Triple-oxygen isotope compositions (Delta'17O) in carbonates serve as valuable proxies for reconstructing past environments, with applications ranging from humidity to hydrological cycles and diagenetic history. High-precision Delta'17O measurements are essential to validate these applications, with the measurement of Delta'17O on CO2 extracted through phosphoric acid digestion of carbonates being a common practice. However, since this process does not achieve 100% oxygen extraction, kinetic isotope fractionation factors, i.e. 1000ln18 alpha CO2/carb and theta CO2/carb (---ln17 alpha CO2/carb/ln18 alpha CO2/carb), are critical for accurately translating the measured Delta'17O to that of the associated carbonate samples. Notably, a significant discrepancy exists between theoretically predicted theta CO2/carb values (e.g. 0.525-0.529) and experimental observations (e.g. 0.5170-0.5230), even though the 1000ln18 alpha CO2/ carb values align between the two approaches. We propose that this discrepancy stems from an incomplete treatment of solvation effects in previous theoretical studies. To test this hypothesis, we conducted quantum mechanical calculations that explicitly incorporate phosphoric acid molecules as solvent species. Molecular clusters consisting of up to 12 phosphoric acid molecules and a single carbonic acid molecule were used to simulate the reaction system. Results show that clusters with 6 or more phosphoric acid molecules are sufficiently large to account for the solvation effects. Along the reaction pathway, two transition states and one intermediate state were identified, which differs from previously proposed reaction mechanisms and is critical for the calculated kinetic isotope fractionation factors. The determined values for 1000ln18 alpha CO2/carb and theta CO2/carb are 11.06 +/- 0.82 (2SE) and 0.5233 +/- 0.0009 (2SE) at 25 degrees C, respectively, and are consistent with experimental results. This consistency highlights the importance of including solvation effects in the theoretical model dealing with phosphoric acid digestion reaction. Resulting temperature dependencies are: 1000ln18 alpha CO2/carb = 3.9358 (+/- 0.0041) & times; 103/T(K)-2.1421(+/- 0.0129) and theta CO2/carb = 1.2019(+/- 0.0172)/T(K) + 0.5192(+/- 0.0001), between 0 degrees C and 100 degrees C. These results suggest that the Delta'17O difference between extracted CO2 and carbonate sample is approximately-50 ppm when 0.5280 is used to calculate Delta'17O and appears relatively insensitive to digestion temperature. The existing discrepancy in Delta'17O for international standards potentially arises from the Delta'17O calibration issue.

The cerium isotope ratio (142Ce/140Ce, commonly denoted as S142Ce) has emerged as a promising tracer to complement the traditional Ce anomaly proxy for reconstructing past marine redox conditions. Although marine carbonates have been proposed as crucial archives of seawater Ce isotopic compositions, the effects of various diagenetic processes on Ce isotope fractionation in marine carbonates remain poorly constrained. Here, we present high-precision and high-resolution S142Ce data from two deep drill cores in the South China Sea that record nearly all typical carbonate diagenetic environments. S142Ce in these carbonates ranges from 0.04%o to 0.35%o, with a mean of 0.19 +/- 0.14%o (2SD). Variations in mean S142Ce values among the different diagenetic zones are minimal: the meteoric diagenetic zone ranges from 0.04%o to 0.34%o (mean = 0.20 +/- 0.16%o, 2SD), the marine diagenetic zone from 0.06%o to 0.28%o (mean = 0.18 +/- 0.14%o, 2SD), and the dolomitization zone from 0.07%o to 0.32%o (mean = 0.19 +/- 0.14%o, 2SD). These values closely overlap with those of unconsolidated carbonate sediments from the primary or weakly diagenetic zone (0.10%o to 0.35%o; mean = 0.20 +/- 0.14%o, 2SD), indicating negligible modification of S142Ce during early diagenesis. An exception occurs in the marine diagenetic zone of the NK-1 core, where we observed anomalously low S142Ce values that correlate with elevated Ce concentrations and (Nd/Yb)SN. We attribute these anomalies to enhanced porewater-rock interaction with volcanic detritus during early diagenesis, which altered the isotopic composition of diagenetic fluids that were subsequently recorded in the carbonates. Overall, our results indicate that Ce isotopes in shallow marine carbonates are largely resistant to early diagenetic alteration, preserving depositional signals across a range of diagenetic conditions. This finding supports the application of carbonate S142Ce as a promising proxy for reconstructing ancient marine redox conditions.

