Carbonate Clumped Isotope Thermometry Research Articles

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

The lack of effective paleogeothermometers restricts the study of the thermal history of carbonate strata. Carbonate clumped isotope thermometry and in-situ U–Pb dating are appropriate for reconstructing the thermal history of carbonate strata. Herein, this study analyzed clumped isotope compositions and U–Pb ages of calcite fabrics, including micrite matrix and veins, from Permian carbonate strata in the eastern Sichuan Basin. U–Pb dating of the micrite matrix was performed to rule out the influence of recrystallization and determine the diagenetic age of the samples. Clumped isotope temperatures of the four micrite matrix samples ranged from 102 °C to 137 °C. The maximum paleotemperature derived from a solid-state reordering model was between 210 °C and 220 °C. From all four samples, three calcite veins yielded crystallization temperatures corresponding to 160 ± 10 °C, 60 ± 10 °C, and 29 ± 4 °C, as determined by clumped isotope and solid-state reordering model analyses. In-situ U–Pb dating provided corresponding crystallization ages of 159 ± 17 Ma, 256.6 ± 5.5 Ma, and 17.86 ± 0.8 Ma. The crystallization ages and temperatures of the three calcite veins align well with the thermal history of the Permian strata, indicating a high degree of accuracy. This study introduces a new strategy for reconstructing the thermal history of carbonate strata. The thermal history can be quantified using the constraints of the crystallization ages and temperatures from different stages of calcite veins or cements. This approach is suitable for determining the maturation progress of the source rock and conducting oil and gas exploration in petroliferous basins.

Burial History and Hydrocarbon Potential of the Harudi and Fulra Limestone of Kachchh, Western India Constrained using Carbonate Clumped Isotope Thermometry

Abstract The clumped isotope paleothermometry is used to estimate minimum burial depth of the mid Eocene Harudi and Fulra Limestone Formations of Kachchh, western India. The Nummulites and other fossil foraminiferal genera were separated for conventional (δ18O, δ13C) and clumped isotopic (Δ47) measurements. The measurements were also made on whole rock (WR) to visualize the different phases of diagenetic alteration, expected depth of burial and capacity to generate hydrocarbon. The measured δ18O and δ13C values of WR samples vary between -3.09 and -2.21‰ and -0.78 and 0.96‰ respectively, with average values of -2.88‰ and 0.16‰. The δ18O and δ13C values of benthic foraminifera vary between -3.52 and -3.20‰ (Average: -3.35±0.013) and -1.04 and 0.81‰ (Average: 0.26±0.76) respectively. The measured Δ47 for the foraminifera translates to temperature of 35 and 49°C and between 26 and 48°C for WR indicating minor diagenetic alteration. The minimum estimates for the burial depth of the studied formations are up to 2 km which is based on the temperature difference between the ecological preference temperature of benthic foraminifera and the diagenetically altered foraminifera and δ13C of WR and foraminifera, indicating that rocks have potential to generate methane.

Hot carbonates deep within the Chicxulub impact structure.

Constraining the thermodynamic conditions within an impact structure during and after hypervelocity impacts is extremely challenging due to the transient thermal regimes. This work uses carbonate clumped-isotope thermometry to reconstruct absolute temperatures of impact lithologies within and close to the ∼66 Myr old Chicxulub crater (Yucatán, México). We present stable oxygen (δ18O), carbon (δ13C), and clumped-isotope (Δ47) data for carbonate-bearing impact breccias, impact melt rock, and target lithologies from four drill cores on a transect through the Chicxulub structure from the northern peak ring to the southern proximal ejecta blanket. Clumped isotope-derived temperatures (T(Δ47)) are consistently higher than maximum Late Cretaceous sea surface temperatures (35.5°C), except in the case of Paleogene limestones and melt-poor impact breccias outside of the crater, confirming the influence of burial diagenesis and a widespread and long-lived hydrothermal system. The melt-poor breccia unit outside the crater is overlain by melt-rich impact breccia yielding a much higher T(Δ47) of 111 ± 10°C (1 standard error [SE]), which likely traces the thermal processing of carbonate material during ejection. Finally, T(Δ47) up to 327 ± 33°C (1 SE) is determined for the lower suevite and impact melt rock intervals within the crater. The highest temperatures are related to distinct petrological features associated with decarbonation and rapid back-reaction, in which highly reactive CaO recombines with impact-released CO2 to form secondary CaCO3 phases. These observations have important climatic implications for the Cretaceous-Paleogene mass extinction event, as current numerical models likely overestimate the release of CO2 from the Chicxulub impact event.

