Geochimica et Cosmochimica Acta最新文献

{"title":"The importance of basaluminite formation on controlling the fate of Al(III) in aqueous solutions at pH 4–5","authors":"Hoda Moradi,&nbsp;Gerhard Furrer,&nbsp;Sergio Carrero,&nbsp;Rosemarie Pöthig,&nbsp;Daniel Rentsch,&nbsp;Ilche Gjuroski,&nbsp;Julien Furrer,&nbsp;Christoph Wanner","doi":"10.1016/j.gca.2026.04.017","DOIUrl":"10.1016/j.gca.2026.04.017","url":null,"abstract":"<div><div>The formation of secondary Al-hydroxysulfates is environmentally important as they regulate aluminum concentrations in acidic environments and may serve as efficient sinks for geogenic toxic elements such as arsenic and selenium. Based on recent studies, nanocrystalline basaluminite [Al₄(OH)₁₀(SO₄) x 5(H₂O)] appears to be the most frequent Al-phase in such settings. A few studies, however, have postulated that secondary Al-hydroxysulfates include the ɛ-Keggin polyoxocation, [Al₁₂(AlO₄)(OH)₂₄(H₂O)₁₂]⁷⁺, commonly referred to as Al₁₃, and that the entire low temperature geochemistry of Al passes through the Al₁₃ metastability field. To better understand what controls the type of Al-hydroxysulfates forming in natural and anthropogenically affected settings, here we present results from titration experiments leading to the formation of Al-hydroxysulfates from solutions with either presence or absence of aqueous Al₁₃. Both the synthesized precipitates as well as the corresponding supernatant solutions were analyzed by standard and advanced analytical techniques including chemical analyses, ²⁷Al Nuclear Magnetic Resonance (NMR) spectroscopy, synchrotron-based high energy X-ray diffraction and subsequent pair distribution analyses, and a kinetic photometric method for the quantification of aqueous Al₁₃. While the chemical composition of the synthesized precipitates are similar, their structures strongly differ depending on the presence or absence of Al₁₃ in solutions. The synthesis from sulfate-bearing solutions with a large compositional range resulted in instantaneous precipitation of nanocrystalline basaluminite. In contrast, crystalline Al₁₃-sulfate formed in experiments where aqueous Al₁₃ was synthesized from AlCl₃ solutions before sulfate was added at a later stage. Based on these results and complimentary geochemical modelling, we conclude that the presence of sulfate leads to a strong complexation of monomeric aluminum. This inhibits the formation of aqueous Al₁₃ and thus the precipitation of Al₁₃-sulfate, while promoting the precipitation of nanocrystalline basaluminite instead. Since sulfuric acid is a primary source of acidity in most natural acidic environments, the formation of aqueous Al₁₃ in nature is likely less important than previously thought. Instead, due to its chemical and structural variability, the formation of basaluminite and its transformation to more stable phases likely control the fate of Al in a large range of natural and anthropogenic settings.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 316-330"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147736500","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Molybdenum isotopic compositions in volcanic rocks from Northeast Japan","authors":"Tatsuya Tamura,&nbsp;Tetsuya Yokoyama,&nbsp;James B. Gill,&nbsp;Takeshi Kuritani,&nbsp;Hikaru Iwamori,&nbsp;Kenta Ueki,&nbsp;Takashi Kudo","doi":"10.1016/j.gca.2026.04.025","DOIUrl":"10.1016/j.gca.2026.04.025","url":null,"abstract":"<div><div>The Mo isotope ratio (<em>δ</em>^98/95Mo value) of subduction zone volcanic rocks has been used as a valuable tracer to elucidate material transport by dehydration of aqueous fluid from the slab and recycling of the dehydrated slab deep in the mantle. However, while some volcanoes, such as oceanic island basalts and continental basalts, have been extensively investigated, studies on across-arc variations from the volcanic front (VF) to the rear arc (RA) in volcanic arcs have been limited. The Northeastern Japan (NEJ) arc, a volcanic arc with a well-developed crust and a large number of volcanoes distributed across the VF to RA regions, is particularly suitable for studying geochemical variations along with the depth of Wadati-Benioff zone.