Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate-Based Standardization

Amplified seasonality in western Europe in a warmer world

Documenting the seasonal temperature cycle constitutes an essential step toward mitigating risks associated with extreme weather events in a future warmer world. The mid-Piacenzian Warm Period (mPWP), 3.3 to 3.0 million years ago, featured global temperatures approximately 3°C above preindustrial levels. It represents an ideal period for directed paleoclimate reconstructions equivalent to model projections for 2100 under moderate Shared Socioeconomic Pathway SSP2-4.5. Here, seasonal clumped isotope analyses of fossil mollusk shells from the North Sea are presented to test Pliocene Model Intercomparison Project 2 outcomes. Joint data and model evidence reveals enhanced summer warming (+4.3° ± 1.0°C) compared to winter (+2.5° ± 1.5°C) during the mPWP, equivalent to SSP2-4.5 outcomes for future climate. We show that Arctic amplification of global warming weakens mid-latitude summer circulation while intensifying seasonal contrast in temperature and precipitation, leading to an increased risk of summer heat waves and other extreme weather events in Europe's future.

Reconciling Southern Ocean fronts equatorward migration with minor Antarctic ice volume change during Miocene cooling

Gradual climate cooling and CO2 decline in the Miocene were recently shown not to be associated with major ice volume expansion, challenging a fundamental paradigm in the functioning of the Antarctic cryosphere. Here, we explore Miocene ice-ocean-climate interactions by presenting a multi-proxy reconstruction of subtropical front migration, bottom water temperature and global ice volume change, using dinoflagellate cyst biogeography, benthic foraminiferal clumped isotopes from offshore Tasmania. We report an equatorward frontal migration and strengthening, concurrent with surface and deep ocean cooling but absence of ice volume change in the mid-late-Miocene. To reconcile these counterintuitive findings, we argue based on new ice sheet modelling that the Antarctic ice sheet progressively lowered in height while expanding seawards, to maintain a stable volume. This can be achieved with rigorous intervention in model precipitation regimes on Antarctica and ice-induced ocean cooling and requires rethinking the interactions between ice, ocean and climate.

Assessment of the accuracy of the 17 O correction algorithm used in δ13 C determinations by CO2 mass spectrometry

RATIONALE

High-accuracy δ¹³ C values are required for observations of greenhouse gases CO₂ and methane, and, consequently, for international reference materials (RMs). Recently, another application, clumped isotope geothermometry of natural carbonates, has demonstrated the requirement for high-accuracy δ¹³ C values. δ¹³ C determinations by mass spectrometry use an ¹⁷ O isobaric correction on m/z 45, where ¹⁷ O abundance is calculated from the measured ¹⁸ O with the ¹⁷ O-¹⁸ O relationship assumed with λ = 0.528. This relationship is the key assumption of the algorithm proposed in 2003 and accepted by IUPAC in 2010. However, to date, this relationship and potential δ¹³ C biases have not been verified using ¹⁷ O measurements.

METHODS

To verify the ¹⁷ O correction and to estimate potential δ¹³ C biases, we compile measured ¹⁷ O data for carbonate RMs, for a range of natural carbonates that are typically analyzed in clumped isotope geothermometry and for CO₂ in isotope equilibrium with natural waters including plants and biota. δ¹³ C biases are calculated based on ¹⁷ O deviation from the ¹⁷ O-¹⁸ O relationship assumed in the ¹⁷ O correction.

RESULTS

To estimate δ¹³ C biases accurately, the VPDB-CO₂ framework for expressing ¹⁷ O excess is defined and linked to the δ¹³ C scale definition. δ¹³ C biases estimated for carbonate RMs are found within ±0.004‰; the biases estimated for natural carbonates and CO₂ in equilibrium with natural waters are mostly within ±0.010‰ (bidirectional distribution around zero). In all cases, the estimated biases are found within the best instrumental uncertainty of modern isotope ratio mass spectrometry (IRMS) (around ±0.014‰, k = 2).

CONCLUSIONS

For the first time, high accuracy of δ¹³ C data obtained by CO₂ mass spectrometry using the ¹⁷ O correction with fixed λ = 0.528 has been demonstrated using measured ¹⁷ O data. δ¹³ C biases estimated are within the best IRMS precision (±0.014‰, k = 2) and can be neglected in most practical applications. To obtain high-quality δ¹³ C data, it is strictly necessary that all data are treated on the VPDB-CO₂ scale.

