Pub Date : 2024-12-13DOI: 10.1016/j.gca.2024.12.009
Melissa S. Schwab, Negar Haghipour, Timothy I. Eglinton
Source-specific lipid biomarkers, such as n-alkanes and n-fatty acids, serve as valuable tracers of past and contemporary exchanges of vegetation-derived organic matter across different carbon pools. While substantial research has explored the incorporation and transformation of vascular plant debris within soils, our understanding of the transfer of biospheric carbon from terrestrial to aquatic systems and along the fluvial continuum remains incomplete. To address this knowledge gap, we conducted a 30-month high-resolution time series study in the subalpine Sihl River watershed, monitoring the abundance and composition of higher plant-derived lipids in the suspended load to examine the response of long-chain n-alkanes and n-fatty acids to hydrological variability, seasonal changes, and climatic disturbances.
{"title":"Hydrodynamic, Seasonal, and Climatic Factors Affecting the Terrestrial-to-Aquatic Export of Plant-Derived Biomarkers","authors":"Melissa S. Schwab, Negar Haghipour, Timothy I. Eglinton","doi":"10.1016/j.gca.2024.12.009","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.009","url":null,"abstract":"Source-specific lipid biomarkers, such as <ce:italic>n</ce:italic>-alkanes and <ce:italic>n</ce:italic>-fatty acids, serve as valuable tracers of past and contemporary exchanges of vegetation-derived organic matter across different carbon pools. While substantial research has explored the incorporation and transformation of vascular plant debris within soils, our understanding of the transfer of biospheric carbon from terrestrial to aquatic systems and along the fluvial continuum remains incomplete. To address this knowledge gap, we conducted a 30-month high-resolution time series study in the subalpine Sihl River watershed, monitoring the abundance and composition of higher plant-derived lipids in the suspended load to examine the response of long-chain <ce:italic>n</ce:italic>-alkanes and <ce:italic>n</ce:italic>-fatty acids to hydrological variability, seasonal changes, and climatic disturbances.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"20 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936033","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-10DOI: 10.1016/j.gca.2024.12.006
Kevin Padilla, Alexander P. Gysi
The mobility of rare earth elements (REE) in natural hydrothermal systems can be assessed using geochemical modeling, which requires reliable thermodynamic data of relevant aqueous species. In this study, we evaluate the controls of pH and temperature on La speciation and the role of hydroxyl complexes in REE transport at hydrothermal conditions. Batch-type hydrothermal solubility experiments were conducted using synthetic La hydroxide powders equilibrated in perchloric acid-based aqueous solutions at temperatures between 150 and 250 °C and starting pH of 2 to 5. The La hydroxide solubility is retrograde with temperature and displays a strong pH dependence with a decrease in La concentrations from acidic to mildly acidic pH spanning between 3 and 5 orders of magnitude (e.g. log La molality of −2.5 to −7.2 at 250 °C). Thermodynamic optimizations using GEMSFITS allow to retrieve the standard partial molal Gibbs energies for the La3+ aqua ion and the formation constants for the La hydroxyl species (i.e., LaOH2+, La(OH)2+, La(OH)30) between 25 and 250 °C. A comparison between the experimentally derived thermodynamic properties with the calculated values from the Helgeson-Kirkham-Flowers equation of state parameters indicates an increased divergence with temperature. Discrepancies in standard partial molal Gibbs energies range between ∼ 1 − 12 kJ/mol and result in a predicted La hydroxide solubility differing by up to 3 orders of magnitude at 250 °C. Speciation calculations indicate a higher stability of La3+ and LaOH2+ over the other La hydroxyl species in the studied pH range of 3.4 to 6. The optimized thermodynamic properties for La aqueous species have important implications for modeling the solubility of REE minerals such as monazite and the mobility of REE in hydrothermal systems.