Oxidation of hydrogen sulfide to sulfate is commonly occurring at the redoxcline of oceans and freshwater systems, producing newly formed sulfate with delta 18O and delta 34S values different from those of the original sulfate. Hydrogen sulfide oxidation leads to the formation of several sulfur oxide species, occurs with various oxidants such as O2 and Fe3+, and proceeds through both biotic and abiotic pathways. A large number of biotic and abiotic sulfide oxidation experiments have been conducted to elucidate the complex pathway using oxygen isotopes. Oxygen atoms in sulfate are derived from both O2 and H2O during sulfide oxidation, which allows us to disentangle the oxidants involved in the reaction and further clarify the detailed mechanism. However, the oxygen isotope behaviors during hydrogen sulfide oxidation, particularly the fractions of oxygen from H2O or O2 being incorporated into sulfate, are inconsistent in literature. Besides, many of the sulfide oxidation experiments have been conducted in low pH condition (around 2), targeting the sulfur cycle in extreme environment such as acid-mine drainages. The low pH condition enhances oxygen exchange with sulfite and water, possibly reducing the fraction of O2-derived oxygen in the product sulfate. To address this issue, we conducted both biotic (using Cupriavidus pinatubonensis JMP134) and abiotic HS- oxidation experiments at pH 8 with 17O-labeled water to quantify the proportion of dissolved O2 in the resulting sulfate. The oxygen isotope analyses revealed that 88.6 +/- 1.1% of the oxygen in product sulfate was derived from dissolved O2 during aerobic abiotic HSoxidation. This fraction exceeds both the 67% concluded in a previous 18O-labeled experimental study and the less than 75% predicted by the abiotic HS- oxidation model, indicating the dominant portion of the sulfate must have been formed via a pathway that bypasses the intermediate sulfite or bisulfite. In addition, we determined the apparent kinetic isotope effect (AKIE) during O2 incorporation into sulfate at 18 epsilon SO4- O2 = -12.2 +/- 0.7 parts per thousand. These results suggest that a large O2 fraction could be incorporated into product sulfate during HS- oxidation at close-to-modern seawater pH condition.

Sulfate produced during the oxidative weathering of reduced sulfur can preserve the triple oxygen isotope signature of atmospheric O-2 (Delta O-'17) through the intermediate step of sulfite oxidation by O-2. Because Delta O-'17 in O-2 varies as a function of atmospheric pO(2) and pCO(2) and, arguably, gross primary productivity, large negative Delta O-'17 values in sulfate have been used to reconstruct past atmospheric composition and biospheric activity. More recently, this framework has been extended to interpret small Delta O-'17 variations in sulfate. For such applications, accurate constraints on both equilibrium and kinetic triple oxygen isotope fractionation during sulfite oxidation by O-2 are essential. While equilibrium fractionation factors have been relatively well constrained, kinetic fractionation factors remain poorly quantified. In this study, we employed density functional theory (DFT) to evaluate kinetic triple oxygen isotope fractionation during sulfite oxidation by O-2, using classical transition state theory. We focused on the elementary reaction SO3- + O-2 -> SO5-, which represents a key O-2 consuming step in the proposed chain reaction mechanism for sulfite oxidation in aqueous solution. Vibrational frequencies of reactants and transition states were calculated at the omega B97XD/6-311G+(2df,2p) level of theory. Our results indicate a substantial normal kinetic O-18 fractionation for O-2 consumption (-21.6 +/- 0.6 parts per thousand) at 25 degrees C, significantly different from experimental estimates (-9.8 parts per thousand to 23.3 parts per thousand). This discrepancy likely arises from variations in O-2 reaction reversibility under different experimental conditions, such as sulfite concentration and O-2 partial pressure. If full isotopic equilibrium is achieved between O-2 corresponding sites in SO5- and ambient O-2, the equilibrium O-18 fractionation is predicted to be -3.5 +/- 0.8 parts per thousand at 25 degrees C, partially reconciling the difference between theoretical and experimental observations. The corresponding theta values for the kinetic and equilibrium scenarios (theta equivalent to ln(17)KIE/ln(18)KIE or ln(17)alpha(eq)/ln(18)alpha(eq)) are 0.5145 +/- 0.0009 and 0.5199 +/- 0.0021, respectively. These results imply that the Delta O-'17 of O-2 incorporated into sulfate could be higher by up to similar to 0.35 parts per thousand relative to ambient O-2, depending on reaction reversibility. Our findings highlight the importance of accounting for O-2 reaction reversibility when interpreting both laboratory and geological Delta O-'17 signatures in sulfate.

