Geochimica et Cosmochimica Acta最新文献

{"title":"Dehydration processes of stagnant slab within the mantle transition zone revealed by geochemistry of olivine-hosted melt inclusions","authors":"Junfu Zhang, Yang Yu, Jie Tang, Feng Wang, Wenliang Xu","doi":"10.1016/j.gca.2026.10.044","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.044","url":null,"abstract":"Nearly 50% of subducted plates may stagnate in the mantle transition zone (MTZ), forming the big mantle wedge (BMW) structure. Fluids or melts released by subducted stagnant slab within the MTZ intensively influence the physical–chemical nature and evolution of the upper mantle in a big mantle wedge system. However, dehydration processes of stagnant slab within the MTZ are unclear. Here, we present geochemical and H-B isotopic data of olivine-hosted melt inclusions from Early Oligocene-Miocene basalts in eastern Northeastern China (ENC), with an aim of constraining the dehydration processes of subducted stagnant slab in the MTZ within the BMW system. The olivine-hosted melt inclusions in OIB-like basalts (33–21 Ma) exhibit low water contents in primary melts (545–965 ppm) and mantle sources (116–202 ppm), coupled with low H<ce:inf loc=\"post\">2</ce:inf>O/Ce ratios (7.96–13.6) and low δD values (−406 ‰ to −191 ‰), implying that the water in mantle sources is mainly contributed by ancient sediment-dominated dehydration in the MTZ. The olivine-hosted melt inclusions in weak-arc like basalts (11 Ma) exhibit relatively high water contents in primary melt (5780–6381 ppm) and mantle source (354–390 ppm), with elevated H<ce:inf loc=\"post\">2</ce:inf>O/Ce ratios (135–166) and upper mantle-like δD values (−119 ‰ to −78 ‰), suggesting that the water in mantle source is mainly contributed by the young lower oceanic crust-dominated dehydration in the MTZ. Additionally, the melt inclusions in the ENC basalts also exhibit variable δ<ce:sup loc=\"post\">11</ce:sup>B values (−17.10 ‰ to −0.08 ‰). The heavy B isotopic compositions of a small amount of melt inclusions from 33 Ma and 11 Ma basalts indicate the contribution of subducted serpentinite dehydration. The water contents in mantle sources of 33 Ma and 21 Ma basalts are lower than those in mantle sources of the early Cenozoic basalts, which is consistent with the transition from an active continental margin setting to trench-arc-basin system and formation of the northeastern (NE) Asian BMW, whereas the increase of water contents in mantle sources from 33 Ma to 11 Ma basalts could reflect the initial stagnance of subducted Pacific slab to formation of the NE Asian BMW as well as the transformation from ancient sediment-dominated dehydration to young lower oceanic crust-dominated dehydration, which is closely related to the evolution of the NE Asian BMW.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"4 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359282","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":"Nanoscale mechanisms of mesocrystal formation during non-classical heterogeneous growth of manganese (hydr)oxides on ferrihydrite","authors":"Yixuan Yang, Qingze Chen, Chi Zhang, Haozhe Ma, Jing Liu, Jianxi Zhu, Runliang Zhu, Hongping He","doi":"10.1016/j.gca.2026.10.046","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.046","url":null,"abstract":"Crystallization by particle attachment (CPA) is an important non-classical pathway of mineral growth that often produces mesocrystals composed of aligned nanoparticles, whose internal interfaces and defects can impart distinctive geochemical reactivity. Given that mineral nucleation and growth in nature commonly occur on pre-existing substrates, a key yet underexplored question is how CPA-mediated mesocrystal growth operates at such heterogeneous interfaces. Herein, we investigated the heterogeneous oxidation of aqueous Mn(II) on ferrihydrite, with a specific focus on the formation of manganese (Mn) (hydr)oxides mesocrystals via CPA. Our results show that Mn (hydr)oxides formed on ferrihydrite grew via oriented assembly into mesocrystalline nanowires. Specifically, Mn (hydr)oxide nanoparticles assembled via end-to-end or side-by-side manners to elongate or widen nanowires, respectively. Misorientations among component nanoparticles led to curved nanowires, whereas straight nanowires resulted from well-aligned interfaces. Mn (hydr)oxide nanoparticles with different crystallographic orientations, and even different mineral phases, can assemble into mesocrystalline nanowires featuring misoriented but crystallographically correlated