Geochimica et Cosmochimica Acta最新文献

{"title":"Biodegradation of low molecular weight organic compounds driving arsenic mobilization in groundwater systems","authors":"Jianyi Jin, Huaming Guo, Zhipeng Gao, Gaoyuan Li, Jun Yao, Yizhi Sheng, Hailiang Dong","doi":"10.1016/j.gca.2025.06.034","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.034","url":null,"abstract":"Although organic matter (OM) has been intensively investigated in high arsenic (As) groundwater, the biodegradable OM molecules contributing to As enrichment is poorly understood. To address this, two multilevel wells (K1 &amp; K2 being located in flat plain and alluvial fan, respectively) from the Hetao Basin were setup to investigate lipid biomarkers and their compound-specific carbon isotopes of dissolved organic matter (DOM) and depth-matched sedimentary organic matter (SOM). We revealed that groundwater As and DOC concentrations exhibited greater in K1 from the flat plain than those in K2 from the alluvial fan. The flat plain aquifer had higher microbe-derived OM inputs in both SOM and DOM, since groundwater and sediment OM from K1 was characterized by lower C/N ratios, lower terrigenous/aquatic ratio values, and higher proportion of low molecular weight (LMW) n-alkane compounds (≤C24) than that from K2. The stronger extent of OM biodegradation appeared in the flat plain aquifers, which was supported through greater contrast of δ<ce:sup loc=\"post\">13</ce:sup>C between short-chain n-alkanes and long-chain n-alkanes in groundwater (Δδ<ce:sup loc=\"post\">13</ce:sup>C<ce:inf loc=\"post\">DOM</ce:inf>) and in sediments (Δδ<ce:sup loc=\"post\">13</ce:sup>C<ce:inf loc=\"post\">SOM</ce:inf>), higher thermal maturity of SOM, less odd-to-even predominance and aquatic alkane alkanol values of DOM and SOM from K1. The DOM of high-As groundwater showed more microbe-derived OM than the depth-matched SOM. Arsenic levels showed positive associations with LMW n-alkanes and Δδ<ce:sup loc=\"post\">13</ce:sup>C<ce:inf loc=\"post\">DOM,</ce:inf> suggesting that LMW n-alkanes were electron donors driving As mobility. More specifically, C22-alkane and C18-alkanoic acid were identified as the primary drivers of As mobility, being evidenced by positive relationships between their concentrations and dissolved As levels. Furthermore, a linear correlation between C22-alkane δ<ce:sup loc=\"post\">13</ce:sup>C and As concentrations reinforced this conclusion. This study provides insights into specific LMW compounds, i.e., C22-alkane and C18-alkanoic acid, elevating dissolved As concentrations in groundwater systems.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"71 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900773","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":"Apatite in situ Lu-Hf and U-Pb geochronology in layered mafic intrusions","authors":"Melissa Kharkongor ,&nbsp;Stijn Glorie ,&nbsp;Adam Abersteiner ,&nbsp;Sarah Dare ,&nbsp;Christopher Kirkland ,&nbsp;David Chew ,&nbsp;Jacob Mulder ,&nbsp;Sarah Gilbert","doi":"10.1016/j.gca.2025.07.027","DOIUrl":"10.1016/j.gca.2025.07.027","url":null,"abstract":"<div><div>Layered Mafic Intrusions (LMI) represent the solidified remnants of basaltic magma chambers and provide important insights for understanding subsurface igneous processes, including those linked to the formation of economic deposits of Fe-Ni-Cu-Co sulphides, platinum group elements (PGE), Cr-Fe-Ti-V-oxides and apatite. However, constraining the timing of emplacement of LMI can be challenging, as datable minerals like zircon have a relatively low abundance in mafic rocks. In this study, we use apatite, a common accessory mineral in mafic rocks that can reach high modal abundances in the most evolved parts of LMI to evaluate the robustness of the <em>in-situ</em> U-Pb and Lu-Hf dating methods by laser ablation – inductively coupled plasma – reaction cell mass spectrometry (LA-ICP-MS/MS) for constraining the timing of LMI petrogenesis. We present <em>in-situ</em> apatite U-Pb and Lu-Hf dates from apatite-rich lithologies from four major LMI localities: the Fe-Ti Boulder Lake North deposit (Duluth Complex, USA), the Upper Zone (Bushveld Complex, South Africa), the Nebo-Babel Intrusion (Giles Complex, Musgrave Province, Australia), and the Proterozoic anorthosite massifs of the Grader Intrusion and Lac Perron deposit (Grenville Province, Canada). Trace element discrimination plots, coupled with petrological data, indicate that apatite from all four localities exhibit typical mafic compositions and are classified as cumulus apatite. The U-Pb dates in apatite either represent primary magmatic ages or exhibit varying degrees of Pb-loss, as seen in the Nebo-Babel intrusion, and in the Bushveld Complex, where the degree of Pb loss is dependent on apatite crystal size. In contrast, the apatite Lu-Hf system is undisturbed and records primary LMI crystallisation ages as demonstrated for the Duluth and Bushveld Complexes, and Nebo-Babel intrusion. In the Grenville Province, the long-lived magmatic systems likely record younger apatite growth ages compared to zircon ages. Hence, this study demonstrates <em>in-situ</em> LA-ICP-MS/MS Lu-Hf of apatite as a robust tool for obtaining primary magmatic ages of LMI.