Isotope Fractionation Research Articles

To date or not to date? A comparison of different 14C pretreatment methods applied to archeological marine shells from Vale Boi (Portugal)

Abstract Mollusk shells are often found in archeological sites, given their great preservation potential and high value as a multipurpose resource, and they can often be the only available materials useful for radiocarbon (14C) dating. However, dates obtained from shells are often regarded as less reliable compared to those from bones, wood, or charcoals due to different factors (e.g., Isotope fractionation, reservoir effects etc.). The standard acid etching pretreatment for mollusk shells is the most used in many 14C laboratories, although another protocol known as CarDS (Carbonate Density Separation) was introduced just over a decade ago. We compare these two methods with two newly proposed methods for intracellular organic matrix extraction. We applied all four methods to samples selected from different archeological layers of the well-known Upper Paleolithic site of Vale Boi, rich in mollusk specimens throughout the stratigraphic sequence. Here we compare our results to previous dates to determine which of these pretreatment methods results in the most reliable 14C dates. Based on the results of this study, all four methods gave inconsistent ages compared to previous dates and the stratigraphic attribution.

Garnet clinopyroxenite formation via amphibole-dehydration in continental arcs: Evidence from Fe isotopes

Lower-crustal garnet clinopyroxenite (sometimes termed “arclogite”) fractionation in thick-crustal (>35 km) arc settings presents a compelling model to explain Fe-depletion trends, high oxygen fugacity, and evidence of recent delamination observed in many continental arcs. However, the origin of the garnet clinopyroxenites via igneous or metamorphic processes remains unclear. Due to the preferential incorporation of light Fe isotopes in garnet relative to clinopyroxene or amphibole, Fe isotopes are ideally suited for studying the effects of garnet fractionation on magmatic systems. Here, we present whole-rock and mineral Fe isotope data from a suite of lower to mid/upper-crustal Andean xenoliths from Mercaderes, Colombia. This data is combined with petrography, major and trace element mineral and whole-rock chemistry, geothermobarometry, and thermodynamic modeling to explore the xenoliths' petrogenesis and the Northern Andes' crustal structure. Whole-rock samples display a narrow range of Fe isotope compositions (δ56Fe = –0.02 to +0.11 ‰), which do not correlate with lithology, chemistry, or pressure-temperature conditions. This result is inconsistent with previous studies predicting the existence of an isotopically light Fe reservoir in the garnet-rich lower Andean crust. Through thermodynamic modeling, we show that the lack of isotopic fractionation in the Mercaderes xenoliths is more consistent with the suite representing a prograde metamorphic sequence, in which amphibole dehydration reactions drive metamorphism of mid/upper-crustal diorite protoliths. While our data do not preclude the presence of garnet clinopyroxenite cumulates at the base of the Andean crust, or that the delamination of such cumulates played an important role in the evolution of the Andes, they do indicate that not all garnet clinopyroxenites are cumulate in origin. Instead, the lower Andean crust represents an amalgamation of igneous and metamorphic rock, with metamorphism of mid-crustal lithologies and partial melting of mafic cumulate roots acting in tandem to drive densification and delamination of the lower crust in a self-feeding mechanism.

Characterization of Phytoplankton-Derived Amino Acids and Tracing the Source of Organic Carbon Using Stable Isotopes in the Amundsen Sea.

We utilized amino acid (AA) and carbon stable isotope analyses to characterize phytoplankton-derived organic matter (OM) and trace the sources of organic carbon in the Amundsen Sea. Carbon isotope ratios of particulate organic carbon (δ13C-POC) range from -28.7‱ to -23.1‱, indicating that particulate organic matter originated primarily from phytoplankton. The dissolved organic carbon isotope (δ13C-DOC) signature (-27.1 to -21.0‱) observed in the sea-ice melting system suggests that meltwater contributes to the DOC supply of the Amundsen Sea together with OM produced by phytoplankton. A negative correlation between the degradation index and δ13C-POC indicates that the quality of OM significantly influences isotopic fractionation (r2 = 0.59, p < 0.001). The AA distribution in the Amundsen Sea (5.43 ± 3.19 µM) was significantly larger than previously reported in the Southern Ocean and was associated with phytoplankton biomass (r2 = 0.49, p < 0.01). Under conditions dominated by P. antarctica (DI = 2.29 ± 2.30), OM exhibited greater lability compared to conditions co-dominated by diatoms and D. speculum (DI = 0.04 ± 3.64). These results highlight the important role of P. antarctica in influencing the properties of OM, suggesting potential impacts on carbon cycling and microbial metabolic activity in the Amundsen Sea.