interfaces. This assembly is rationalized by approximate lattice correspondence at particle junctions and further supported by molecular dynamics simulations revealing energetically favorable interfacial configurations. The assembly of Mn (hydr)oxide nanoparticles was likely initiated by their detachment from ferrihydrite surfaces, driven by the accumulation and release of stress and strain at mismatched Mn (hydr)oxide-ferrihydrite interfaces. Revealing numerous new atomic details of Mn (hydr)oxide CPA, our study links the assembly behaviors of Mn (hydr)oxide nanoparticles to the development of their mesocrystalline structures. We also integrate our new findings with previous studies and synthesize the mechanisms by which different mineral substrates regulate CPA of Mn (hydr)oxides. These findings advance the understanding of heterogeneous growth of Mn (hydr)oxides on iron (hydr)oxides at the nanoscale and contribute to a more integrated view of coupled Mn-Fe cycling in the environment. Our study also highlights the critical roles of substrate mineral surfaces in non-classical mineral growth.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"25 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359284","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":"Beyond bulk sediments: extracting salinity signals from Sr/Ba ratios in complex marginal-marine settings","authors":"Xia Zhang, Xiaotian Zhang, Yue Cai, Rong Hu, Guyue Tan","doi":"10.1016/j.gca.2026.11.003","DOIUrl":"https://doi.org/10.1016/j.gca.2026.11.003","url":null,"abstract":"Sedimentary successions in modern to sub-modern marginal-marine environments are valuable archives of climatic and environmental change. However, paleosalinity reconstruction in these settings is often compromised by lithological heterogeneity, complex sediment sources, hydrodynamic processes, and diagenetic overprinting. Using a well-constrained postglacial succession (∼15 ka to present) from the Qiantang River coastal system, we test and refine Sr/Ba-based salinity proxies, that have been widely used in fine-grained siliciclastic successions. To investigate occurrence modes and interfering factors of Sr and Ba, we analyzed bulk sediments, grain-size fractions, and selective chemical extracts using dilute acetic acid (HAc), ammonium carbonate (AC), sodium acetate (NaAc), and ammonium acetate (AA). Our results show that Sr/Ba ratios in the &lt;10 μm fractions and in the NaAc and AC extracts broadly track environmental variations, as they are primarily controlled by environment-sensitive phases, particularly exchangeable fractions. Yet these ratios cannot fully resolve key salinity boundaries, such as depositional-interval contacts or the maximum flooding surface (MFS), due to overprints from detrital provenance, hydrodynamic sorting, and early diagenesis. To address this, we tested the Sr&lt;ce:inf loc=\"post\"&gt;AC&lt;/ce:inf&gt;/Ba&lt;ce:inf loc=\"post\"&gt;HAc&lt;/ce:inf&gt; ratio, which corrects for detrital provenance and diagenetic effects. This ratio exhibits heightened sensitivity across depositional boundaries, and enables discrimination between freshwater (&lt;0.35), brackish (0.35–1.50), and marine (&gt;1.50) environments. Nevertheless, this ratio tends to overestimate salinity in high-energy settings, where clay winnowing preferentially removes exchangeable Ba. We therefore apply a correction using the relative proportion of the coarse-grained sensitive end-member (FEI). The FEI-normalized Sr&lt;ce:inf loc=\"post\"&gt;AC&lt;/ce:inf&gt;/Ba&lt;ce:inf loc=\"post\"&gt;HAc&lt;/ce:inf&gt; ratio shows enhanced sensitivity to relative sea-level variations and successfully identifies the MFS. This refined multi-proxy framework is well-suited for modern to sub-modern marginal-marine settings where exchangeable fractions are largely preserved, and confounding influences can be corrected through integration with sedimentological analyses. For ancient or diagenetically altered successions, where exchangeable fractions are severely modified but within a closed system, bulk Sr/Ba proxies in fine-grained successions remain a promising alternative, provided that carbonate-rich and/or barite-bearing intervals are treated with caution. This conclusion is supported by the general consistency observed between the Si/Al-normalized Sr/Ba&lt;ce:inf loc=\"post\"&gt;&lt;10μm&lt;/ce:inf&gt; ratios and the refined exchangeable proxies. Overall, this multi-proxy approach circumvents confounding factors and offers practical utility for paleoenvironmental and