Dual carbonate clumped isotope (Δ47-Δ48) measurements constrain different sources of kinetic isotope effects and quasi-equilibrium signatures in cave carbonates

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO2 degassing. More recently, studies have shown that mass-47 and mass-48 CO2 from acid digested carbonate minerals (Δ47 and Δ48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO2, temperature) for (dis)equilibrium using Δ47-Δ48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ47-Δ48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ47-predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothem samples, we find Δ47-Δ48 values consistent with CO2 degassing effects, however, the majority of samples with KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopic buffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ47 and Δ48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ47 and Δ48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest average departures from equilibrium (ΔΔ47¯ = −0.032 ± 0.007, ΔΔ48¯ = −0.104 ± 0.035, where ΔΔi is the measured value – the equilibrium value). This study shows how the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects.

A correction scheme for calcium carbonate clumped isotope (Δ47) thermometric equation depending on sample preparation technique

The CO2 preparatory methods implemented during carbonate-clumped isotope analysis for the acid digestion of carbonate with the goal of high sample throughput yielded multiple empirical relationships for the thermometry. These methods varied significantly from its original practice of carbonate reaction at 25 °C using sealed vessel method to automated quick performance acid drip or common acid bath method at 70 °C or 90 °C temperatures, respectively; these approaches differed noticeably. Technical development replacing primitive reaction protocols, introduced different reaction conditions, causing significant differences in the chemical reaction procedure and CO2 trapping, which caused variation in the values of slope and intercept for the linear regression equations governing carbonate-clumped isotope(Δ47) distribution with carbonate growth temperatures. These studies include the Δ47 measurement and analysis of either laboratory-grown or natural carbonates with precise knowledge of their precipitation/depositional temperatures. However, the discrepancies in the existing universal calibration schemes remained poorly understood despite adopting an identical data correction protocol. This is explained here by the reaction kinetics and CO2 collection methodologies adopted during experimentation. The present study investigated the slope and intercept values of the published carbonate clumped isotope thermometry equations expressed in the accepted Absolute Reference Frame (ARF in CDES) at 25 °C after accounting for the acid correction factor. We observe a systematic shift in the mean slope and intercept values of 0.0154(±0.007) and 0.153(±0.0686) ‰ for 70 °C reaction experiments and offset of 0.0181(±0.008) and 0.197(±0.079) ‰ for the experiments conducted at 90 °C by using the acid drip and/or Common Acid bath method, respectively from the slope and intercept values of the calibration equations proposed using sealed vessel method at 25 °C reaction temperature. The mean values for slope and intercept are compared using ANOVA and paired f-test. These correction factors for slopes and intercepts will allow the transformation of clumped isotope values at different temperatures into ARF scale at 25 °C and enable accurate deduction of temperature for carbonate samples. Correction factors proposed here account for variations in the sample preparation techniques arising due to different reaction temperatures, mechanisms and vapor pressure in the chamber for isotopic exchange reaction to happen for smaller or prolonged time intervals.

Applicability of carbonate clumped isotope thermometry in the Tuchang-Jentse geothermal field