Monitoring weekly δ 13С variations along the cambium-xylem continuum in the Canadian eastern boreal forest.

Intra-annual variations of carbon stable isotope ratios (δ13C) in different tree compartments could represent valuable indicators of plant carbon source-sink dynamics, at weekly time scale. Despite this significance, the absence of a methodological framework for tracking δ13C values in tree rings persists due to the complexity of tree ring development. To fulfill this knowledge gap, we developed a method to monitor weekly variability of δ13C in the cambium-xylem continuum of black spruce species [Picea mariana (Mill.) BSP.] during the growing season. We collected and isolated the weekly incremental growth of the cambial region and the developing tree ring from five mature spruce trees over three consecutive growing seasons (2019-2021) in Simoncouche and two growing seasons (2020-2021) in Bernatchez, both located in the boreal forest of Quebec, Canada. Our method allowed for the creation of intra-annual δ13C series for both the growing cambium (δ13Ccam) and developing xylem cellulose (δ13Cxc) in these two sites. Strong positive correlations were observed between δ13Ccam and δ13Cxc series in almost all study years. These findings suggest that a constant supply of fresh assimilates to the cambium-xylem continuum may be the dominant process feeding secondary growth in the two study sites. On the other hand, rates of carbon isotopic fractionation appeared to be poorly affected by climate variability, at an inter- weekly time scale. Hence, increasing δ13Ccam and δ13Cxc trends highlighted here possibly indicate shifts in carbon allocation strategies, likely fostering frost resistance and reducing water uptake in the late growth season. Additionally, these trends may be related to the black spruce trees' responses to the seasonal decrease in photosynthetically active radiation. Our findings provide new insights into the seasonal carbon dynamics and growth constraints of black spruce in boreal forest ecosystems, offering a novel methodological approach for studying carbon allocation at fine temporal scales.

The soil-air interfacial migration process of volatile PFAS at the contaminated sites: Evidence from stable carbon isotopes with CSIA

Volatile per- and polyfluoroalkyl substances (PFAS) are prone to transport among various environmental media, with the soil-air interfacial migration process being an important pathway that significantly influences their environmental fate. To assess the migration and transformations of target volatile PFAS at contaminated site using compound-specific stable isotope analysis (CSIA), it is necessary to understand the isotopic fractionation that occurs during their transfer from soil to air. We have established methods for pre-treatment and GC/CSIA analysis methods of target volatile PFAS in soil and air samples and ensured the accuracy of carbon isotope analysis. GC/IRMS δ13C measurements showed optimal precision at instrumental response above 1.35–2.75 Vs, with recommended minimum on-column C levels of 1.67–5.00 nmol for target volatile PFAS. Stable carbon isotope fractionation factors related to the soil-air interfacial migration process for target volatile PFAS were determined by performing laboratory simulations. The observed εsoil-air values are all negative, suggesting that the soil-air interfacial migration process for target volatile PFAS is kinetic fractionation, the removal of molecules containing lighter isotopes. By comparing the simulated and experimentally observed δ13C (‰) values of target volatile PFAS, we found consistent trends in the soil and inverse trends in the air. These δ13C (‰) values and the related isotope fractionation model provide valuable insights into the isotopic behavior of target volatile PFAS during soil-air interfacial migration process, aiding in the assessment of their environmental fate.

Presence of microplastics enhanced the toxicity of silver nanoparticles on the collembolan Folsomia candida