Equilibrated Gas and Carbonate Standard-Derived Dual (Δ47 and Δ48) Clumped Isotope Values

Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of m/z 47 CO2 for geothermometry, but theoretical calculations and recent experiments indicate paired analysis of the m/z 47 (13C18O16O) and m/z 48 (12C18O18O) isotopologues (referred to as Δ 47 and Δ 48 ) can be used to study non-equilibrium isotope fractionations and refine temperature estimates. We utilize 5,448 Δ 47 and 3,400 Δ 48 replicate measurements of carbonate samples and standards, and 183 Δ 47 and 195 Δ 48 replicate measurements of gas standards from 2015 to 2021 from a multi-year and multi-instrument data set to constrain Δ 47 and Δ 48 values for 27 samples and standards, including Devils Hole cave calcite, and study equilibrium Δ 47 -Δ 48 , Δ 47 -temperature, and Δ 48 -temperature relationships. We compare results to previously published findings and calculate equilibrium regressions based on data from multiple laboratories. We report acid digestion fractionation factors, Δ * 63 - 47 and Δ * 64 - 48 , and account for their dependence on the initial clumped isotope values of the mineral.

North Atlantic surface ocean warming and salinization in response to middle Eocene greenhouse warming

Quantitative reconstructions of hydrological change during ancient greenhouse warming events provide valuable insight into warmer-than-modern hydrological cycles but are limited by paleoclimate proxy uncertainties. We present sea surface temperature (SST) records and seawater oxygen isotope (δ18Osw) estimates for the Middle Eocene Climatic Optimum (MECO), using coupled carbonate clumped isotope (Δ47) and oxygen isotope (δ18Oc) data of well-preserved planktonic foraminifera from the North Atlantic Newfoundland Drifts. These indicate a transient ~3°C warming across the MECO, with absolute temperatures generally in accordance with trace element (Mg/Ca)-based SSTs but lower than biomarker-based SSTs for the same interval. We find a transient ~0.5‰ shift toward higher δ18Osw, which implies increased salinity in the North Atlantic subtropical gyre and potentially a poleward expansion of its northern boundary in response to greenhouse warming. These observations provide constraints on dynamic ocean response to warming events, which are consistent with theory and model simulations predicting an enhanced hydrological cycle under global warming.

Temperature Dependence of Clumped Isotopes (∆47) in Aragonite

Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1-18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1-850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1-18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.

Cenozoic evolution of deep ocean temperature from clumped isotope thermometry

Characterizing past climate states is crucial for understanding the future consequences of ongoing greenhouse gas emissions. Here, we revisit the benchmark time series for deep ocean temperature across the past 65 million years using clumped isotope thermometry. Our temperature estimates from the deep Atlantic Ocean are overall much warmer compared with oxygen isotope-based reconstructions, highlighting the likely influence of changes in deep ocean pH and/or seawater oxygen isotope composition on classical oxygen isotope records of the Cenozoic. In addition, our data reveal previously unrecognized large swings in deep ocean temperature during early Eocene acute greenhouse warmth. Our results call for a reassessment of the Cenozoic history of ocean temperatures to achieve a more accurate understanding of the nature of climatic responses to tectonic events and variable greenhouse forcing.

Combined use of conventional and clumped carbonate stable isotopes to identify hydrothermal isotopic alteration in cave walls

Alteration of conventional carbonate stable isotopes (δ¹⁸O, δ¹³C) in cave walls has been shown to be a useful tool to identify cave formation driven by deep-seated processes, i.e., hypogene karstification. If combined with a prior information on the paleowater stable isotope composition, further insights can be obtained on the temperature and the source of the paleowater. Clumped isotope composition (Δ₄₇) of carbonates is an independent measurement of temperature, and if combined with the conventional stable isotopes, can provide information on the paleowater stable isotope composition. On the example of Provalata Cave (N. Macedonia), we apply for the first time, both conventional and clumped stable isotope analysis, and identify two different isotope alteration trends, reflecting two distinct hydrothermal events: an older, hotter one, where isotope alteration was likely related to isotope diffusion, lowering the δ¹⁸O values of the carbonate; and a younger one, related to the cave formation by low-temperature CO₂-rich thermal waters, with dissolution-reprecipitation as the alteration mechanism, causing decrease in δ¹⁸O values, and unexpected increase in δ¹³C values. The findings are further corroborated by additional insight from optical petrography and cathodoluminescence microscopy, as well as fluid inclusion analysis of secondary calcite crystals related to the cave forming phase.

