Geochimica et Cosmochimica Acta最新文献

{"title":"Experimental constraints on stable potassium (K) isotope fractionation during phase separation in NaCl–KCl–H2O and KCl–H2O systems: implications for the K isotope composition of seafloor hydrothermal vent fluids","authors":"Soisiri Charin ,&nbsp;Guy N. Evans ,&nbsp;Xinyang Chen ,&nbsp;Yanlu Xing ,&nbsp;Tianyu Chen ,&nbsp;William E. Seyfried Jr ,&nbsp;Xin-Yuan Zheng","doi":"10.1016/j.gca.2025.07.014","DOIUrl":"10.1016/j.gca.2025.07.014","url":null,"abstract":"<div><div>Phase separation is a ubiquitous process in marine hydrothermal systems, significantly influencing the chemical and isotopic composition of vent fluids. Understanding its effects on elemental partitioning and isotopic fractionation is essential for using vent fluid chemistry as an accurate predictor of the chemical and physical conditions at depth in the oceanic crust. Here, we report the first laboratory experimental study investigating stable potassium isotope (⁴¹K/³⁹K) fractionation during phase separation of alkali (Na, K) chloride fluids under temperature and pressure conditions relevant to natural hydrothermal systems. Two distinct fluid compositions were tested in our experiments at 400 °C, including a mixed NaCl–KCl solution with a starting Na/K molar ratio of 10, and a pure KCl solution. This compositional difference allows for evaluating the role of Na on K isotope fractionation between coexisting vapor and liquid phases. For the NaCl–KCl–H₂O system, all experiments showed preferential enrichment of light K isotopes in the vapor phase during phase separation, yielding an average K isotope fractionation factor of −0.12 ‰ (±0.04 ‰, 1SD). In sharp contrast with the NaCl–KCl–H₂O experiments, the Na-free KCl–H₂O experiments revealed no measurable K isotope fractionation during phase separation. These results suggest that Na plays a critical role in modifying K bonding environments, likely through the formation of multi-cation polynuclear species in the concentrated liquid phase. The coexistence of the multi-cation polynuclear species in the liquid and compositionally simpler K free ions or KCl°(aq) ion pairs in the vapor may account for the observed K isotope fractionation in the NaCl–KCl–H₂O system. The absence of Na in the KCl–H₂O system prevents significant differences in K bonding environments between vapor and liquid phases, thereby precluding measurable K isotope fractionation. Using the K isotope fractionation factor quantified here, a Rayleigh fractionation model can successfully explain the previously unexplained low δ⁴¹K values (∼−0.8 ‰) reported for vapor-dominated fluids at Main Endeavour Field (NE Pacific Ocean). These results highlight the importance of considering phase separation alongside seawater interaction with the oceanic crust to fully understand K isotope variations in hydrothermal systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 13-24"},"PeriodicalIF":4.5,"publicationDate":"2025-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144703121","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}

{"title":"Adsorption and coarsening of gold nanoparticles in iron (oxyhydr)oxides lead to high-grade gold mineralization","authors":"Feng-Xia Huang ,&nbsp;Ya-Fei Wu ,&nbsp;Katy Evans ,&nbsp;Anthony E. Williams-Jones ,&nbsp;Xin-Fu Zhao ,&nbsp;Yong Wei ,&nbsp;Wei Feng ,&nbsp;Jian-Wei Li","doi":"10.1016/j.gca.2025.07.013","DOIUrl":"10.1016/j.gca.2025.07.013","url":null,"abstract":"<div><div>Many hydrothermal gold systems within Earth’s crust develop high-grade ores that contain gold-rich iron (oxyhydr)oxides including hematite (α-Fe₂O₃) and goethite (α-FeOOH). The origin of these iron (oxyhydr)oxides and their role in gold enrichment, however, remain unclear. This study presents detailed micrometer- to nanometer-scale mineralogical and compositional studies of gold-rich iron (oxyhydr)oxides in a high-grade gold deposit at Jinying, China. Our results show that nanoparticles of goethite and gold formed from oxidizing fluids, which might be derived from interactions between magmatic-hydrothermal fluid and hematite-rich wall rocks and infiltration of oxygenated meteoric water. Goethite nanoparticles formed acicular crystals via aggregation and oriented attachment and then transformed into porous hematite via solid-state transformation. Gold nanoparticles that were initially adsorbed on the goethite surfaces via electrostatic interaction and surface adsorption were released during the transformation of