Rocky planets formed through the differentiation of planetesimals into metal‑rich cores and silicate mantles, yet direct evidence linking these complementary reservoirs remains scarce. Here we show that IIIE irons and howardite-eucrite-diogenite (HED) meteorites are genetically linked, by integrating mineralogical, isotopic, and rare earth element (REE) analyses of the silicate-bearing Aletai IIIE meteorite. The silicates display Fe/Mn-Fe/Mg correlations, concordant oxygen isotope compositions, and strongly fractionated REE patterns consistent with those of HED meteorites. A pronounced negative Eu anomaly in orthopyroxene marks the onset of plagioclase saturation and buoyant segregation during early magma ocean crystallization, supported by thermodynamic modelling. These findings establish a genetic link between magmatic irons and differentiated achondrites, suggesting that IIIE irons and HEDs share a common genetic heritage, and providing direct physical evidence for core-mantle reservoir connections in the early Solar System. IIIE irons share geochemical characteristics with howardite-eucrite-diogenite meteorites suggesting both may have formed from differentiated planetesimal parent bodies, according to analyses on a silicate-bearing inclusion in the Aletai IIIE iron meteorite.

The isotopic composition of metals in marine carbonates provides a powerful archive of past seawater chemistry, yet these primary signals can be modified by diagenetic alteration and vital effects associated with biomineralization in biogenic carbonates. This study investigates the behaviors of calcium (δ44/40Ca) and lithium (δ7Li) isotopes across a diverse set of shallow-marine carbonate systems, including microbialites from Hamelin Pool in Shark Bay, Western Australia, and bulk carbonate sediments and biogenic corals from the Nansha Islands, South China Sea, to evaluate how depositional conditions, mineralogy, and early diagenesis govern their isotopic compositions. The microbialites exhibit low average values of δ44/40Ca (–1.22 ± 0.23‰, 2σ, n = 8) and δ7Li (+21.1 ± 3.5‰, 2σ, n = 8), with their offsets from modern seawater values (δ44/40CaSW = 0‰; δ7LiSW ≈ 31‰) primarily controlled by carbonate mineralogy. A positive correlation between δ44/40Ca and δ7Li within different individual microbialites is also identified, which is not regulated by vital effects, salinity, or basin restriction, but likely reflects the modest diagenetic alteration of primary metastable aragonites. Compared with microbialites, bulk carbonate sediments show elevated average values of δ44/40Ca (–1.09 ± 0.22‰, 2σ, n = 23) and δ7Li (+24.0 ± 1.3‰, 2σ, n = 30), indicating mixing of primary aragonite and high-Mg calcite in these sediments. In contrast, the shallow-water corals exhibit higher average δ44/40Ca values (–1.02 ± 0.14‰, 2σ, n = 4) and lower δ7Li values (+18.0 ± 0.9‰, 2σ, n = 4), which we interpret as the influence of vital effects associated with isotopic fractionations during transmembrane transport of cations between seawater and the calcifying fluid for coral biomineralization. The agreement of δ44/40Ca and δ7Li data from this study with published shallow-water carbonate records suggests that the isotopic compositions of primary carbonate sediments are predominantly controlled by carbonate mineralogy, while additional processes, particularly vital effects in biogenic carbonates and early diagenetic alteration in carbonate sediments, can further influence these isotopic signatures. This underscores the need to account for carbonate mineralogy, vital effects, and diagenetic alteration when reconstructing coeval seawater signals from ancient carbonates, which can be done using multi-proxy isotope datasets and a modeling framework. Further, when carbonate mineralogy and diagenesis are well constrained, microbialites may be used to reliably reconstruct δ44/40Ca and δ7Li signatures of Precambrian seawater, as their isotopic signatures are largely insensitive to salinity, basin restriction, and vital effects.