There is growing interest in interactions of microplastics (MPs) with other pollutants. However, there is limited understanding of the combined effects of MPs and silver nanoparticles (AgNPs) on nontarget soil organisms. This work aimed to examine the effects of exposure to various AgNPs' concentrations alone (0, 0.1, 1, 10, 100, 1000 mg·kg-1, 50 nm) and in combination with polyvinyl chloride microplastics (PVC MPs, 80-250 μm) at 0.1% concentration for 28 days on reproduction, Ag accumulation, C/N ratio, and isotopic fractionation of the standard soil fauna collembolan Folsomia candida. Results showed that compared to the AgNPs exposure alone, the presence of MPs significantly reduced reproduction by 51.4% and markedly increased Ag content in collembolans by 87.7% at 1000 mg·kg-1 AgNPs, which evidenced a synergistic effect. Co-exposure to MPs and AgNPs resulted in a noticeable reduction in the C/N ratio in F. candida body tissues by 9.90% and 5.27% at 1 and 10 mg·kg-1 AgNPs, respectively, showing additive and synergistic effects. Additionally, this co-exposure altered stable isotope fractionation, with the highest increments of δ15N by 32.3% and inhibition of δ13C by 2.62%, demonstrating the turnover of nutrients shift in the collembolan tissues. Collectively, this study demonstrates that con-current exposure to environmentally relevant concentration of MPs and relatively high doses of AgNPs synergistically induces toxic effects on F. candida, leading to Ag accumulation and reproduction decline. These findings imply that MPs could alter collembolans' responses to AgNPs exposure, potentially enhancing the metal ions' bioavailability in soil environments and posing ecotoxicological threats to soil-dwelling organisms.

Impact of Rhizosphere Biostimulation on Cd Transport and Isotope Fractionation in Cd-Tolerant and Hyperaccumulating Plants Based on MC-ICP-MS and NanoSIMS.

Phytoremediation efficiency can be enhanced by regulating rhizosphere processes, and the Cd isotope is a useful approach for deciphering Cd transport processes in soil-plant systems. However, the effects of adsorption and complexation on Cd isotope fractionation during the rhizosphere processes remain unclear. Here, we cultivated the Cd hyperaccumulator Sedum alfredii and Cd-tolerance Sedum spectabile in three different soils with citric acid applied as a degradable rhizosphere biostimulant. Cellular elemental distributions in the tissues and Cd isotope compositions were determined through NanoSIMS and MC-ICP-MS, respectively. Cd precipitation/adsorption on cell walls and intracellular regional distribution were the main mechanisms of Cd tolerance in S. spectabile. Plant roots became enriched with heavier Cd isotopes relative to the surrounding soils upon increasing secretion of rhizosphere organic acids. This indicates that organic matter with O and N functional groups preferentially chelates heavy Cd isotopes. In addition, Cd isotope fractionation between roots and shoots varies within the three soils, which could be due to the influence of protein and metallothionein contents in roots and leaves. The finding indicates that sulfur-containing ligands preferentially chelate light Cd isotopes. This study suggests that organic ligands play a vital role in Cd isotope fractionation and consequent hyperaccumulation of soil-plant systems.

Elucidating the Fate of the Organochlorine Pesticide Chlordecone under Abiotic Reductive and Oxidative Processes: Kinetics, Transformation Products, and C vs Cl Isotope Fractionation.

Pollution of French West Indies (FWI) soils by the organochlorine pesticide chlordecone poses environmental and societal concerns due to its long-term persistence. Assessing chlordecone degradation remains challenging due to analytical constraints to identify transformation products. Here, multielement compound-specific isotope analysis (ME-CSIA) was used to identify changes in stable isotope signatures of chlordecone produced during abiotic transformation reactions under reducing and oxidative conditions. Effective chlordecone transformation was shown in reactions with zerovalent iron (ZVI), vitamin B12 and sodium sulfide (VSS), alkaline ascorbic acid (AA), and sodium persulfate activated by microwave irradiation (MWPS). Significant enrichment of 13C and 37Cl was observed in all abiotic reactions, with εC,bulk and εCl,bulk values ranging from -4.3 ± 0.4‰ to -2.3 ± 0.2‰ and from -2.6 ± 0.4‰ to -1.3 ± 0.3‰, respectively. Distinct mechanisms were evidenced in dual isotope plots, resulting in Λ values of 1.17 ± 0.28 for ZVI, 1.26 ± 0.50 for VSS, 2.06 ± 0.30 for AA, and 2.90 ± 0.50 for MWPS. Two major products were formed, 10-monohydrochlordecone and 8-monohydrochlordecone. Cl-CSIA data suggested that the first Cl substitution at the C10-position likely produced secondary Cl isotope effects (via nucleophilic substitution). Overall, results suggest that ME-CSIA can help quantify in situ chlordecone degradation, distinguishing between different ongoing degradation mechanisms and fingerprinting pollutant sources from chlordecone formulations (Curlone).