Alpine permafrost could account for a quarter of thawed carbon based on Plio-Pleistocene paleoclimate analogue

Estimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Δ₄₇-T) indicate warmer MAAT (~1.2 °C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Δ₄₇-T indicate ~8.1 °C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3–3.0 Ma), ~60% of alpine permafrost containing ~85 petagrams of carbon may be vulnerable to thawing compared to ~20% of circumarctic permafrost. This estimate highlights ~25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas.

The stability of permafrost carbon is poorly understood. Here the authors use Plio-Pleistocene clumped isotope reconstructions from the Tibetan Plateau and climate simulation to determine that ~85 petagrams of alpine carbon is vulnerable to thawing.

Carbon Geochemistry of the Active Serpentinization Site at the Wadi Tayin Massif: Insights From the ICDP Oman Drilling Project: Phase II

A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO₂. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400-m deep) and BA3A (300-m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro-fractures and grain boundaries, causing large-scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with δ ¹⁸O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.

Uranium isotopes as a possible tracer of terrestrial authigenic carbonate

The concentration and isotopic composition of uranium (δ²³⁸U, ²³⁴U/²³⁸U activity ratio) in combination with traditional isotopes (δ¹⁸O, δ¹³C) were examined as potential tracers of authigenic carbonate formation in a karst aquifer. The U concentration and ²³⁴U/²³⁸U activity ratios in the tufa-precipitating sections of two connected karst rivers (Krka and Zrmanja, Croatia) decreased downstream in water and in precipitated carbonate due to active self-purification processes, i.e. adsorption of isotopically lighter U(VI) on mineral particles, sedimentation and co-precipitation with carbonate. The isotopic composition of carbonate in tufa mostly resembled the ²³⁴U/²³⁸U activity ratio and the δ²³⁸U values of dissolved U in water but was also affected by the presence of detrital carbonate flushed into the river from soil and weathered bedrock. This interpretation was supported by the δ¹⁸O and δ¹³C values of tufa, which were shifted out of equilibrium with river water and dissolved in organic carbon and in their isotopic signature, which showed the presence of lithic carbonate. Large fluctuations of the δ²³⁸U values of water, leachable U (eluted in acetic acid buffered with Na-acetate) and residual U fraction could not be fully explained by available data due to the overlapping U isotopic signatures of leachable (mainly carbonate) and residual fractions of soil, bedrock and tufa. Therefore, a long-term, systematic, seasonal and event-based observation of the isotopic composition of dissolved and suspended particulate U in water is necessary. Nevertheless, the U isotopes were found to have the potential to be used as identifiers of authigenic carbonate and the storage of CO₂ in terrestrial river sediments, to improve knowledge on fluxes within local and global biogeochemical carbon cycle.

Marine temperatures underestimated for past greenhouse climate

Understanding the Earth's climate system during past periods of high atmospheric CO₂ is crucial for forecasting climate change under anthropogenically-elevated CO₂. The Mesozoic Era is believed to have coincided with a long-term Greenhouse climate, and many of our temperature reconstructions come from stable isotopes of marine biotic calcite, in particular from belemnites, an extinct group of molluscs with carbonate hard-parts. Yet, temperatures reconstructed from the oxygen isotope composition of belemnites are consistently colder than those derived from other temperature proxies, leading to large uncertainties around Mesozoic sea temperatures. Here we apply clumped isotope palaeothermometry to two distinct carbonate phases from exceptionally well-preserved belemnites in order to constrain their living habitat, and improve temperature reconstructions based on stable oxygen isotopes. We show that belemnites precipitated both aragonite and calcite in warm, open ocean surface waters, and demonstrate how previous low estimates of belemnite calcification temperatures has led to widespread underestimation of Mesozoic sea temperatures by ca. 12 °C, raising estimates of some of the lowest temperature estimates for the Jurassic period to values which approach modern mid-latitude sea surface temperatures. Our findings enable accurate recalculation of global Mesozoic belemnite temperatures, and will thus improve our understanding of Greenhouse climate dynamics.