goethite into hematite. The released gold was re-enriched within the small pore in hematite and grew into irregular micron-sized particles via aggregation and Ostwald ripening. These results highlight the previously undocumented impact of the formation and transformation of iron (oxyhydr)oxides on gold accumulation in forming some types of high-grade gold mineralization related to iron formation. This study provides significant new insights into the enrichment of gold nanoparticles in iron (oxyhydr)oxides that are common in a variety of oxidizing ore-forming environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 1-12"},"PeriodicalIF":4.5,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144664931","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}

{"title":"Dominant environmental variables affecting brGMGT-derived temperature proxies differ in lake surface sediments and sediment cores","authors":"Jie Wu, Qiangqiang Kou, Caiming Shen, Xinxin Wang, Liping Zhu, Huan Yang, Shucheng Xie","doi":"10.1016/j.gca.2025.07.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.012","url":null,"abstract":"Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a suite of orphan membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), have been proposed as promising tools for quantitative paleotemperature reconstruction in lakes. However, the applicability of brGMGT-based temperature proxies and the influence of salinity and dissolved oxygen (DO) concentration on brGMGT distributions remain largely unexplored, limiting their use as reliable and quantitative paleoclimate proxies in sediment cores. To address these uncertainties, we examined brGMGT concentrations and distributions in surface sediments from West China lakes with varying temperatures and salinities, as well as transects in Lake Yangzonghai featured by a wide gradient of DO concentration. Our results show that brGMGTs are abundant and primarily autochthonous in lake surface sediments, with minimal contributions from surrounding soils. Temperature was found to be the primary factor influencing brGMGT distributions. The brGMGT-based proxy brGMGTII, newly proposed in this study, shows significant correlations with mean annual air temperature (MAAT) in surface sediments from both West China and East African lakes (R<ce:sup loc=\"post\">2</ce:sup> = 0.90 and 0.91, respectively; both <ce:italic>p</ce:italic> &lt; 0.01), underscoring its potential as paleotemperature indicator. BrGMGTII calibrations for temperature reconstructions were established for these regions, yielding RMSE (Root mean squared error) values of 2.8 °C and 2.6 °C, respectively. Additionally, high salinity appears to reduce the methylation degree of brGMGTs, whereas low DO concentration may increase the relative abundance of brGMGTs vs. brGDGTs, though DO shows a complex effect on brGMGTI and brGMGTII. Further analysis of brGMGTs in a Holocene sediment core from Lake Yangzonghai in China reveals that these brGMGT-derived temperature proxies inferred unrealistic temperature trends and absolute values. This is likely due to (1) non-temperature factors, particularly DO concentration, which may exert a stronger influence on brGMGT distributions in sediment core than temperature itself; and (2) the unique mechanism by which brGMGTs respond to temperature, i.e., shifts in bacterial communities between high- and low-temperature brGMGT producers rather than a homeoviscous adaptation of membrane lipids.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"38 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622045","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}

{"title":"Nickel isotopic variations in Ni–Cu sulfide deposits: from theoretical predictions to natural observations","authors":"Zhuang Ma, Shui-Jiong Wang, Wenzhong Wang, Matthew J. Brzozowski, Yun Zhao, Shengchao Xue, Ruipeng Li, Shangguo Su, Jie Yang","doi":"10.1016/j.gca.2025.07.008","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.008","url":null,"abstract":"Extremely light and highly variable <ce:italic>δ</ce:italic><ce:sup loc=\"post\">60/58</ce:sup>Ni values have been observed in sulfide-bearing rocks globally, making Ni isotopes a potential tracer for the genesis of magmatic sulfide deposits. However, the underlying mechanisms that cause the large Ni isotopic variation in sulfides remain unclear. In this study, we first conduct first-principles calculations to determine the reduced partition function ratios of <ce:sup loc=\"post\">60</ce:sup>Ni/<ce:sup