Geochimica et Cosmochimica Acta最新文献

{"title":"Anaerobic oxidation of methane from abandoned oil and gas wells leaking into aquifers","authors":"Samuel W. Shaheen, Max K. Lloyd, Eric E. Roden, Susan L. Brantley","doi":"10.1016/j.gca.2025.08.039","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.039","url":null,"abstract":"As oil and gas wells in sedimentary basins worldwide are increasingly abandoned, the legacy wells can remain as conduits for deeper fluids to reach shallow aquifers. To investigate impacts of mixing between the deep and shallow subsurface, we characterized the geochemistry of artesian flows and groundwater discharges associated with abandoned wellbores in the oldest commercially developed oil and gas basin (Appalachian Basin, U.S.A.). Across 18 sites, we observed evidence that basin brines and gases migrate upwards from oil- and gas-producing formations and mix with meteoric waters at shallow depths. All the observed groundwater chemistries were methane-rich and reducing, but some sites were also characterized by high concentrations of dissolved metals, which we hypothesized to derive from the anaerobic oxidation of methane near leaking wellbores. To explore this, we characterized microbial assemblages in groundwaters, conducted laboratory incubation experiments, and developed a reactive transport model integrating our results. We detected evidence for both brine contamination and anaerobic methanotrophic archaea in field samples. Our observations of these archaea in field samples, alongside observations from laboratory incubations of the consortia, provide evidence for active anaerobic oxidation of methane (AOM) in aquifers affected by gas and oil extraction. We also discovered that dissolved gas compositions and the microbial community assemblages are consistent with both methanogenesis and methanotrophy affecting fluids that migrate via abandoned wellbores. We observed complex patterns in the carbon isotopes of hydrocarbons, including divergent trends in the isotopic composition of methane and ethane. These patterns may derive from non-traditional isotopic effects of AOM at low energetic yields in freshwater environments, particularly where aquifer geochemistry or long fluid residence times produce low concentrations of electron acceptors. Simulating AOM coupled to iron- or sulfate-reduction within methane migration pathways using a reactive transport model, we show that metal-rich and metal-poor waters can be easily explained by a chemical divide driven by availability of electron acceptors (iron versus sulfur respectively) in the presence of AOM. Our results emphasize that legacy oil and gas operations not only leak natural gases that contaminate the atmosphere, but also release salts and reducing equivalents into groundwater that deleteriously affect water quality.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"72 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145059706","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":"Substrate chemistry trumps mineral protection in governing temperature sensitivity of organic carbon mineralization in saline lake sediments","authors":"Beichen Wang ,&nbsp;Xiaoxi Sun ,&nbsp;Zhen Yang ,&nbsp;Xinyi Li ,&nbsp;Bingfu Yao ,&nbsp;Min Cai ,&nbsp;Tianran Sun ,&nbsp;Jian Yang ,&nbsp;Jibin Han ,&nbsp;Xiying Zhang ,&nbsp;Hongchen Jiang","doi":"10.1016/j.gca.2025.08.040","DOIUrl":"10.1016/j.gca.2025.08.040","url":null,"abstract":"<div><div>Organic carbon sequestered in lake sediments represents a crucial but vulnerable reservoir within the global carbon cycle. However, the temperature sensitivity of organic carbon mineralization in saline and alkaline lake sediments, particularly under global warming, remains poorly understood. This study addresses this knowledge gap by employing microcosm incubation, coupled with absorption spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, to investigate the temperature-dependent mineralization dynamics of organic carbon in saline lake sediments sourced from the Qinghai-Tibet Plateau. The results revealed that, although lake sediments are rich in mineral−bound organic carbon, the stability of organic carbon under warming conditions is primarily governed by substrate concentration and molecular composition, rather than mineral protection. Specifically, increases in organic carbon concentration and the proportion of labile organic matter significantly enhance mineralization activity and its temperature sensitivity (Q₁₀), thereby accelerating the turnover of sedimentary organic carbon pools, and challenging the classical Carbon Quality-Temperature (CQT) hypothesis. Moreover, dissolved and insoluble organic carbon pools exhibited comparable temperature response patterns, together contributing to the accumulation of recalcitrant organic carbon at high temperature conditions. This study systematically disentangles the relative contributions of substrate properties and mineral protection to organic carbon stability and elucidates temperature-driven shifts in sediment carbon pools. These findings provide critical insights into the vulnerability and future trajectory of lake sediment organic carbon reservoirs under climate warming scenarios.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 81-90"},"PeriodicalIF":5.0,"publicationDate":"2025-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144933302","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":"Zn isotopic variations in ore-related granite and ore minerals in a magmatic-hydrothermal deposit","authors":"Youqiang Qi, Ruizhong Hu, Haifeng Fan, Jun-Hong Zhao","doi":"10.1016/j.gca.2025.08.038","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.038","url":null,"abstract":"Metal isotopes are powerful tools for investigating the migration and distribution of ore-forming elements. Zinc (Zn) is predominantly transported as chloride complexes in fluids, and its valence state remains relatively unaffected by redox conditions, thus Zn isotopes can provide valuable insights into the formation of magmatic-hydrothermal ore deposits. However, significant uncertainties persist regarding Zn isotopic fractionation in magmatic-hydrothermal systems during exsolution of hydrothermal fluids from silicic melts and crystallization of Zn-bearing minerals. The Lengshuikeng Ag–Pb–Zn deposit, one of the largest magmatic-hydrothermal Ag deposits in South China, was formed associated with the Mesozoic granitoid magmatism, and thus serves as an ideal natural laboratory for investigating Zn isotopic variations during the granite magmatism and hydrothermal mineralization. Systematic Zn isotope analyses were conducted on hydrothermal Zn-bearing ores, gangue minerals, and ore-related and unrelated felsic rocks. Sphalerite is the main ore mineral, which can be divided into two generations based on the “chalcopyrite disease” texture and Fe content variations. The early-formed sphalerite displays lighter Zn isotopic compositions (δ<ce:sup loc=\"post\">66</ce:sup>Zn = 0.05 ‰ to 0.30 ‰) and higher Fe contents (6 wt% – 10 wt%) than the late-formed ones (δ<ce:sup loc=\"post\">66</ce:sup>Zn = 0.30 ‰ to 0.60 ‰, Fe &lt; 6 wt%), which probably resulted from Rayleigh-type Zn isotopic fractionation between sphalerite and ore-forming fluid. The ore-forming fluid was derived from large-scale exsolution of volatiles under a cumulative mode. Fe-Mn carbonate minerals are the main Zn-bearing gangue minerals, and they coexist with, and have similar δ<ce:sup loc=\"post\">66</ce:sup>Zn values (0.24 ‰–0.41 ‰) to those of the late sphalerite. The estimated δ<ce:sup loc=\"post\">66</ce:sup>Zn value of the initial ore-forming fluid is around 0.26 ‰, which is lower than those of the ore-related and unrelated felsic rocks (0.23 ‰–0.69 ‰), further supporting the role of fluid exsolution during the Zn isotopic fractionation. Based on the early experimental data and our theoretical modeling, a new Zn-isotope fractionation model has been reconstructed during the processes from Zn-bearing fluids released from the felsic melt to Zn-bearing mineralization. Our findings reveal that hydrothermal fluids are richer in Zn and have lighter Zn isotopic compositions than their felsic residual melts (i.e., solidified granites), and the Zn isotopic fractionation is influenced by amounts of the released fluids and volatile components (i.e., Cl<ce:sup loc=\"post\">-</ce:sup> and HS<ce:sup loc=\"post\">-</ce:sup>) in magmatic-hydrothermal systems. Therefore, Zn isotopes serve as powerful and sensitive indicators for tracing mineralization processes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"13 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145009050","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":"Magmatic Cl-H2O contents, fluid extraction and porphyry fertility: Evidence from zircon and its apatite inclusions","authors":"Wenting Huang, Madeleine C.S. Humphreys, David J. Colby, Michael J. Stock, Jian Zhang, Zexian Cui, Xiao-Ping