Coda of the snowball: combined U-Pb LA-ICPMS dating of calcite-after-aragonite crystal fans and clumped isotope thermometry of Ediacaran cap carbonates

Abstract The “dolomite problem” refers to the scarcity of dolomite in Cenozoic marine environments compared with its abundance in earlier strata. This discrepancy has been attributed to changes in marine environments or to insufficient thermal maturity required for dolomite formation. We measured carbon, oxygen, and clumped isotope compositions of carbonate rocks from the Albian Nahal Me'arot reef complex (north Israel), where dolomite is confined to back‐reef lagoon facies and associated with reflux‐brine dolomitization, a process thought responsible for large volumes of dolomite in Phanerozoic rocks. Results show a negative trend between TΔ 47 (29–54°C) and δ 18 O dolomite (−3.25 to −1.35 ‰VPDB), consistent with burial alteration, and support a two‐stage process: (a) proto‐dolomite formation near the lagoon floor, and (b) recrystallization during burial. Together with compilations of Mg/Ca ratios in Cenozoic marine carbonates predicting more dolomite than observed directly, our results suggest that insufficient thermal maturity has limited dolomite formation in Cenozoic strata.

Our understanding of the long-term behaviour of global climate and the Antarctic ice sheet relies heavily on the oxygen isotopic composition of marine calcite (δ18Ocalcite), which reflects a combination of ocean temperature and the amount of water stored in ice sheets. On the basis of δ18Ocalcite, the Antarctic ice sheet has been interpreted as extremely dynamic in the Oligocene, 34–23 million years ago. Yet, the proposed continental-scale ice volume changes are challenging to reproduce with models and may be overestimated owing to a larger influence of temperature on the deep-sea δ18Ocalcite than previously assumed. Here we present the first Oligocene record of orbital variability in deep ocean temperature based on benthic foraminiferal clumped isotope thermometry, a method affected only by temperature and independent of seawater chemistry. We find large, eccentricity-paced temperature variations of up to 4 °C, sufficient to explain the δ18Ocalcite cycles without requiring continental-scale ice volume changes. This finding is consistent with the simulated stability of the Antarctic ice sheet, highlighting the importance of robust independent temperature reconstructions. Our results show that the temperature in the deep Southern Ocean, and possibly globally, is highly sensitive to the seasonal distribution of insolation in an Oligocene-like climate state warmer than today.

Accurate predictions of the future climate response to CO2 depend on the ability of climate models to simulate past analog warmer climates, like the Miocene. However, one key unresolved issue in paleoclimate modeling is reproducing the pronounced high-latitude warmth and relatively flat latitudinal temperature gradients inferred from proxy records. Here, we use clumped isotope thermometry—a method that sidesteps limitations of conventional proxies—on pure coccolith calcite from a high-latitude North Atlantic site, extending from the Mid Miocene to the Quaternary. Coccolith-derived clumped isotope temperatures are on average ~9°C lower than alkenone estimates, representing the first proxy dataset to align with Miocene model outputs and calling into question the prevailing paradigm of pronounced high latitude amplification. This record highlights the need to continuously reevaluate proxy interpretations to achieve both reliable trends and absolute temperature values, while providing a more optimistic perspective of future high latitude climate response to CO2 emissions.

