Equilibrium Fractionation Research Articles

Variations in triple oxygen isotope of speleothems from the Asian monsoon region reveal moisture sources over the past 300 years

Characterization of moisture origins is crucial to understanding hydroclimatic processes. Nevertheless, documenting past atmospheric moisture contents and their sources remains challenging partially due to insufficient moisture-tracing proxies. Here, we present triple oxygen isotope compositions in 21 cave speleothems from monsoonal Asia to examine spatial moisture origin differences in eastern Asia over the past 300 years. Our data suggest an isotopic equilibrium fractionation during speleothem formation, and thus parent water 17O anomalies (Δ′17O) values reconstructed from speleothems preserve information on moisture origins and recycling across space. Notably, speleothem Δ′17O records exhibit a distinct geographical distribution, with low values in central-eastern and southern China, high values in northwestern and northern China, and relatively high values in the Yangtze River region. This spatial pattern underscores the diverse influence of different moisture sources regulated by regional hydrological circulation, and demonstrates the potential use of speleothem Δ′17O in reconstructing the moisture cycle at spatial scales.

Compositions of hydrogen and oxygen isotopes of precipitation in Xiamen, southeast China coast: Seasonal variations, synoptic processes, and typhoons impact

AbstractIn this study, the δ2H and δ18O values of 162 precipitation samples (including 33 typhoon‐related samples), collected in Xiamen, southeast China coast, during June 2018 to August 2019, were investigated and analysed. The findings indicate an obvious seasonal variation in the δ2H and δ18O, primarily influenced by the East Asia Monsoon system with notable impact from typhoon occurrences during summer months. The synoptic time‐series variation of δ2H and δ18O exhibits greater complexity: three distinct types of isotopic variations were observed, indicating a clear influence from re‐evaporation processes and continuing equilibrium fractionation during condensation. The variation patterns of δ2H and δ18O were dominated by the local meteorological parameters that govern the re‐evaporation, whereas the overall isotopic composition of precipitation was primarily regulated by the moisture sources. The unique atmospheric structure and dynamic processes of typhoons led to a three‐stage variation of δ2H and δ18O in precipitation as the distance between the typhoon's centre and the study area changed. The consistent configuration of typhoon structure prompts analogous staged changes in different typhoon‐related precipitation. The trajectory of moisture source for typhoon‐related precipitation demonstrates a clear spiral structure (excluding typhoon Yutu), revealing notable impacts from typhoon events.

First-principles calculations of equilibrium barium isotope fractionation among silicate minerals

Barium isotopes have been increasingly applied to trace igneous processes, but the applications are hindered by the lack of equilibrium Ba isotope fractionation factors between silicate minerals. In this study, the reduced partition function ratios (RPFRs) of the major Ba-hosting silicate minerals, including muscovite, phlogopite, tremolite, richterite, microcline, albite, and epidote, as well as witherite and barite, are obtained using first-principles calculations. The effect of Ba concentration on the RPFR of those silicate minerals was examined, and no significant effect was observed in the natural range of Ba concentrations. Our results show a heavy Ba isotope enrichment order of tremolite > epidote > albite > witherite ∼ richterite ∼ microcline ∼ barite ∼ muscovite ∼ phlogopite. The RPFRs are controlled by the force constant of Ba and affected by the BaO bond length. The lattice environment of substituted ions controls the equilibrium Ba isotope fractionation factors of trace elements. For instance, the replacement of Ba2+ with Mg2+ or K+ in phlogopite introduces an approximately 0.16‰ offset of Ba isotope ratios (138Ba/134Ba) at 1000 K. In general, light Ba isotopes are preferentially enriched in crystalline positions with larger ion radii. Equilibrium Ba isotope fractionation among silicate minerals is up to 0.08‰ at 1000 K, which can be well resolved by the current analytical precision (∼0.05‰ for δ138/134Ba). This study provides the calculated equilibrium fractionation factors, which are the key to understanding Ba isotope fractionation in natural rocks.