Inter- and Intra-Annual Variations in Oak Tree Ring δ13C Values across Different Elevations and Their Climatic Responses in Qinling Mountains

The Qinling Mountains, serving as a natural geographical and climatic boundary in China, require comprehensive climatic records to elucidate the trends in climate changes across the country. While stable isotopes in tree rings are widely employed to indicate historical environmental changes, investigations into tree ring isotopes in the Qinling Mountains, particularly within the widespread broad-leaf oaks, remain limited. In this study, we investigated both intra- and inter-annual variations in the δ13C values of tree rings and their correlations with climatic signals over the past two decades for Quercus aliena var. acuteserrata, a dominant species among oak trees on the main peak of the Qinling Mountains. Our results reveal that responses to climate differ among altitudes and individual trees, with trees at higher altitudes exhibiting higher sensitivity to extreme climate, which is low temperatures and rainfall fluctuations during the growth period in intra-annual δ13C variations. Furthermore, the positive correlations are observed between temperature during growing season and both tree growth and the inter-annual δ13C variations. However, the climate signal appears to be hampered by oak-specific factors, such as intense competition among individuals and the age of trees. Therefore, we suggest a more rigorous selection of sampling and propose further investigations into isotopic fractionation processes in oaks for future studies.

Petrogenesis of Montana, USA Sapphires Inferred from Oxygen Isotopes and Zircon Inclusions

Abstract Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e., crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U-Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ18O values, from -3 to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0 to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4 to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 3‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U-Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U-Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.

Light δD apatites reveal deep origin water in North China Craton intracontinental granites and basalts

Water is essential to the formation of intracontinental granites, but its origin remains elusive. Here we address this scientific problem by analyzing D/H isotopes of apatites, hydrous minerals in Jurassic and Early Cretaceous granites and basalts from eastern North China Craton, where water was previously interpreted as derived from subducting slab. Results reveal extremely low δD values in pristine Early Cretaceous granitic (−203‰ to −127‰) and basaltic (−197‰ to −107‰) apatites, contrasting with relatively high δD values (−137‰ to −47‰) in Jurassic granites. Given the depth-dependent D/H isotopic fractionation during slab dehydration and high-water contents in coeval primitive mafic magmas, the Early Cretaceous magma water is attributed to the stagnant slab within the mantle transition zone. Secular change in the depth of water aligns with steepening of subducting Paleo-Pacific plate from Jurassic to Early Cretaceous, demonstrating the potential of apatite H isotopes in tracing water origin in granites and basalts.

Coupled iron and heavy metal accumulation in karst soils in Southwestern China: Iron isotope perspective

Heavy metals (HMs) are abundant in the karst soils of Southwest China, posing significant health risks to millions of people. Iron (Fe) (hyr)oxides serve as critical carriers of HMs in these soils; however, the processes governing Fe oxide formation and transformation associated with HM accumulation during carbonate weathering in karst region is less understood. In this study, we present Fe isotope compositions from a carbonate–derived profile to investigate the major factors controlling Fe migration. In the saprolite layer, strong correlations between δ56Febulk and the proportions of extracted FeNH2OH.HCl or Feresidue fractions suggest that the formation of goethite and phyllosilicate may be responsible for variations in δ56Febulk. The positive correlations between δ56FeNH2OH.HCl values and HM concentrations in this layer suggest an enhanced capacity for HM fixation by goethite in these soils. In contrast, the fractionation of Fe isotopes in the soil layer appears to be influenced by vegetation, as indicated by the correlation between total organic carbon and δ56Febulk. The negative correlations between δ56Febulk values and HM concentrations in the soil layer likely indicate that vegetation litter enhances the retention capacity of Fe oxides for HMs. This study highlights the trajectory of Fe and its connection to HM accumulation in karst soil with high geological background levels.

