Δ47 Values Research Articles

Primary to post-depositional microbial controls on the stable and clumped isotope record of shoreline sediments at Fayetteville Green Lake.

Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post-depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturation insitu, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ18O, δ13C, and Δ47 values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth-based changes of porewater carbonate chemistry to microbial community composition, insitu carbon cycling (using δ13C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ13C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite-each with a different origin. Authigenic micrite is interpreted to have precipitated insitu from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ13Ccarb) and clumped isotope values (Δ47) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments.

Dual clumped isotopes (Δ47 and Δ48) reveal non-equilibrium formation of freshwater cements

Low-Mg calcite, precipitating from meteoric fluids, is a common mineral that forms in a variety of near-surface diagenetic environments. However, recent studies, based on a combination of analyses of δ18O and Δ47 values, have suggested that this mineral might form in disequilibrium and consequently yield kinetic bias in Δ47-derived temperatures and fluid δ18O values. Here, we use dual clumped isotope proxies (Δ47 and Δ48) to investigate the influence of kinetic isotope effects within different meteoric diagenetic zones of Holocene and Pleistocene carbonates from the southern Florida and the Dominican Republic. In the Miami Oolite, the primary aragonite ooids and secondary low-Mg calcite cements in the bulk sample were separated from each other and their isotopic compositions (δ13C, δ18O, Δ47 and Δ48 values) were measured. The Δ47 and Δ48 values of the separated aragonite are consistent with the modern ooid sediments and in approximate equilibrium with the surface seawater. In contrast, the low-Mg calcite cement shows the higher Δ48 and lower Δ47 values, than expected, with the disequilibrium arising as a result of CO2 degassing in the vadose zone. Such deviations of Δ47 and Δ48 values are also observed in low-Mg calcite vadose cements in the Dominican Republic. While low-Mg calcites formed in the lower freshwater phreatic zone in the Dominican Republic have the Δ47- and Δ48-derived temperatures close to expected, the same mineral forming near the water-table and upper phreatic zone shows much higher Δ48-derived temperatures (up to ∼ 90 °C). The possible origin of such elevated temperatures can be attributed to non-equilibrium processes caused by changes in pH and pCO2, mediated by microbial sulfate reduction. Such differential kinetic behavior of Δ48 values between vadose and phreatic zones could be used as a proxy marker for the presence and the location of a water-table. This study demonstrates the great potential of dual clumped isotopes in the investigation of meteoric diagenesis and will help understand the alteration of ancient sequences and the interpretation of stable C isotope trends that they contain.

Seep carbonate clumped isotopes revealing ocean warming-induced gas hydrate dissociation

The dissociation of marine methane hydrates due to the increased ocean temperature may be widespread in the context of global warming. However, direct evidence for this phenomenon is limited. Here, we reported the carbon and oxygen isotopes, mineralogy, elements, and clumped isotopes (Δ47) of coring seep carbonates (MIS5e) to explore the influence of temperature changes on gas hydrate stability. The extremely low δ13C and high δ18O values of bulk carbonates (average, −48.1‰ and 4.4‰, respectively) indicated the authigenic carbonates formed from the anaerobic oxidation of methane and gas hydrate dissociation. An anaerobic environmental condition with intense methane seepages was confirmed by the dominant aragonite, Mo enrichment (MoEF averaged as 312), and positive Ce anomalies (averaged as −0.03). The Δ47 values ranged from 0.714 to 0.745, indicating a higher bottom-water temperature (averaged at 7.5 °C) during MIS5e compared with the current seafloor temperature (5.4 °C). This study provides direct evidence for the dissociation of gas hydrate triggered by ocean warming, presenting new insights into potential temperature-induced gas hydrate instability due to global climate warming.