{"title":"The solubility of La hydroxide and stability of La3+ and La hydroxyl complexes at acidic to mildly acidic pH from 25 to 250 °C","authors":"Kevin Padilla, Alexander P. Gysi","doi":"10.1016/j.gca.2024.12.006","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.006","url":null,"abstract":"The mobility of rare earth elements (REE) in natural hydrothermal systems can be assessed using geochemical modeling, which requires reliable thermodynamic data of relevant aqueous species. In this study, we evaluate the controls of pH and temperature on La speciation and the role of hydroxyl complexes in REE transport at hydrothermal conditions. Batch-type hydrothermal solubility experiments were conducted using synthetic La hydroxide powders equilibrated in perchloric acid-based aqueous solutions at temperatures between 150 and 250 °C and starting pH of 2 to 5. The La hydroxide solubility is retrograde with temperature and displays a strong pH dependence with a decrease in La concentrations from acidic to mildly acidic pH spanning between 3 and 5 orders of magnitude (e.g. log La molality of −2.5 to −7.2 at 250 °C). Thermodynamic optimizations using GEMSFITS allow to retrieve the standard partial molal Gibbs energies for the La<ce:sup loc=\"post\">3+</ce:sup> aqua ion and the formation constants for the La hydroxyl species (i.e., LaOH<ce:sup loc=\"post\">2+</ce:sup>, La(OH)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">+</ce:sup>, La(OH)<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">0</ce:sup>) between 25 and 250 °C. A comparison between the experimentally derived thermodynamic properties with the calculated values from the Helgeson-Kirkham-Flowers equation of state parameters indicates an increased divergence with temperature. Discrepancies in standard partial molal Gibbs energies range between ∼ 1 − 12 kJ/mol and result in a predicted La hydroxide solubility differing by up to 3 orders of magnitude at 250 °C. Speciation calculations indicate a higher stability of La<ce:sup loc=\"post\">3+</ce:sup> and LaOH<ce:sup loc=\"post\">2+</ce:sup> over the other La hydroxyl species in the studied pH range of 3.4 to 6. The optimized thermodynamic properties for La aqueous species have important implications for modeling the solubility of REE minerals such as monazite and the mobility of REE in hydrothermal systems.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"75 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841533","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.12.001
W. Abouchami, F. Wombacher, S.J.G. Galer
Early pioneering studies of Apollo lunar soils revealed a geochemical dichotomy reflecting a dominance of mare and highland lithologies, with variable additions of Procellarum KREEP Terrane material. Here, we use the moderately volatile element cadmium to identify the sources and processes responsible for mass-dependent Cd stable isotope variations in the lunar regolith. In addition, capture of thermal neutrons by 113Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.
{"title":"Cadmium isotope fractionation and neutron capture effects in lunar samples","authors":"W. Abouchami, F. Wombacher, S.J.G. Galer","doi":"10.1016/j.gca.2024.12.001","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.001","url":null,"abstract":"Early pioneering studies of Apollo lunar soils revealed a geochemical dichotomy reflecting a dominance of mare and highland lithologies, with variable additions of Procellarum KREEP Terrane material. Here, we use the moderately volatile element cadmium to identify the sources and processes responsible for mass-dependent Cd stable isotope variations in the lunar regolith. In addition, capture of thermal neutrons by <ce:sup loc=\"post\">113</ce:sup>Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"63 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841588","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.12.002
S. Nemiah Ladd, Daniel B. Nelson, Blake Matthews, Shannon Dyer, Romana Limberger, Antonia Klatt, Anita Narwani, Nathalie Dubois, Carsten J. Schubert
Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ2H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ2H values of their lipids. Despite identical source water δ2H values, lipid δ2H values ranged from −455 ‰ to −52 ‰, incorporating variability associated with chemical compound classes and taxonomic groups. In particular, green algae synthesized fatty acids with higher δ2H values than other taxa, cyanobacteria synthesized phytol with relatively low δ2H values, and diatoms synthesized sterols with higher δ2H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ2H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ2H values for generic fatty acids, relatively stable δ2H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ2H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, with both datasets indicating large taxon-specific changes that are unlikely to be driven by bacterial heterotrophy. This suggests that measuring δ2H values of multiple lipids from sediment and calculating 2H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.
{"title":"Taxon-specific hydrogen isotope signals in cultures and mesocosms facilitate ecosystem and hydroclimate reconstruction","authors":"S. Nemiah Ladd, Daniel B. Nelson, Blake Matthews, Shannon Dyer, Romana Limberger, Antonia Klatt, Anita Narwani, Nathalie Dubois, Carsten J. Schubert","doi":"10.1016/j.gca.2024.12.002","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.002","url":null,"abstract":"Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ<ce:sup loc=\"post\">2</ce:sup>H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ<ce:sup loc=\"post\">2</ce:sup>H values of their lipids. Despite identical source water δ<ce:sup loc=\"post\">2</ce:sup>H values, lipid δ<ce:sup loc=\"post\">2</ce:sup>H values ranged from −455 ‰ to −52 ‰, incorporating variability associated with chemical compound classes and taxonomic groups. In particular, green algae synthesized fatty acids with higher δ<ce:sup loc=\"post\">2</ce:sup>H values than other taxa, cyanobacteria synthesized phytol with relatively low δ<ce:sup loc=\"post\">2</ce:sup>H values, and diatoms synthesized sterols with higher δ<ce:sup loc=\"post\">2</ce:sup>H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ<ce:sup loc=\"post\">2</ce:sup>H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ<ce:sup loc=\"post\">2</ce:sup>H values for generic fatty acids, relatively stable δ<ce:sup loc=\"post\">2</ce:sup>H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ<ce:sup loc=\"post\">2</ce:sup>H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, with both datasets indicating large taxon-specific changes that are unlikely to be driven by bacterial heterotrophy. This suggests that measuring δ<ce:sup loc=\"post\">2</ce:sup>H values of multiple lipids from sediment and calculating <ce:sup loc=\"post\">2</ce:sup>H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"13 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884631","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.11.033
Yujian Wang, Jingao Liu