Temperature and oxygen isotope composition of fluids, which are critical for reconstructing thermal structures and characterizing hydrothermal fluids in a geothermal system, can be obtained using carbonate clumped isotope analysis. Ideally, carbonate clumped isotope compositions (Δ47) recording the thermal fluid temperatures and compositions for fault-associated veins and rhombic calcite crystals inside open cracks demonstrate previous tectonic activities and present reservoir conditions, respectively. We studied the clumped isotope geochemistry of veins and travertines in outcrops and cuttings from exploration wells in the Tuchang–Jentse geothermal field in Taiwan. The Δ47-based temperatures (Δ47-T; 161−8+8 to 238−14+16 °C) and calculated δ18Ofluid values (+2.0 to +5.0‰) of vein calcites were the highest, indicating mixing of the magmatic/metamorphic fluids with the meteoric water after water–rock interaction in the deep thermal fluid cycle and migration to shallow areas by fault activity. Rhombic calcites with high Δ47-T (142−6+6 to 185−4+5 °C) and low calculated δ18Ofluid (−9.1 to −5.3‰) correspond to downhole temperatures and measured δ18Ofluid of thermal fluids in the exploration wells, respectively, suggesting that the latest thermal fluid was of meteoric water origin. Travertines near faults have low calculated δ18Ofluid (−8.9 to −1.0‰) and Δ47-T that are above air temperatures (32−9+9 to 95−9+10 °C), implying that the local groundwater was subsequently heated by or mixed with deep fluid. Carbonate clumped isotope thermometry application demonstrates that the carbonate clumped isotope thermometer not only precisely estimates temperature in the carbonate-dominated system using drill cutting samples during the exploration phase, but also reveals the spatial and temporary evolution of geothermal fluids used as a reference for future site selection planning.

Fluid flow evolution revealed by carbonate clumped isotope thermometry along the fractures in a complex salt dome setting: Study case (Jebel Madar, Oman)

A better understanding of constraining paleo fluid circulation along fractures in tectonically complex regions is essential from the scientific perspective and in exploiting sustainable geo-energies such as geothermal resources, but it remains a challenge. Jebel Madar in Oman as a salt dome is an ideal case study because of the complexity of its tectonic evolution and diagenetic process history. Here, we investigated the Cretaceous carbonate-encasing rock of salt domes and calcite-infilling fractures, making them an ideal case study for paleo fluid flow. The objectives were to test two different hypotheses: (1) to verify the previous study based on seismic data that the Hawasina Nappe had been priorly emplaced at the study location and was subsequently completely eroded, and (2) that the complex history of paleo-fluid flow existed in the fractures of the salt dome, and that this system was not yet fully constrained by previous studies, and also including accounting for the presence of any kinetic fractionation. To achieve our objectives, we applied carbonate clumped isotopes on different generations of veins as well as on the carbonate cement of the host rock itself to reconstruct paleo temperatures and the isotopic composition of the paleo-fluids. The carbonate clumped isotope temperatures indicate that calcite vein precipitation occurred between 48 and 171 °C. Assuming that our carbonate clumped isotopes data faithfully capture the recrystallization temperature of the limestone, the geothermal gradient during the Campanian can be estimated at 33.5 °C km−1, implying that the Hawasina Nappe must indeed have been present at the location. We also argue that the evolution of fluid flow in the study area comprises two distinct episodes of fluid circulation. The first episode led to the precipitation of high-temperature calcites followed by closed-system recrystallization during the local emplacement of the Hawasina Nappe. The second episode of fluid circulation occurred at lower temperatures with calcite members representing meteoric diagenetic fluids with evidence of the kinetic fractionation of the clumped isotopes signature in our samples. Our result demonstrates that the new paleotemperature data present new constraints of paleo fluid circulation along the fractures in a salt-domed setting and, importantly, confirms for the first time that the Hawasina Nappe was present at the site of Jebel Madar in the Late Cretaceous.

Physicochemical Conditions of the Devonian-Jurassic Continental Deep Biosphere Tracked by Carbonate Clumped Isotope Temperatures of Granite-Hosted Carbonate Veins

Previous studies have shown that microorganisms thrive in oligotrophic fracture systems, and metabolisms include consumption and production of methane. In the Laxemar, Götemar, and Forsmark areas of Sweden, ancient microbial activity has previously been demonstrated by large δ13CVPDB variability of carbonate vein infillings within granitic host rocks. Here, we apply carbonate clumped isotope thermometry to reconstruct the temperature of precipitation of the carbonate within these veins. The carbonate clumped isotope temperatures indicate that mineralization took place between 46°C and 108°C, in line with previously published fluid inclusion data (<50°C to 113°C). The new carbonate clumped isotope data more accurately narrows temperatures at the lower end of this range as fluid inclusions are not easily applicable below 80°C, a temperature regime of high importance for paleobiological reconstructions. Our results demonstrate that large volumes of biogenic carbonate cement were formed from low-temperature microbial processes in low-salinity water, succeeding calcite of deep brine origin. The known burial and thermal history of the region combined with our carbonate clumped isotope data place the microbial activity at the end of the Variscan orogeny and later events (e.g., Jurassic). Thus, carbonate clumped isotope thermometry reduces uncertainties in deep biosphere studies by providing more accurate temperature constraints in the low-temperature regime, the biogenic processes, and the origin of the diagenetic fluids.