</div><div>In this study, we measured the major and trace element abundances as well as the Mo, Sr, Nd, and Pb isotopic compositions of Quaternary volcanic rock samples from eight volcanoes in NEJ and three volcanoes in the Izu arc, a type of intra-oceanic arc, to gain a deeper understanding of the Mo isotopic variations in island arcs. The mean <em>δ</em>^98/95Mo value of the NEJ-VF samples is higher than that of the depleted mantle (DM), and the variation is larger than that of the other volcanic arcs such as the Izu, Mariana, and Kamchatka arcs. The <em>δ</em>^98/95Mo values of the NEJ-VF samples do not show clear correlations with any geochemical tracers, including major element abundances, trace element ratios, or radiogenic isotope compositions, ruling out crustal assimilation and magma differentiation as the principal cause of the observed <em>δ</em>^98/95Mo variation. Rather, the variation most likely reflects the <em>δ</em>^98/95Mo diversity in the primary magmas that in turn reflects heterogeneity of the <em>δ</em>^98/95Mo value in the slab-derived fluid. On the other hand, in the NEJ-RA region, both <em>δ</em>^98/95Mo and Pb isotope ratios show two distinct systems. The <em>δ</em>^98/95Mo values in NEJ-RA samples reflect either the along-arc variation associated with the change in amounts of slab-derived fluid, or the heterogeneous distribution of recycled materials in the upper mantle beneath the RA.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 419-444"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148553770","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 neodymium isotopic fractionation during plate subduction: insights from back-arc basin basalts","authors":"Fang Liu ,&nbsp;Lei Li ,&nbsp;Hongli Zhu ,&nbsp;Xin Li ,&nbsp;Yajun An ,&nbsp;Zhaofeng Zhang","doi":"10.1016/j.gca.2026.03.024","DOIUrl":"10.1016/j.gca.2026.03.024","url":null,"abstract":"<div><div>Plate subduction transports surface materials into the mantle, altering its chemical and isotopic composition. As an emerging geochemical proxy, stable neodymium (Nd) isotopes hold great potential for tracing recycled subducted materials; however, their isotopic fractionation behavior during subduction remains poorly constrained. Back-arc basin basalts (BABB), which range compositionally from MORB- to arc-like due to variable contributions of subducted fluids and sediments, provide an ideal natural laboratory to investigate the behavior of stable Nd isotopes in subduction zones. In this study, we present high-precision stable Nd isotopic composition (<em>δ</em>^146/144Nd) of twenty-one fresh basaltic glasses from the Woodlark Basin, Vate Trough, and Lau Basin in the southwestern Pacific. Our results reveal that samples exhibit narrow variations in <em>δ</em>^146/144Nd ranging from −0.048‰ to −0.005‰ with an average of −0.022 ± 0.018‰ (2SD, n = 21), similar to that of the Bulk Silicate Earth (BSE, −0.024 ± 0.031‰, 2SD) and global mid-ocean ridge basalts (MORB) reported in literature (−0.025 ± 0.013‰, 2SD). <em>δ</em>^146/144Nd exhibited no correlation with indices of magma differentiation (MgO, CaO/Al₂O₃, Sr/Nd, Eu/Eu*) or partial melting (Nb/Yb, Sm_N/Yb_N), indicating that magmatic processes exert a negligible influence on Nd isotopic fractionation. Additionally, <em>δ</em>^146/144Nd showed no resolvable correlations with subduction indicators (e.g., Ce/Pb, Nb/U, Ba/La, and Th/Yb). However, for BABB from the Lau Basin, weak but resolvable correlations were observed between <em>δ</em>^146/144Nd and ⁸⁷Sr/⁸⁶Sr (r² = 0.40) and ¹⁴³Nd/¹⁴⁴Nd (r² = 0.41). The binary mixing model demonstrates that this relationship cannot be solely explained by the addition of recycled sediments. Instead, fluids derived from the altered oceanic crust (AOC) play a key role in generating the subtle Nd isotopic variations observed in the Lau Basin BABB. Our model suggests that &lt;10% contribution of an AOC-sediment mixture (95–100% AOC) to the mantle source can explain the stable Nd isotopic variation of BABB. Therefore, we conclude that both AOC and sediments significantly influence the Nd isotopic composition of BABB, demonstrating that stable Nd isotopes have the potential to trace subduction components.