• Radiometrically dated tuffs reveal a 34 to 27 Ma old sedimentary record in SW Montana. • Δ 47–48 analysis shows disequilibrium and NO 2 contaminants in some calcretes. • Unaffected calcretes show a 10 ± 1 °C cooling during the early Oligocene. Cooling during the Eocene-Oligocene Transition (EOT; 34 Ma) and a shift towards open habitats during the end of the Oligocene (∼26 Ma) are key characteristics of the paleoclimatic history of western North America. Yet, the paleo-temperature evolution during the rest of the Oligocene remains poorly constrained. Here, we present a new temperature record from the high-elevation North American Cordillera (Sage Creek Basin; SW Montana, USA) covering 34 to 27 Ma as revealed by zircon U-Pb geochronology of four volcanic tuffs. High-precision dual clumped isotope thermometry (Δ 47 and Δ 48 ) is used to identify calcretes unbiased by NO 2 contaminants and kinetic effects. The Δ 47 values of these calcretes show no major temperature change across the EOT, but instead gradual cooling of 10 ± 1 °C during the early Oligocene (32 ± 1 to 29 ± 2 Ma). Protracted cooling after, rather than abrupt temperature changes during the EOT, may explain the lack of mammalian turnover in North American fossil assemblages compared with other continents. Reconstructed water oxygen isotope compositions remain unchanged during the early Oligocene cooling, indicating no major surface uplift at this location. Furthermore, global climate reconstructions show only a minor decrease in atmospheric CO 2 concentrations at this time. The mechanisms driving this Oligocene cooling thus remain elusive, but may be related to land surface feedbacks operating in the high-elevation North American Cordillera. Given the large magnitude of the cooling that we observe in SW Montana, these mechanisms should be considered in climate model simulations and proxy reconstructions of high-elevation regions.

Abstract. Estimates of surface ocean temperatures in the past are essential for evaluating the sensitivity of Earth's surface temperature to higher atmospheric CO2 concentrations such as those characterizing the Miocene. However, in the higher latitude regions, many proxy-based temperature estimates suggest extreme warmth, which imply much lower latitudinal temperature gradients than can be simulated by most coupled general circulation climate models under enhanced greenhouse gas forcing. This discrepancy implies either systematic biases in temperature proxy interpretation or the absence of key feedback processes in models. Here, we use a new approach to estimate high southern latitude surface ocean temperatures using clumped isotope thermometry in coccoliths – calcite plates precipitated in the surface ocean by the calcifying phytoplankton group coccolithophores. We present new determinations of the clumped isotope ratio in well-preserved coccoliths spanning the last 15 million years, extracted and purified from a sediment core located just south of the modern subtropical front (Ocean Drilling Program Site 1088, 41° S). Coccolith clumped isotopes reveal a 10 °C decline in temperatures at this location over the last 15 million years, and over the last 11 Ma of overlapping records the magnitude of cooling is similar to that estimated from the degree of undersaturation of alkenone biomarkers. However, the temperatures derived from coccolith clumped isotopes are 8–12 °C cooler than those estimated from alkenones, even though both are biosynthesised by the same organisms and therefore must reflect an identical production depth and season. This implies that some of the model-proxy mismatch may be due to unresolved issues in proxy interpretation. We propose that at this site, calibration biases lead to alkenone sea surface temperature estimates up to 5 °C too warm, whereas coccoliths reflect temperatures at the production depth which is several degrees cooler than the sea surface. The influence of secondary diagenetic carbonate precipitation at the seafloor is constrained to contribute a cold bias of 2 °C or less on the clumped isotope temperature for most samples.

The clumped isotope paleothermometer (Δ 47 ) has been used to reconstruct temperatures from various biogenic carbonate archives. Calibration studies demonstrate that some biogenic carbonates precipitate in Δ 47 equilibrium and record growth temperatures accurately (e.g., many bivalve mollusks), while others appear to exhibit disequilibrium, or ‘vital’, effects and yield isotopically reconstructed temperatures that are biased (e.g., shallow-water corals). These studies have largely excluded marine gastropods, so it is not known whether they tend to precipitate their shells in or out of isotopic equilibrium. In this study, we present seasonal-scale δ 18 O and seasonally targeted Δ 47 and Δ 48 measurements from modern marine gastropods representing 8 genera and 10 species, reconstructing apparent growth temperatures and screening for equilibrium precipitation. We find that most marine gastropods appear to precipitate in Δ 47 and Δ 48 equilibrium and faithfully record environmental temperatures, making them suitable for Δ 47 -paleothermometry. A few gastropods ( Caviturritella/Turritella sp., Campanile symbolicum , Megastraea undosa ) appear to precipitate out of Δ 47 equilibrium, though these disequilibrium signatures may partially be explained by differences between actual growth temperatures and instrumental calibration temperatures ( Caviturritella/Turritella sp., M. undosa ) or differences between inner and outer layer precipitation ( C. symbolicum ). We present new Δ 47 -temperature data for 2 Middle Eocene Campanile giganteum fossils collected from the Paris Basin and discuss how to interpret the results in the context of our modern samples. Finally, in conducting this calibration we demonstrate paired Δ 47 /Δ 48 as an effective tool to screen for disequilibrium precipitation in marine gastropods.