Frontiers of Carbonate Clumped Isotope Thermometry

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 Δi is 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 Δi values 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 Δi values 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 Δ638 measurements 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.

Calcite‐cemented concretions in non‐marine sandstones: An integrated study of outcrop sedimentology, petrography and clumped isotopes

AbstractCalcite‐cemented concretions can reduce reservoir quality and form important low‐permeability baffles for fluid flow in sandstone hydrocarbon carrier beds/reservoirs. Understanding the origin and distribution patterns of concretions has important implications for characterizing reservoir heterogeneity and developing fields. The origin of calcite concretions in non‐marine sandstones lacking detrital carbonate to source the cement is poorly studied. The practical difficulty of obtaining the precipitation temperature of the concretions prevents the precise determination of the source of cement components, solute flux, pore water type and the geochemical microenvironments in which cement precipitated. Carbonate clumped isotope thermometry can give unique constraints to the temperature of concretionary cement precipitation. This study focused on outcrops of the Lower Cretaceous non‐marine Qingshuihe Formation in the Urho area, on the north‐western margin of the Junggar Basin (West China). Four depositional facies associations were recognized, corresponding to channel, mid‐channel bar, abandoned channel and overbank floodplain environments. Calcite concretions occur only in sandstones and include spherical, ellipsoidal and tabular forms. They are isolated and/or mutually aligned in layers that tend to be either at the top, base, or within channel and mid‐channel sand bodies. The clumped isotope temperatures for the concretionary calcite range from 31 to 45°C with a standard error of <3°C. The calcite has highly variable, low δ13C values ranging from −16.91 ± 0.01 to −2.93 ± 0.02‰ (Vienna Pee Dee Belemnite). The 13C‐depleted bicarbonate source was linked to biodegradation of migrated oils at shallow burial owing to tectonic uplift and erosion. Calcite δ18O values are very consistent and fall between −12.30 ± 0.04 to −9.79 ± 0.31‰ (Vienna Pee Dee Belemnite). Introduction of meteoric water was a dominant mechanism for the pronounced 18O‐depletion in the oxygen isotopes of pore water, along with water–rock interaction during alteration of volcaniclastic materials in sandstones. Meteoric groundwater flushed the Qingshuihe sandstones as confined aquifers down the regional palaeo‐slope, biodegrading the migrated oils and enabling a sufficient solute flux for precipitation of concretions.

Late Mesozoic elevation history of the north Taihang Mountains, China: Constraints from clumped isotope geochemistry

A series of tectonic events during the late Mesozoic, including the Yanshanian Orogeny and the Early Cretaceous tectonic extension, reshaped the tectonic regime and landscape of North China (NC). During this period, the Taihang Mountains (THM) became a prominent geomorphic and ecological separation zone, which separates NC into regions with different landforms, tectonic settings, and climatic regimes. The uplift history of the THM during the late Mesozoic is of great interest, since it is critical to constrain the geodynamic and regional climatic evolution of NC. In this study, we report the first carbonate clumped isotope thermometry (Δ47) data for paleosol carbonates from this region to constrain the paleoelevation of the north THM during the late Mesozoic. Our result indicates that the north THM was almost certainly 2.0 ± 0.8 km elevation at ca. 160 Ma associated with crust thickening and shortening. After the collapse of the eastern North China Craton (NCC), the north THM maintained high elevation at ca. 113 Ma (2.7 ± 0.7 km). The findings suggest that the present topographic features of NC may have been established in the Early Cretaceous. Finally, our study suggests elevation decrease at the end of the Early Cretaceous (ca. 110−100 Ma), which is possibly related to isostatic adjustment caused by asthenosphere upwelling and partial crustal thinning in the central NCC.