loc=\"post\">58</ce:sup>Ni (10<ce:sup loc=\"post\">3</ce:sup>lnβ of <ce:sup loc=\"post\">60</ce:sup>Ni/<ce:sup loc=\"post\">58</ce:sup>Ni) for major Ni-doped sulfide minerals, including chalcopyrite, pentlandite, pyrite, and pyrrhotite. Our calculations demonstrate that the 10<ce:sup loc=\"post\">3</ce:sup>lnβ of <ce:sup loc=\"post\">60</ce:sup>Ni/<ce:sup loc=\"post\">58</ce:sup>Ni values of pyrite and pyrrhotite are sensitive to Ni concentration, and the 10<ce:sup loc=\"post\">3</ce:sup>lnβ of <ce:sup loc=\"post\">60</ce:sup>Ni/<ce:sup loc=\"post\">58</ce:sup>Ni values of sulfides increase in the order chalcopyrite ≈ pyrrhotite &lt; pentlandite &lt; pyrite at natural Ni abundance levels. We then investigate the Ni isotopic variability of sulfide minerals from different types of magmatic Ni–Cu sulfide deposits. Despite the significant Ni isotopic variability observed among the different sulfide minerals, the isotopic differences between pyrrhotite and pentlandite fall along the theoretically calculated equilibrium isotope fractionation line. Chalcopyrite exhibits a progressive enrichment in light Ni isotopes as its Fe/(Fe + Cu) ratio decreases, suggesting that its Ni isotopic composition varies with sulfide liquid fractionation over a wide range of temperatures. Sulfides in the Tulaergen deposit (orogenic setting) have systematically lighter Ni isotopic compositions compared to sulfides in the Hulu (orogenic setting) and Jinchuan (intraplate setting) deposits. These differences may be associated with the different mineralizing processes that operated to form these deposits. The parental magma to the Tulaergen deposit was relatively oxidized, leading to the dissolution of a large amount of isotopically light sulfides in the source of the parent melts, whereas the parental magmas to the Hulu and Jinchuan deposits were comparatively reduced and segregated sulfide liquid prior to emplacement in the crust. Our first-principles calculations and measured Ni isotopic compositions of sulfides in ore samples from magmatic sulfide deposits provide foundational information for the future application of Ni isotopes to the petrogenetic assessment of these economically significant critical metal deposits.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"19 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621778","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}

{"title":"Corrigendum to “Mineral-melt calcium isotope fractionation factors constrained using ab initio molecular dynamics simulations and their implications to calcium isotope effects during partial melting in the upper mantle”. [Geochim. Cosmochim. Ac. 396 (2025) 51–70]","authors":"Yonghui Li ,&nbsp;Justin Hardin ,&nbsp;Wenzhong Wang ,&nbsp;Zhongqing Wu ,&nbsp;Shichun Huang","doi":"10.1016/j.gca.2025.07.005","DOIUrl":"10.1016/j.gca.2025.07.005","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Page 253"},"PeriodicalIF":4.5,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621779","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}

{"title":"Stable Mo isotopic fractionation during complexation onto hematite surface: Molecular insights from Ab-initio MD and X-ray absorption spectroscopy","authors":"Xinyu Wang ,&nbsp;Ruilin Wang ,&nbsp;Luo Li ,&nbsp;David M. Sherman ,&nbsp;Tao Yang ,&nbsp;Mingcai Hou ,&nbsp;Yoshio Takahashi ,&nbsp;Wei Li","doi":"10.1016/j.gca.2025.06.031","DOIUrl":"10.1016/j.gca.2025.06.031","url":null,"abstract":"<div><div>Iron (hydr)oxides are key sorbents of molybdenum (Mo) in a wide range of environmental settings. Among them, hematite (α-Fe₂O₃) is the most stable iron oxide under surficial conditions; however, the extent and molecular-scale mechanisms of Mo isotopic fractionation on hematite across variable pH conditions have remained poorly constrained. In this study, we conducted a series of Mo adsorption experiments at pH 5.0–9.0 to investigate isotopic fractionation and identified two distinct fractionation regimes under acidic and alkaline conditions. Mo isotopic fractionation remained relatively constant between pH 4.0 and 7.0, with Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} ≈ 2.10‰, indicating minimal pH sensitivity. Above pH 7, however, fractionation decreased markedly, with values declining to 1.80‰ at pH 8 and 1.37‰ at pH 9.