Abstract Reconstructing atmospheric CO2 concentration in the Late Miocene is crucial for understanding the relationship between greenhouse gas concentrations and climate change in a warmer‐than‐modern world. Both δ11B‐based and alkenone‐εp‐based CO2 estimates feature uncertainties due to poorly constrained past seawater chemistry, and algal physiological processes, respectively. Additionally, both proxies estimate CO2[aq], so they require reliable surface ocean temperatures to calculate solubility and atmospheric CO2. To evaluate proxy coherence, in this study we generate new records of alkenone εp and δ11B, from the Western Tropical Atlantic ODP Site 926 during the Late Miocene. We provide surface ocean temperature estimates from coccolith clumped isotope thermometry, alkenone undersaturation ratios, and planktonic foraminiferal Mg/Ca ratios. The warm temperatures estimated from our new clumped isotope records, together with alkenone temperatures >29°C, confirm warm tropics, and provide constraints on the assumptions of seawater Mg/Ca and dissolution corrections for foraminiferal Mg/Ca SST estimates. New alkenone εp CO2 estimates at 926 yield generally similar CO2 levels (400 ppm ± 100 ppm) as the new and published δ11B‐based CO2 records (500 ppm ± 100 ppm) for the site, and are similar to published alkenone εp CO2 records (500 ppm ± 100 ppm) from the South Atlantic ODP Site 1088. However, over the 7.3 to 7.8 Ma interval, the CO2 values from εp are 100–200 ppm lower than other records, which may reflect uncertainties in estimation of δ13CDIC from planktic foraminifera or variations in algal physiology. We evaluate which proxy indicators can best predict variations in algal physiology which may bias the εp‐based CO2 reconstructions in this interval at Site 926.

ABSTRACT Clumped isotopic thermometry of carbonate minerals is a valid method for revealing the thermal history of sedimentary basins. This method has been successfully applied to basins with carbonate strata, whereas its application in basins composed of clastic strata is limited. This study focused on calcite cements in the upper Permian to Triassic terrestrial clastic strata in the Junggar Basin, northwestern China. Petrological, elemental geochemical and clumped isotopic analyses were conducted in combination with vitrinite reflectance analysis and forward thermal modelling. The studied strata contain multiple generations of calcite cement: early‐ and late‐stage calcite. Relatively high δ 13 C values (−6.2‰ to −0.8‰), high δ 18 O values (−15.9‰ to −11.3‰) and low clumped isotopic temperatures (T(∆ 47 ): 31°C–43°C) suggest that the Permian and Triassic early‐stage calcite precipitated during the penecontemporaneous stage. Considering the high MnO contents (2.22%~14.05%), extremely low δ 13 C values (−60.5‰ to −38.4‰) and high T(∆ 47 ) values (95°C–132°C), the late‐stage calcite in the Triassic rocks is explained as the product of the oxidation of hydrocarbons by high‐valence Mn/Fe oxides during mesodiagenesis. The high δ 13 C values (−10.2‰ to −10.7‰) indicate that the late‐stage calcite in the Permian rocks is the product of the decarboxylation of organic acids. Constrained by the T(∆ 47 ) values of the early‐ and late‐stage calcite and forward kinetic modelling, the maximum temperature of the upper Permian is confined to 150°C during the Late Jurassic. The thermal gradient of the study area exhibited an overall decreasing trend from 40°C·km −1 in the late Permian to 22°C·km −1 in the Cenozoic. The results are 2°C–4°C per km higher than those of previous works based on vitrinite reflectance and apatite fission track annealing. This research demonstrates that the combination of clumped isotope thermometry of multistage carbonate cements and kinetic modelling can quantitatively reveal a basin's thermal history.