Fluid-mediated exchange reaction induces clumped isotope resetting: Insights from aragonite dolomitization experiments

Carbon and oxygen isotopic exchange between external and internal fluids of carbonate minerals can reset the clumped isotope composition (Δ47) of paleotemperature archives. Understanding the nature of this exchange during carbonate diagenetic alterations is essential for correctly interpreting the paleoenvironmental significance of Δ47-derived temperatures. Through batch hydrothermal experiments on aragonitic coral carbonates, we examine the kinetic signatures of clumped isotope fractionation from the initial disequilibrium to final equilibrium states during the dolomitization of the aragonite. By analyzing oxygen isotope fractionation and Sr partitioning between carbonate and fluid, we simulate the effect of exchange reactions on the Δ47 values. As the resetting feature of Δ47 values in our aragonite samples disagrees with the fractionation pattern observed in previous aragonite solid-state reordering experiment, we rule out the effect of the solid-state reordering. Our findings demonstrate that isotope exchanges with internal or external fluids coexist, particularly during the early stage of aragonite self-recrystallization. The results of this study indicate that the Δ47 value of coral aragonite is susceptible to internal exchange reactions in Mg-bearing solutions at 160–220 °C. Additionally, by modeling the bulk recrystallization and internal fluid-mineral exchange and evaluating their relative importance, we determine that the internal exchange reaction results in a nonlinear correlation between δ18O and Δ47 values. Importantly, we highlight the previously unrecognized diagenetic scenario in which the internal fluid-mineral exchange effect on the Δ47 value can outweigh the bulk recrystallization effect with external fluid coexisting. Consequently, our findings emphasize the need for a comprehensive assessment on the reaction kinetics for different types of recrystallization effects, to support the application of clumped isotope proxies for altered carbonate samples in diagenetic environments.

Carbonate clumped isotopes and blocking temperatures of marbles from the Backbone Range, Taiwan

Clumped isotopic compositions (abundance of 13C16O18O in CO2, expressed by Δ47) in massive calcite marble (CM) and dolomite marble (DM) were reported to refine our understanding of greenschist-facies metamorphism during the still on-going arc-continent collision from the northern, central, and southern localities along the northern-central Tailuko belt of the Backbone Range of Taiwan. Black calcite marble (BCM), white calcite marble (WCM), and DM lenses with cataclastic texture were analyzed. We observed that the carbonate clumped isotope reordering is independent of the grain size of minerals and is less likely affected by lattice defects and active stress. The averaged Δ47 values for CMs and DMs are 0.308 ± 0.004 ‰ (n = 26) and 0.235 ± 0.003 ‰ (n = 13), respectively, giving blocking temperatures of 211 ± 6 °C and 332 ± 8 °C. The blocking temperatures infer mean cooling rates of 614 °C/m.y. and 8.8 × 104 °C/m.y for calcites and dolomites, respectively, estimated based on the available models and clumped reordering mechanisms. We noted that the variations in cooling rates widely differ with different reordering models, especially with different temperature calibrations. Some cooling rate estimates are in agreement with those based on zircon/apatite thermochronological studies. Further refinements on Δ47 - temperature calibration, reordering mechanism and estimation of thermodynamic constants, in addition to improvement in analytical precision, are needed for practical applications of carbonate clumped thermometry as a tectonic speedometer for metamorphic terranes.

Clumped Isotope Reordering and Kinetic Differences in Co-Hosted Calcite and Dolomite Minerals throughout Burial Diagenesis and Exhumation

The clumped isotope paleo-thermometer has become a valuable proxy for the burial history reconstruction of carbonate formations. To maximise the accuracy of these reconstructions, post-depositional alterations, such as recrystallisation and Δ47 isotope exchange reactions, must be understood. In this study, we examine the isotopic behaviour of calcites and early dolomite samples from the same stratigraphic intervals, and thus with similar burial history. This approach provides additional constraints on the kinetics of Δ47 reordering in dolomite during exhumation. Clumped isotope measurements were performed on 19 calcites and 15 early dolomites from the Permian, Jurassic, and Cretaceous periods from four locations in Oman spanning different burial regimes. The calcite and dolomite samples were collected from the rock matrix, based on the assumption that fine material was more susceptible to recrystallisation. Our results show that calcites and dolomites record different Δ47 values despite being subjected to the same thermal history. The maximum Δ47 temperature recorded in dolomites (181 ± 13 °C) corresponds to the oldest and most deeply buried Permian rock. This value is approximately 35 °C higher than those measured in the co-located and coeval calcite matrix (145 ± 14 °C). This discrepancy suggests that calcite and dolomite have different kinetic parameters. Our data confirm (1) that dolomite Δ47 values are more resistant to alteration during burial and exhumation than Δ47 calcite values, and (2) that dolomite has a higher Δ47 closing temperature than calcite during cooling. The presence of two mineral phases with distinct kinetic parameters in the same stratigraphic unit provides additional constraints on models of burial and uplift. In addition, mineralogical data coupled with Δ47 and burial depths suggest that the progressive development of dolomite cation ordering is driven by temperature elevation, as previously suggested.