Secular variations in the composition of the subcontinental lithospheric mantle (SCLM) are intimately controlled by multiple geological processes, including diverse melting mechanisms and complex metasomatic processes. We present comprehensive analyses of whole rock and mineral chemistry, along with Re-Os isotopic system of the Eastern Pyrenean orogenic mantle massifs, supported by quantitative modeling through alphaMELTS thermodynamic software. The Eastern Pyrenean lherzolites display consistent negative correlations of TiO2/Al2O3, TiO2 versus MgO and relatively high and unfractionated heavy rare earth elements. These chemical features shed light on the diverse melting mechanisms responsible for these rocks, encompassing passive continuous melting in regions of lithospheric extension and decompression melting linked to the upwelling asthenospheric mantle. Some refractory harzburgites exhibit elevated TiO2/Al2O3, TiO2 contents and moderate light rare earth element enrichment. This suggests chromatographic metasomatism due to the reactive porous flow of evolved melts/fluids in the upper part of the lithosphere during their ascent to the surface. An isochron analogy between 187Os/188Os and Al2O3 yields an age of ∼ 1.5 Ga, constraining the timing of partial melting responsible for the formation of SCLM beneath the Eastern Pyrenean region. We then expand the scope of our study to encompass on– and off-cratonic SCLM on a global scale. This expanded analysis explores the variations in melting mechanisms across different tectonic settings and geological epochs and scrutinizes the role of diverse metasomatic processes in shaping the characteristics of the lithospheric mantle and its longevity on a broad scale. Silicate metasomatism typically produces fertile peridotites at the asthenosphere-lithosphere boundary, making them vulnerable to thermomechanical erosion, whereas cryptic metasomatism, commonly observed in refractory mantle rocks induced by evolved melts at decreasing melt-rock ratios (e.g., hydrocarbon-bearing silicate melt, carbonatite melt etc.), generally forms enrichment of highly incompatible elements but less significant influence on the mineral assemblages and major element geochemistry of the on-cratonic refractory SCLM, making them remain stable for extended periods.
{"title":"Deciphering multiple episodes of partial melting, metasomatic and remelting processes in the Eastern Pyrenean orogenic mantle massif","authors":"Yujian Wang, Jingao Liu","doi":"10.1016/j.gca.2024.11.033","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.033","url":null,"abstract":"Secular variations in the composition of the subcontinental lithospheric mantle (SCLM) are intimately controlled by multiple geological processes, including diverse melting mechanisms and complex metasomatic processes. We present comprehensive analyses of whole rock and mineral chemistry, along with Re-Os isotopic system of the Eastern Pyrenean orogenic mantle massifs, supported by quantitative modeling through alphaMELTS thermodynamic software. The Eastern Pyrenean lherzolites display consistent negative correlations of TiO<ce:inf loc=\"post\">2</ce:inf>/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf>, TiO<ce:inf loc=\"post\">2</ce:inf> versus MgO and relatively high and unfractionated heavy rare earth elements. These chemical features shed light on the diverse melting mechanisms responsible for these rocks, encompassing passive continuous melting in regions of lithospheric extension and decompression melting linked to the upwelling asthenospheric mantle. Some refractory harzburgites exhibit elevated TiO<ce:inf loc=\"post\">2</ce:inf>/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf>, TiO<ce:inf loc=\"post\">2</ce:inf> contents and moderate light rare earth element enrichment. This suggests chromatographic metasomatism due to the reactive porous flow of evolved melts/fluids in the upper part of the lithosphere during their ascent to the surface. An isochron analogy between <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os and Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf> yields an age of ∼ 1.5 Ga, constraining the timing of partial melting responsible for the formation of SCLM beneath the Eastern Pyrenean region. We then expand the scope of our study to encompass on– and off-cratonic SCLM on a global scale. This expanded analysis explores the variations in melting mechanisms across different tectonic settings and geological epochs and scrutinizes the role of diverse metasomatic processes in shaping the characteristics of the lithospheric mantle and its longevity on a broad scale. Silicate metasomatism typically produces fertile peridotites at the asthenosphere-lithosphere boundary, making them vulnerable to thermomechanical erosion, whereas cryptic metasomatism, commonly observed in refractory mantle rocks induced by evolved melts at decreasing melt-rock ratios (e.g., hydrocarbon-bearing silicate melt, carbonatite melt etc.), generally forms enrichment of highly incompatible elements but less significant influence on the mineral assemblages and major element geochemistry of the on-cratonic refractory SCLM, making them remain stable for extended periods.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"201 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841587","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.12.005
Yiren Duan, Hongtao He, Wenchao Liu, Wenxian Gou, Zhao Wang, Peng Liu, Jing Zhang, Caroline L. Peacock, Wei Li
Calcite plays a pivotal role in regulating the mobility and fate of zinc (Zn) in natural environments. Despite its significance, the mechanism of Zn sorption on calcite surfaces, particularly the transitional dynamics from surface adsorption to precipitation, remains unclear. This research studied the sorption behavior of Zn on calcite across a wide range of reaction times, pH values, and Zn concentrations. The underlying sorption mechanisms were examined through a combination of Zn stable isotope measurements and X-ray absorption fine structure (XAFS) spectroscopy. At pH 6.5 and a low Zn concentration of 5 μM, the surface coverage reached 0.9 μmol/m2, accompanied by a pronounced Δ66Znsorbed-aqueous of + 0.40 ‰, which is indicative of a tetrahedral inner-sphere surface complexation mechanism. Conversely, at pH ≥ 7.5 and a higher Zn concentration (100 μM), the surface coverage surpassed 57.6 μmol/m2, resulting in diminished Zn isotope fractionation (+0.20 ‰), suggesting the formation of hydrozincite precipitates. These results, integrated with the XAFS analysis, revealed a continuous transition from inner-sphere tetrahedral surface complexes to hydrozincite precipitates as the pH and/or Zn concentration increased. Notably, the sensitivity of Zn isotope fractionation to distinct Zn sorption mechanisms was supported by an inverse linear relationship between Zn isotope fractionation and the Zn-O bond distance. This study significantly advances our understanding of Zn sorption mechanisms on calcite by demonstrating that the surface of calcite may have catalyzed hydrozincite precipitation when the bulk solution was undersaturated with respect to hydrozincite. The synergistic application of Zn stable isotopes and XAFS spectroscopy provides a robust framework for probing metal-mineral interactions under environmentally relevant conditions.