relative sea-level reconstruction in complex modern to sub-modern marginal","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"81 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359285","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":"Potassium isotope dichotomy in hydrogenetic ferromanganese crusts: phase-specific archives of seawater signatures or authigenic processes?","authors":"Jiayang Chen, Weiqiang Li, Tianyu Chen, Yang Qu","doi":"10.1016/j.gca.2026.11.004","DOIUrl":"https://doi.org/10.1016/j.gca.2026.11.004","url":null,"abstract":"Stable potassium (K) isotopes offer valuable insights into K cycles at various scales. Hydrogenetic ferromanganese (Fe-Mn) crusts, precipitated from seawater and enriched in K, could serve as potential marine K isotopic archives. To evaluate this potential, we conducted detailed investigations of the elemental and isotopic characteristics of K in two hydrogenetic Fe-Mn crusts from the northwestern Pacific Ocean floor over the past 80 million years. Sequential extractions reveal two major K-bearing phases: exchangeable K mainly in Mn oxyhydroxides (62.8 ± 8.9%) and structural K in silicates (32.6 ± 9.3%), with other phases contributing less than 5%. The δ<ce:sup loc=\"post\">41</ce:sup>K values of the exchangeable K, ranging from −0.09 ± 0.04‰ to 0.35 ± 0.05‰, are higher than those of silicate K, which range from −0.71 ± 0.06‰ to −0.34 ± 0.05‰. Notably, the δ<ce:sup loc=\"post\">41</ce:sup>K values of exchangeable K in the two crusts exhibit inconsistent temporal variations. This indicates that exchangeable K in Fe-Mn crusts is susceptible to local diagenetic processes, undermining its reliability as a proxy for long-term seawater δ<ce:sup loc=\"post\">41</ce:sup>K changes. In contrast, the δ<ce:sup loc=\"post\">41</ce:sup>K values of silicate K show consistent temporal trends, correlating with changes in distances between the sample sites from the Hawaiian hotspot. This suggests that the isotopic ratios of silicate-bound K in Fe-Mn crusts can record seafloor authigenic processes, and the growth of authigenic minerals in Fe-Mn crusts appears to be mainly influenced by basaltic volcanic detritus from hotspots. This study reveals a fundamental elemental and isotopic dichotomy of K in Fe-Mn crusts, demonstrating that 1) hydrogenetic Fe-Mn crusts are unlikely to be robust recorders of seawater K isotope evolution, and 2) silicate-bound K can be a sensitive tracer for seafloor authigenic processes and hotspot-related volcanism.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"10 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359283","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":"Volcanically driven redox stratification and N–S–C cycling perturbation in a Late Triassic lacustrine system: controls on carbon sequestration efficiency","authors":"Xianxu Fang, Kelai Xi, Yingchang Cao, Xiujuan Wang, Guanghui Yuan, Yuan You, Miruo Lin, Ke Li, Chao Liang, Keyu Liu, Fang Hao","doi":"10.1016/j.gca.2026.11.001","DOIUrl":"https://doi.org/10.1016/j.gca.2026.11.001","url":null,"abstract":"Water-column redox structure fundamentally governs ecosystem dynamics, biogeochemical cycling, and ultimately sedimentary responses. Redox stratification recorded in deep-time organic-rich lacustrine shales was frequently perturbed by geological events, particularly volcanism, which enhanced deoxygenation and nutrient loading under recurrent volcanic inputs. However, the biogeochemical pathways by which ecological stress developed and persisted remain poorly constrained. Here we investigate the Upper Triassic Chang 7<ce:inf loc=\"post\">3</ce:inf> Member of the Yanchang Formation in the Ordos Basin, a continuous succession of organic-rich lacustrine shales deposited under episodic volcanic influence. Integrating sedimentological, paleontological and multi-proxy geochemical records, we reconstruct the vertical redox structure of the paleolake and assess its response to contrasting volcanic input fluxes. Our results reveal a stable three-layer redox stratification (oxic, suboxic, and euxinic) that couples nitrogen–sulfur–carbon biogeochemical cycling via a cascade of aerobic respiration, denitrification, anammox, Fe–Mn reduction, and sulfate reduction. Positive δ<ce:sup loc=\"post\">15</ce:sup>N<ce:inf loc=\"post\">org</ce:inf> values provide direct evidence for a chemocline that restricts denitrification and anammox to the suboxic layer without interference from sulfate reduction below. As volcanic input flux increased from low to high, δ<ce:sup