Abstract Fluid flow in fault zones is generally controlled by fault activity, yet understanding how such activity influences fluid flow processes within associated damage zones remains a significant challenge. This study reconstructs a multi-episodic fluid flow history spanning >200 m.y. within Late Triassic tight sandstone-hosted strike-slip fault zones. This reconstruction innovatively integrates U-Pb dating of calcite, fluid inclusion analysis, clumped isotope thermometry, and organic geochemistry. This study focuses on the Jinghe Oilfield in the Ordos Basin, China, where fractured reservoirs in fault damage zones host the main oil accumulations. Widely developed microfractures associated with fault activity facilitate fluid flow within the damage zones. Calcite cementation, oil charge, and secondary alteration of oil occurred successively in the fault zones. The tightness of the sandstone reservoirs primarily resulted from early calcite cementation. The accumulation of early low maturity oil followed early cementation in the fractures in the damage zones, while higher-maturity oil mainly accumulated later in sandstone pores during the late Early Cretaceous. Although regional uplift began in the Late Cretaceous, the adjustment and alteration (e.g., oil remigration and biodegradation) of previous oil accumulations in the fault zones occurred only when the faults were reactivated at ca. 5.8–3.7 Ma. Consequently, the oil retained in fractures underwent more intensive biodegradation than the oil within the sandstone pores. These results further indicate that fault-controlled fluid flow can be episodic, with transient activation events occurring during prolonged tectonic uplift (e.g., ~100 m.y.). Oil within fault zones can still be pristine and not biodegraded unless intense fault activity occurred, at temperatures of ~47 °C during cooling from a maximum reservoir temperature of ~110–120 °C (corresponding to a burial depth of ~2000 m) prior to continuous uplift.

ABSTRACTBeef calcite veins in the Green River Formation of the Uinta Basin, Utah, were geochemically characterised to test two hypotheses: (1) that beef calcite veins can form during extensional tectonism and (2) that fluid overpressure can develop in open or partially restricted hydrologic systems. Laser ablation U–Pb geochronology yielded three precipitation ages, with the most precise at 24.8 ± 4.8 Ma (2σ), consistent with maximum burial of the formation and coinciding with uplift of the Uinta Basin segment of the Colorado Plateau. Clumped isotope thermometry indicates precipitation temperatures between 55°C and 72°C—substantially lower than the estimated host rock temperatures of 110°C to 140°C based on a ~30°C/km geothermal gradient. δ13C and δ18O values of beef calcite range from 1.6‰ to −1.2‰ and −10.7‰ to −11.5‰ (VPDB), respectively, with calculated δ18O of the precipitating fluid (VSMOW) ranging from −3.3‰ to −5.2‰. These values are consistent with a mixed meteoric and shallow connate water source, suggesting the downward invasion of cold, evolved meteoric fluids along faults and fractures during post‐Laramide extensional tectonic deformation. The overpressure required for beef calcite formation may have been generated by hydraulic head associated with these downward‐migrating fluids and the subsequent lateral displacement of basin brines along stratigraphic interfaces beneath regionally continuous mudstone and evaporite seals.

Knowledge of the formation temperatures of geological deposits is essential for investigating their genesis. Oxygen isotope thermometry (OIT), using the temperature dependence of oxygen isotope fractionation between host carbonate mineral and mineral-forming water trapped in fluid inclusions, and clumped isotope thermometry, based on the degree of 13C and 18O clumping, are receiving increasing interest. However, only a few studies have applied combinations of these methods, and their databases are limited. In this study, we compare OIT and clumped isotope temperatures obtained for 18 samples from Mesozoic to early Cenozoic calcite veins. Our analysis indicates that the formation temperatures were preserved in the clumped isotopic compositions (16–45 °C), whereas the OIT temperatures were shifted to lower temperatures (− 2 to 33 °C). An OIT temperature shift occurred, due to a retrograde oxygen isotope exchange between the fluid inclusion water and the host calcite. These results imply that the retrograde isotope exchange should be taken into consideration, even for low-temperature carbonate deposits, if a sufficiently long time is available.