Satellite-Based Distribution of Inverse Altitude Effect of Global Water Vapor Isotopes: Potential Influences on Isotopes in Climate Proxies

The widely-distributed altitude effect of stable isotopes in meteoric water, i.e., the negative correlation between stable hydrogen (or oxygen) isotope compositions and altitude, is the theoretical basis of isotope paleoaltimetry in climate proxies. However, as many recent local observations have indicated, the inverse altitude effect (IAE) in meteoric water does exist, and the regime controlling IAE is still unclear on a global scale. Based on a remote sensing product of the Infrared Atmospheric Sounding Interferometer (IASI), we examined the global frequency of IAE in water vapor isotopes, and the possible influences on isotopes in precipitation and climate proxies. According to the satellite-based δD values in water vapor at 2950 m and 4220 m above sea level, frequent IAEs are observed on a daily scale in North Africa, West and Central Asia, and North America, and IAEs are more likely to occur during the daytime than during the nighttime. We also converted water vapor δD to precipitation δD via equilibrium fractionation and then analyzed the potential presence of IAE in precipitation, which is more associated with climate proxies, and found that the spatial and temporal patterns of water vapor can be transferred to the precipitation. In addition, different thresholds of δD difference were also tested to understand the impact of random errors. The potential uncertainty of the changing isotope and altitude gradient should be considered in paleo-altitude reconstructions.

The Impact of Soil Tension on Isotope Fractionation, Transport, and Interpretations of the Root Water Uptake Origin

AbstractThe new isotope module in HYDRUS‐1D can be used to infer the origin of root water uptake (RWU), a suitable dynamic indicator for agriculture and forest water management. However, evidence shows that the equilibrium fractionation between liquid water and water vapor within the soil is affected not only by soil temperature but also by soil tension. How soil tension affects isotope transport modeling and interpretations of the RWU origin is still unknown. In this study, we evaluated three fractionation scenarios on model performance for a field data set from Langeoog Island: (a) no fractionation (Non_Frac), (b) the soil temperature control on equilibrium fractionation as described by the standard Craig‐Gordon equation (CG_Frac), and (c) CG_Frac plus the soil tension control on equilibrium fractionation (CGT_Frac). The model simulations showed that CGT_Frac led to more depleted isotopic compositions of surface soil water than CG_Frac. The vertical origin of RWU was estimated using the water balance (WB) calculations and the Bayesian mixing model (SIAR). While the former directly used water flow outputs, the latter used as input simulated isotopic compositions (using different fractionation scenarios) of RWU and soil water. Both methods provided similar variation trends with time and depth in different soil layers' contributions to RWU. The contributions of all soil layers interpreted by the CGT_Frac scenario were always between Non_Frac and CG_Frac. The temporal origin of RWU was deduced from particle tracking (PT, releasing one hypothetical particle for individual precipitation event and tracking its movement based on the water balance between particles) and a virtual tracer experiment (VTE, assigning a known isotope composition to individual precipitation event and tracking its movement based on the cumulative isotope flux). Both methods revealed similar variation trends with time in drainage and root zone (RZ) travel times. The interpreted drainage and RZ travel times were generally ranked as Non_Frac > CGT_Frac > CG_Frac. Overall, the factors considered in the standard CG equation dominated isotope fractionation, transport, and interpretations of the RWU origin. Isotope transport‐based methods (SIAR, VTE) were more computationally demanding than water flow‐based methods (WB, PT).

A set of methods to evaluate the below-cloud evaporation effect on local precipitation isotopic composition: a case study for Xi'an, China

Abstract. When hydrometeors fall from an in-cloud saturated environment toward the ground, especially in arid and semiarid regions, below-cloud processes may heavily alter the isotopic composition of precipitation through equilibrium and non-equilibrium fractionations. If these below-cloud processes are not correctly identified, they can lead to misinterpretation of the precipitation isotopic signal. To correctly understand the environmental information recorded in the precipitation isotopes, qualitatively analyzing the below-cloud processes and quantitatively calculating the below-cloud evaporation effect are two important steps. Here, based on 2 years of synchronous observations of precipitation and water vapor isotopes in Xi'an, China, we compiled a set of effective methods to systematically evaluate the below-cloud evaporation effect on local precipitation isotopic composition. The ΔdΔδ diagram is a tool to effectively diagnose below-cloud processes, such as equilibration or evaporation, because the isotopic differences (δ2H; d-excess) between the precipitation-equilibrated vapor and the observed vapor show different pathways. By using the ΔdΔδ diagram, our data show that evaporation is the major below-cloud process in Xi'an, while snowfall samples retain the initial cloud signal because they are less impacted by the isotopic exchange between vapor and solid phases. Then, we chose two methods to quantitatively characterize the influence of below-cloud evaporation on local precipitation isotopic composition. One is based on the raindrop's mass change during its falling (hereafter referred to as method 1), and the other is dependent on the variations in precipitation isotopic composition from the cloud base to the ground (hereafter referred to as method 2). By comparison, we found that there are no significant differences between the two methods in evaluating the evaporation effect on δ2Hp, except for snowfall events. The slope of the evaporation in proportion to the variation in δ2H (Fi/Δδ2H) is slightly larger in method 1 (1.0 ‰ %−1) than in method 2 (0.9 ‰ %−1). Additionally, both methods indicate that the evaporation effect is weak in autumn and heavy in spring. Through a sensitivity test, we found that in two methods, relative humidity is the most sensitive parameter, while the temperature shows different effects on the two methods. Therefore, we concluded that both methods are suited to the investigation of the below-cloud evaporation effect, while in method 2, other below-cloud processes, such as supersaturation, can still be included. By applying method 2, the diagnosis of below-cloud processes and the understanding of their effects on the precipitation isotopic composition will be improved.