Carbonate clumped isotope analysis (Δ47 ) of 21 carbonate standards determined via gas-source isotope-ratio mass spectrometry on four instrumental configurations using carbonate-based standardization and multiyear data sets

RATIONALE

Clumped isotope geochemistry examines the pairing or clumping of heavy isotopes in molecules and provides information about the thermodynamic and kinetic controls on their formation. The first clumped isotope measurements of carbonate minerals were first published 15 years ago, and since then, interlaboratory offsets have been observed, and laboratory and community practices for measurement, data analysis, and instrumentation have evolved. Here we briefly review historical and recent developments for measurements, share Tripati Lab practices for four different instrument configurations, test a recently published proposal for carbonate-based standardization on multiple instruments using multi-year data sets, and report values for 21 different carbonate standards that allow for recalculations of previously published data sets.

METHODS

We examine data from 4628 standard measurements on Thermo MAT 253 and Nu Perspective IS mass spectrometers, using a common acid bath (90°C) and small-sample (70°C) individual reaction vessels. Each configuration was investigated by treating some standards as anchors (working standards) and the remainder as unknowns (consistency standards).

RESULTS

We show that different acid digestion systems and mass spectrometer models yield indistinguishable results when instrument drift is well characterized. For linearity correction, mixed gas-and-carbonate standardization or carbonate-only standardization yields similar results. No difference is observed in the use of three or eight working standards for the construction of transfer functions.

CONCLUSIONS

We show that all configurations yield similar results if instrument drift is robustly characterized and validate a recent proposal for carbonate-based standardization using large multiyear data sets. Δ₄₇ values are reported for 21 carbonate standards on both the absolute reference frame (ARF; also refered to as the Carbon Dioxide Equilibrated Scale or CDES) and the new InterCarb-Carbon Dioxide Equilibrium Scale (I-CDES) reference frame, facilitating intercomparison of data from a diversity of labs and instrument configurations and restandardization of a broad range of sample sets between 2006, when the first carbonate measurements were published, and the present.

A calibration equation between Δ48 values of carbonate and temperature

RATIONALE

Information on the temperature of formation or alteration of carbonate minerals can be obtained by measuring the abundance of the isotopologues 47 and 48 (Δ₄₇ and Δ₄₈ values) of CO₂ released during acid dissolution. The combination of these two proxies can potentially provide a greater insight into the temperature of formation, particularly if the carbonate minerals form by non-equilibrium processes.

METHODS

We have precipitated calcium carbonates at seven temperatures between 5 and 65°C and measured their δ⁴⁸ values using a Thermo-253 plus isotope ratio mass spectrometer. The values were transformed to Δ₄₈ values in the conventional manner and then converted to the carbon dioxide equilibrium scale.

RESULTS

Using the Δ₄₈ values, we have established an empirical calibration between temperature and Δ₄₈ values: [Formula: see text] CONCLUSIONS: The calibration line produced allows the determination of the temperature of natural carbonates using the Δ₄₈ values and agrees with the measurements of the Δ₄₇ and Δ₄₈ values of some carbonates assumed to have formed under equilibrium conditions.

A high-resolution record of early Paleozoic climate

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygen-isotope paleothermometry. Bulk-rock [Formula: see text] datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro-Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk [Formula: see text] due to low fluid exchange. Consequently, both sequences preserve temporal trends in [Formula: see text] Motivated by this result, we compile a global suite of bulk rock [Formula: see text] data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock [Formula: see text] with brachiopod and conodont [Formula: see text] trends through time. Given evidence that the [Formula: see text] value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely least-altered, [Formula: see text] values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early-Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events.

InterCarb: A Community Effort to Improve Interlaboratory Standardization of the Carbonate Clumped Isotope Thermometer Using Carbonate Standards

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I-CDES) values for Intercarb-Carbon Dioxide Equilibrium Scale.