Research Hotspots in and Progress of Stable Isotopic Techniques Applied in Tracing Mine Water Pollution and Its Environmental Impact: A Bibliometric and Visualization Analysis from 1998 to 2023

Stable isotope techniques have become a critical tool for tracking mine water and identifying its contamination. In order to explore in depth the research hotspots and trends in stable isotope technology in the study of mine water and the environmental pollution it induces, the Web of Science Core Collection (WoSCC) database of the relevant literature in this field from 1998 to 2023 was used for visual bibliometric analysis by applying CiteSpace software (version 5.7R5). The results showed that the periodical literature in this field shows a fluctuating upward trend. In the cooperation network of country and institution, the centrality of the United States was as high as 0.74 and 0.23, much higher than that of other countries, which means that in terms of the institutions, the number of publications, and the status of research, the United States is ahead of other countries. China’s research started later than the United States’s but is developing rapidly. Although its importance and influence in this research field are only slightly lower than those of the United States, China still needs to improve its cooperation with other countries and regions. The research hotspots in this area center around identifying and understanding pollution processes, studying mine water sources and mixing, exploring the evolution of water chemistry and its isotopic composition, and investigating the environmental impacts of mine water. Innovative isotope-tracing methods and techniques, isotope fractionation mechanisms, sources of sulfate, and their impact on the water environment will remain the focus of the forthcoming research phase. This study uses bibliometrics to systematically summarize the research hotspots and trends in stable isotope techniques in mine water problems in terms of their footprint in the academic literature, which is of great significance for the utilization of water resources in mine drainage and pollution control in mines.

The behavior of nickel isotopes during mantle melting

Mantle-derived mafic rocks show relatively large variations in nickel (Ni) isotopes and mostly isotopically light compared to the bulk silicate Earth (BSE). Whether this signature is due to the source heterogeneity or controlled by melting processes has been a debatable issue. Here, we analyzed Ni isotopic compositions of 18 intraplate basalts from the Sabzevar region, northern Iran and 16 serpentinized peridotites from Cyprus and South China. The Sabzevar basalts were likely sourced from a sulfur-free/barren mantle domain with recycled pyroxenites involved, manifested by their high Cu contents (127–265 ppm), FeT/Mn (58.2–77.4) and Zn/FeT ratios (11.9–16.4). The Ni isotopic compositions of the Sabzevar basalts (average δ60/58Ni = +0.15 ± 0.08 ‰; 2SD) are slightly heavier than the BSE value (average δ60/58Ni = +0.11 ± 0.06 ‰; 2SD), consistent with equilibrium Ni isotope fractionation during mantle silicate melting as predicted by ionic model calculations. Our new data of serpentinized peridotites (average δ60/58Ni = +0.16 ± 0.06 ‰; 2SD), together with previously reported data for oceanic sediments and metabasalts, suggest that recycled lithologies have mantle-like or relatively heavy Ni isotopic compositions. Thus, the isotopically light Ni in mafic rocks is unlikely to be caused by crustal recycling-induced mantle heterogeneity. Rather, the light Ni isotopic signature is caused by the dissolution of recycled sulfides into the mantle melts. We suggest two sulfide dissolution models (“dissolve and go” and “dissolve and equilibrium”) to describe the δ60/58Ni and Cu systematics in terrestrial basalts. In both models, the initial sulfide content (Sinitial) and oxygen fugacity (fO2) exert a major control on Ni isotopic compositions of resulting melts. These two parameters vary among different geological settings. Basalts derived from mantle sources with high Sinitial and fO2 are characterized by light Ni isotopic compositions, whereas basalts sourced from sulfur-free/barren and reduced mantle domains are likely characterized by heavy Ni isotopic compositions, aligning with the characteristic observed in natural samples.

Effects of CO2 on the Nitrogen Isotopic Composition of Marine Diazotrophic Cyanobacteria

AbstractBiological N2 fixation has been crucial for sustaining early life on Earth. Very negative δ15N values detected in Archean sediments, which are not observed in present‐day environments, have been attributed to the low efficiency of proto‐nitrogenases. Alternatively, variations in early atmospheric CO2 may also play a role. Here we examine the effects of CO2 concentrations on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii, which utilize Mo‐Fe nitrogenase (the most common form of the enzyme). Our results show that these organisms produce biomass with δ15N values up to ∼3‰ lower under both decreased and elevated CO2 concentrations compared to modern levels (∼380 μatm). These deviations from modern CO2 levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N2 fixation. This study offers an alternative explanation for the observed fluctuations in geological δ15N records and provides new insights into the past nitrogen cycle on Earth.