loc=\"post\">13</ce:sup>C<ce:inf loc=\"post\">org</ce:inf> shifted from negative excursions reflecting internal recycling of <ce:sup loc=\"post\">12</ce:sup>C to positive excursions driven by Rayleigh fractionation. This shift marks the transition from weakened to efficient carbon sequestration. These contrasting redox states demonstrate that volcanic forcing regulated carbon sequestration through nutrient fertilization that elevated primary productivity, but it was the integrity of redox stratification that ultimately determined carbon burial efficiency. Accordingly, we develop an integrated redox‑ecological model that integrates the strength of external volcanic forcing, the feedbacks of internal biogeochemical cycling, and the efficiency of lacustrine carbon sequestration into a coherent system. This model provides a process-based explanation of how this integrity mediates ecological stress and governs carbon sequestration efficiency. Drawing on this deep-time eutrophic lake system, this study identifies critical redox thresholds governing ecosystem responses to nutrient enrichment and deoxygenation in modern lakes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"15 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-11-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359286","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":"Lithospheric mantle derivation and depth-dependent endowment of Neoarchean orogenic gold in the Yilgarn Craton","authors":"Qingfei Wang, David I. Groves, Runsheng Yin, Shiyu Liu, Carl Brauhart, Lin Yang, Huajian Li, Neal J. McNaughton, Kunfeng Qiu, Yongbo Peng, Jun Deng","doi":"10.1016/j.gca.2026.10.041","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.041","url":null,"abstract":"The Yilgarn Craton represents one of the largest Precambrian orogenic gold provinces globally. The Kalgoorlie and Kurnalpi terranes, dominated by mesozonal-epizonal deposits, host the bulk of the gold, whereas the South West and Youanmi terranes, dominated by hypozonal deposits, are far less endowed. The source of auriferous fluids has long been debated between models involving crustal metamorphic devolatilization at mid-crustal levels and magmatic-hydrothermal processes near the crust-mantle boundary, whereas the causes for the marked regional endowment contrast remain poorly understood. Here we present integrated Hg, multiple S, and Sr isotope data to reveal that these four terranes share a common lithospheric mantle source and that their contrasting gold endowment relates to distinct depth-controlled mineralization processes. Non-zero Δ<ce:sup loc=\"post\">33</ce:sup>S values of gold-related sulfides (−0.09 to 0.81 %) are comparable to those of granites across all terranes. This implies that lower-crustal components were involved in the source, arguing against a model involving a mid-crustal metamorphic fluid source. The Δ<ce:sup loc=\"post\">199</ce:sup>Hg values of native gold and sulfides range from negative to positive and center near zero, indicating that lithospheric mantle metasomatized by oceanic sediments with significant input of continental crust is the most plausible source for the auriferous fluid. This is supported by elevated (<ce:sup loc=\"post\">87</ce:sup>Sr/<ce:sup loc=\"post\">86</ce:sup>Sr)<ce:inf loc=\"post\">i</ce:inf> ratios of hydrothermal apatite. Critically, Δ<ce:sup loc=\"post\">199</ce:sup>Hg values are similar between the well-endowed Kurnalpi Terrane and the poorly endowed South West and Youanmi terranes, confirming a common lithospheric mantle source. In striking contrast, δ<ce:sup loc=\"post\">202</ce:sup>Hg values are markedly higher in mesozonal-epizonal deposits (0.11–1.79 %) than in hypozonal deposits. This distinction is attributed to fluid phase separation during mesozonal and epizonal mineralization at middle-upper crustal levels, whereas isotopically light Hg was preferentially partitioned into the vapor phase and lost, leaving the residual fluids enriched in heavy Hg isotopes. In contrast, the greater depths of hypozonal mineralization effectively inhibited such phase separation. Collectively, these interpretations suggest that the primary control on gold endowment is not source heterogeneity but rather the current exposed crustal depths of the terranes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"1 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359306","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":"Dissolved organic matter decouples Fe(II) oxidation from arsenic immobilization: radical localization, mineral