Stable iridium isotopes (191Ir, 193Ir) offer novel potential for investigating geological processes, yet their application in natural systems remains hindered by poorly constrained fractionation behaviors. To establish a predictive framework, we present the systematic first-principles calculations of equilibrium Ir isotope fractionation (273.15–5000 K) for Ir-bearing phases, concurrently evaluating mass-dependent and nuclear volume effects. Our results demonstrate that mass-dependent fractionation dominates over nuclear volume fractionation in most species, but the contribution of latter to the overall isotope fractionation increases with increasing temperature. Critically, we establish a systematic enrichment sequence for 193Ir during igneous processes: metallic Ir (Ir0) < sulfides (Ir3+, Ir4+) < silicates (Ir1+, Ir2+) and halides (Ir3+, Ir4+) < oxides (Ir2+, Ir3+, Ir4+). Beyond conventional factors (temperature, oxidation state, bond length, and electron density), we further identify the significance of bonding atom properties (electronegativity, ionic radius, effective ionic charge), non-bonded interactions, and structural constraints in influencing equilibrium isotope fractionation. This work provides a fundamental understanding of Ir isotope fractionation behaviors, enabling reliable interpretation of natural isotopic variations and advancing Ir isotopes as tracers for geological processes.

Oxygen isotope compositions in ancient seawater (delta O-18(SW) and Delta O-'17(SW)) are key proxies for deciphering past climate and tectonic regimes. Despite various reconstruction attempts, a consensus on their history remains elusive. Chert, valued for its diagenetic stability, offers a promising archive, particularly through triple oxygen isotope analysis. However, Precambrian cherts frequently exhibit deviations in delta O-18 and Delta O-'17 from triple oxygen isotope equilibrium, complicating interpretations. Here, we propose that this deviation results from the coprecipitation of oxygen-bearing minerals (detrital silicates, authigenic clays, and barite), each with distinct isotopic signatures. To test this hypothesis, we analyzed Mesoproterozoic chert samples from the Yangzhuang and Wumishan formations in North China. Our results show a significant correlation between chert delta O-18 and detrital silicate content, with coexisting oxygen-bearing minerals driving deviations in delta O-18 and Delta O-'17 from equilibrium. The delta O-18 and Delta O-'17 values of the purest, most pristine chert samples suggest that Mesoproterozoic seawater had a delta O-18 similar to that of modern seawater (i.e. similar to -1 parts per thousand, ice free world seawater), a conclusion further supported by our oxygen balance model. This study demonstrates that using pure chert or an oxygen balance model is essential for accurately reconstructing ancient delta O-18(SW).