The application of dual clumped isotope thermometer (Δ47 and Δ48) to the understanding of dolomite formation

Abstract Although several studies have shown the possibility of kinetic isotopic effects during the precipitation of carbonates using the dual clumped isotope proxy (Δ47 and Δ48), this approach has not yet been applied to the study of dolomite. We present Δ47 and Δ48 values of two types of Cenozoic Bahamian dolomites formed by distinctly different mechanisms. One dolomite, present in a core drilled on the island of San Salvador, is believed to have formed from normal seawater in the open system, while the other, taken from the Clino core drilled on the margin of the Great Bahama Bank, was precipitated associated with microbial sulfate reduction in the closed system. The Δ48-derived temperatures of San Salvador dolomites are consistent with their Δ47-temperatures, which are in turn close to the expected value of normal seawater. In contrast, dolomites from Clino show slightly elevated Δ47-derived temperatures (~15 °C higher than the expected) and significantly increased Δ48-temperatures (~145 °C) that are positively correlated with the sulfur isotopes of carbonate associated sulfate, in excess of +30‰. Such an association suggests a non-equilibrium process driven by microbial sulfate reduction, which elevates the alkalinity and depresses the pH of dolomitizing fluids. Notably, the greater amplitude of disequilibrium in Δ48 relative to Δ47 values demonstrates the sensitivity of the Δ48 proxy to kinetic isotope effects in dolomite. Conclusively, this study shows the potential of the dual clumped isotope proxy to identify microbial-mediated dolomitization in modern and ancient records.

Clumped isotope analysis of (bio)apatite using a Kiel IV device in long‐integration dual‐inlet mode

RationaleClumped isotope (Δ47) analysis of bioapatite‐derived CO2 is a powerful tool to determine body temperatures of extinct vertebrates. The common acid bath technique in combination with dual‐inlet‐based mass spectrometric measurements has been the preferred method of choice for this purpose, but the large amount of material necessary and the presence of secondary calcite represent obstacles.MethodsWe analyzed the Δ47 composition of carbonate‐bearing (bio)apatites using a Kiel IV device, which – in general – allows a reduction of sample replicate size by a factor of ~40 over dual‐inlet‐based techniques. The Kiel IV device was tested in two different modes: without and with additional water sinks for improved water removal. Furthermore, we tested a pretreatment technique based on 1 M acetic acid (pH = 5) to selectively remove secondary calcite from the carbonate‐bearing (bio)apatite phase.ResultsSignificantly lower Δ47 values were obtained for a given bioapatite after the installation of the two water sinks. With this setup, Δ47 of (bio)apatites followed a temperature relationship that is indistinguishable from the unified one for pure carbonates, provided a dentine sample, rich in organic matter, was excluded. The original bioapatite Δ47 value was restored from a bioapatite/calcite mixture if the mixed material was treated for 1 h with 1 M acetic acid (pH = 5).Conclusions(Bio)apatites having low organic matter content such as enamel(oid) can be analyzed accurately for Δ47 using a Kiel IV equipped with water sinks that ensure effective removal of water. Secondary calcite can be effectively removed from carbonate‐bearing apatite by pretreatment with 1 M acetic acid (pH = 5) for 1 h.