Experimental and theoretical investigations of stable Sr isotope fractionation during its incorporation in aragonite

Strontium partitioning and isotope fractionation between aragonite and fluid have been determined experimentally at low values of the fluid saturation state (Ω) with respect to this mineral (1.1 ≤ Ωaragonite ≤ 2.2), and the measured isotope fractionation has been compared with the results of first-principles simulations. For the latter, Density Functional Theory (DFT) was used for estimation of the equilibrium Sr isotope fractionation between aragonite and Sr2+(aq). The obtained results suggest that, for values of Ωaragonite close to unity, the apparent distribution coefficient of Sr in aragonite (DSr,aragonite=XSrCO3XCaCO3(CaSr)fluid) exhibits values higher than one that rapidly decrease at increasing aragonite growth rate. Under equilibrium conditions (i.e. Ωaragonite=1) a DSr,aragoinite value of 2.7 can be extrapolated. Additionally, for aragonite growth rates rp ≤ 10−8.0±0.2 (mol/m2/s) the Sr isotope fractionation between aragonite and the fluid (i.e. Δ88/86Sraragonite-fluid) shows a constant value of −0.1±0.05‰, whereas it decreases to −0.40‰ when the growth rate increases to 10−7.7(mol/m2/s). The surface reaction kinetic model (SRKM) developed by DePaolo (2011) has been used to describe the dependence of DSr,aragonite and Δ88/86Sraragonite-fluid on mineral growth rate. In this model the best fit for DSr,aragonite and Δ88/86Sraragonite-fluid were 4 and −0.01‰, respectively, whereas the kinetic isotope fractionation factor for Δ88/86Sraragonite-fluid was −0.6‰. The results of first-principles calculations yield an equilibrium Sr isotope fractionation factor of −0.04‰ which is in excellent agreement with the experimental value of the present study. These results are the first experimental measurements of Sr isotope fractionation during inorganic aragonite precipitation as a function of growth rate and the first DFT calculations of Sr equilibrium fractionation in the aragonite-fluid system. The results of this study provide new insight into the mechanisms controlling stable Sr isotope composition in aragonite, which has implications for using Sr isotopes for paleo-reconstructions of natural archives, particularly those of abiogenic origin.

Link Mo isotopes to the sources of the Paleoproterozoic Mo mineralization in the Qinling orogen

Longmendian and the Zhaiwa are the two oldest Mo deposits in the Qinling orogenic belt, China, which is one of the most important Mo provinces in the world with numerous and diverse types of Mo deposits that experienced multistaged mineralization. Here we investigated Mo isotope compositions in molybdenite and rocks from the Longmendian and the Zhaiwa Paleoproterozoic Mo deposits to link Mo isotopes to the sources of Mo metal. The significant Mo isotope variation in the Longmendian and the Zhaiwa Mo deposits (δ98Mo = −0.51‰ to + 1.01‰ and −0.37‰ to +0.53‰, respectively, relative to NIST-3134) may be controlled by Rayleigh and equilibrium fractionation. The different distributions of Mo isotope compositions of molybdenite in the two deposits may be caused by the physiochemical differences in ore-forming systems. The high Mo content of the Taihua Supergroup and the Xiong’er Group, combined mean values of Mo isotopes between molybdenite and whole rocks suggested that the two stratigraphic units would be the main contributor of Mo for the Paleoproterozoic Mo mineralization that mixed with small proportions of isotopically lower values of the mantle.