{"title":"Coupling stable isotope analyses and X-ray absorption spectroscopy to investigate the molecular mechanism of zinc sorption by calcite","authors":"Yiren Duan, Hongtao He, Wenchao Liu, Wenxian Gou, Zhao Wang, Peng Liu, Jing Zhang, Caroline L. Peacock, Wei Li","doi":"10.1016/j.gca.2024.12.005","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.005","url":null,"abstract":"Calcite plays a pivotal role in regulating the mobility and fate of zinc (Zn) in natural environments. Despite its significance, the mechanism of Zn sorption on calcite surfaces, particularly the transitional dynamics from surface adsorption to precipitation, remains unclear. This research studied the sorption behavior of Zn on calcite across a wide range of reaction times, pH values, and Zn concentrations. The underlying sorption mechanisms were examined through a combination of Zn stable isotope measurements and X-ray absorption fine structure (XAFS) spectroscopy. At pH 6.5 and a low Zn concentration of 5 μM, the surface coverage reached 0.9 μmol/m<ce:sup loc=\"post\">2</ce:sup>, accompanied by a pronounced Δ<ce:sup loc=\"post\">66</ce:sup>Zn<ce:inf loc=\"post\">sorbed-aqueous</ce:inf> of + 0.40 ‰, which is indicative of a tetrahedral inner-sphere surface complexation mechanism. Conversely, at pH ≥ 7.5 and a higher Zn concentration (100 μM), the surface coverage surpassed 57.6 μmol/m<ce:sup loc=\"post\">2</ce:sup>, resulting in diminished Zn isotope fractionation (+0.20 ‰), suggesting the formation of hydrozincite precipitates. These results, integrated with the XAFS analysis, revealed a continuous transition from inner-sphere tetrahedral surface complexes to hydrozincite precipitates as the pH and/or Zn concentration increased. Notably, the sensitivity of Zn isotope fractionation to distinct Zn sorption mechanisms was supported by an inverse linear relationship between Zn isotope fractionation and the Zn-O bond distance. This study significantly advances our understanding of Zn sorption mechanisms on calcite by demonstrating that the surface of calcite may have catalyzed hydrozincite precipitation when the bulk solution was undersaturated with respect to hydrozincite. The synergistic application of Zn stable isotopes and XAFS spectroscopy provides a robust framework for probing metal-mineral interactions under environmentally relevant conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841534","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.12.003
Jörg Tittel, Volker Lüderitz, Sabine Radke, Yvonne Rosenlöcher, Oliver J. Lechtenfeld
Organic carbon (OC) in rivers is one of the most rapidly recycled carbon pools. A significant proportion of OC is remineralized and contributes to the globally relevant CO2 emissions of river networks. However, there is no consensus on the mechanisms that determine which compounds are remineralized. Previous studies found that OC older than the mean age of the source was decomposed by aquatic microorganisms. Here, we study the radiocarbon (Δ14C) age of dissolved OC (DOC) that is decomposed in laboratory experiments across a range of stream bulk DOC ages. Stream DOC was collected from small forested catchments under summer dry flow, average flow and storm flow conditions. The decomposed DOC was neither consistently older nor younger than the original stream DOC. The Δ14C of respiratory CO2 increased with the Δ14C of stream DOC (P = 0.006, N = 16). However, the slope of the regression was small (0.20 ± 0.06) and the dependence was weak (R2 = 0.43). Moreover, the age range of respired DOC (modern to 950 years BP) was considerably narrower than the age range of stream DOC (modern to 3880 years BP). In further experiments, we used leachates of catchment soil from 0-8 cm and 8–20 cm depth and a 1:1 mixture of the two depths as initial DOC. Again, the increase in Δ14C-CO2 as a function of Δ14C-DOC was significant (R2 = 0.74, P = 0.028, N = 6), but the slope was small (0.13 ± 0.04) and the age range of respired DOC was narrow (modern to 280 years BP) compared to initial leachate DOC (600 to 3400 years BP). Fourier transform ion cyclotron resonance mass spectrometry showed that the dissolved organic matter (DOM) starting material from the deeper soil layer was characterized by smaller, more reduced (i.e. aliphatic) molecules as compared to the DOM from the shallower soil layer. However, similar (small, unsaturated, oxygen-rich) CHO molecules were consumed regardless of DOM source. The narrow age ranges of respired DOC suggest that intrinsic chemical quality sets the limits for which compounds can be utilized under given geochemical conditions. However, strategies of microorganisms to optimize growth (optimal foraging) may modulate their specific substrate choice, as indicated by the dependence of the age of respired OC on the age composition of the original DOC.