Xia, Jingjing Zhu, Juan Liao, Huaying Liang","doi":"10.1016/j.gca.2025.08.031","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.031","url":null,"abstract":"The formation of giant porphyry deposits requires the accumulation of up to billions of tons of volatiles (H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O, Cl, and S) in an evolving &lt;ce:italic&gt;trans&lt;/ce:italic&gt;-crustal magma system, followed by their focused release to the hydrothermal system via volatile exsolution in the upper crustal magma chamber. However, it remains unclear whether ore-forming and ore-barren magmatic pulses derived from the same parental magma system differ in initial volatile compositions or exsolution conditions. This uncertainty hinders our comprehension of how massive volatiles enrich and deliver the key drivers of porphyry mineralization. In this work, we investigate zircon water contents and zircon-hosted apatite volatile compositions of the pre-ore (109.7 ± 0.8 Ma), &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-ore (103.1 ± 1.1 Ma), and post-ore (98.1 ± 1.1 Ma) intrusions of the Zijinshan ore field (ZOF). These data, along with numerical modelling, allow us to illustrate the volatile evolution of the magmatic system which sourced the largest Cretaceous porphyry Cu-Mo deposit in South China. Although geochemically similar, these co-sourced magmas exhibit distinct volatile compositions and evolution patterns. Zircon-hosted apatite inclusions from the &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-ore porphyry show highly variable Cl (0.14–2.94 wt%) and F (0.04–3.37 wt%) contents, whereas apatite from the pre-ore and post-ore intrusions displays more restricted halogen variation (0.62–2.70 wt% Cl, and 0.41–1.56 wt% F). Numerical modelling of apatite evolutionary trends suggests that the &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-ore magma had the highest initial melt H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O (∼5.0 wt%) and Cl (∼1300 ppm) contents, exsolving fluids with average aggregated salinity of 3.24 ± 0.57 wt% NaCl&lt;ce:inf loc=\"post\"&gt;eq&lt;/ce:inf&gt; at the deepest level. These results align with the zircon OH&lt;ce:sup loc=\"post\"&gt;–&lt;/ce:sup&gt; contents and biotite Al-thermobarometers, which indicate the highest saturation pressure (∼211 MPa) for the &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-ore porphyry. We suggest that fluid exsolution from magma with moderate Cl content at &gt; 200 MPa optimises Cl-Cu extraction efficiency and promotes the formation of connected fluid networks throughout the magma chamber, facilitating upward fluid migration and subsequent mineralisation. In contrast, modelling indicates that the pre-ore magma had an intermediate initial melt H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O (∼ 4.0 wt%) but the lowest Cl (∼700 ppm) contents, generating less saline fluids (&lt; 3.0 wt% NaCl&lt;ce:inf loc=\"post\"&gt;eq&lt;/ce:inf&gt;), which hampered metal extraction. The post-ore magma, despite relatively high initial Cl (∼1100 ppm) and comparable fluid salinity (3.36 ± 1.40 wt% NaCl&lt;ce:inf loc=\"post\"&gt;eq&lt;/ce:inf&gt;), had the lowest H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O (3.72 ± 0.70 wt%) and saturated at the lowest pressure (∼78 MPa), limiting Cu extraction, exsolved fluid amount, and focused fluid flux. Our findings indicate that the &lt;ce:ita","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"27 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145009046","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":"Implications of sanidine K/Ca compositions for 40Ar/39Ar geochronology","authors":"Paul R. Renne","doi":"10.1016/j.gca.2025.08.018","DOIUrl":"10.1016/j.gca.2025.08.018","url":null,"abstract":"<div><div>Relative concentrations of K and Ca in minerals are easily obtained as byproducts of the ⁴⁰Ar/³⁹Ar dating process, and are useful in revealing the ranges of K/Ca in feldspars of volcanic ejecta. Most important for geochronology is the alkali feldspar sanidine. The K/Ca of sanidines obtained from Ar isotope data and reported in the literature are often imprecise and/or inaccurate, hence recommendations are given for producing and reporting data to enhance their utility for clarifying the petrogenetic implications. Data of suitable quality from the literature reveal that large volume ash flow tuffs display the greatest range of K/Ca in sanidines that in many cases include