Application of ostracod-based carbonate clumped-isotope thermometry to paleo-elevation reconstruction in a hydrologically complex setting: A case study from the northern Tibetan Plateau

Paleo-elevation estimates based on the oxygen and hydrogen isotopes of meteoric waters are commonly unreliable in hydrologically complex settings such as the northern Tibetan Plateau (TP), an area of confluence of the East Asian monsoon and mid-latitude Westerlies. Here, we explore a novel application of carbonate clumped-isotope thermometry for estimation of paleo-elevations based on a robust correlation of altitude with modern summer lake-surface temperatures that is independent of moisture sources and recycling processes. Using shallow-lake ostracod species, we estimate a summer lake-surface temperature of ∼ 20 ℃ for the Late Miocene Qaidam Basin of the northern TP. Paleo-elevation calculations yield basin floor estimates of ∼ 3475 m (for ∼ 11 Ma, based on a conventional geochronology model) or ∼ 3375 m (for ∼ 7.0 Ma, based on a revised geochronology model), each with an uncertainty of ±∼950 m, yielding elevations that are close to or slightly higher than the present-day elevation of the study site (∼3200 m). This study highlights the potential of ostracod-based carbonate clumped-isotope thermometry to reconstruct paleo- elevations in hydrologically complex areas of the TP and other montane systems.

“Roll-Front” Mass Transfer of Carbonate Cations in Carlin-Type Gold Deposits: Insights from UV-Fluorescent Calcite Veins

Abstract Hydrothermal fluids dissolve and precipitate vast quantities of carbonate during the formation of Carlin-type gold deposits. This mass transfer results in extensive volumes of decarbonatized rock and hydrothermal calcite with distinct δ18O and δ13C signatures, occurring as both veins and wall-rock pseudomorphs. However, the processes which cause the fluids to switch between carbonate-dissolving and carbonate-precipitating are not well understood. Here, we present a model for the formation of ore-stage calcite through the pseudomorphic replacement of pre-existing calcite based on a detailed study of UV-fluorescent (UVF) calcite veins from the Nadaleen trend Carlin-type gold deposits in Yukon. The UV fluorescence in the veins is a visual indicator of their high-Mn, low-Fe chemistry, which we use to develop our model. The proximity of UVF calcite veins to ore-stage alteration and the realgar within the veins indicate that they formed during Au mineralization. Vein orientation and crosscutting relationships with faults and dikes suggest they formed by replacing pre-existing calcite veins that developed during folding. Cathodoluminescence responses, showing the selective replacement of pre-ore calcite by Mn-rich calcite, support this hypothesis. We propose that differences in Mn, Sr, and Mg content between pre-existing calcite and UVF calcite drive this replacement reaction as the fluids approach calcite saturation. UVF calcite veins have fluid-dominated δ13C and δ18O compositions, suggesting that the fluids dissolved fluid-buffered hydrothermal carbonate upstream along the flow path. In our model, unreactive and high-permeability decarbonatized rock allows upstream fluids to reach reactive calcite at the edge of the dissolution zone, where they replace the calcite with ore-stage calcite pseudomorphs. Over several iterations of this process, the fluids repeatedly precipitate and dissolve ore-stage calcite, moving it downstream as a roll front, which concentrates Mn at the edge of the dissolution zone. We suggest that this mass transfer also occurs at the deposit scale to form zoned carbonate alteration along the flow path. Additionally, we present carbonate clumped isotope thermometry results that indicate UVF calcite veins formed at temperatures of ~140°C, although higher temperatures of up to 300°C are possible if the veins exhumed slowly after their formation. Calculated fluid δ13C and δ18O values at these temperatures suggest a meteoric fluid source, although the isotopic data are not inconsistent with a magmatic source if fluid temperatures were ≥200°C because less isotopic fractionation occurs at higher temperatures.