Abstract Historically, clumped isotope thermometry (T(∆47)) of soil carbonates has been interpreted to represent a warm‐season soil temperature based dominantly on coarse‐grained soils. Additionally, T(∆47) allows the calculation of the oxygen isotope composition of soil water (δ18Ow) in the past using the temperature‐dependent fractionation factor between soil water and pedogenic carbonate, but previous work has not measured δ18Ow values with which to compare to these archives. Here, we present clumped isotope thermometry of modern soil carbonates from three soils in Colorado and Nebraska, USA, that have a fine‐to‐medium grain size, contain clay, and are representative of many carbonate‐bearing paleosols preserved in the rock record. At two of the three sites, Briggsdale, CO and Seibert, CO, T(∆47) overlaps with mean annual soil temperature (MAST), and the calculated δ18Ow overlaps within uncertainty with measured δ18Ow at carbonate bearing depths. At the third site, in Oglala National Grassland, NE, mean T(∆47) is 8–11°C warmer than MAST, and the calculated δ18Ow has a significantly higher isotope value than any observations of δ18Ow. At all three sites, even in the fall season, δ18Ow values at carbonate bearing depths overlap with spring rainfall δ18Ow, and there is little to no evaporative enrichment of δ2Hw and δ18Ow values. These data challenge long‐held assumptions that all pedogenic carbonate records a warm‐season bias, and that δ18Ow at carbonate‐bearing depths is affected by evaporative enrichment.

Abstract Lacustrine, riverine and spring carbonates represent archives of terrestrial climates and their geochemistry has been used to study palaeoenvironments. Clumped isotope thermometry is an emerging tool that has been applied to freshwater carbonates. Limited work has been done to evaluate comparative relationships between clumped isotopes and temperature in different types of modern freshwater carbonates. This study assembles an extensive calibration data set with 135 samples of modern freshwater carbonates from 96 sites and constrains the relationship between independent observations of water temperature and the clumped isotopic composition of carbonates (denoted by Δ47), including new measurements, and recalculates published data in accordance with current community‐defined standard values. For temperature reconstruction, the study reports a composite freshwater calibration and material‐specific calibrations for biogenic carbonates (freshwater gastropods and bivalves), fine‐grained carbonate (e.g. micrites), biologically mediated carbonates (microbialites and tufas) and travertines. Material‐specific calibration trends show a convergence of slopes that are in agreement with recently published syntheses, but statistically significant differences in intercepts occur between some materials (e.g. some biogenics, fine‐grained carbonates). These differences may arise due to unresolved seasonal biases, kinetic isotope effects and/or varying degrees of biological influence. The impact of different calibrations is shown through application to new data for glacial and deglacial age travertines from Austria and published data sets. While material‐specific calibrations may yield more accurate results for biogenic and fine‐grained carbonate samples, the use of material‐specific and the composite freshwater calibrations generally produces values within 1.0–1.5°C of each other, and typically fall within calibration uncertainty given limitations of precision.

