fresh water snails (Melanopsis spp.) grown in the waters of the Sea of Galilee and Hula Valley, in the north of Israel. Modern shells, grown at known temperatures agree with the Δ47-T calibration of Zaarur et al. (2013). Fossil Melanopsis shells from 2 locations, Gesher Bnot Ya’aqov (at the southern tip of the Hula Valley) and the Sea of Galilee provide a temperature record for the region during the time interval of the past 20 kyrs. Glacial temperatures are ∼5 °C cooler than mid-Holocene and ∼3 °C cooler than modern, similar to other records in the region. These Δ47-derived temperatures are combined with δ18O of the shell carbonate to calculate the oxygen isotopic composition of the habitat waters. Contrary to global trends and other regional records, reconstructed δ18Owater values increase from the late glacial through the Holocene. This reversed signal reflects a decrease in the relative contribution of snowmelt to the watershed post-LGM and a transition to a more rain dominated inflow. A fairly constant difference in δ18Owater values between the Hula Valley and Sea of Galilee waters, suggests that the hydrological relationship of the two water bodies had remained constant, with the temperature changes playing only a minor role in the extent of evaporation of the Sea of Galilee relative to the Hula.

Carbonate bearing materials, such as foraminifera, mollusks shells, or speleothems, have the potential to preserve geochemical and isotopic signatures reflecting the environmental conditions at the time they formed. Beyond the conventional δ18O, information from triple oxygen isotopes (reported as 17Oexcess, which is defined as (ln (δ17O/1000 + 1) − λref ln (δ18O/1000 + 1)) × 106) in these archives promises to be a valuable tool to reconstruct past hydrological processes. The goal of this study is to determine the triple oxygen isotope fractionation between CaCO3 and H2O under well-constrained laboratory conditions. We performed laboratory experiments to precipitate CaCO3 polymorphs, either calcite or aragonite, at temperatures between 10 and 35 °C. We then evaluated the effect of polymorphism, temperature, and solution concentration on the 17Oexcess of CO2 extracted from these carbonates and the 17O isotopic fractionation (17α) between water and CaCO3. The obtained values of 18α and 17α between CO2 extracted from CaCO3 and parent water allow us to calculate the fractionation slope θ (=ln17α/ln18α). Our observations suggest that θ is indistinguishable at temperatures of 10 and 27 °C, but is slightly lower at 35 °C. The lower value at 35 °C may be related to disequilibrium during these experiments. We found that θ is independent of polymorph and of solution concentration, indicating that 17Oexcess is less sensitive than δ18O to these geochemical parameters and can thus be a robust proxy for reconstructing 17Oexcess of parent water.

D.E. and H.P.A. acknowledge support from Yale University and the Yale Analytical and Stable Isotope Center and from Grant 171/16 of the Israel Science Foundation. W.R. and L.J.C. were supported by NWO Grant ALW 822 01 009. The Tanzania Commission for Science and Technology (COSTECH) and the Tanzania Petroleum Development Corporation supported the Tanzania Drilling Project (TDP) which recovered the Tanzanian specimens. Laser-ablation ICPMS work at RHUL was partly funded by a 2014 Natural Environment Research Council (NERC) Capital Equipment Grant (Ref. CC073). The Research Foundation Flanders is acknowledged for financial support (to P.S.). M.Z. acknowledges support from the Netherlands Earth System Science Center and Horizon 2020 Grant MSCA-IF-2014 655073. Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models. Postprint

Clumped isotope measurements aim to quantify some statistical properties of the isotopologue population in a given sample, which requires prior knowledge of the absolute isotopic abundance ratios in reference materials such as VSMOW or VPDB. In the case of CO2, matters are further complicated by the need to define a mass-dependent fractionation law linking the three stable isotopes of oxygen. Conversion from raw mass spectrometric data to mass-47 anomalies (Δ47) thus relies on four external parameters: the (13C/12C) ratio of VPDB, the (17O/16O) and (18O/16O) ratios of VSMOW (or VPDB-CO2), and the slope of the triple oxygen isotope line (λ). Here we investigate the influence that these isotopic parameters exert on measured Δ47 values, using (a) real-world data corresponding to seven months of measurements; (b) simple simulations based on randomly generated data; (c) precise comparisons between water-equilibrated CO2 samples and between carbonate standards believed to share quasi-identical Δ47 values; and (d) reprocessing of two carbonate calibration data sets with different slopes of Δ47 versus T. We demonstrate that the use of different sets of isotopic parameters generally produces systematic offsets as large as 0.04‰ in measured Δ47 values, even after following the established standardization procedures. What's more, even using a single set of isotopic parameters can produce intra- and inter-laboratory discrepancies in measured Δ47 values, if some of these parameters are inaccurate, and depending on the isotopic compositions of the standards used for conversion to the absolute scale of Dennis et al. (2011), these errors should correlate strongly with either δ13C or δ18O, or more weakly with both. Based on measurements of samples expected to display quasi-identical Δ47 values, such as 25 °C water-equilibrated CO2 with different carbon and oxygen isotope compositions, or high-temperature standards ETH-1 and ETH-2, we conclude that the traditional set of isotopic parameters used in all early clumped isotope studies produces large, systematic errors controlled by the relative bulk isotopic compositions of samples and standards. These errors are likely to be one of the key factors responsible for current inter-laboratory discrepancies, but cannot easily explain the conflicting carbonate calibration slopes obtained by different groups. By contrast, the isotopic parameters of Brand et al. (2010) appear to yield accurate Δ47 values regardless of bulk isotopic composition. Based on these findings, we offer recommendations aiming to minimize errors related to the choice of isotopic parameters.