Calibration of the carbonate clumped isotope thermometer of land snail shells

The application of carbonate clumped isotopes (Δ47) as a temperature proxy has been widely investigated in many types of carbonate materials, including biogenic carbonates such as foraminifera and corals. However, their application to land snails is relatively rare and it remains controversial whether shell carbonate Δ47 is impacted by biologically-driven fractionation. These uncertainties may stem from the lack of precise temperature-controlled calibration samples in previous natural-environment-based studies. In this study we cultured two species of land snail with different environmental tolerances: the thermo-humidiphilous Achatina fulica (A. fulica), and the cold-aridiphilous Cathaica fasciola (C. fasciola), under strictly controlled temperature conditions, covering the growth temperature range of 15–33 °C. The results show that the shell Δ47 values for both A. fulica and C. fasciola had statistically significant negative correlations with the culturing temperature within their respective optimum temperature ranges (A. fulica: 25–33 °C, C. fasciola: 15–25 °C). However, shell Δ47 values for A. fulica at lower temperatures (20 °C in this study) and those for C. fasciola showed a negative deviation from the expected equilibrium values. We attribute the lower-than-equilibrium values to the existence of a vital effect in individual snail species, and more specifically, to the CO2 degassing associated with dehydration/dehydroxylation during carbonate precipitation in the snail's calcifying fluid.

Revisiting Oxygen‐18 and Clumped Isotopes in Planktic and Benthic Foraminifera

AbstractForaminiferal 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.

δ13 C and δ18 O heterogeneities in carbonates: Nonlinear mixing in the application of dual-carbonate-clumped isotope thermometer.

Dual-carbonate-clumped isotope thermometry assumes any departure from Δ47 and Δ48 co-equilibrium is due to disequilibrium processes; however, the effects of endmember mixing have not been evaluated for Δ48 . We show that variations in δ13 C and δ18 O values within a sample can lead to offsets in Δ47 and Δ48 that can be mistaken for kinetic fractionation. A numerical mixing model was developed to calculate the Δ48 and Δ47 values of samples with heterogeneous isotope compositions. The model was used to test a variety of possible endmember mixing scenarios and produce a dataset of mixing offsets in both Δ48 and Δ47 . Different mixing patterns arise from endmember mixing, with different patterns between Δ48 and Δ47 . Δ47 can be both positively and negatively offset from equilibrium values by mixing; however, Δ48 can only be offset in a positive direction, producing an underestimate of formation temperature. The overall results suggest that endmember mixing can mimic kinetic fractionation caused by CO2 degassing in dual-clumped isotope measurements. Mixing between endmembers can result in patterns that resemble that of CO2 degassing. However, these effects require a variation of greater than 5‰ in endmember δ18 O values to have a significant effect relative to measurement errors on Δ48 with the detection limits of modern mass spectrometers. Δ47 remains more sensitive to endmember mixing effects and will display measurable mixing effects at 2‰ variation or less in endmember isotopic values.

Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry

The stable oxygen and clumped isotope composition of brachiopod calcite are important proxies for the reconstruction of Phanerozoic seawater temperatures and δ18O values. The utility of brachiopods as a temperature archive is nonetheless challenged by indications that their shells precipitate out of isotopic equilibrium with ambient seawater, though the origin of disequilibrium effects has remained elusive. Here, we apply the recently developed dual clumped isotope thermometer (i.e., the simultaneous measurement of Δ47 and Δ48 values in CO2 derived from the acid digestion of carbonates) to modern and fossil brachiopods to investigate disequilibrium signatures recorded in brachiopod calcite. We present evidence that disequilibrium signatures are derived from the combination of two rate-limiting processes: the hydration/hydroxylation of CO2, and, potentially, the diffusion of HCO3−/CO32− in water. An empirical correction for clumped isotope disequilibrium is presented, based on correlation between measured disequilibrium offsets in the Δ47 and Δ48 values of modern brachiopods (Δ47disequilibrium = –0.88 × Δ48disequilibrium + 0.02). Dual clumped isotope thermometry of Eocene age brachiopods from Seymour Island (∼65 °S) yields temperatures in agreement with previously reported Δ47 derived temperatures from coeval bivalves. In contrast, uncorrected Δ47 derived temperatures from the same brachiopods are 6–11 °C colder. Measurement of dual clumped isotope composition alongside δ18O values of brachiopod carbonate should also enable more accurate reconstruction of seawater-δ18O. In comparison with group-specific Δ47-temperature calibrations, which correct for an average kinetic bias in Δ47, dual clumped isotope thermometry accounts for the magnitude of disequilibrium inherent to an individual sample, facilitating more reliable paleotemperature and seawater-δ18O reconstructions.