Rate-dependent effects of state-specific sodium channel blockers in cardiac tissue: Insights from idealized models

A common side effect of pharmaceutical drugs is an increased propensity for cardiac arrhythmias. Many drugs bind to cardiac ion-channels in a state-specific manner, which alters the ionic conductances in complicated ways, making it difficult to identify the mechanisms underlying pro-arrhythmic drug effects. To better understand the fundamental mechanisms underlying the diverse effects of state-dependent sodium (Na+) channel blockers on cellular excitability, we consider two canonical motifs of drug-ion-channel interactions and compare the effects of Na+ channel blockers on the rate-dependence of peak upstroke velocity, conduction velocity, and vulnerable window size. In the literature, both motifs are referred to as “guarded receptor,” but here we distinguish between state-specific binding that does not alter channel gating (referred to here as “guarded receptor”) and state-specific binding that blocks certain gating transitions (“gate immobilization”). For each drug binding motif, we consider drugs that bind to the inactivated state and drugs that bind to the non-inactivated state of the Na+ channel. Exploiting the idealized nature of the canonical binding motifs, we identify the fundamental mechanisms underlying the effects on excitability of the various binding interactions. Specifically, we derive the voltage-dependence of the drug binding time constants and the equilibrium fractions of channels bound to drug, and we then derive a formula that incorporates these time constants and equilibrium fractions to elucidate the fundamental mechanisms. In the case of charged drug, we find that drugs that bind to inactivated channels exhibit greater rate-dependence than drugs that bind to non-inactivated channels. For neutral drugs, the effects of guarded receptor interactions are rate-independent, and we describe a novel mechanism for reverse rate-dependence resulting from neutral drug binding to non-inactivated channels via the gate immobilization motif.

No biological effect on magnesium isotope fractionation during stromatolite growth

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

Are Large Sulfur Isotope Variations Biosignatures in an Ancient, Impact-Induced Hydrothermal Mars Analog?

Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first in situ δ34S data from Mars at Gale crater display a ∼75‰ range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δ34S fractionations >21‰ as indicative of a biological origin, and studies of δ34S at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δ34S, total organic carbon wt%, and scanning electron microscope imaging) on multiple lithologies from the Nördlinger Ries impact crater, focusing on hydrothermally altered impact breccias and associated sedimentary lake-fill sequences to determine whether the δ34S properties define a biosignature. The differences in δ34S between the host lithologies may have resulted from thermochemical sulfate reduction, microbial sulfate reduction, hydrothermal equilibrium fractionation, or any combination thereof. Despite abundant samples and instrumental precision currently exclusive to Earth-bound analyses, assertions of biogenicity from δ34S variations >21‰ at the Miocene Ries impact crater are tenuous. This discourages the use of δ34S as a biosignature in similar environments without independent checks that include the full geologic, biogeochemical, and textural context, as well as a comprehensive acknowledgment of alternative hypotheses.

Quantifying Raindrop Evaporation Deficit in General Circulation Models from Observed and Model Rain Isotope Ratios on the West Coast of India

Raindrop evaporation is an important sub-cloud process that modifies rainfall amount and rainwater isotope values. Earlier studies have shown that various general circulation models (GCMs) do not incorporate this process properly during the simulation of water isotope ratios (oxygen and hydrogen). Our recent study has demonstrated that an inadequate estimation of this process for the Indian Summer Monsoon (ISM) results in significant biases (model-observed values) in the simulation of various GCMs on a monthly scale. However, a quantitative estimation was lacking. The magnitude of raindrop evaporation depends upon ambient humidity and temperature, which vary considerably during the ISM. Consequently, the isotope biases would also vary over various time scales. The present study aims to investigate the magnitude of the monthly scale variation in raindrop evaporation in the simulations and its causal connection with the corresponding variation in isotope biases. Towards this, we compare an 11-year-long (1997–2007) dataset of rain isotope ratios (both oxygen and hydrogen) from an Indian station, Kozhikode (Kerala), obtained under the Global Network of Isotopes in Precipitation (GNIP) programme of the International Atomic Energy Agency (IAEA) with the corresponding outputs of two isotope-enabled nudged GCMs—ISOGSM and LMDZ4. The raindrop evaporation fractions are estimated for 44 ISM months (June–September) of the study period using the Stewart (1975) formalism. Using a simple condensation–accretion model based on equilibrium fractionation from vapour, obtained from two adopted vapour isotope profiles, we estimate the liquid water isotope ratios at the cloud base. Considering this water as the initial rain, the raindrop evaporation fractions are estimated using the observed oxygen and hydrogen isotope ratios of Kozhikode surface rain samples. The estimated fractions show strong positive correlations with the isotope biases (R2 = 0.60 and 0.66). This suggests that lower estimates of raindrop evaporation could be responsible for the rain isotope biases in these two GCMs.