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 407-417"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147465113","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":"Controls on the cadmium isotopic compositions of marine sediments in the oligotrophic Western Pacific","authors":"Zheyuan Zhang ,&nbsp;Tao Li ,&nbsp;Tianyu Chen ,&nbsp;Gang Yang ,&nbsp;Jianfan Zheng ,&nbsp;Yanguang Liu","doi":"10.1016/j.gca.2026.03.039","DOIUrl":"10.1016/j.gca.2026.03.039","url":null,"abstract":"<div><div>The oceanic biogeochemical cycle of cadmium (Cd) is classically interpreted through its nutrient-like distribution, yet major sinks balancing its isotopic budget remain debated. While Cd sequestration in sub-oxic/anoxic margin sediments has been emphasized, the role of vast expanses of oxic sediments is poorly constrained. This study presents Cd concentrations and stable isotopic compositions (<em>δ</em>^114/110Cd) of surface sediments from the oligotrophic Western Pacific (CJ09), spanning water depths of 2,100–5,580 m, to identify Cd hosts and assess their significance in the global Cd cycle. Results show significant enrichment of non-detrital Cd, primarily hosted in carbonate minerals, organic matter, and Mn oxides, with their relative proportions shifting systematically with depth. Specifically, near-complete loss of carbonate-derived Cd is observed during carbonate dissolution at depth, with limited re-adsorption by other phases. The bulk sedimentary Cd isotopic composition decreases from +0.14 ± 0.06‰ in shallow sediments (2,100–2,700 m) to +0.05 ±0.04‰ in intermediate sediments (4,700–5,580 m), and further to 0.00 ± 0.06‰ in deep sediments. Although Mn oxides dominate non-detrital Cd at deep sediments, the constrained <em>δ</em>^114/110Cd of Mn oxides (+0.13 ± 0.05‰) is significantly higher than bulk <em>δ</em>^114/110Cd values. Mass balance calculations further reveal an exceptionally light Cd isotopic composition (<em>δ</em>^114/110Cd &lt;  −0.4‰) for the organic fraction to account for the low bulk <em>δ</em>^114/110Cd values of deep sediments. This light isotope signature likely originates from the combined effects of rapid export via zooplankton fecal pellets, preferential adsorption of light Cd isotopes onto organic matter, and the delivery of a refractory, isotopically light subsurface particulate Cd pool to the seafloor in the oligotrophic Western Pacific. A first-order flux estimation suggests that Cd burial in these oxic, pelagic clays represents a non-negligible sink (∼2–18% of total input), which is characterized by isotopically lighter Cd compared to deepwater. This finding highlights that oxic sediments in the open ocean constitute a significant light isotopic sink, which must be incorporated into models to fully resolve the marine Cd isotopic mass balance.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 294-307"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147587135","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":"Mechanistic insights into quinone-enhanced hydroxyl radical production during the oxygenation of Fe(II) minerals","authors":"Yingxiao Tian ,&nbsp;Rong Chen ,&nbsp;Peng Zhang ,&nbsp;Songhu Yuan","doi":"10.1016/j.gca.2026.03.033","DOIUrl":"10.1016/j.gca.2026.03.033","url":null,"abstract":"<div><div>Hydroxyl radical (•OH) production from the oxidation of Fe(II) minerals by O₂ has important implications in various environmental processes. However, the role of co-existing quinones in •OH production remains poorly understood. This study systematically investigated the influence of quinones with different Eh on •OH production during the oxygenation of Fe(II) minerals. Results showed that the presence of quinones significantly enhanced •OH production at pH 6.8, thereby elevating the degradation efficiency of phenol. This enhancement was strongly dependent on the redox state, type and concentration of quinones. Specifically, reduced quinones facilitated greater •OH production than the oxidized forms. Additionally, quinones with higher redox potentials (Eh) and higher concentrations exhibited more pronounced enhancement effects.</div><div>In the absence of quinones, •OH was mainly produced through the Haber-Weiss mechanism in the Fe(II) mineral system. In the presence of quinones, the electron transfer pathways responsible for •OH production shifted. Reduced quinones directly reduced O₂ to H₂O₂, thereby facilitating •OH production. Further, reduced quinones competed with Fe(II) minerals to decompose H₂O₂ into •OH, and the •OH yields from H₂O₂ decomposition by reduced quinones (9.4 − 9.9%) were significantly higher than those observed for Fe(II) minerals (2.9 − 7.1%). For oxidized quinones, their reduction by Fe(II) minerals produced reduced quinones, which subsequently contributed to •OH production through the two pathways described above. In addition, the oxidized quinones can also facilitate electron transfer from highly reactive Fe(II) minerals to O₂, thereby enhancing •OH production. During