This paper reports the chemical and isotopic compositions of Oued Chebeika 002 (OC 002). High-precision, mass-independent O, Fe, and Ti isotope data confirm that it is the first desert-derived CI chondrite. Trace elemental data indicate that the sample experienced minimal terrestrial weathering, making it a significant supplement for the lack of CI samples representing the compositions of Solar System and bulk planets. Notably, we observed 58Fe excess in OC 002, indicating Earth and CI chondrites have different mass-independent Fe isotope compositions. We also present high-precision oxygen isotope data for seven ‘CY’ chondrites. All ‘CY’ chondrites exhibit two distinct groups in their mass-independent oxygen isotope compositions (Δ17O), suggesting they originated from two separate parent bodies. This interpretation is consistent with their differing petrology, as well as variations in metal elemental and isotopic compositions. CI1T chondrites share Δ17O values with CI chondrites, implying a common parent body. In contrast, CY2 chondrites have Δ17O values distinct from those of CM chondrites. Furthermore, CI1T and CI chondrites have Δ17O values similar to those measured in Ryugu samples, suggesting they may have accreted in similar regions of the solar system. However, any genetic relationship between CI-Ryugu and Bennu remains uncertain and requires further investigation.

Earth's surface underwent stepwise oxygenation before persistently reaching modern levels late in its history1-5, but the details of this transition remain unclear5-16. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Δ'17Osulfate), a proxy linked to the atmospheric partial pressure of O2 ( p O 2 )17-19. This record, together with existing sedimentary Δ33S data20-22, demonstrates a 2-Gyr transition characterized by generally low, fluctuating p O 2 between an O2-free state before 2.4 billion years ago (Ga) and a modern p O 2 state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Δ'17Osulfate and sulfate-δ34S during major negative carbonate-δ13C excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing p O 2 , which may have driven episodic ocean oxygenation through an increased atmospheric O2 influx. This process intensified the oxidation of marine organics and reduced-sulfur species, while triggering temporary p O 2 drawdowns as negative feedback15. These findings support a dynamic, lengthy co-oxygenation history for the atmosphere and oceans-marked by long-term positive coupling and short-term negative feedbacks-offering a coherent explanation for the anomalous Neoproterozoic carbon cycles23,24 and the protracted, episodic rise of complex life25-27.

A synthesis of global barite sulfate isotope data from approximately 635 million years ago, at the end of a global glaciation, undermines the hypothesis that river sulfate was the primary carrier of the distinctive 17O-depleted atmospheric O2 signature of the time. Instead, an aqueous H2S oxidation model on the shelf emerges as a compelling alternative, though it demands extensive validation across multiple fronts by the scientific community.

Kinetic isotope effects occurring during carbonate precipitation impact the accuracy of paleoclimate records. One rate-limiting process is CO2 absorption, which, at high pH, primarily proceeds through hydroxylation, where aqueous CO2 and OH- react to form HCO3-. In this study, we investigated the triple oxygen isotope fractionation (O-18/O-16 and O-17/O-16) associated with the hydroxylation reaction. We experimentally determined the extent of the kinetic isotope effects superimposed on the reacting CO2 and OH- during hydroxylation, using high-pH precipitation reactions. These results were compared to the equilibrium triple oxygen isotope fractionation between H2O and OH- modeled using quantum chemical computations. Our quantum chemical modeling confirms the previously suggested equilibrium theta(eq.)(H2O/OH-) of 0.5296 at 25 degrees C. Our empirical data show a kinetic isotope effect superimposed on the OH- involved in the hydroxylation reaction of approximately -16.4 parts per thousand for the fractionation of O-18/O-16 and a corresponding slope of 0.514 in triple oxygen isotope space. The theta(effective)(H2O/OH-) for the fractionation between water and the reacting OH- is 0.523. In order to identify and potentially correct kinetic isotope effects in carbonates, it is important to understand the fractionations occurring during precipitation. Our results enable a more accurate modeling of kinetic isotope effects in triple oxygen isotope space, particularly the slope of hydroxylation. The theta(hydroxylation) is dependent on the isotope composition of the ambient water, CO2, and the temperature. We estimate that theta(hydroxylation) is approximately 0.532 for carbonates growing in 25 degrees C seawater.