disorder and colloid stabilization","authors":"Junhao Niu, Valentina Erastova, Xinghao Wang, Yanwei Wang, Juan Gao","doi":"10.1016/j.gca.2026.10.045","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.045","url":null,"abstract":"Arsenic contamination of paddy soils and shallow aquifers threatens drinking-water safety and staple-crop production worldwide, and its severity is set by Fe-As coupling at subsurface oxygenation fronts. Where oxygen periodically infiltrates reduced layers, Fe(II) oxygenation generates reactive oxygen species (ROS) and fresh Fe(III) (oxyhydr)oxides that jointly oxidize mobile As(III) and sequester the resulting As(V). How dissolved organic matter (DOM), a ubiquitous constituent of redox-dynamic environments, disrupts this natural attenuation circuit remains insufficiently resolved at the molecular scale. We examined straw-derived DOM across C/Fe molar ratios of 0–10 during aerobic oxidation of 1 mM Fe(II) in synthetic solutions at pH 5.5–7.5, combining radical quenching, kinetic modelling, XRD, Fe K-edge XAS and STEM-EELS. Converging quenching and modelling evidence indicates that As(III) oxidation proceeded predominantly via hydroxyl radicals generated at the Fe-mineral – water interface (·OH<ce:inf loc=\"post\">surf</ce:inf>) rather than via the freely dissolved pool. Increasing DOM progressively relocated ·OH generation to aqueous Fe(II)-DOM complexes, where radicals were scavenged before reaching surface-associated As(III); the Fe(II) utilization efficiency for As(III) oxidation accordingly declined from 0.33% to 0.13%. A transition near C/Fe ≈ 5 for rice straw-derived DOM separated two geochemical regimes: below it, micron-scale lepidocrocite aggregates settled and retained arsenic, whereas above it, ferrihydrite–organic nanocolloids of 160–360 nm carrying approximately 40% of total As remained suspended. We identify that DOM controls As fate through coupled effects on the spatial partitioning of radical generation, iron mineral transformation, and colloidal stabilization, and propose C/Fe stoichiometry as a governing chemical parameter for predicting arsenic fate in redox-fluctuating environments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"22 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359303","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":"Manganese mineralization driven by dynamic oceanic redox fluctuations: insights from chromium isotopes","authors":"Qiang Chen, Xuefei Liu, Qingfei Wang, Fangge Chen, Xuefei Sun, Lei Liu, Jun Deng","doi":"10.1016/j.gca.2026.10.042","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.042","url":null,"abstract":"Sedimentary manganese (Mn) deposits are valuable archives of paleoceanographic redox conditions and biogeochemical cycles, yet the mechanisms controlling Mn precipitation remain debated. Here, we present mineralogical, geochemical, and Cr isotopic (δ<ce:sup loc=\"post\">53</ce:sup>Cr) data from the Late Carboniferous Ortokarnash Mn deposit (NW China) to constrain its genesis and link redox fluctuations to Mn mineralization. Mn-bearing layers are hosted in organic-rich marlstones with δ<ce:sup loc=\"post\">53</ce:sup>Cr values of 0.46 ± 0.03‰, comparable to those of modern euxinic sediments (Cariaco Basin: 0.53 ± 0.21‰). Together with low redox-sensitive element abundances and abundant fine-grained framboidal pyrite, these signatures indicate wall-rock deposition under euxinic conditions. In contrast, relict primary Mn oxides within Mn carbonates, elevated δ<ce:sup loc=\"post\">53</ce:sup>Cr values (average 1.08‰), and positive Ce anomalies indicate that Mn carbonates formed via diagenetic reduction of Mn oxides originally precipitated in oxic bottom waters. We link these redox oscillations to episodic ventilation and self-regulation of restricted basins during transgressions. Large-scale sea-level rises introduced oxygenated surface waters, transiently oxygenating the basins, while subsequent upwelling of Mn(II)-rich anoxic deep waters drove rapid Mn(II) oxidation. Enhanced nutrient supply during transgressions increased primary productivity and benthic oxygen consumption, promoting diagenetic Mn-oxide reduction and Mn-carbonate formation in anoxic pore waters. This study highlights repeated regional redox fluctuations as a key driver of major Mn carbonate deposits, establishing them as robust archives of local marine redox oscillations throughout Earth’s history.