the reaction course, redox cycling between reduced and oxidized quinones may further magnify the enhancement through these mechanisms. The kinetic model showed that quinone-mediated pathways account for a substantial fraction of •OH formed in Fe(II) mineral-quinone systems. Linear free energy relationships (log<em>k</em> = a + b<em>E</em>_h) further revealed a quantitative relationship between quinone Eh and the rate constants for the oxidation of reduced quinones by O₂ and H₂O₂, as well as for the reduction of oxidized quinones by Fe(II) minerals. The findings highlight the important role of the interaction between Fe(II) minerals and quinones in mediating •OH production in the subsurface.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 250-265"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495454","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":"Hematite records palaeo-seawater Zn isotopic compositions: Evidence from Zn isotopic fractionation during adsorption onto and incorporation into Fe (hydr)oxides","authors":"Xinran Yan ,&nbsp;Chuanwei Zhu ,&nbsp;Wenrui Song ,&nbsp;Yunzhu Wu ,&nbsp;Lisheng Gao ,&nbsp;Wentao Wang ,&nbsp;Hui Yin ,&nbsp;Haifeng Fan ,&nbsp;Hanjie Wen","doi":"10.1016/j.gca.2026.03.030","DOIUrl":"10.1016/j.gca.2026.03.030","url":null,"abstract":"<div><div>The Zn isotopic compositions of Fe (hydr)oxides have significant potential for reconstructing seawater Zn isotopic compositions in the geological past. However, the Zn isotopic fractionations between adsorbed/structural and aqueous Zn species during the formation of ferric hydroxides and their subsequent transformation into hematite remain poorly constrained. In this study, experiments were conducted to quantify the Zn isotopic fractionation during adsorption onto hematite, coprecipitation with ferrihydrite, and transformation of ferrihydrite into hematite. Adsorption experiments onto hematite indicate that heavy Zn isotopes are adsorbed preferentially onto the solid phase by equilibrium fractionation, with the fractionation (Δ⁶⁶Zn_{adsorbed‒aqueous}) of 0.29‰ ± 0.03‰ being independent of the Zn concentrations and pH. In Zn‒Fe coprecipitation experiments, preferential enrichment of heavy Zn isotopes into the ferrihydrite structure resulted in an average fractionation (Δ⁶⁶Zn_{structural‒aqueous}) of 0.22‰ ± 0.07‰, which is attributed to indirect incorporation into the structure after adsorption onto the surface. The ferrihydrite–hematite transformation experiments show that adsorbed and structural Zn are released during ferrihydrite dissolution and subsequently incorporated into newly formed hematite or re-adsorbed onto its surface. Heavy Zn isotopes that were previously adsorbed onto ferrihydrite or partially incorporated into its structure are released back into solution during the transformation, whereas the isotopic signature of structural Zn in the ferrihydrite is partially preserved in the resulting hematite. The Zn isotopic fractionation between solids and solutions is better described by equilibrium fractionation at low <em>f</em>_solid (&lt;80%) and by Rayleigh fractionation at higher <em>f</em>_solid, yielding an average isotopic fractionation (Δ⁶⁶Zn_{structural‒aqueous}) of 0.03‰ ± 0.03‰ at low <em>f</em>_solid (&lt;80%). This suggests that no isotopic fractionation occurs between structural and aqueous Zn species during the transformation of ferrihydrite into hematite, regardless of the molar Zn/Fe ratio and pH at low <em>f</em>_solid (&lt;80%). These results provide new insights into the Zn isotopic systematics of Fe (hydr)oxides in archives such as Fe formations and ferromanganese crusts, and highlights their utility as reliable proxies for reconstructing ancient seawater Zn isotopic compositions and constraining Zn biogeochemical cycling and marine primary productivity.