Dual clumped (Δ47-Δ48) isotope data for amorphous carbonates and transformation products reveal a novel mechanism for disequilibrium clumped isotope effects

Amorphous precursors to minerals have been observed in laboratory materials and in nature, including across diverse phyla. These metastable phases allow for the incorporation of cations at higher concentrations than classical crystallization pathways, thus, their chemistry and behavior have implications in an array of disciplines. Currently, little is known about the isotopic composition of the anion in amorphous carbonates and how isotopic values evolve during transformation into a mineral. Here, we examined the evolution of isotopic values in amorphous carbonates and mineral transformation products to identify the potential origins of disequilibrium isotopic compositions in carbonate minerals that form from an amorphous precursor. We measured dual carbonate clumped isotopes (13C18O16O - Δ47; 12C18O18O - Δ48), bulk stable isotope ratios (δ13C, δ18O), and chemical and structural data throughout the transformation of amorphous calcium magnesium carbonate (ACMC) into high Mg-calcite (HMC) over 1 year, with crystallization occurring in solutions from 10 to 60 °C. The Δ47, Δ48, and δ18O values evolved significantly during transformation, indicating dissolution of ACMC and reprecipitation of HMC. After crystallization, the Δ47 and Δ48 values achieved a disequilibrium steady state, while δ18O values continued to evolve. For the fully crystallized HMC samples, the low temperature samples formed at 10 °C had the greatest extent of oxygen isotope disequilibrium (measured value – equilibrium value = −39‰); conversely; the greatest clumped isotope disequilibrium was observed in high temperature samples formed at 40 and 60 °C (0.068‰ for Δ47, 0.072‰ for Δ48). These results are consistent with a new potential mechanism of disequilibrium clumped isotope values in carbonate minerals. Specifically, the dissolution of ACMC during transformation causes disequilibrium oxygen and clumped isotope values in the dissolved inorganic carbon (DIC) pool. The extent of isotopic disequilibrium in DIC during transformation is temperature dependent, and is recorded by the forming mineral. Isotopic results may also reflect mixing effects, as scanning electron microscopy (SEM) showed ACMC and HMC existing simultaneously during transformation, indicating that ACMC likely progressively dissolves and reprecipitates as the DIC pool isotopically evolves. This may result in heterogeneous isotopic values in HMC. In total, these data suggest a highly dynamic localized environment could exist in biomineralizing organisms and abiotic systems that utilize amorphous precursors to form carbonate minerals, potentially resulting in isotopic values that are not representative of formation temperature.

Synthetic Witherite for Standardisation of Clumped Isotope (Δ47) Measurements

Carbonate clumped isotope, quantified by Δ47 value, has wide application potential in Earth science. A recently proposed carbonate standardisation scheme based on community‐available reference materials (i.e., ETH‐1–4) has proven to improve interlaboratory comparability in Δ47 measurement. However, the shortage of these reference materials may be problematic in future; therefore, developing traceable and supplementary in‐house reference materials may become necessary, especially when supplies run out. Here, we describe a method of producing synthetic witherite carbonates with variable bulk isotopic compositions and clumped isotope ordering states, for which Δ47 values were determined together with ETH carbonate reference materials. Results indicate that the witherite carbonates allow effective corrections for instrumental linearity and scale variations, improving long‐term reproducibility. Additionally, by normalising witherite Δ47 values to agreed ETH carbonate values on the Intercarb‐Carbon Dioxide Equilibrium Scale, the witherite reference materials can be used to determine Δ47 values without using equilibrated gases and reduce the consumption of ETH carbonates.

Assessing the effects of embedding resins on carbonate stable and clumped isotope analyses.