</div><div>Ab initio molecular dynamics (AIMD) simulations indicate that Mo is predominantly present as a five-fold coordinated complex at the water–hematite interface. This structural motif is supported by Mo K-edge X-ray absorption near-edge structure (XANES) spectra, which show that the local coordination environment of sorbed Mo differs from that of MoO₃ (octahedral geometry) and cannot be explained by a simple mixture of tetrahedral and octahedral species. Extended X-ray absorption fine structure (EXAFS) analysis yields an average Mo–O coordination number of 4.3 ± 0.9 at pH = 5.0 and 4.7 ± 0.9 at pH = 7.0, consistent with prior observations (5.3 ± 0.6), thereby corroborating the predominance of a five-fold Mo species.</div><div>Theoretical isotopic fractionation for this five-fold complex, calculated as Δ⁹⁸Mo_{[Mo(aq)-Mo(five-fold complex)]} = 2.01 ‰ at 20 °C, agrees well with our experimental results (Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} = 1.899 ± 0.322 ‰) and previous data (2.19 ± 0.54 ‰). A two-component model incorporating both five-fold and tetrahedral Mo complexes successfully reproduces the observed pH-dependent fractionation trend. Collectively, these findings demonstrate that Mo isotope fractionation on hematite is strongly pH-dependent and governed by changes in surface complexation geometry, with implications for the application of Mo isotopes as tracers in both natural and anthropogenically influenced systems, from continental weathering profiles to aqueous transport pathways.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 52-66"},"PeriodicalIF":4.5,"publicationDate":"2025-07-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144605582","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}

{"title":"Closing in on carbonate uranium isotopes as a paleo-redox proxy based on continuous records from Late Oligocene to Holocene island carbonates","authors":"Guolin Xiong, Feifei Zhang, Yibo Lin, Guang-Yi Wei, Na Li, Mengchun Cao, Xuexue Jia, Jian Wang, Wen Yan, Hai Cheng, Shu-Zhong Shen","doi":"10.1016/j.gca.2025.07.011","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.011","url":null,"abstract":"Understanding the fractionation of uranium isotopes (&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U/&lt;ce:sup loc=\"post\"&gt;235&lt;/ce:sup&gt;U, commonly denoted as δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U) during carbonate diagenesis is crucial for quantitatively reconstructing the variations in global seafloor anoxic extent throughout Earth’s history using marine carbonates. While the existing diagenetic framework is primarily based on the Bahamian carbonate platform, its robustness and applicability need validation in other regions. The reliability of proxies for recognizing U isotope alteration during diagenesis remains inadequate due to the lack of comprehensive investigations into the influence of diagenetic stages, environments, and redox conditions on U isotope fractionation. In this study, island carbonate samples were continuously collected from two deep drill cores (∼600 m XK-1 core and ∼ 1000 m NK-1 core) in the South China Sea, spanning from the Late Oligocene to Holocene (ca. 25 Ma to present). These samples, which underwent intensive meteoric and marine diagenesis, were notably devoid of organic matter, providing an excellent opportunity to validate the Bahamian diagenetic framework and to explore new geochemical tools for recognizing U isotope alterations during diagenesis. The observed significant positive offset from contemporaneous seawater in the South China Sea (0.24 ‰ ± 0.24 ‰, 1SD, n = 152) is similar to that found in the Bahamas (0.24 ‰ ± 0.15 ‰, 1SD), indicating that U isotope fractionation during early diagenesis is likely consistent on a global scale. The δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U values associated with syndepositional diagenesis in the South China Sea exhibit a uniform offset of 0.25 ‰ ± 0.07 ‰ (1SD), consistent with those observed in the Bahamas (0.26 ‰ ± 0.10 ‰, 1SD), suggesting uranium isotope fractionation during syndepositional diagenesis occurs extensively and consistently across different redox environments in shallow-water carbonates. However, the δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U values undergo further alteration during post-depositional diagenesis, with these values during post-depositional marine diagenesis being mainly determined by the redox conditions of the diagenetic fluids. Based on the sequential enrichment of vanadium (V)–uranium (U) concentrations, we classified the redox conditions of the diagenetic fluids within marine diagenetic environments into upper suboxic