Magnesium geochemistry of authigenic carbonate at marine cold seep

Cold seeps, featured by their extremely methane-rich sedimentary environments, play a significant role in the geological history and are common in marine sediments across the seafloor. Primary dolomite, possibly mediated by microorganisms, can be widely discovered in methane-rich environments. Hence, cold seeps may provide new insights into the ‘dolomite problem’, which has confused geologists for decades. Magnesium isotope geochemistry of seep carbonates contributes to the understanding of the dolomite formation mechanism in marine environments. In this paper, magnesium geochemical characteristics of carbonates in modern sediments are summarized, along with rare researches on magnesium isotopes of seep carbonates. Methane vigorously interacts with sulfate by anaerobic oxidation of methane at cold seeps, producing vast amounts of dissolved sulfide which can significantly promote dolomitization of seep carbonates. Compared with temperature, alkalinity, mineralogy, etc., the competition between rapid carbonate precipitation rates and aqueous ligands may be the main factor of the magnesium fractionation at cold seeps, which is controlled by the kinetic effect. The range of magnesium isotopes of seep carbonates is narrow (from -3.46‰ to -2.36‰), and an upper limit of magnesium content seems to exist. This characteristic may be a good indicator for identifying dolomitization related to anaerobic oxidation of methane. Whereas, mechanisms of magnesium isotope fractionation and dolomitization at cold seeps remain unclear, necessitating more natural samples tested, stimulated calculation and laboratory experiment.

Experimental Study on the Impact of Sorption-Desorption on Nitrate Isotopes

Nitrate pollution is a global environmental problem, and mean nitrate levels have risen by an estimated 36% in global waterways since 1990. Tracing nitrate sources is important for water quality management, and nitrate isotopes (δ15N-NO3− and δ18O-NO3−) are commonly used for this purpose because of the different isotopic compositions of different sources. However, the impact of nitrate sorption on matrix and desorption from matrix on N and O isotopic composition of nitrate in liquid phase has not been well clarified. To explore the mechanism for the changes in nitrate concentration and isotopes in liquid phase during sorption and desorption, this study took a shale sample (enriched in clay minerals and commonly exposed in the Earth), conducted a series of laboratory experiments for nitrate sorption and desorption, and studied the impact of sorption and desorption on nitrate N and O isotopic composition in liquid phase. The results showed that the shale sample exhibited a rapid sorption and desorption rate for nitrate in the surface water samples, with the nitrate concentration in the solution decreasing from 14.3 mg/L to 4.1 mg/L within 5 min. The sorption data fit the Langmuir model better than that of the Freundlich model. The maximum possible sorption (Qmax) for the shale sample was estimated to be 46 μg/g. Preliminary laboratory experiments showed that changes in δ15N-NO3− values were not obvious, and changes in δ18O-NO3− values in liquid phase were minor during sorption and desorption of the shale sample, suggesting that nitrogen isotopic fractionation can be neglected, and the sorption of nitrate by the shale sample has a very limited impact on the distribution of nitrate isotopes in liquid phase. However, the impact of nitrate desorption on the nitrate isotopes in liquid phase depends on the isotopic composition of exchangeable nitrate in the solid phase, which may be related to antecedent water–rock interactions. This study provides important information for elucidating the evolution mechanism of nitrate and its isotopic compositions following sorption-desorption, and is conducive to revealing the nitrogen cycle law in the environment.

Isotopic Fractionation during Sublimation of Low Porosity Ice

The magnitude of isotopic fractionation during sublimation of ice remains poorly constrained. Field and laboratory studies that span decades have persistently shown conflicting results. A better understanding of fractionation during sublimation is needed to improve interpretation of alpine hydrology, glaciology, paleoclimate and planetary histories that rely on stable isotopic records stored in icy reservoirs. At the core of the problem is the question of whether sublimation occurs as a layer-by-layer process with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. We present results from an experiment where we suspended ice spheres in temperatures ranging from −25 to -10 °C in an unsaturated atmosphere and used a Rayleigh distillation model to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, isotope fractionation (103lnα18O of ~ −0.6 ‰ and 103lnα2H between −3 and − 4 ‰ or α = 0.999, α = 0.994 to 0.997, respectively) was found during the sublimation of ice. The results definitively indicate the presence of fractionation yet the values are an order of magnitude smaller than would be predicted for equilibrium fractionation at this temperature and humidity. By assuming a porosity for the ice of 0.0005 % - which is typical for ice frozen under these conditions - we estimate a sufficient increase in diffusivity relative to solid ice to explain the observed fractionation values. The results help to reconcile how fractionation estimates during sublimation could vary between experimental and observational studies where porosity is not controlled for and can vary substantially across a continuum from porous firn layers to low porosity ice deep in glaciers.