Temperature regimes during formation of Miocene island dolostones as determined by clumped isotope thermometry: Xisha Islands, South China Sea

Carbonate clumped isotope thermometry is a relatively new type of analysis that has been used to obtain a better understanding of the temperature and salinity of the water that mediated the development of dolostones in the Cenozoic successions of isolated oceanic islands. There is, however, still some uncertainty about the factors that could modify the clumped isotopes even in shallow burial conditions and thereby affect the temperatures and salinities derived from them. Here, focus is placed on a 210.5 m thick succession of Miocene dolostones that is found on the Xisha islands in the South China Sea that have experienced rapid subsidence since the Early Miocene. These dolostones are typically formed of crystals that have a “dirty” core that is encased by a clear outer rim. The formation of the “dirty cores”, the primary replacement dolomite, was probably mediated by normal seawater in near-surface conditions, whereas the later cements formed at higher temperatures from slightly evaporated seawater. Given that it is impossible to separate the “dirty cores” from the clear rimes for analytical purposes, it must be recognized that the clumped isotopes and other geochemical data (e.g., δ18O, 87Sr/86Sr) are averages of these two types of dolomite. This also means that the development of dolomite in these dolostones involved two successive stages of dolomite development that was probably related to sea-level cycles. Despite these issues, clumped isotope thermometry can still provide valuable temperature and salinity information for understanding the dolomitization processes that led to the development of island dolostones.

Structural Diagenesis of a Salt Dome Revealed by Carbonate Clumped Isotope Thermometry: Study Case (Jebel Madar, Oman)

Structural Diagenesis of a Salt Dome Revealed by Carbonate Clumped Isotope Thermometry: Study Case (Jebel Madar, Oman)

Clumped isotopes in modern marine bivalves

Oxygen-isotope measurements of fossil carbonates remain the most common method for paleoclimatic temperature reconstructions. A well-known limitation of this approach is the influence of the oxygen isotope composition of water in which mineralization occurs, which may vary significantly through space and time, and is often difficult to constrain precisely. Carbonate clumped-isotope thermometry is an alternative approach applicable to many carbonates. It is based on measurements of Δ47 (a tracer of small statistical anomalies in the abundance of rare, doubly-substituted carbonate isotopologues), and requires no independent information on the oxygen-isotope composition of parent waters. Here, we report new calibration observations of clumped isotopes in four species of calcitic marine bivalves (A. colbecki, N. cochlear, S. cucullata, M. gigas) from various ecosystems including coastal and deep-sea environments, with calcification temperatures ranging from −2 °C to 27 °C and very different amplitudes of seasonal temperature variability. At two localities with large seasonal temperature variability, calcification time intervals were constrained using a sclerochronological approach to test whether seasonal gradients of temperature can be accurately quantified based on Δ47 measurements.Our results indicate that the mature bivalves we analyzed have clumped-isotope compositions entirely consistent with earlier calibration studies processed in the I-CDES reference frame and based on biogenic/abiotic/synthetic materials. By contrast, juvenile M. gigas oysters yield substantially lower Δ47 values than expected based on their calcification environments, suggesting that their early growth phase is associated with yet poorly understood isotopic biases affecting both δ18Ο and Δ47 values. The link between seawater temperatures and bivalve Δ47 values is thus potentially applicable to seasonal reconstructions, but only if shell sections formed in cold seasons are precisely identified and precisely sampled, and taking into account that winter calcification is likely to be biased due to reduced growth rate. Moreover, the excellent agreement between our observations and the existing I-CDES calibrations further demonstrates the efficacy of the I-CDES standardization approach, and adds to the evidence that many different types of carbonates conform to statistically indistinguishable relationships between Δ47 and crystallization temperature.

Carbonate clumped isotope thermometry of fault rocks and its possibilities: tectonic implications from calcites within Himalayan Frontal Fold-Thrust Belt

Disentangling the temperature and depth of formation of fault rocks is critical for understanding their rheology, exhumation, and the evolution of fault zones. Estimation of fault rock temperatures mostly relies on conventional geothermometers of metamorphic minerals and element partitioning analysis, which are largely inapplicable in shallow crustal fault rocks. Here, we demonstrate the applicability of the carbonate clumped isotope thermometer in low-grade carbonate-bearing fault rocks from the Himalayan frontal wedge (northwest India). Coalescing carbonate clumped isotope thermometry and calcite e-twin morphology allows us to constrain the temperature and depth of formation of the two main thrusts of the Himalayan frontal wedge, the Nahan thrust (170 ± 10 °C; 6–7 km depth), and the Main Boundary thrust (262 ± 30 °C; 10–11 km depth). The integration of the adopted analytical techniques can promote the application of calcite-based clumped isotope thermometry to the fault zone processes and refinement of shallow crustal fault zone models.