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 433-445"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495459","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":"Do magmatic sill intrusions simply liberate the sulphur of the sedimentary reservoirs?","authors":"Alban Cheviet ,&nbsp;Flavien Choulet ,&nbsp;Martine Buatier ,&nbsp;Christophe Galerne ,&nbsp;Wolfgang Bach ,&nbsp;Wolf-Achim Kahl ,&nbsp;Armelle Riboulleau ,&nbsp;Lucie Pastor","doi":"10.1016/j.gca.2026.02.043","DOIUrl":"10.1016/j.gca.2026.02.043","url":null,"abstract":"<div><div>Sedimentary basins are at the interface between deep (lithosphere) and superficial (hydrosphere, atmosphere and biosphere) reservoirs, with the capacity to trap large quantities of volatile elements (e.g. S, C, H, Cl, F) over time through sedimentation and diagenesis. These volatiles can be released abruptly due to contact metamorphism associated to the emplacement of magmatic intrusions in the sediments, further disturbing the geochemical cycles in the atmosphere and biosphere. The interactions between magma and sediments during sill emplacement are complex and include contact metamorphism and contamination processes, but also the assimilation of sediments into the sill. In this study, we examine the impact of magma-sediment interactions on the sulphur cycle. The Ringvent sill in the Guaymas Basin drilled during Expedition IODP 385, is a funnel-shaped sill that emplaced in soft sediments at the subseafloor, representing an exceptional in-situ natural laboratory for studying magma-sediment interactions. The petrological and geochemical study of the sediments, magmatic rocks and magma-sediment mixtures reveals a high concentration of pyrite at the magma-sediment interfaces. Multiple types of pyrite indicate different sulphur-trapping processes occurring during the emplacement of the sill. Trace elements and isotopic (δ³⁴S) analyses were used to identify the different sources of sulphur and to better constrain the processes. We demonstrate that the assimilation of sediment and the contamination of porewater in the Ringvent sill enables the formation of these different pyrite types from the time of sill emplacement until to late hydrothermal circulations, which remain still active. Such processes enable the storage of a minimum of 700,000 tonnes of sulphur within the sill, while there is minimal evidence of the release of volatiles in the surrounding area, indicating that, under certain conditions, these systems can function as a significant sulphur sink.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 195-218"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147393097","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":"Iron (Fe) isotopes decipher Fe fate in acid mine drainage within a karst watershed","authors":"Yuhui Liu ,&nbsp;Yafei Xia ,&nbsp;Jing Sun ,&nbsp;Lei Song ,&nbsp;Ting Gao ,&nbsp;Meng Qi ,&nbsp;Chengshuai Liu","doi":"10.1016/j.gca.2026.03.035","DOIUrl":"10.1016/j.gca.2026.03.035","url":null,"abstract":"<div><div>The fate of Fe in acid mine drainage (AMD) profoundly affects water quality and pollutant migration. In karst watersheds, although it is acknowledged that carbonate buffering strongly controls the fate of Fe in AMD, the impacts of hydrochemical variations induced by karst hydrology and anthropogenic remediation measures remain poorly understood. This study employs Fe isotopes to elucidate the Fe sources, transformation pathways, and controlling factors in a karst watershed impacted by AMD. Results reveal that dissolved Fe(II) is primarily derived from the oxidative dissolution of coal-hosted pyrite. As AMD migrates underground, the enrichment of heavy Fe isotopes in shaft sediments (Δ⁵⁶Fe_{sediment-water} = +0.86‰) indicates additional isotope fractionation during subsequent Fe(II) oxidation-precipitation, as supported by mass-balance calculations. Furthermore, following anthropogenic source reduction measures (e.g., grout sealing), the δ⁵⁶Fe values of outfall waters exhibit a transient shift (&lt;1 year) toward lighter signatures. Partial least squares structural equation modeling (PLS-SEM) confirms that this isotopic lightening is driven by temporary enhancement of carbonate buffering that subsequently elevates the Fe(II) oxidation ratio. At the surface, Δ⁵⁶Fe_{sediment–water} values are notably high at acidic outfalls (+0.85‰ to +1.49‰), yet decrease (+0.18‰ to +0.94‰) upon mixing with alkaline karst river water. These different isotope fractionations reflect the sensitivity of Fe(II) oxidation to pH, as alkalinity promotes a higher oxidation ratio shifting the fractionation mechanism from an oxidation-controlled to a precipitation-dominated regime. Collectively, these results establish a process-dependent framework for Fe isotope fractionation in AMD-impacted watershed and highlight the limitation of carbonate attenuation under both natural and engineered hydrological perturbations.