{"title":"Invariable selection of compounds from organic matter by stream microbes","authors":"Jörg Tittel, Volker Lüderitz, Sabine Radke, Yvonne Rosenlöcher, Oliver J. Lechtenfeld","doi":"10.1016/j.gca.2024.12.003","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.003","url":null,"abstract":"Organic carbon (OC) in rivers is one of the most rapidly recycled carbon pools. A significant proportion of OC is remineralized and contributes to the globally relevant CO<ce:inf loc=\"post\">2</ce:inf> emissions of river networks. However, there is no consensus on the mechanisms that determine which compounds are remineralized. Previous studies found that OC older than the mean age of the source was decomposed by aquatic microorganisms. Here, we study the radiocarbon (Δ<ce:sup loc=\"post\">14</ce:sup>C) age of dissolved OC (DOC) that is decomposed in laboratory experiments across a range of stream bulk DOC ages. Stream DOC was collected from small forested catchments under summer dry flow, average flow and storm flow conditions. The decomposed DOC was neither consistently older nor younger than the original stream DOC. The Δ<ce:sup loc=\"post\">14</ce:sup>C of respiratory CO<ce:inf loc=\"post\">2</ce:inf> increased with the Δ<ce:sup loc=\"post\">14</ce:sup>C of stream DOC (<ce:italic>P</ce:italic> = 0.006, <ce:italic>N</ce:italic> = 16). However, the slope of the regression was small (0.20 ± 0.06) and the dependence was weak (<ce:italic>R</ce:italic><ce:sup loc=\"post\">2</ce:sup> = 0.43). Moreover, the age range of respired DOC (modern to 950 years BP) was considerably narrower than the age range of stream DOC (modern to 3880 years BP). In further experiments, we used leachates of catchment soil from 0-8 cm and 8–20 cm depth and a 1:1 mixture of the two depths as initial DOC. Again, the increase in Δ<ce:sup loc=\"post\">14</ce:sup>C-CO<ce:inf loc=\"post\">2</ce:inf> as a function of Δ<ce:sup loc=\"post\">14</ce:sup>C-DOC was significant (<ce:italic>R</ce:italic><ce:sup loc=\"post\">2</ce:sup> = 0.74, <ce:italic>P</ce:italic> = 0.028, <ce:italic>N</ce:italic> = 6), but the slope was small (0.13 ± 0.04) and the age range of respired DOC was narrow (modern to 280 years BP) compared to initial leachate DOC (600 to 3400 years BP). Fourier transform ion cyclotron resonance mass spectrometry showed that the dissolved organic matter (DOM) starting material from the deeper soil layer was characterized by smaller, more reduced (i.e. aliphatic) molecules as compared to the DOM from the shallower soil layer. However, similar (small, unsaturated, oxygen-rich) CHO molecules were consumed regardless of DOM source. The narrow age ranges of respired DOC suggest that intrinsic chemical quality sets the limits for which compounds can be utilized under given geochemical conditions. However, strategies of microorganisms to optimize growth (optimal foraging) may modulate their specific substrate choice, as indicated by the dependence of the age of respired OC on the age composition of the original DOC.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"123 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884589","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-07DOI: 10.1016/j.gca.2024.12.004
Xiaoyu Zhou, Ratul Banerjee, Laurie Reisberg, Sisir K. Mondal
Highly siderophile element (HSE) contents of komatiites have been widely used to estimate the HSE composition of Earth’s mantle. However, the interpretation of existing komatiite data is controversial, with some authors arguing that the Archean deep mantle komatiite source was impoverished in HSE due to slow admixture of a late accretion component, while others invoke a melting process that would allow observed komatiite abundances to be obtained from a mantle source with present-day HSE abundances. To obtain insight into this issue, we present new HSE abundance data for komatiites from the Gorumahishani greenstone belt of the Singhbhum Craton, eastern India. Our Sm-Nd and Re-Os isotope data indicate a ∼3.5 Ga age for these little-studied rocks, which provide extreme examples of Al-depleted and Ti-depleted komatiite varieties, juxtaposed over a short-length scale. The calculated parental melt compositions for the Al-depleted komatiites have 2.7 ± 0.2 ppb Ru, 3.4 ± 0.2 ppb Pt, and 3.2 ± 0.6 ppb Pd, whereas, for the Ti-depleted type these values are 4.4 ± 0.3 ppb Ru, 3.2 ± 0.6 ppb Pt, 3.0 ± 0.5 ppb Pd. These concentrations are similar to those found in most Archean komatiites at >3.4 Ga. For the Al-depleted samples, these values would correspond to mantle abundances equivalent to ∼38 % of modern Bulk Silicate Earth (BSE) Ru contents and ∼24 and ∼21 % of BSE Pt and Pd contents, respectively, if it is assumed that simple extrapolation of the measured values to the MgO content of fertile peridotite provides an adequate approximation of the HSE composition of the BSE. To examine the alternative model that the low contents of Ru, Pd and Pt in Gorumahishani komatiites could be obtained from a mantle source with BSE-like HSE contents, we apply a simple two-stage critical melting model using current experimental HSE partitioning coefficients. The Ru abundances of the Gorumahishani Al-depleted komatiitic magmas can be produced from the pooled melts of a fertile source with BSE-like Ru and S contents during the first melting stage. The Ru abundances of the Ti-depleted komatiitic magmas can then be produced from remelting the residue left by this first melting stage. On the other hand, Pt and Pd abundances cannot be successfully modelled for either the Al-depleted or the Ti-depleted komatiites using available partition coefficients, though our current understanding of Pt and Pd partitioning after sulfide exhaustion is limited. The use of komatiites to characterize the abundance and distribution of HSE in the early mantle critically depends on developing a better understanding of the partitioning behaviors of these elements between mantle sources and komatiitic magmas.