Regulation of deep carbon degassing by gas-rock-water interactions in a seismic region of Southern Italy

This study is focused on fluids characterization and circulations through the crust of the Irpinia region, an active seismic zone in Southern Italy, that has experienced several high-magnitude earthquakes, including a catastrophic one in 1980 (M = 6.9 Ms). Using isotopic geochemistry and the carbon‑helium system in free and dissolved volatiles in water, this study aims to explore the processes at depth that can alter pristine chemistry of these natural fluids. Gas-rock-water interactions and their impact on CO2 emissions and isotopic composition are evaluated using a multidisciplinary model that integrates geochemistry and regional geological data. By analyzing the He isotopic signature in the natural fluids, the release of mantle-derived He on a regional scale in Southern Italy is verified, along with significant emissions of deep-sourced CO2. The proposed model, supported by geological and geophysical constraints, is based on the interactions between gas, rock, and water within the crust and the degassing of deep-sourced CO2. Furthermore, this study reveals that the Total Dissolved Inorganic Carbon (TDIC) in cold waters results from mixing between a shallow and a deeper carbon endmember that is equilibrated with carbonate lithology. In addition, the geochemical signature of TDIC in thermal carbon-rich water is explained by supplementary secondary processes, including equilibrium fractionation between solid, gas, and aqueous phases, as well as sinks such as mineral precipitation and CO2 degassing. These findings have important implications for developing effective monitoring strategies for crustal fluids in different geological contexts and highlight the critical need to understand gas-water-rock interaction processes that control fluid chemistry at depths that can affect the assessment of the CO2 flux in atmosphere. Finally, this study highlights that the emissions of natural CO2 from the seismically active Irpinia area are up to 4.08·10+9 mol·y−1, which amounts is in the range of worldwide volcanic systems.

Model-independent bubble wall velocities in local thermal equilibrium

Accurately determining bubble wall velocities in first-order phase transitions is of great importance for the prediction of gravitational wave signals and the matter-antimatter asymmetry. However, it is a challenging task which typically depends on the underlying particle physics model. Recently, it has been shown that assuming local thermal equilibrium can provide a good approximation when calculating the bubble wall velocity. In this paper, we provide a model-independent determination of bubble wall velocities in local thermal equilibrium. Our results show that, under the reasonable assumption that the sound speeds in the plasma are approximately uniform, the hydrodynamics can be fully characterized by four quantities: the phase strength αn , the ratio of the enthalpies in the broken and symmetric phases, Ψ n , and the sound speeds in both phases, cs and cb . We provide a code snippet that allows for a determination of the wall velocity and energy fraction in local thermal equilibrium in any model. In addition, we present a fit function for the wall velocity in the case cs = cb = 1/√(3).

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C3 grass-atmospheric drivers and physiological relations.

The 18 O enrichment (Δ18 O) of leaf water affects the Δ18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18 O of bulk leaf water (Δ18 OLW ) and leaf sucrose (Δ18 OSucrose ). We grew Lolium perenne (a C3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol-1 ) as factors, and determined Δ18 OLW , Δ18 OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf ), stomatal conductance (gs ) and mesophyll conductance to CO2 (gm ). The Δ18 O of photosynthetic medium water (Δ18 OSSW ) was estimated from Δ18 OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio ). Δ18 OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18 Oe ) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2 ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18 OLW was a poor proxy for Δ18 OSucrose , mainly due to opposite Δ18 O responses of nonphotosynthetic tissue water (Δ18 Onon-SSW ) relative to Δ18 OSSW , driven by atmospheric conditions.