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 266-282"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495452","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":"Source-specific carbon dioxide production and acetoclastic methanogenesis upon anaerobic degradation of dissolved organic matter","authors":"Yongqiang Zhou ,&nbsp;Jiayi Wu ,&nbsp;Lei Zhou ,&nbsp;Yunlin Zhang ,&nbsp;Kyoung-Soon Jang ,&nbsp;Liwei Zhang ,&nbsp;José R. Paranaíba ,&nbsp;Christopher C. Keneally ,&nbsp;Justin D. Brookes ,&nbsp;David P. Hamilton ,&nbsp;Erik Jeppesen","doi":"10.1016/j.gca.2026.03.037","DOIUrl":"10.1016/j.gca.2026.03.037","url":null,"abstract":"<div><div>Lakes receive and process diverse sources of dissolved organic matter (DOM), which contribute differently to the production of carbon dioxide (CO₂) and methane (CH₄) within the system. Here, we isolated DOM from soil, algae, submerged macrophytes, and sewage, then tracked their anaerobic fate for 56 days with high-frequency isotope and ultrahigh-resolution mass spectrometry analyses. Biolabile macrophyte-, sewage-, and algal-DOM lost approximately 80% of DOC within 56 days, leading to CO₂ concentrations of up to 1422 µmol L⁻¹ and CH₄ concentrations of up to 1.59 µmol L⁻¹ for the macrophyte-DOM group. These concentrations were 6 to 9-fold higher than those found for soil-DOM and 30 to 90-fold higher than their initial levels. The calculated carbon isotope fractionation factor (α_C) decreased from initial values of 1.05–1.10 to 1.03–1.04 by day 56, and δ¹³C-CH₄ rose to −40‰, indicating the immediate dominance of the acetoclastic methanogenic pathway. Extreme gradient boosting models identified δ¹³C-DOC as a key predictor of both CO₂ and CH₄ concentrations. Degradation of 1 mg algal DOC yielded 47 µmol CO₂ and 0.04 µmol CH₄, surpassing soil-derived yields by 7- and 4-fold, respectively. By quantifying yield coefficients and identifying δ¹³C-DOC as a key predictor, we move beyond qualitative patterns; the immediate onset of acetoclastic methanogenesis across all sources provides a mechanistic framework for modeling emissions from mixed DOM pools in natural systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 283-293"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147496673","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":"Monsoonally hydrological control on seasonally riverine magnesium isotopes in the tectonically-active Tibetan Plateau","authors":"Yang Xu ,&nbsp;Zhangdong Jin ,&nbsp;Fei Zhang ,&nbsp;Long-Fei Gou ,&nbsp;Chenzi Li ,&nbsp;Chenyang Jin ,&nbsp;Li Deng","doi":"10.1016/j.gca.2025.10.012","DOIUrl":"10.1016/j.gca.2025.10.012","url":null,"abstract":"<div><div>Chemical weathering plays a crucial role in Earth’s geochemical cycles, affecting the climate and surface environment. Magnesium isotope ratios (δ²⁶Mg) are emerging as a powerful tracer of chemical weathering processes, but the impact of hydrology on δ²⁶Mg variations in river systems remains still controversial. This study aims to investigate the seasonal variations of riverine δ²⁶Mg over a full hydrological year in the upper Min Jiang, a tectonically active zone on the eastern Tibetan Plateau, to address how the monsoonal hydrology affects Mg cycling. Weekly water samples were collected at the Zhenjiangguan station and analyzed for Mg isotopes, with the δ²⁶Mg values ranging from –1.20‰ to –1.00‰. Sensitive to storm events, riverine δ²⁶Mg show rapid increases with increasing water discharge and suspended sediment concentrations. After ruling out the influences of lithology and secondary mineral formation through Mg lattice incorporation, we attribute the rapid fluctuations in riverine δ²⁶Mg to Mg²⁺–Na⁺ exchange processes occurring on clay inter-layer sites or surfaces, which are regulated by monsoonal hydrology. Moreover, correlation of time-series riverine δ²⁶Mg data with runoff (<em>r</em>² = 0.50) in the tectonically-active Tibetan Plateau further confirms hydrological control on riverine δ²⁶Mg by instantaneous high suspended sediment concentration and rapid water–rock/particulate interaction. Our findings suggest that hydrological variability, driven by monsoonal runoff, is a key modulator of Mg sources and isotopic fractionation in tectonically-active river systems, particularly during storm periods with high-runoff, which is essential for deciphering climate effects on the global Mg cycle.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"423 ","pages":"Pages 354-366"},"PeriodicalIF":5.0,"publicationDate":"2026-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145397967","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}