The widespread occurrence of ferroan anorthosites and other magmatic rock types in Apollo lunar samples directly contributed to the construction and development of modern planetary formation models. If the Vesta magma ocean (VMO) model is analogous to the lunar magma ocean model, anorthosites should be present as a late crystallization product of the Vesta magma ocean. However, the lack of anorthositic meteorites unequivocally linked to Vesta or its parent body casts doubts on the existence of a past VMO and to our knowledge of Vesta's formation and early evolution. Here, we report a newly discovered ferroan anorthosite, Northwest Africa 15118, consisting entirely of anorthite (similar to 94 vol%) and orthopyroxene (similar to 6 vol%). We show that the mineral chemical compositions, whole-rock oxygen isotope composition (Delta O-17 = -0.243 +/- 0.014 parts per thousand), as well as chromium isotope composition in chromite (epsilon Cr-54 = -0.74 +/- 0.14), of Northwest Africa 15118 overlaps with howardite-eucrite-diogenite meteorites, indicating a Vestan origin for this meteorite. The occurrence of ferroan anorthosite with prominent positive Eu anomalies supports a primary anorthositic crust layer in Vesta, thus validating the VMO model.

The largest negative inorganic carbon isotope excursion in Earth's history, namely the Ediacaran Shuram Excursion (SE), closely followed by early animal radiation, has been widely interpreted as a consequence of oceanic oxidation. However, the primary nature of the signature, source of oxidants, and tempo of the event remain contested. Here, we show that carbonate-associated sulfate (CAS) from three different paleocontinents all have conspicuous negative 17O anomalies (Δ'17OCAS values down to -0.53‰) during the SE. Furthermore, the Δ'17OCAS varies in correlation with its corresponding δ34SCAS and δ18OCAS as well as the carbonate δ13Ccarb, decreasing initially followed by a recovery over the ~7-Myr SE duration. In a box-model examination, we argue for a period of sustained water-column ventilation and consequently enhanced sulfur oxidation in the SE ocean. Our findings reveal a direct involvement of mass-anomalously 17O-depleted atmospheric O2 in marine sulfate formation and thus a primary global oceanic oxygenation event during the SE.

Air-water gas transfer largely influences the geochemical and biogeochemical cycles of essential atmospheric components (e.g. O-2 and CO2), in which gas molecular diffusion in water is recognized as the rate limiting step. Isotope compositions in these gas molecules are useful tools to quantify this mass transfer process, in which diffusional isotope fractionation factors (i.e. alpha(diff)) are the key intrinsic parameters. These alpha(diff)s are often determined by gas transfer experiments with large uncertainties because the roughness of water surface can affect the interpretation of experimental data. In this study, molecular dynamic simulations were employed to investigate directly the diffusional isotope fractionation for singly and doubly substituted isotopologues of H-2, N-2, and O-2. The results show that diffusional isotope fractionation factors are dependent on both the molecular mass and moment of inertia, which is consistent with previous findings for polyatomic molecules rather than for monoatomic ones. When comparing with the kinetic isotope fractionation (i.e. alpha(k)) determined by gas transfer experiments, I found that alpha k is likely close to (alpha(diff))(1/2) within errors (i.e. alpha(k) = (alpha(diff))(1/2)), rather than to (alpha(diff))(2/3) that has often been employed to calculate alpha(diff )using alpha(k) in literature (i.e. alpha(k) = (alpha(diff))(2/3)). If this is the case, the results further indicate that the nuclear quantum effect is not significant when alpha(diff) is of interest. With these findings, I determined the isotope fractionation relationship theta for different O-2 isotopologues to be 0.5100 +/- 0.0002 and 1.9535 +/- 0.0013 respectively for (17)theta(diff) (equivalent to ln(17)alpha(diff)/ln(diff)(18 alpha)) and (36)theta(diff) (equivalent to ln(36)theta(diff)/ln(18)alpha(diff)) as an example.(C) 2022 Elsevier Ltd. All rights reserved.