{"title":"Source composition or melting effect: New evidence from Archean komatiites concerning the origin of low highly siderophile element abundances in Earth’s mantle","authors":"Xiaoyu Zhou, Ratul Banerjee, Laurie Reisberg, Sisir K. Mondal","doi":"10.1016/j.gca.2024.12.004","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.004","url":null,"abstract":"Highly siderophile element (HSE) contents of komatiites have been widely used to estimate the HSE composition of Earth’s mantle. However, the interpretation of existing komatiite data is controversial, with some authors arguing that the Archean deep mantle komatiite source was impoverished in HSE due to slow admixture of a late accretion component, while others invoke a melting process that would allow observed komatiite abundances to be obtained from a mantle source with present-day HSE abundances. To obtain insight into this issue, we present new HSE abundance data for komatiites from the Gorumahishani greenstone belt of the Singhbhum Craton, eastern India. Our Sm-Nd and Re-Os isotope data indicate a ∼3.5 Ga age for these little-studied rocks, which provide extreme examples of Al-depleted and Ti-depleted komatiite varieties, juxtaposed over a short-length scale. The calculated parental melt compositions for the Al-depleted komatiites have 2.7 ± 0.2 ppb Ru, 3.4 ± 0.2 ppb Pt, and 3.2 ± 0.6 ppb Pd, whereas, for the Ti-depleted type these values are 4.4 ± 0.3 ppb Ru, 3.2 ± 0.6 ppb Pt, 3.0 ± 0.5 ppb Pd. These concentrations are similar to those found in most Archean komatiites at >3.4 Ga. For the Al-depleted samples, these values would correspond to mantle abundances equivalent to ∼38 % of modern Bulk Silicate Earth (BSE) Ru contents and ∼24 and ∼21 % of BSE Pt and Pd contents, respectively, if it is assumed that simple extrapolation of the measured values to the MgO content of fertile peridotite provides an adequate approximation of the HSE composition of the BSE. To examine the alternative model that the low contents of Ru, Pd and Pt in Gorumahishani komatiites could be obtained from a mantle source with BSE-like HSE contents, we apply a simple two-stage critical melting model using current experimental HSE partitioning coefficients. The Ru abundances of the Gorumahishani Al-depleted komatiitic magmas can be produced from the pooled melts of a fertile source with BSE-like Ru and S contents during the first melting stage. The Ru abundances of the Ti-depleted komatiitic magmas can then be produced from remelting the residue left by this first melting stage. On the other hand, Pt and Pd abundances cannot be successfully modelled for either the Al-depleted or the Ti-depleted komatiites using available partition coefficients, though our current understanding of Pt and Pd partitioning after sulfide exhaustion is limited. The use of komatiites to characterize the abundance and distribution of HSE in the early mantle critically depends on developing a better understanding of the partitioning behaviors of these elements between mantle sources and komatiitic magmas.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"7 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841535","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-05DOI: 10.1016/j.gca.2024.11.032
Lei Lu, Jing Sun, Yu Dai, Yaoyu Zhou, Haojie Cui, Ming Lei, Huihui Du
As an emerging contaminant, tungsten (W) displays unexpectedly high mobility in soil despite its extremely low solubility, challenging current scientific understanding. This paradox underscores the limited knowledge regarding the specific W species responsible for its high mobility. In this study, a series of field and incubation experiments were conducted across multiple soil types to investigate the distribution and speciation of W in soil porewater, widely known as ″the most mobile fraction″. Ultrafiltration analysis revealed that W in soil porewater predominantly existed in colloidal-size (5 kDa–0.45 μm) phases rather than the ″truly-dissolved″ phase (<5 kDa). Especially in deeper soil layers approaching shallow groundwater, colloidal W content exceeded 93 %. XANES spectra showed that colloidal W was mainly in the hexavalent state (WVI), and insoluble W metal (W0) entering the soil could rapidly oxidize into WVI through biotic or abiotic processes. Advanced characterizations, including STEM-EDS-SAED, SEM-EDS, and SR-XRF, identified aluminosilicate mineral colloids as the primary carrier for W in soil porewater. Within these W-bearing aluminosilicate mineral colloids, W was primarily present as Al2(WO4)3 precipitates with a W–Al distance of ∼ 3.64 Å, as confirmed by EXAFS. Additionally, a minor fraction of silicotungstates was also detected in the colloidal fraction using XAS and STEM-EDS-SAED. These two species were further substantiated through geochemical modeling and density functional theory (DFT) analysis. Importantly, this study hypothesizes that the associations of W with aluminosilicate mineral colloids and silicotungstates are widespread across different soil types. The finding suggests that colloid-associated W mobility is a dominant yet previously overlooked process, helping to explain why W, despite its low solubility, exhibits exceptionally high mobility in soils.