Burial-Deformation History of Folded Rocks Unraveled by Fracture Analysis, Stylolite Paleopiezometry and Vein Cement Geochemistry: A Case Study in the Cingoli Anticline (Umbria-Marche, Northern Apennines)

Unravelling the burial-deformation history of sedimentary rocks is prerequisite information to understand the regional tectonic, sedimentary, thermal, and fluid-flow evolution of foreland basins. We use a combination of microstructural analysis, stylolites paleopiezometry, and paleofluid geochemistry to reconstruct the burial-deformation history of the Meso-Cenozoic carbonate sequence of the Cingoli Anticline (Northern Apennines, central Italy). Four major sets of mesostructures were linked to the regional deformation sequence: (i) pre-folding foreland flexure/forebulge; (ii) fold-scale layer-parallel shortening under a N045 σ1; (iii) syn-folding curvature of which the variable trend between the north and the south of the anticline is consistent with the arcuate shape of the anticline; (iv) the late stage of fold tightening. The maximum depth experienced by the strata prior to contraction, up to 1850 m, was quantified by sedimentary stylolite paleopiezometry and projected on the reconstructed burial curve to assess the timing of the contraction. As isotope geochemistry points towards fluid precipitation at thermal equilibrium, the carbonate clumped isotope thermometry (Δ47) considered for each fracture set yields the absolute timing of the development and exhumation of the Cingoli Anticline: layer-parallel shortening occurred from ~6.3 to 5.8 Ma, followed by fold growth that lasted from ~5.8 to 3.9 Ma.

Bacterial and abiogenic carbonates formed in caves-no vital effect on clumped isotope compositions

Speleothems (dominated by cave-hosted carbonate deposits) are valuable archives of paleoclimate conditions. As such, they are potential targets of clumped isotope analyses that may yield quantified data about past temperature variations. Clumped isotope analyses of stalagmites, however, seldom provide useful temperature values due to various isotope fractionation processes. This study focuses on the determination of the microbially induced vital effect, i.e., the isotope fractionation processes related to bacterial carbonate production. A cave site with biologically mediated amorphous calcium carbonate precitation was selected as a natural laboratory. Calcite deposits were farmed under a UV lamp to prevent bacterial activity, as well as under control conditions. Microbiological analyses and morphological investigations using scanning electron microscopy showed that the UV lamp treatment effectively reduced the number of bacterial cells, and that bacterial carbonate production strongly influenced the carbonate’s morphology. Stable oxygen isotope analyses of calcite and drip waters, as well as clumped isotope measurements revealed that, although most of the studied carbonates formed close to oxygen isotope equilibrium, clumped isotope Δ₄₇ values varied widely from equilibrium to strongly fractionated data. Site-specific kinetic fractionations played a dominant role in the distribution of Δ₄₇ values, whereas bacterial carbonate production did not result in a detectable clumped isotope effect.

Factors controlling the oxygen isotopic composition of lacustrine authigenic carbonates in Western China: implications for paleoclimate reconstructions

In the carbonate-water system, at equilibrium, the oxygen isotopic composition of carbonate is dependent not only on the temperature but also on the isotopic composition of host water in which the carbonate is formed. In this study, lake surface sediment and water samples were collected from 33 terminal lakes in Western China to evaluate controls on the oxygen isotopic composition of lacustrine authigenic carbonates (δ¹⁸O_carb) and its spatial distribution. Our results show that water oxygen isotopic composition (δ¹⁸O_water) rather than lake summer water temperature (T_water), is the main determinant of δ¹⁸O_carb, irrespective of whether oxygen isotope equilibrium is achieved. There are significant linear correlations between δ¹⁸O_carb and elevation, as well as that between δ¹⁸O_carb and latitude for lakes located on the Tibetan Plateau. In Western China, the spatial distribution of δ¹⁸O_carb is consistent with that of δ¹⁸O_water, and is ultimately controlled by the isotopic composition of local precipitation (δ¹⁸O_{precipitation}) that depends on the source of water vapor. Therefore, changes in δ¹⁸O_carb can be predominantly interpreted as variations of δ¹⁸O_water, which in turn represent changes in δ¹⁸O_{precipitation} for paleoclimate reconstructions in this region, and may be relevant to studies of other areas.