anomalously old crystals. In some such cases there is a positive correlation between age and K/Ca, suggesting that the older grains are antecrysts or variably degassed xenocrysts. Modelling of five sanidine-bearing magmas with the rhyolite-MELTS code shows that high K/Ca sanidines observed in many cases are not in equilibrium with their host magmas at P-T-X_H2O conditions at which the magma is &lt;60 % solid by volume, hence they would be uneruptible. The exceptions are a dome lava and a relatively small volume pyroclastic flow. Actual data from these latter extrusions show unusually restricted ranges of sanidine K/Ca, and in the case of the dome, statistically homogeneous ⁴⁰Ar/³⁹Ar ages. Sanidines with K/Ca &gt; 200 in subalkaline magmas are most likely to occur in large volume explosive eruptions that disrupt and mobilize otherwise uneruptible crystal mushes, which in some cases resided in partial ⁴⁰Ar* retention conditions, i.e. cold storage, prior to eruption. Analysis of exsolution lamellae would be useful for clarifying the mechanism for retention of pre-eruptive ⁴⁰Ar* retention, e.g. in cases where cold storage is suspected. Sanidines with K/Ca &gt; 200 from moderately to highly peralkaline magmas are likely to represent autocrysts that were in equilibrium with their host magma at eruptible degrees of crystallinity. Determining relative concentrations of Na in alkali feldspars in addition to K and Ca would extend the petrogenetic information attainable from modified ⁴⁰Ar/³⁹Ar experiments; a potentially viable method using nucleogenic Ne isotopes is outlined.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 91-104"},"PeriodicalIF":5.0,"publicationDate":"2025-08-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144989368","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":"Revisiting the role of amphibole fractionation in generating intermediate-silicic arcs: Geochemical and geophysical perspectives from cumulate roots","authors":"Ming-Jian Li, Yun-Chuan Zeng, Ji-Feng Xu, Feng Huang, Xi-Jun Liu, Jian-Lin Chen","doi":"10.1016/j.gca.2025.08.036","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.036","url":null,"abstract":"The formation of intermediate to silicic magmas in subduction zones is fundamental to understanding the genesis of Earth’s continental crust. A prominent hypothesis posits that fractionation of amphibole-rich package from basaltic magmas in the mid-to-lower crust plays a central role in this process. This model assumes the widespread presence of amphibole-rich cumulates—such as hornblendites—within the mid- to lower crust beneath volcanic arcs; however, this assumption remains largely untested. Here, we integrate geochemical analyses and thermodynamic modelling to evaluate the equilibrium melt compositions and geophysical properties of olivine hornblendites from central Tibet and analogous rocks worldwide. Petrological, mineralogical, and isotopic analyses of spatially associated and coevally formed olivine hornblendite, gabbro, and andesite from an Early Cretaceous arc in central Tibet reveal that amphibole fractionation plays a cryptic role in generating some arc andesites, despite amphibole is not present as a phenocryst phase. Melts in equilibrium with amphibole in Tibetan and global hornblendites are predominantly andesitic (mean SiO<ce:inf loc=\"post\">2</ce:inf> = 62 wt%). Additionally, these olivine and pyroxene hornblendites exhibit relatively high seismic velocities (Vp &gt; 7 km/s) and are primarily rooted in the mid-crust. However, large volumes of such high-velocity cumulates have not detected by geophysical studies beneath the middle crust of central Tibet or other volcanic arcs globally. Our findings indicate that amphibole formed via peritectic reactions produces melts with SiO<ce:inf loc=\"post\">2</ce:inf> contents rarely exceeding 65 wt%, implying that more silicic magmas likely require additional processes, such as magma mixing and/or partial melting of pre-existing crust.