and lower suboxic conditions in the South China Sea. The uranium isotope offset from contemporaneous seawater is nearly negligible under upper suboxic conditions but progressively increases by about 1 ‰ due to the authigenic U(IV) enrichment under lower suboxic conditions. However, the uranium isotope offset (0.18 ‰ ± 0.12 ‰, 1SD) during eogenetic meteoric diagenesis is primarily inherited from the earlier syndepositional diagenesis. Our new data from the South China Sea confirm a general diagenetic offset of 0.24 ‰ ± 0.24 ‰ (1SD) between carbonates and seawater, ","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"45 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622046","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}

{"title":"Molecular insights into Yb(III) speciation in sulfate-bearing hydrothermal fluids from X-ray absorption spectra informed by ab initio molecular dynamics","authors":"Xiaodong Zhao, Duo Song, Sebastian Mergelsberg, Micah Prange, Daria Boglaienko, Zihua Zhu, Zheming Wang, Carolyn I. Pearce, Chengjun Sun, Kevin M. Rosso, Xiaofeng Guo, Xin Zhang","doi":"10.1016/j.gca.2025.07.010","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.010","url":null,"abstract":"Rare earth elements (REEs) are critical for advanced technologies, yet in hydrothermal aqueous solutions the molecular level details of their interaction with ligands that control their geochemical transport and deposition remain poorly understood. This study elucidates the coordination behavior of Yb<ce:sup loc=\"post\">3+</ce:sup> in sulfate-rich hydrothermal fluids using <ce:italic>in situ</ce:italic> extended X-ray absorption fine structure (EXAFS) spectroscopy and <ce:italic>ab initio</ce:italic> molecular dynamics (AIMD) simulations. By integrating multi-angle EXAFS with AIMD-derived constraints, we precisely resolve Yb<ce:sup loc=\"post\">3+</ce:sup> coordination structures and ligand interactions under hydrothermal conditions. At room temperature, Yb<ce:sup loc=\"post\">3+</ce:sup> is coordinated by five water molecules and two sulfate ligands (coordination number, CN = 8), forming a distorted square antiprism geometry. Increasing temperature induces progressive dehydration, reducing the hydration shell and favoring stronger sulfate complexation. At 200°C, sulfate ligands reorganize around Yb<ce:sup loc=\"post\">3+</ce:sup>, shifting its geometry to a capped octahedron (CN = 7). At 300 °C, sulfate binding dominates, leading to structural reorganization that parallels the onset of sulfate mineral precipitation, consistent with the retrograde solubility of REE sulfates. These findings provide direct molecular-scale evidence that sulfate acts as both a transport and deposition ligand, critically influencing REE mobility in geochemical environments. Our results can also help to refine thermodynamic models of REE speciation in high-temperature hydrothermal fluids and improve our understanding of REE ore formation processes in nature.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622049","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}

{"title":"Exploring the affinity and selectivity of sedimentary Mackinawite (FeS) towards natural organic matter","authors":"Kathryn Balind, Milad Ezzati, Aude Picard, Yves Gélinas","doi":"10.1016/j.gca.2025.07.009","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.009","url":null,"abstract":"While iron oxides have been thoroughly explored in terms of their ability to sorb and sequester organic carbon (OC) in sediments, the role of iron sulfide (Fe-S) minerals in the long-term sequestration of OC remains poorly defined. In this study, we assessed the affinity of different types of natural organic matter (NOM) towards synthetic Fe-S minerals using sorption isotherms. We found affinities and sorption capacities varying in the following order: plankton NOM &gt; corn leaves NOM &gt; aged terrestrial NOM. Scanning electron microscopy showed that NOM increases the size and surface area of Fe-S aggregates, likely also influencing their surface reactivity. High NOM contents in Fe-S minerals protected Fe(II) from oxidation after exposure to atmospheric oxygen. Analysis of the synthetically prepared Fe-S-NOM complexes by synchrotron scanning transmission X-ray microscopy (STXM) coupled to near-edge X-ray absorption fine-edge structure (NEXAFS) spectroscopy revealed strong interactions between Fe-S minerals and NOM extracted from plankton, specifically with amide and carboxylic functional groups. We