Geological control on carbon isotope equilibrium and kinetic fractionation of CH4-CO2-HCO3− in microbial coalbed and shale gas systems

Microbial coalbed gas (CBG) and shale gas (SG), predominately composed of methane (CH4) and carbon dioxide (CO2), are important economic resources and potent greenhouse gases. Although isotopic equilibrium of CH4 and CO2 has been observed in microbial CBG and SG basins, it is difficult to judge under what geological conditions equilibrium is achieved. Moreover, the effects of CO2 dissolution on the isotopic fractionation process need to be considered. We use data from eight microbial CBG and SG basins to discuss the geological conditions in which equilibrium and kinetic isotopic fractionation in CH4-CO2-HCO3− system is achieved. Based on isotopic equilibrium temperatures calculated using computer codes developed in MatLab software, we show that, in deep and closed reservoirs, the CH4-CO2 and CH4-HCO3− are close to carbon isotope equilibrium. In contrast, in shallow and open reservoirs, they are in disequilibrium. The CO2-HCO3− is in disequilibrium in most reservoirs. We propose that both low free energy gradients and long coexisting time of CH4 and CO2/HCO3− are necessary to attain isotopic equilibrium. However, it is difficult to accurately estimate the timescale for attaining isotope equilibrium among them. In general, a closed and deep CBG/SG reservoir is likely to be geologically and geochemically stable over long timescales, favoring isotopic equilibrium of CH4-CO2 and CH4-HCO3−. However, a shallow and open reservoir is unfavourable for their isotopic equilibrium due to shorter timescales for the coexistence of CH4-CO2-HCO3−. Using data from systems close to equilibrium, we estimated the percentage of CO2 in total CH4 and CO2 in CBG reservoirs in various basins to be from 27% to 50%, where methanogenesis is mainly by CO2 reduction. This is significantly higher than the CO2 content (1% to 15%) in gaseous CH4 and CO2 in these basins but is consistent with those (36% to 48%) from culture experiments for coal conversion by methanogenesis. Further study shows that 53–99% of the CO2 formed during CBG generation has dissolved into groundwaters to form dissolved inorganic carbon (DIC) in CBG reservoirs. We propose that CO2 dissolution likely has significantly affect the abundance and isotopic compositions of gaseous CO2 in subsurface.

Formation pathways and source apportionments of inorganic nitrogen-containing aerosols in urban environment: Insights from nitrogen and oxygen isotopic compositions in Guangzhou, China

Air quality issues caused by PM2.5-induced persistent extreme haze episodes have become increasingly serious in urban environments in recent years. Secondary water-soluble inorganic sulfate (SO42−), nitrate (NO3−) and ammonium (NH4+) ions are the major components of PM2.5. However, the contributions of inorganic nitrogen species, especially nitrate, to PM2.5 have greatly increased during haze episodes in many cities in China. Therefore, better understanding of their emission sources and formation pathways holds the key to controlling urban PM2.5 pollution more efficiently and effectively. In this study, water-soluble ionic characteristics and isotopic compositions and sources of NH4+ and NO3−, as well as NO3− formation pathways were determined in PM2.5 aerosol samples collected in Guangzhou, China, during 2015–2018. The PM2.5 concentrations varied from 30.5 to 189.8 μg⋅m−3 and their mean values were highest in spring (111.1 μg⋅m−3) and lowest in summer (63.5 μg⋅m−3). The δ15N–NH4+ and δ15N–NO3- values ranged from +4.5 to +20.2‰ and from +4.8 to +14.8‰, respectively, with their mean values being highest in winter (14.0‰ and 9.5‰, respectively) and lowest in summer (10.4‰ and 7.1‰, respectively). The seasonal δ15N variability was mainly attributed to isotopic equilibrium fractionation, and partly due to the changes in NH3 and NOx sources. The average δ18O–NO3- value of 63.0‰, ranging seasonally from +58.6‰ in summer to +68.0‰ in spring, suggests that NO2 +⋅OH pathway played a vital role (70.3–85.9%) in NO3− formation. The Bayesian isotope mixing model results revealed fossil fuel combustion sources as dominant sources of atmospheric NH3 and NOx. This study suggests that more effort should be devoted to reduce NH3 and NOx from combustion-related processes and highlights the importance of δ18O–NO3- analysis for exploring variations of nitrate formation pathways in urban atmospheres.

Antimony Isotope Fractionation Revealed from EXAFS during Adsorption on Fe (Oxyhydr)oxides.