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"27 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145009052","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":"Modeling the early Earth: Core formation in the nebular era does not guarantee a high [formula omitted]He/[formula omitted]He ratio","authors":"Madelyn Sita, Marvin Osorio, Colin Jackson, Sujoy Mukhopadhyay","doi":"10.1016/j.gca.2025.08.022","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.022","url":null,"abstract":"Ocean island basalts (OIBs) sourced from mantle plumes contain a high <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He component, marking the lower mantle as a potential reservoir for primordial, less degassed, material. Some of these same samples have been observed to contain low <ce:sup loc=\"pre\">182</ce:sup>W/<ce:sup loc=\"pre\">184</ce:sup>W isotope ratios, which suggest the formation of high <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He reservoirs occurred during the early stages of Earth’s formation and point to the core as, potentially, the ultimate source of high <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He materials. We developed a computational model to investigate parameters that affect the time-integrated He/(U+Th) ratio present in the core in order to establish the conditions during planetary formation that favor the formation of a high <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He reservoir in the core. The parameters investigated are representative of the processes responsible for transporting primordial <ce:sup loc=\"pre\">3</ce:sup>He from the nebular atmosphere and the refractory elements U and Th from the silicate magma ocean into the protoplanets’ differentiated core. The parameters investigated include the radius of the protoplanet, timescale of accretion (<mml:math altimg=\"si111.svg\" display=\"inline\"><mml:msub><mml:mrow><mml:mi>τ</mml:mi></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>c</mml:mi><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:math>), optical opacity of the atmosphere (<mml:math altimg=\"si112.svg\" display=\"inline\"><mml:mi>κ</mml:mi></mml:math>), amount of Si in the bulk planet (<mml:math altimg=\"si145.svg\" display=\"inline\"><mml:mi>ϕ</mml:mi></mml:math>), depth of magma ocean-core equilibration, magma ocean thermal gradient, and the metal-silicate partition coefficient of He (D<mml:math altimg=\"si4.svg\" display=\"inline\"><mml:msub><mml:mrow></mml:mrow><mml:mrow><mml:mi>H</mml:mi><mml:mi>e</mml:mi></mml:mrow></mml:msub></mml:math>). The model results, obtained through random sampling of the parameter space, indicated that protoplanets which undergo relatively slow accretion during the lifetime of the solar nebula but still reach sizes larger than 4500 km, protoplanets with optically thin atmospheres, and protoplanets that maintain relatively shallow and cool magma oceans will preferentially develop high <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He cores. Overall, Earth’s core could serve as a reservoir for primordial helium, but current parameter space makes the core’s <ce:sup loc=\"pre\">3</ce:sup>He/<ce:sup loc=\"pre\">4</ce:sup>He ratio highly uncertain.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"35 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144924084","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":"Molybdenum isotopic fractionation during assimilation by unicellular cyanobacteria Synechococcus elongatus FACHB-1061","authors":"Xiaohui Ma, Yingnan Zhang, Ziyao Fang, Junjie Fang, Tiantian Sha, Liping Qin","doi":"10.1016/j.gca.2025.08.035","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.035","url":null,"abstract":"Molybdenum is essential for life, but the low Mo concentration in paleo-oceans restricted its bioavailability, especially for cyanobacteria, which contributed oxygen and organic compounds to the paleo-oceans. However, the Mo assimilation mechanism of cyanobacteria and the response to different ambient Mo concentrations remain unclear. Molybdenum isotopic fractionation is sensitive to changes in the bonding environment and thus was used to investigate Mo assimilation processes in cyanobacteria. In this study, we measured the Mo isotopic compositions of <ce:italic>Synechococcus elongatus</ce:italic> grown in media with different Mo concentrations. Results show that at low Mo concentrations (30 nmol/L − 1500 nmol/L), <ce:italic>Synechococcus elongatus</ce:italic> exhibits a significantly lighter Mo isotopic composition relative to solution during the rapid growth phase (mean Δ<ce:sup loc=\"post\">98</ce:sup>Mo<ce:inf loc=\"post\">cell-medium</ce:inf> = −2.20 ± 0.25 ‰), which gradually rises to a heavier equilibrium value (mean Δ<ce:sup loc=\"post\">98</ce:sup>Mo<ce:inf loc=\"post\">cell-medium</ce:inf> = −1.30 ± 0.32 ‰) during the stationary phase. In contrast, at high Mo concentrations (e.g., 10000 nmol/L), the Mo isotopic composition is significantly heavier than