作为一种新出现的污染物，钨（W）尽管溶解度极低，但在土壤中却表现出出人意料的高流动性，这对当前的科学认识提出了挑战。这一悖论凸显了人们对造成其高迁移率的特定钨物种的了解有限。在这项研究中，我们在多种土壤类型中进行了一系列现场和培养实验，以研究 W 在土壤孔隙水中的分布和种类，孔隙水被广泛称为 "流动性最强的部分"。超滤分析表明，土壤孔隙水中的 W 主要存在于胶体尺寸（5 kDa-0.45 μm）相中，而不是″真正溶解″相（<5 kDa）中。特别是在接近浅层地下水的较深土层中，胶体 W 的含量超过 93%。XANES 光谱显示，胶体 W 主要处于六价状态（WVI），进入土壤的不溶性金属 W（W0）可通过生物或非生物过程迅速氧化成 WVI。通过 STEM-EDS-SAED、SEM-EDS 和 SR-XRF 等先进的表征方法，确定了铝硅酸盐矿物胶体是土壤孔隙水中 W 的主要载体。经 EXAFS 证实，在这些含 W 的铝硅酸盐矿物胶体中，W 主要以 Al2(WO4)3 沉淀形式存在，W-Al 间距为 3.64 Å。此外，利用 XAS 和 STEM-EDS-SAED 方法还在胶体部分检测到了一小部分硅钨酸盐。地球化学模型和密度泛函理论（DFT）分析进一步证实了这两种物质。重要的是，本研究假设 W 与铝硅酸盐矿物胶体和硅钨酸盐的关联广泛存在于不同的土壤类型中。这一发现表明，与胶体相关的钨迁移率是一个主要的过程，但却被人们所忽视，这有助于解释为什么钨的溶解度很低，但在土壤中却表现出极高的迁移率。
{"title":"Association of tungsten with aluminosilicate mineral colloids and silicotungstates in soil porewaters: Insights into the unexpectedly high tungsten mobility in soil","authors":"Lei Lu, Jing Sun, Yu Dai, Yaoyu Zhou, Haojie Cui, Ming Lei, Huihui Du","doi":"10.1016/j.gca.2024.11.032","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.032","url":null,"abstract":"As an emerging contaminant, tungsten (W) displays unexpectedly high mobility in soil despite its extremely low solubility, challenging current scientific understanding. This paradox underscores the limited knowledge regarding the specific W species responsible for its high mobility. In this study, a series of field and incubation experiments were conducted across multiple soil types to investigate the distribution and speciation of W in soil porewater, widely known as ″the most mobile fraction″. Ultrafiltration analysis revealed that W in soil porewater predominantly existed in colloidal-size (5 kDa–0.45 μm) phases rather than the ″truly-dissolved″ phase (<5 kDa). Especially in deeper soil layers approaching shallow groundwater, colloidal W content exceeded 93 %. XANES spectra showed that colloidal W was mainly in the hexavalent state (W<ce:sup loc=\"post\">VI</ce:sup>), and insoluble W metal (W<ce:sup loc=\"post\">0</ce:sup>) entering the soil could rapidly oxidize into W<ce:sup loc=\"post\">VI</ce:sup> through biotic or abiotic processes. Advanced characterizations, including STEM-EDS-SAED, SEM-EDS, and SR-XRF, identified aluminosilicate mineral colloids as the primary carrier for W in soil porewater. Within these W-bearing aluminosilicate mineral colloids, W was primarily present as Al<ce:inf loc=\"post\">2</ce:inf>(WO<ce:inf loc=\"post\">4</ce:inf>)<ce:inf loc=\"post\">3</ce:inf> precipitates with a W–Al distance of ∼ 3.64 Å, as confirmed by EXAFS. Additionally, a minor fraction of silicotungstates was also detected in the colloidal fraction using XAS and STEM-EDS-SAED. These two species were further substantiated through geochemical modeling and density functional theory (DFT) analysis. Importantly, this study hypothesizes that the associations of W with aluminosilicate mineral colloids and silicotungstates are widespread across different soil types. The finding suggests that colloid-associated W mobility is a dominant yet previously overlooked process, helping to explain why W, despite its low solubility, exhibits exceptionally high mobility in soils.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"39 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142804467","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-04DOI: 10.1016/j.gca.2024.11.009
Getachew Agmuas Adnew, Thomas Röckmann, Thomas Blunier, Christian Juncher Jørgensen, Sarah Elise Sapper, Carina van der Veen, Malavika Sivan, Maria Elena Popa, Jesper Riis Christiansen
Clumped isotopes of methane (CH<ce:inf loc="post">4</ce:inf>), specifically <mml:math altimg="si74.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">3</ce:inf>D and <mml:math altimg="si75.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">2</ce:inf>D<ce:inf loc="post">2</ce:inf>, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH<ce:inf loc="post">4</ce:inf> provenance based on bulk isotopes. However, interpreting the origin of CH<ce:inf loc="post">4</ce:inf> using isotopes becomes challenging when the isotopic signature is altered by post-generation processes. In this study, we measured, for the first time, the bulk and clumped isotopic composition of sub-glacial CH<ce:inf loc="post">4</ce:inf> samples. These samples were collected from the air-filled headspace of the glacier portal (ice cave) at the edge of the Isunnguata Sermia glacier (ISG), located at the western margin of the Greenland ice sheet (GrIS). Our goal was to identify the processes underlying the sub-glacial production and potential processing of CH<ce:inf loc="post">4</ce:inf>. The <mml:math altimg="si74.