A lack of knowledge about antimony (Sb) isotope fractionation mechanisms in key geochemical processes has limited its environmental applications as a tracer. Naturally widespread iron (Fe) (oxyhydr)oxides play a key role in Sb migration due to strong adsorption, but the behavior and mechanisms of Sb isotopic fractionation on Fe (oxyhydr)oxides are still unclear. Here, we investigate the adsorption mechanisms of Sb on ferrihydrite (Fh), goethite (Goe), and hematite (Hem) using extended X-ray absorption fine structure (EXAFS) and show that inner-sphere complexation of Sb species with Fe (oxyhydr)oxides occurs independent of pH and surface coverage. Lighter Sb isotopes are preferentially enriched on Fe (oxyhydr)oxides due to isotopic equilibrium fractionation, with neither surface coverage nor pH influencing the degree of fractionation (Δ123Sbaqueous-adsorbed). Limited Fe atoms are present in the second shell of Hem and Goe, resulting in weaker surface complexes and leading to greater Sb isotopic fractionation than with Fh (Δ123Sbaqueous-adsorbed of 0.49 ± 0.004, 1.12 ± 0.006, and 1.14 ± 0.05‰ for Fh, Hem, and Goe, respectively). These results improve the understanding of the mechanism of Sb adsorption by Fe (oxyhydr)oxides and further clarify the Sb isotope fractionation mechanism, providing an essential basis for future application of Sb isotopes in source and process tracing.

Site-specific isotope effect: Insights from equilibrium magnesium isotope fractionation in mantle minerals

The equilibrium Mg isotope fractionation factors (lnα) derived from both theoretical calculation and experimental studies are not consistent with the values from natural sample observation. Different cation sites, which have identical coordinate numbers but different bond strengths, contribute equally to lnα in theoretical calculations. These non-equivalent sites might complicate the inter-mineral equilibrium isotope fractionations. Here, we investigate reduced partition function ratios (103lnβ) of non-equivalent Mg cation sites (e.g., M1, M2, and M3) in forsterite, orthoenstatite, clinoenstatite, tremolite, omphacite, pyrope and spinel to explore the site-specific effect on the intra- and inter-mineral equilibrium isotope fractionation. Based on all-electron first-principles calculations, 103lnβ in different cation sites decreases in the following sequence of spinelM1 > tremoliteM3 > orthoenstatiteM1 > forsteriteM1 > omphaciteM1 > tremoliteM1 > tremoliteM2 > clinoenstatiteM1 > forsteriteM2 > orthoenstatiteM2 > clinoenstatiteM2 > pyropeM1. These findings suggest that cation sites with larger polyhedron volumes prefer light Mg isotopes to those with smaller polyhedron volumes inside minerals, and the different contribution of intra-mineral cation sites could result in significant variations of equilibrium Mg isotope fractionation between minerals. The range of lnα between minerals induced by specific sites is defined as the equilibrium isotope fractionation domain. The site-corrected lnα can provide better evaluation of equilibrium fractionation factor of Mg isotopes between spinel and olivine and clarify the equilibrium state between mantle minerals. In addition, we establish spinel-olivine and omphacite-pyrope Mg isotope geothermometers based on the site-specific effect. This study suggests that the site-specific effect could be prevalent in solid materials and thus important for other isotope systems.

Thermodynamic modeling of the Nb-Ni system with uncertainty quantification using PyCalphad and ESPEI

The Nb–Ni system is remodeled with uncertainty quantification (UQ) using software tools of PyCalphad and ESPEI (the Extensible, Self-optimizing Phase Equilibria Infrastructure) with the presently implemented capability of modeling site fraction based on Wyckoff positions. The five- and three-sublattice models are used to model the topologically close pack (TCP) μ-Nb7Ni6 and δ-NbNi3 phases according to their Wyckoff positions. The inputs for CALPHAD-based thermodynamic modeling include the thermochemical data as a function of temperature predicted by first-principles and phonon calculations based on density functional theory (DFT), ab initio molecular dynamics (AIMD) simulations, together with phase equilibrium and site fraction data in the literature. In addition to phase diagram and thermodynamic properties, the CALPHAD-based predictions of site fractions of Nb in μ-Nb7Ni6 agree well with experimental data. Furthermore, the UQ estimation using the Markov Chain Monte Carlo (MCMC) method as implemented in ESPEI is applied to study the uncertainty of site fraction in μ-Nb7Ni6 and enthalpy of mixing (ΔHmix) in liquid.