that at low concentration (Δ<ce:sup loc=\"post\">98</ce:sup>Mo<ce:inf loc=\"post\">cell-medium</ce:inf> = −1.13 ± 0.13 ‰) during the rapid growth phase and decreases to a lighter equilibrium value (Δ<ce:sup loc=\"post\">98</ce:sup>Mo<ce:inf loc=\"post\">cell-medium</ce:inf> = −1.57 ± 0.16 ‰) during the stationary phase. The results suggest the coexistence of two distinct Mo transport systems in <ce:italic>Synechococcus elongatus</ce:italic>. Most of the Mo is assimilated by <ce:italic>Synechococcus elongatus</ce:italic> via the ModABC transport system, resulting in significant kinetic isotopic fractionation during transmembrane transport during the rapid growth phase. When ambient Mo concentration increases, low-affinity Mo transport systems are activated, reducing this kinetic fractionation. However, as the cyanobacteria grow, transmembrane transport reaches equilibrium under all ambient Mo concentrations during the stationary phase, where the Mo isotopic fractionation is controlled by the synthesis of Mo-containing enzymes. This study reveals that cyanobacteria dynamically adapt their Mo uptake pathways to varying Mo concentration conditions, exhibiting distinct isotopic fractionation patterns. The findings provide new insights into early microbial metal utilization and a potential isotopic tool for reconstructing paleo-ocean biogeochemistry.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"41 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145059705","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":"Global cycling of vanadium isotopes from multiple ocean water masses and a restricted euxinic basin","authors":"Siqi Li, Sune G. Nielsen, Jeremy D. Owens","doi":"10.1016/j.gca.2025.08.037","DOIUrl":"https://doi.org/10.1016/j.gca.2025.08.037","url":null,"abstract":"To date, there have been only six vanadium isotope data (δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V) points for open ocean seawater samples from four locations that lack vertical or spatial constraints. To provide more robust constraints on global seawater δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V signatures and explore the potential spatial deviations due to local/regional biogeochemical processes, we measured the V concentrations ([V]) and δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V values of seawater samples collected in the South Atlantic Ocean (GEOTRACES 40°S transect) and in the restricted euxinic Black Sea (MedBlack GEOTRACES cruises in 2013, R/V &lt;ce:italic&gt;Pelagia&lt;/ce:italic&gt; cruise 64PE373; “euxinic” indicates free hydrogen sulfides in the water column). The South Atlantic sampling profiles intersect surface (∼5 m) to bottom waters (∼4500 m), which cover the major global ocean water masses that participate in large-scale ocean circulations. The two seawater samples in the Black Sea were collected at 100 and 150 m, across the anoxic-to-euxinic chemocline where dissolved V is proposed to be removed from the water column by sinking particulates. The results reveal relatively constant seawater V concentrations in deep water masses below the euphotic zone (&gt;100 m) and a minor [V] depletion of ∼6 % observed in the surface seawater samples (&lt;100 m, within the euphotic zone) in the South Atlantic Ocean. The depletion is likely due to biological processes but is also modified by seawater mixing processes. The deep ocean water masses, except for the Antarctic Bottom Water, present relatively constant δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V values averaged at 0.27 ‰ ± 0.14 ‰ (2SD). In contrast, the Antarctic Bottom Water, which forms from the Antarctic continental margin, presents a relatively lighter seawater δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V value at 0.03 ‰, which may result from the local influence of the weathering inputs from the Antarctica continent but also requires further verification. Meanwhile, the surface seawater samples in the euphotic zone (&lt;100 m) present heavier values averaged at 0.42 ‰ ± 0.08 ‰ (2SD) along with the minor depletion of seawater [V] relative to deeper water masses, which suggests a preferential uptake of &lt;ce:sup loc=\"post\"&gt;50&lt;/ce:sup&gt;V during biological processes. The two Black Sea seawater samples show relatively low [V] values of around 16 nmol/kg in the shallow water columns and lighter seawater δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V values at 0.17 ‰ and −0.07 ‰, which should reflect a