also attempted to identify and characterize interactions between OC and Fe-S minerals in natural sulfidic sediments from the St. Lawrence Estuary and the Saguenay Fjord, hence linking our work on synthetic iron sulfides to what is occurring in natural environments, although this effort proved more challenging owing to the presence of Fe(III) minerals even at depth and the difficulty in distinguishing FeS from other Fe(II) minerals. We present depth concentration profiles of dissolved OC, iron, and sulfur in the liquid-phase (pore water) along with speciation data from sequential extractions of sulfur in the solid-phase collected from sediment cores. We found a clear association between mixed Fe(II)/Fe(III) minerals and OC in sediments, which, combined with the results of the synthetic FeS experiment, suggests that Fe-S minerals can promote OC sequestration in sediments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"1 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621780","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}

{"title":"Uranium reduction in modern and ancient marine carbonate settings – insights from anaerobic U extractions and high-energy resolution X ray spectroscopy","authors":"Kasper Primdahl Olesen, Elvira Bura-Nakic, Ivan N. Pidchenko, Kristina O. Kvashnina, Tais W. Dahl","doi":"10.1016/j.gca.2025.07.006","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.006","url":null,"abstract":"In the marine environment, hexavalent uranium, U<ce:sup loc=\"post\">6+</ce:sup>, is incorporated into primary carbonate minerals with the same isotopic composition (δ<ce:sup loc=\"post\">238</ce:sup>U) as the seawater in which they are formed. Yet, modern marine carbonate sediments carry heavier U isotope compositions. This enrichment of heavy U isotopes has been linked to biogenic U reduction in and below the Fe-reducing zone inside the sediment. Still, the oxidation state(s) of uranium in marine carbonate sediments undergoing syndepositional diagenesis has never been measured before. Here, we 1) present an anaerobic ion chromatographic technique based on the TEVA® resin to chemically separate and quantify abundances of tetravalent U<ce:sup loc=\"post\">4+</ce:sup> and hexavalent U<ce:sup loc=\"post\">6+</ce:sup> fractions in the carbonate, and 2) compare the results from ion chromatography to U L3 edge HERFD-XANES spectroscopic measurements of the total U in sediments to 3) estimate U oxidation states of fresh carbonate sediments from a modern seawater-fed lake and ancient limestones. We find that our anaerobic extraction technique can provide credible evaluations of reduced U<ce:sup loc=\"post\">4+</ce:sup> and oxidized U<ce:sup loc=\"post\">6+</ce:sup> contents, applicable to carbonate sediments and rocks. Our results show that U resides both in reduced and oxidized states in modern carbonate sediments and ancient carbonate rocks. By comparing air-exposed, oven-dried samples to samples always kept under strictly anaerobic condition, we find that the majority of authigenic U in modern carbonate sediments resides in oxidation-sensitive phases that accumulate with sediment depth, instead of being structurally bound in carbonate minerals (aragonite and calcite) We propose a model to account for the observed trends in U oxidation state, U phase associations, and U isotope fractionation, where a substantial fraction of U in the sediments is likely delivered via microbial reduction and precipitated as a non-crystalline, reduced form near the sediment–water interface. We suggest these oxidation-sensitive reduced U species participate in redox cycling where some U is re-oxidized and perhaps bio-reduced again later, for example in the presence of Fe(III) mineral surfaces that undergo reductive dissolution with depth. Simultaneously, a continued incorporation of recalcitrant and isotopically light (i.e. <ce:sup loc=\"post\">238</ce:sup>U-depleted) U from the pore fluids into diagenetic carbonate may occur. The determination of U oxidation states in modern carbonates in this study helps to bridge a gap in our knowledge of how U isotope signals are affected by <ce:italic>syn</ce:italic>-sedimentary diagenetic U transformations, opening new avenues for understanding sedimentary U cycling and improving the δ<ce:sup loc=\"post\">238</ce:sup>U paleo redox proxy.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"13 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621781","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}