Δ47 based clumped isotope thermometry has enabled reconstruction of Earth's surface temperatures independent of the source of oxygen within the carbonate. It has been postulated that carbonate samples can contain contaminants that cause isobaric interferences, compromising measured Δ47 values and reconstructed temperatures. The exact nature of contaminants and isobaric interferents, however, largely remained unidentified.Here, we compare theoretically predicted contamination vectors with measured Δ47-Δ48 values and measured NO2 abundances in the CO2 evolved from phosphoric acid digestion of carbonates in a common acid bath at 90 °C. We show that nitrate-derived NO2 constitutes a serious isobaric interferent for the extracted CO2. During acid digestion, nitrate decomposes to NO and NO2. Both compounds are not effectively removed during subsequent purification of carbonate-derived CO2 using cryogenic traps (−80 °C) and gas chromatography (packed Porapak Q column at −15 °C), generating a bias in measured Δ47 and Δ48 values. In dual clumped isotope space, biased samples plot along a slope of −0.3 that is characteristic for variable sub-ppm contributions of NO2+ to CO2+ in the ion source. Measured NO2 concentrations in the analyte-grade CO2 correspond to observed biases in Δ47 and Δ48 values if preferential ionization of CO2 over NO2 is taken into account.Nitrate contamination occurs in a synthetic calcite precipitated using Ca(NO3)2, a pedogenic carbonate nodule, a plasma-ashed echinoid spine, a bioapatite (Greenland shark dentine), and in ETH-3 (a recently assigned anchor for Δ47 analysis of carbonates). Sequential bleaching tests reveal that nitrate contaminant and NO2+ bias can be effectively removed if carbonate samples are pre-treated overnight with 3 wt-% sodium hypochlorite (NaOCl). NO2+ bias in ETH-3-derived CO2 and its effective removal through bleaching is also indicated in a completely different analytical setup that makes use of individual reaction vessels, acid digestion at 70 °C, cryogenic traps at −60 °C and a static Porapak Q trap at −30 °C.Considering that NO2+ bias is observed in two fairly conventional analytical setups, we strongly recommend that each laboratory tests to which extent their setup is affected. Unless independent evidence is given that NO2+ bias is irrelevant for a specific setup, ETH-3 should be bleached and further systematic sequential bleaching tests be carried out on unknown samples in order to avoid any isotopic bias. Our high-precision long-term Δ47 (CDES 90) values for ETH-1 and ETH-2 (Bernecker et al., 2023) and for bleached ETH-3 exactly confirm recently assigned Δ47-I-CDES values for these standards, demonstrating that ETH-3 – in the short term – could be replaced by its bleached counterpart for accurate I-CDES data normalization. In the long term, it should be replaced by a low-temperature carbonate anchor devoid of contaminants.

Abstract Tectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

Mineralogical and environmental effects on the δ13C, δ18O, and clumped isotope composition of modern bryozoans

Freshwater mussels preserve valuable information about hydrology, climate, and population dynamics, but developing seasonal chronologies can be problematic. Using clumped isotope thermometry, we produced high-resolution reconstructions of modern and historic (~ 1900) temperatures and δ18Owater from mussel shells collected from an impounded river, the Brazos in Texas, before and after damming. We also performed high-resolution growth band analyses to investigate relationships between mussel growth rate, rainfall, and seasonal temperature. Reconstructed δ18Owater and temperature vary little between the modern (3R5) and historic shell (H3R). However, a positive relationship between reconstructed δ18Owater and growth rate in H3R indicates that aside from diminished growth in winter, precipitation and flow rate are the strongest controls on mussel growth in both modern and pre-dam times. Overall, our results demonstrate (1) the impact, both positive and negative, of environmental factors such as flow alteration and temperature on mussel growth and (2) the potential for clumped isotopes in freshwater mussels as a paleohydrology and paleoclimate proxies in terrestrial environments.

The thermal history of Permian carbonate strata reconstructed with clumped isotopes and U–Pb dating: Eastern Sichuan Basin, SW China

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) are excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained from these methods. However, subaqueous carbonate deposits may provide reliable data, as kinetic fractionations are less significant in underwater environments. In this study, subaqueous speleothems, whose formation temperatures were directly measured in the water, were investigated. Additionally, temperatures calculated from the oxygen isotope fractionations between calcite and fluid inclusion-hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated fluid inclusion temperatures within the analytical precisions. Carbonate deposits formed at elevated temperatures (~50°C or above) may undergo post-formational calcite-water oxygen isotope exchange, altering the composition of the inclusion-hosted water. In contrast, subaqueous speleothems formed at about 20-25°C appear to preserve the primary isotopic compositions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, making them valuable paleotemperature archives.