Embedding resins are widely used to fix carbonates for high-precision sample preparation and high-resolution sampling. However, these embedding materials are difficult to remove after sample preparation and are known to affect the accuracy of carbonate stable isotope analyses. Nevertheless, their impact on clumped isotope analysis, which is particularly sensitive to contamination artifacts, has so far not been tested. The observation that running resin-containing samples decreased the reproducibility of clumped isotope values for internal laboratory carbonate standards and increased the external standard deviation (SD 0.061-0.088‰) compared to the long-term observations (0.034‰), prompted us to set up an experiment to test the influence of resin addition on instrument performance. Here we analyzed the stable and clumped isotope composition of a pure calcium carbonate standard (ETH-4) mixed with three types of embedding resins in 2:1 and 1:1 proportions. Our aim was to assess how resin addition affects isotope analyses. We found that none of the stable isotopic values were significantly different. The δ13 C values were -10.22 ± 0.07‰ (mean ± SD) for pure ETH-4, while the δ13 C values of ETH-4 mixed with embedding resins in 2:1 and 1:1 proportions were -10.21 ± 0.06‰ and -10.18 ± 0.06‰, respectively (p> 0.05). The δ18 O values were -18.82 ± 0.11‰ for pure ETH-4 versus -18.81 ± 0.09‰ and -18.82 ± 0.08‰ for 2:1 and 1:1 ETH-4:resin mixtures, respectively (p> 0.05). Given the large uncertainty in our results, we did not find significant differences between different mixtures in the carbonate clumped isotope values (Δ47 ), with 0.458 ± 0.107‰, 0.464 ± 0.086‰, and 0.417 ± 0.089‰ in pure ETH-4 and ETH-4 with 2:1 and 1:1 resin mixtures, respectively (p> 0.05). However, a resin-related bias in the results might be masked by the large uncertainty. The measured ETH-4 values in our study are similar to the InterCarb values (δ13 C = -10.20‰, δ18 O = -18.81‰, Δ47 = 0.450‰, InterCarb-Carbon Dioxide Equilibrium Scale). However, the external SD of Δ47 in sessions measuring ETH-4 with resins is higher than in sessions without deliberate resin addition for the same measuring period. We find that the potential contamination from the resin addition leads to a larger variability for Δ47 values in sessions measuring ETH-4 including resins. We therefore recommend purification of embedded samples using a contamination trap with Porapak prior to analysis, if possible, or avoiding resins during sample preparation and workup, as well as monitoring the measurement quality during and after sessions with samples containing embedding resins.

Dual carbonate clumped isotopes (Δ47-Δ48) constrains kinetic effects and timescales in peridotite-associated springs at the Cedars, Northern California

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO2-charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca2+ and OH– enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO3 at surface springs. This environment enables us to apply the recently developed Δ47-Δ48 dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO2 transformation in a region with the potential for geological CO2 sequestration. We analyzed CaCO3 recovered from various localities and identified significant kinetic fractionations associated with CO2 absorption in a majority of samples, characterized by enrichment in Δ47 values and depletion in Δ48 values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ47: 0.163‰, ΔΔ48: −0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 h to achieve isotopic equilibria. The Δ48/Δ47 slope of samples from the Cedars (−3.223 ± 0.519) is within the range of published theoretical values designed to constrain CO2 hydrolysis-related kinetic fractionation (−1.724 to −8.330). The Δ47/δ18O slope (−0.009 ± 0.001) and Δ47/δ13C slope (−0.009 ± 0.001) are roughly consistent with literature values reported from a peridotite in Oman of −0.006 ± 0.002 and −0.005 ± 0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to study CO2-absorption processes in applied systems, including sites of interest for geological sequestration.

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Coevolutions of terrestrial temperature and monsoonal precipitation amounts from the latest Pleistocene to the mid-Holocene in Japan: Carbonate clumped isotope record of a stalagmite

Quantitative paleotemperature reconstruction is a challenging and important issue in paleoenvironmental studies, for which carbonate clumped isotope (Δ47) thermometry is a promising approach. Here we analyzed Δ47 values from 66 layers of OT02 stalagmite from Ohtaki Cave in central Japan, covering two separate time intervals (2.6–8.8 and 34.8–63.5 ka) to reconstruct terrestrial temperature and meteoric δ18O records. The average Δ47 temperatures of the Holocene interval and the latest Pleistonece interval were 16.3 °C ± 5.6 °C and 9.7 °C ± 4.6 °C, respectively. The Δ47 thermometry also revealed that the cold intervals (5 °C–10 °C) correspond to the Heinrich stadials HSs4–6, and the warm interval (up to 19.9 °C ± 6.0 °C) in middle Holocene (approximately 6–5 ka) accompanied by the Hypsithermal climate optimum. We also reconstructed past meteoric δ18O by subtracting the temperature effect from stalagmite δ18O. Average meteoric δ18O was −8.2‰ ± 1.0‰ vs. VSMOW in the Holocene interval and − 8.8‰ ± 0.8‰ in the latest Pleistocene interval. Over centennial timescales, meteoric δ18O was more negative during colder periods, such as Heinrich stadials and a cooling event around 7 ka, and less negative in warmer periods, such as Hypsithermal warming. A temperature dependency of total 18O fractionation from sea water to precipitation is a likely reason for the negative correlation between temperature and meteoric δ18O. Additionally, East Asian summer monsoon (EASM) brought larger rainfall of less negative δ18O during the warm periods, whereas larger snow/rainfall of more negative δ18O brought from East Asian winter monsoon (EAWM) in colder periods. The relative influences from EASM and EAWM were changing in a centennial timescale. δ18O of OT02 had reflected changes in terrestrial temperature and meteoric δ18O, which are both strongly related to EASM and EAWM.