combination of the regional hydrological cycle and biogeochemical processes. This work now confirms the conservative nature of V isotopes in large-scale deep ocean circulations and provides a more robust framework for interpreting the marine sedimentary δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V variations. Furthermore, the distinguishable deviations of seawater δ&lt;ce:sup loc=\"post\"&gt;51&lt;/ce:sup&gt;V values due to the biological processes in the euphotic zone or weathering inputs i","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"52 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145009051","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":"Mo isotope chemostratigraphy of the 2.87 Ga Red Lake Carbonate Platform and implications for incipient oxidative Mo cycling during the Archean","authors":"Amandine L.E. Migeon ,&nbsp;Munira Afroz ,&nbsp;Laureline A. Patry ,&nbsp;Pierre Bonnand ,&nbsp;Pierre Sans-Jofre ,&nbsp;Philip W. Fralick ,&nbsp;Stefan V. Lalonde","doi":"10.1016/j.gca.2025.08.019","DOIUrl":"10.1016/j.gca.2025.08.019","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Prior to 2.5 billion years ago, the Earth system was dramatically different from today, and the Archean oceans were largely anoxic except for local oxygen oases in shallow seas. This study reports a complete molybdenum (Mo) stable isotope chemostratigraphy based on two correlative cored drill holes through Earth’s earliest thick carbonate platform, preserved in the 2.87-billion-year-old (Ga) Ball Assemblage of the Red Lake Greenstone Belt (Ontario, Canada). Previous studies have suggested the presence of an oxygen oasis at this site based on negative Ce anomalies and Mo isotope data obtained from outcrop samples (McIntyre and Fralick, 2017; Thoby et al., 2019), which is further supported by La-Ce geochronological data indicating that the Ce anomalies are syngenetic (&lt;span&gt;&lt;span&gt;Patry et al., 2025&lt;/span&gt;&lt;/span&gt;).&lt;/div&gt;&lt;div&gt;&lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;98/95&lt;/sup&gt;Mo values across diverse lithologies, including stromatolitic dolostones, microbialitic limestones, oxide-facies banded iron formations, black shales, and sulfidic shales, range from −2.22 ‰ to 0.53 ‰, clearly demonstrating important redox-driven Mo isotope fractionation as far back as 2.87 Ga. Despite non-negligible authigenic Mo enrichments in carbonate and iron formation (IF) samples relative to the crust, Mo concentrations determined by isotope dilution remain significantly below crustal values (mean values of ca. 300 ppb and 60 ppb for IF and carbonates, respectively). Mo stable isotopes in carbonate samples are generally unfractionated from crustal values, indicating a small Mesoarchean seawater Mo reservoir of near-zero &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;98/95&lt;/sup&gt;Mo composition. The few outcrop carbonate samples previously analyzed from Red Lake showing heavy Mo isotope compositions (Thoby et al., 2019) may therefore represent a false positive for the presence of O&lt;sub&gt;2&lt;/sub&gt; due to recent (&lt;20 Ka) surface contamination. In contrast, shales and IF are generally characterized by isotopically lighter values, consistent with fractionation during Mo adsorption onto Mn-/Fe-oxides or partial uptake by reducing sediments, respectively, from a dissolved seawater reservoir that was near 0 ‰. The systematic presence of mild positive Ce anomalies in all samples showing the lightest Mo isotope compositions point to a role for Mn-oxide shuttling during deposition, consistent with previous reports of the establishment of an oxygen oases at Red Lake. Combined, this evidence suggests that any oxidative processing of Mo occurring during the deposition of the Red Lake platform did not result in the establishment of an isotopically heavy seawater Mo reservoir, as occurs today. Instead, the results point to a largely unfractionated seawater Mo reservoir derived from weathering under low O&lt;sub&gt;2&lt;/sub&gt; conditions and/or from hydrothermal sources. To investigate the conditions enabling such a reservoir, we employ a steady-state isotope mass balance model to examine the Mesoarchean Mo cycle. Our results suggest that, despit","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 322-337"},"PeriodicalIF":5.0,"publicationDate":"2025-08-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144924085","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}