Cold spells in the Nordic Seas during the early Eocene Greenhouse

The early Eocene (c. 56 - 48 million years ago) experienced some of the highest global temperatures in Earth’s history since the Mesozoic, with no polar ice. Reports of contradictory ice-rafted erratics and cold water glendonites in the higher latitudes have been largely dismissed due to ambiguity of the significance of these purported cold-climate indicators. Here we apply clumped isotope paleothermometry to a traditionally qualitative abiotic proxy, glendonite calcite, to generate quantitative temperature estimates for northern mid-latitude bottom waters. Our data show that the glendonites of the Danish Basin formed in waters below 5 °C, at water depths of <300 m. Such near-freezing temperatures have not previously been reconstructed from proxy data for anywhere on the early Eocene Earth, and these data therefore suggest that regionalised cool episodes punctuated the background warmth of the early Eocene, likely linked to eruptive phases of the North Atlantic Igneous Province.

The early Eocene was characterized by exceptionally high global temperatures and no polar ice. Here, clumped isotope paleothermometry of glendonite calcite from the Danish Basin shows that these were formed in waters below 5 °C, indicating that regionalised cool episodes punctuated the background warmth of the early Eocene.

Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures

Surface temperature is a fundamental parameter of Earth’s climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆₄₇ and ∆₄₈ measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.

Some palaeotemperature proxies suffer from inaccuracies related to kinetic fractionations occurring during carbonate mineral growth. Here, the authors show that dual clumped isotope thermometry can identify the origin of these kinetic biases and allows for the reconstruction of accurate environmental temperatures.

Massive formation of early diagenetic dolomite in the Ediacaran ocean: Constraints on the "dolomite problem"

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO₃)₂] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the "dolomite problem." Accordingly, the genesis of these dolostones is usually attributed to burial-hydrothermal dolomitization of primary limestones (CaCO₃) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (T_Δ47) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our T_∆47 record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our T_Δ47 record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δ¹⁸O values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.

Analytical effects on clumped isotope thermometry: Comparison of a common sample set analyzed using multiple instruments, types of standards, and standardization windows

RATIONALE

Carbonate clumped isotope geothermometry is being increasingly used in multiple disciplines in the geosciences. However, potential interlaboratory issues are arising from different standardization procedures that may contribute to the multiple Δ₄₇ -temperature calibrations reported in the literature. We investigate this issue by comparing a common temperature calibration sample set across three different mass spectrometers, using multiple standardization methods.

METHODS

The same temperature calibration sample set was analyzed on three different mass spectrometers. Several standardization methods were utilized, including the use of carbonate versus gas standards, and different types of background correction were applied to the raw data.

RESULTS

All standardization types applied resulted in statistically indistinguishable Δ₄₇ -temperature slopes, with the exception of standardization calculations that did not correct for background effects. Some instruments and standardizations showed different intercepts relative to each other. The use of carbonate standards improved comparability between different instruments relative to gas standards.

CONCLUSIONS

Our results show that background effects are the largest factor potentially affecting Δ₄₇ results, and there may be an improvement in interlaboratory precision using carbonate standards. Critically, all techniques used for standardizing Δ₄₇ results converge on a common slope as long as background effects are properly corrected. The use of carbonate standards is recommended as a component of standardization procedures.

Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ47 ) measurements: Insights from a long-term dataset

RATIONALE

Carbonate clumped isotope (Δ₄₇ ) thermometry examines the temperature-dependent excess abundance of the ¹³ C-¹⁸ O bond in the carbonate lattice. Inconsistent temperature calibrations and standard values have been reported among laboratories, which has led to the use of equilibrated gases and carbonate standards for standardization. Furthermore, different acid fractionation factors and isotopic parameter sets have been proposed for improving inter-laboratory data comparability. However, few long-term datasets have been generated to explore the effects of these factors on the long-term reproducibility of Δ₄₇ data within a laboratory.