Quantifying mass-dependent isotope fractionation and nuclear field shift effects for the light rare Earth elements in hydrous systems

The improving sensitivity of mass-spectrometers has opened the potential of using stable isotope signatures of the rare Earth elements (REE) as geochemical tracers. However, thus far only limited studies have utilised REE stable isotopes, despite resolvable variations having been observed in a range of systems. An interesting but poorly explored area remains variations in aqueous environments across a range of temperatures including seawater, marine sediments, ion adsorption deposits or hydrothermal systems. Furthermore, the magnitudes and competing effects of mass-dependent isotope fractionation and mass-independent nuclear field shift effects, which can be significant factor for heavy elements especially at high temperatures, remain poorly understood.To contribute to a holistic understanding of REE isotope signatures, we calculated reduced partition function ratios (as 103lnβ) for mass-dependent and nuclear field shift effects for aqueous La, Ce, and Nd complexes from first principles. Theoretical calculations were compared with experimental data, and this demonstrates that calculations involving a combination of two explicit hydration shells and additional implicit solvation effects are required to accurately describe the coordination and molecular vibrations of aqueous complexes. This data was then combined with speciation modelling of seawater and hydrothermal fluids to assess isotope fractionation in hydrous systems. The predicted magnitude of isotope fractionation is significant in low (T = 25 °C) temperature systems (Δ139/138LaMax-Min = 0.20 ‰, Δ142/140CeMax-Min = 0.33 ‰, Δ146/144NdMax-Min = 0.34 ‰) and remains above currently achievable analytical uncertainties even at high (T = 500 °C) temperatures (±0.015 ‰ for δ146/144Nd; ±0.040 ‰ for δ142/140Ce). Unless oxidation occurs, which is only relevant for Ce in terrestrial environments, nuclear field shift effects are negligible even at temperatures up to 500 °C. The large difference in nuclear charge radius between 140Ce and 142Ce strongly enriches the lighter Ce isotope in the higher oxidation state (103lnβCe(IV)-Ce(III) = −0.45 at 25 °C) which almost completely cancels out the opposing mass-dependent effect (103lnβCe(IV)-Ce(III) = +0.46 at 25 °C). This means that variations in δ142/140Ce isotope signatures cannot easily be related to oxidation in ancient or recent environments.

Molybdenum isotope behavior during subduction zone metamorphism

The molybdenum (Mo) isotope composition (defined as δ98Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions.Mo isotope compositions of the metasedimentary rocks decrease with increasing metamorphic grade, from a δ98Momean of −0.04 ± 0.13 ‰ (1σ, n = 3) for the lawsonite albite facies to −0.38 ± 0.07 ‰ (1σ, n = 5) for the epidote–amphibolite facies. The highest grade [amphibolite facies] samples show a slight uptick in δ98Mo values, with a mean of −0.19± 0.14 ‰ (1σ, n = 4). When coupled with major and trace element variations, the changes in δ98Mo appear to reflect metamorphic effects rather than sedimentary source rock heterogeneity. The positive relationship between δ98Mo and [Mo] and negative relationship with Ce/Mo argue for progressive loss of Mo with isotope fractionation during increasing P-T conditions; the reversal of this trend, seen in the increase in δ98Mo between the epidote amphibolite and amphibolite facies may reflect a subsequent partial re-fertilization by fluids carrying isotopically heavy Mo. δ98Mo values in the mélange across all grades are highly variable, ranging from −1.75 to −0.19, consistent with large scale mobilization of Mo with isotope fractionation. A metasomatized amphibolite block and reaction rind show similar light isotope compositions and signs of Mo depletion and transport. Together these three whole-rock data sets demonstrate pervasive open system behavior and mobility of Mo in the Catalina Schist. LA-ICP-MS Mo measurement of minerals in the differing metasedimentary metamorphic grades indicates that Fe oxides and/or hydroxides, titanite, rutile, and epidote are important reservoirs of Mo but when comparing mineral Mo content and modal abundance to whole-rock Mo concentrations, several samples were found to have “missing Mo”, something that has been observed in other Mo inventory studies. We attribute this to sample heterogeneity between the thin section and whole-rock powder scales.