Towards disentangling climatic and tectonic changes of southernmost Africa using strontium isotope stratigraphy and clumped isotope thermometry

Abstract Upper Cretaceous-Cenozoic marine sequences preserved between 30 and 350 masl across southern South Africa record a complex history of climatic and tectonic changes. In this study, we measure the strontium (Sr) isotope composition of fossil shark teeth, echinoderms, corals and oyster shells to chronostratigraphically constrain the ages of these sequences. The method requires careful petrographic screening and micro-drilling of the samples to avoid possible alteration by diagenesis. To assess palaeoenvironmental effects in the shells we measured the Mg/Ca elemental ratios and O isotope values using electron microprobe analysis (EMPA) and secondary ion mass spectrometry (SIMS). In addition, we employed carbonate clumped isotope thermometry to test palaeotemperatures reconstruction. The analysis of recent to modern stromatolites by clumped isotopes yields an average temperature of 20.2°C, in agreement with present day observations. The fossil oyster shells suggest a warmer (23.0°C) seawater palaeotemperature, possibly due to major deglaciation and sea-level rise during the Neogene. We also find that transgressions occurred above 200 to 350 m elevation during: 1) the Campanian-Maastrichian (~75 Ma); 2) the mid-Oligocene (32 to 26 Ma); and 3) the Messinian-Zanclean (6 to 5 Ma). These three episodes are linked to well-known variations in global sea level and regional tectonic processes that could have affected the continental margin differently. The most recent transgression coincides with a maximum global sea-level rise of ~50 m at ca. 5.3 Ma, and a worldwide plate kinematic change around 6 Ma, which in Eurasia led to the closure of the Mediterranean Sea. In the Eastern Cape of South Africa, the new dates of analyzed oyster shells constrain a minimum uplift rate of ca. 150 m/Myr during this tectonic activity. The results have important implications for robust calibration of relative sea level in southern Africa.

Carbonate Standards Ensure Better Paleothermometers

A community effort finds that carbonate standards eliminate the interlaboratory differences plaguing carbonate clumped-isotope thermometry studies.

Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti, Tanzania

The isotopic composition of paleosol carbonates is used extensively to reconstruct past vegetation, climate, and altimetry, but poor constraints on soil evaporation and temperature have limited the utility of oxygen isotopes in the studies. Recent advances in carbonate clumped isotope thermometry (TΔ47) allow for independent controls on temperature, but the influence of evaporation remains unresolved. However, the sensitivity of 18O-17O-16O distributions to kinetic fractionation makes it possible to use triple oxygen isotopes (Δ′17O) to track evaporation in water. Recent work shows the sensitivity of Δ′17O to evaporation in lakes and lacustrine carbonates, but little is known about variation of Δ′17O in soil carbonates and their potential to track evaporation. For this study, we sampled soils across an aridity gradient in the Serengeti, Tanzania to evaluate how soil carbonate Δ′17O tracks soil water evaporation. Modern soil carbonates were collected from 11 sites across a transect of the Serengeti Ecosystem where mean annual precipitation and aridity index range from 499 to 846 mm yr−1 and 0.33 to 0.55, respectively. δ13C values range from −2.7 to 1.8‰ and reflect C4 dominated grasslands, whereas δ18O values of soil carbonates vary by ∼8‰ along a gradient in aridity. TΔ47 from these soil carbonates average 23°C (1σ± 4°C), which does not vary significantly across sites or with depth, likely due to minimal annual variation in temperature at the equator. Using these temperatures for each carbonate, reconstructed δ18O values of soil water are up to 6‰ higher than δ18O values of local precipitation and springs, indicating considerable soil water evaporation. The Δ′17O values of these soil carbonates range from −162 to −106 per meg and decrease as both aridity and δ18O values increase. Our results support the hypothesis that soil water evaporation drives the variance in δ18O and Δ′17O of soil carbonate in arid climates, demonstrating the potential for soil carbonate Δ′17O to track paleoaridity and constrain interpretations of paleosol carbonate δ18O records.