METHODS

Four standards (ISTB-1, NBS-19, GBWO4416, and GB04417) were analyzed as unknowns by isotope ratio mass spectrometry from 2015 to 2019. The values of Δ₄₇ were calibrated using the ETH standards. We investigated the Assonov, Brand, and Gonfiantini isotope parameter sets for carbon and oxygen isotopes, as well as two correction schemes of equilibrated gas and carbonate standardization, using the same sample measurements to determine which procedures enhanced reproducibility. ISTB-1 (calcite) and ZK312-346W (dolomite) were measured to determine the 90°C acid fractionation factor.

RESULTS

The corrected 90°C acid fractionation factors are 0.076 ± 0.008‰ for ISTB-1 and 0.077 ± 0.009‰ for ZK312-346W. The choice of isotope parameter set had no significant influence on final Δ₄₇ values in this study. However, using the Assonov parameters to calculate Δ₄₇ values improved the reproducibility of the results. The use of carbonate standards improved reproducibility through time compared with the use of equilibrated gases for standardization.

CONCLUSIONS

At 90°C, the acid fractionation factors of calcite and dolomite are statistically indistinguishable. We find an insignificant effect from changing the isotope parameter set, suggesting that the choice of isotope parameter set among laboratories is not a major factor affecting inter-laboratory reproducibility. We find that using carbonate standards improved the reproducibility of results, suggesting that the use of carbonate standards may help to achieve inter-laboratory comparability of results in future studies.

Geochemistry and Microtextures of Vein Calcites Pervading the Izu-Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu-Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB-like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion-fed and crystallization pressure-driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB-hosted low-T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35-33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite-hosted low-T (<30 °C) vein calcites precipitated from seawater that was modified by fluid-rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low-T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20-30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.

Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy

The high precision measurement of doubly substituted ("clumped") isotopologues in CO₂ is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the ¹²C¹⁶O₂ + ¹³C¹⁶O¹⁸O ⇆ ¹³C¹⁶O₂ + ¹²C¹⁶O¹⁸O exchange reaction. CO₂ samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 μmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ_16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.

Most Earth-surface calcites precipitate out of isotopic equilibrium

Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that ¹⁸O/¹⁶O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium ¹⁸O/¹⁶O fractionation between calcite and water (¹⁸α_cc/w). Here, we constrain the equilibrium relations linking temperature, ¹⁸α_cc/w, and clumped isotopes (Δ₄₇) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium ¹⁸α_cc/w values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect Δ₄₇ in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.

Isotopic thermometry of carbonate minerals postulates that their composition reflects thermodynamic equilibrium constants. Here the authors constrain equilibrium relationships between temperature, ¹⁸O/¹⁶O and clumped isotopes and find that most natural calcites form out of isotopic equilibrium.

Compositional and beam-size-dependent effects on pressure baseline in clumped isotope mass spectrometry

RATIONALE

The analysis of carbonate samples for the application of clumped isotopes to paleoclimate reconstruction necessitates smaller beam intensities. However, there is a relationship between beam intensity and pressure-dependent baseline (PBL), and therefore between beam intensity and the correction for PBL. Here we explain the relationship between PBL and beam intensity to develop a better correction protocol and an improved understanding of clumped isotope mass spectrometry.

METHODS

We describe a beam size experiment using our Isoprime isotope ratio mass spectrometer in which samples of the carbonate standard IAEA-C1 were analyzed at 30, 50, and 70 nA to establish an optimal protocol and a new method to correct for PBL using the theoretical constraint of invariable Δ₄₇ over a range of δ⁴⁷ (bulk isotope composition) values. We also explore the effects of both over- and under-correction of PBL on equilibrated and heated gas samples to understand the effect of mis-correction of PBL.

RESULTS

The results of our beam size experiments showed that a direct measurement of the baseline consistently introduced variability to measurements of the Δ₄₇ of heated gases, equilibrated gases, and carbonate standards. These results necessitated a new protocol to account for PBL in our system. Our new approach flattens the reference frame line slope to 0 and, importantly, reduces the variability of data points about the heated gas line. We also describe, for the first time, an empirically derived description of the compositional effect of PBL.

CONCLUSIONS

A seemingly small change in our isotope ratio mass spectrometer resulted in a better understanding of PBL, for which we have developed an empirically based correction protocol to apply. Our new protocol has the potential to reduce analytical time for laboratories measuring PBL, and supports the need for carbonate mineral-based clumped isotope standards.