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"125 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-10-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149359302","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":"Species-specific strontium distribution and environment in coccolithophores revealed by X-ray fluorescence and spectroscopy","authors":"H.J.M. Greene, J.M. Walker, G. Langer","doi":"10.1016/j.gca.2026.10.040","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.040","url":null,"abstract":"The Sr/Ca ratio of coccoliths is the most developed coccolithophore-related geochemical proxy; used to reconstruct coccolithophore productivity. A process-based understanding of this proxy is limited, because Sr partitioning models are mostly not independently constrained. It has recently been discovered that the lopadoliths of <ce:italic>Scyphosphaera apsteinii</ce:italic> display stripes of different Sr concentrations, contrary to model predictions. In this study we use XRF imaging to investigate the distribution of Sr across coccoliths from different coccolithophore families. We measured coccoliths from two Coccolithales species, <ce:italic>Coccolithus braarudii</ce:italic> and <ce:italic>Calcidiscus leptoporus</ce:italic>, and two Zygodiscales, <ce:italic>Pontosphaera syracusana</ce:italic> and <ce:italic>Helicosphaera carteri</ce:italic>. Only <ce:italic>P. syracusana</ce:italic> showed Sr stripes akin to the ones in <ce:italic>S. apsteinii.</ce:italic> Thus, we hypothesize that Sr stripiness is an invention of the Pontosphaeraceae. This family produces high Sr muroliths, whereas the other species investigated here produce comparatively low Sr placoliths. All species investigated so far, however, incorporate Sr into Ca sites of the calcite lattice in coccolith calcite, as opposed to other inorganic or organic phases, as evidenced by XANES. We conclude that the Pontosphaeraceae feature a more complex calcification mechanism than placolith bearing species, including time-dependent Sr partitioning, a process that is not captured by our current models","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"13 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-10-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149292273","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 organic matter fractionation and arsenic sequestration by Fe(III)-clay-humic coprecipitation","authors":"Hongling Bu, Hanjie Chen, Ziming Xu, Dong Liu, Peng Yuan, Manjia Chen, Shujie Hu, Fangyuan Meng, Chengshuai Liu","doi":"10.1016/j.gca.2026.10.038","DOIUrl":"https://doi.org/10.1016/j.gca.2026.10.038","url":null,"abstract":"Coprecipitation of iron(III), clay minerals, and organic matter (OM) is a ubiquitous interfacial process in aquatic-terrestrial critical zones including wetlands, riverine sediments, and estuaries, where it governs organic carbon cycling and the subsequent environmental fate of metalloids such as arsenic (As). Molecular fractionation of dissolved organic matter (DOM) during this coprecipitation modulates carbon persistence, and the resulting composites control subsequent As sequestration, yet the mechanisms by which clay minerals shape DOM fractionation and the resulting As sequestration remain unclear. Here we use a ternary model system of Fe(III), montmorillonite (Mnt), and humic substance (HS) to investigate how clay alters Fe–OM coprecipitation and subsequent As uptake. We find that Mnt surfaces promote the heterogeneous nucleation of poorly crystalline ferrihydrite in intimate spatial association with clay edges, producing a defective Fe–OM–Mnt interface that imposes strong molecular selectivity on DOM removal. ESI-FT-ICR-MS shows that CHONS-rich ligands are preferentially scavenged from solution, while aliphatic-like molecules remain in the residual DOM. The same interface that sorts DOM also controls As sequestration. TEM-EDS mapping shows that As is spatially associated with clay-derived Si and Al domains within the composite, rather than segregated as discrete iron oxide particles. We propose that As binds to edge-anchored Fe sites bonded to the Mnt surface through ≡Mnt–O–Fe–OOC–HS linkages, rather than to bulk ferrihydrite. These findings indicate that clay minerals are not inert spectators in Fe–OM coprecipitation but active participants that restructure the interfacial environment and thereby alter the partitioning of both organic carbon and As","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"50 4-5 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2026-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149292274","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}