Clumped isotope analysis of zoned calcite cement, Carboniferous, Isle of Man

AbstractSequential analyses of δ13C, δ18O and Δ47 values of calcite and dolomite deposited in millimetre‐sized cavities are reported from the Ronaldsway Member packstones, Isle of Man. The Ronaldsway brachiopods have δ13C values of ca +2.3‰ and δ18O values of ca −7.2‰; carbon is like predicted Carboniferous values, while oxygen values are more negative. The brachiopods show preserved microstructure but have marginal alteration and a streaky cathodoluminescence pattern. Crinoid ossicles have δ13C values of ca +2.3‰ and one with a δ18O value of ca −3.1‰, compatible with Carboniferous marine precipitates; three samples have δ18O values of ca −6.5‰ and are 18O‐depleted. Calcite stages 1 and 2 have δ13C values ca +3.2‰ and δ18O values ca −2.5‰, compatible with Carboniferous sea water. Stage 1 and 2 have non‐luminescent to orange CL zones. Stage 1 and early stage 2 contain red luminescent dolomite micro crystals generated during Mg calcite stabilisation. The Δ47 values for stage 1 and 2 cements indicate temperatures of 86 and 105°C that occurred after the stabilisation of Mg calcite. Stage 3–8 zoned cements preserve their original growth surfaces and their δ13C and δ18O values suggest precipitation during burial and exhumation. The Δ47 values of the brachiopods and crinoids indicate temperatures between 85 and 140°C indicating they were either recrystallised at high temperatures or affected by solid state reordering. To evaluate these alternatives two quantitative models, water–rock reaction and reordering models are performed. The allochems and cements are progressively altered by porewater towards the fluid‐buffered behaviour. The quantitative evaluation of calcite and dolomite solid‐state reordering suggests the elevated clumped isotopic temperatures are produced by interaction with hydrothermal fluids. This study improves understanding by applying previously untried techniques; further Δ47 data and quantifying elemental variations would help further interpretation but the poorly documented post‐depositional history is a drawback.

Experimental constraints on siderite clumped isotope thermometry

We present results from chemostatic siderite precipitation experiments conducted using a modified constant composition method at temperatures between 15 and 75 °C and compare these results with previous temperature relationships to obtain a composite low temperature relationship for siderite clumped isotope ratios (Δ47) from 8.5 to 75 °C:Δ47=0.0364±0.004∗106T2+0.1672±0.046±95%CL,R2=0.708The slope of this line is statistically indistinguishable from those reported for aragonite, calcite, and dolomite clumped isotopes, but differs in its intercept. To test whether this difference results from mineralogical differences in phosphoric acid fractionation factors (AFF) between siderite and Ca-Mg carbonates, we heated siderite to 625 °C at 1 GPa, which yielded a statistically indistinguishable Δ47 value from that predicted by published clumped isotope calibrations populated by Ca-and Mg-carbonate data at higher temperatures. Thus, the apparent difference between siderite clumped isotope behavior and that of other carbonate minerals may not be well explained by differences in AFF. Despite differences in precipitation methods, our results are entirely consistent with published inorganic and microbially-mediated experimental siderite clumped isotope data, suggesting that siderite Δ47-temperature dependence differs from Ca-Mg carbonates for reasons unrelated to precipitation methods. Our isotopic results are also consistent with published isotopic measurements on natural siderites precipitated at known environmental temperatures. Our analysis leads us to conclude that supersaturation-related kinetic effects likely play an important role in clumped isotope fractionation in natural and experimentally precipitated siderite.