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">3</ce:inf>D and <mml:math altimg="si75.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">2</ce:inf>D<ce:inf loc="post">2</ce:inf> values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The <mml:math altimg="si75.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">2</ce:inf>D<ce:inf loc="post">2</ce:inf> values are close to those of atmospheric CH<ce:inf loc="post">4</ce:inf>. The elevated <mml:math altimg="si75.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">2</ce:inf>D<ce:inf loc="post">2</ce:inf> values can be attributed to the alteration of the source’s isotope signal by aerobic oxidation. This conclusion is supported by previous studies at this site, which reported the presence of methanotrophic bacteria and dissolved oxygen close to saturation in the meltwater. Our results confirm that the correlation between <mml:math altimg="si74.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc="post">3</ce:inf>D and <mml:math altimg="si75.svg" display="inline"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></
{"title":"Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet","authors":"Getachew Agmuas Adnew, Thomas Röckmann, Thomas Blunier, Christian Juncher Jørgensen, Sarah Elise Sapper, Carina van der Veen, Malavika Sivan, Maria Elena Popa, Jesper Riis Christiansen","doi":"10.1016/j.gca.2024.11.009","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.009","url":null,"abstract":"Clumped isotopes of methane (CH<ce:inf loc=\"post\">4</ce:inf>), specifically <mml:math altimg=\"si74.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">3</ce:inf>D and <mml:math altimg=\"si75.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">2</ce:inf>D<ce:inf loc=\"post\">2</ce:inf>, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH<ce:inf loc=\"post\">4</ce:inf> provenance based on bulk isotopes. However, interpreting the origin of CH<ce:inf loc=\"post\">4</ce:inf> using isotopes becomes challenging when the isotopic signature is altered by post-generation processes. In this study, we measured, for the first time, the bulk and clumped isotopic composition of sub-glacial CH<ce:inf loc=\"post\">4</ce:inf> samples. These samples were collected from the air-filled headspace of the glacier portal (ice cave) at the edge of the Isunnguata Sermia glacier (ISG), located at the western margin of the Greenland ice sheet (GrIS). Our goal was to identify the processes underlying the sub-glacial production and potential processing of CH<ce:inf loc=\"post\">4</ce:inf>. The <mml:math altimg=\"si74.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">3</ce:inf>D and <mml:math altimg=\"si75.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">2</ce:inf>D<ce:inf loc=\"post\">2</ce:inf> values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The <mml:math altimg=\"si75.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">2</ce:inf>D<ce:inf loc=\"post\">2</ce:inf> values are close to those of atmospheric CH<ce:inf loc=\"post\">4</ce:inf>. The elevated <mml:math altimg=\"si75.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">2</ce:inf>D<ce:inf loc=\"post\">2</ce:inf> values can be attributed to the alteration of the source’s isotope signal by aerobic oxidation. This conclusion is supported by previous studies at this site, which reported the presence of methanotrophic bacteria and dissolved oxygen close to saturation in the meltwater. Our results confirm that the correlation between <mml:math altimg=\"si74.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup></mml:math>CH<ce:inf loc=\"post\">3</ce:inf>D and <mml:math altimg=\"si75.svg\" display=\"inline\"><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"30 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841590","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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