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 234-256"},"PeriodicalIF":5.0,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890552","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":"Biomarker proxies for redox-controlled biodegradation: Deciphering organic matter preservation and carbon cycling","authors":"Minghao Wu ,&nbsp;Xiaomei Wang ,&nbsp;Kun He ,&nbsp;Jin Su ,&nbsp;Shuichang Zhang","doi":"10.1016/j.gca.2025.07.030","DOIUrl":"10.1016/j.gca.2025.07.030","url":null,"abstract":"<div><div>Organic matter degradation is a vital component of the carbon cycle, which has been extensively studied in modern marine and lacustrine systems. However, the degradation process in their deep-time counterpart is poorly understood due to the lack of suitable proxies. We propose using biomarker concentrations for tracing biodegradation and carbon cycle that happened in the deep-time lacustrine systems, based on two hypotheses: 1) Hopanes will be enriched against total organic carbon (TOC) if the original organic matter has undergone significant reworking, as hopanoids are more recalcitrant than many organic compounds and are derived from heterotrophic bacteria during degradation; 2) The biomarker concentrations relative to sedimentary rock should positively correlate with TOC if the original organic matter undergoes limited biodegradation, while strong reworking will decouple biomarker concentrations and TOC. To test our hypotheses, we focus on the Songliao Basin in Northeast Asia, where the Cretaceous Qingshankou Formation records lacustrine expansion and contraction offering an exceptional natural laboratory to investigate the relationship among redox conditions, biodegradation, and biomarker concentrations. Our study shows that in more oxidized conditions (low TOC and Mo_EF), hopane concentrations relative to TOC (hopane/TOC) can be two orders of magnitude higher than those in more reducing conditions (high TOC and Mo_EF) indicating that hopane concentration reflects the intensity of redox-controlled biodegradation. We also analyze the relationships between biomarker concentrations (biomarker/rock) and TOC from the perspective of the microbial communities living in the ancient lacustrine system. The biomarkers, such as sterane, pristane, phytane, and gammacerane, generated by microbes living above the oxycline or in the transitional zone show no correlation with TOC, whereas the aryl isoprenoids generated beneath the oxycline correlate well with TOC. The distinct correlation patterns suggest that organic matter produced beneath the oxycline undergoes limited biodegradation and predominates the sedimentary organic matter, whereas organic matter generated from surface water is strongly reworked and could be the secondary contributor to sedimentary organic matter. We deduced that biodegradation is stronger in lacustrine than in marine environments after further compiling previously reported hopane concentrations of black shales and modern sediments. Our study highlights the application of biomarker concentrations for evaluating biodegradation and understanding carbon transfer from the photic zone to sediments in geological records.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"405 ","pages":"Pages 148-161"},"PeriodicalIF":5.0,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900772","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":"Constraining oxygen isotope exchange kinetics between organic compounds and water using electrospray ionization Orbitrap mass spectrometry, and implications for the oxygen isotopic compositions of meteoritic organics","authors":"Surjyendu Bhattacharjee,&nbsp;John M. Eiler","doi":"10.1016/j.gca.2025.07.029","DOIUrl":"10.1016/j.gca.2025.07.029","url":null,"abstract":"<div><div>Oxygen isotopic composition of organic compounds is an important tracer of processes and environmental conditions, with applications ranging from prebiotic organic synthesis in the early solar system to paleoclimate reconstruction. To meaningfully interpret the oxygen isotopic signature of such molecules, we need to consider their rate of oxygen exchange with water, e.g., during aqueous alteration on asteroidal parent bodies or residence on the Earth. In this study, we experimentally constrained the kinetics of oxygen isotope exchange between water and ketones (acetone, cyclopentanone) or carboxylates (acetate, butyrate) near neutral pH between 274.7–373 K. We incubated them in ¹⁸O rich water, and measured their ¹⁸O/¹⁶O ratios after incubation using an electrospray ionization Orbitrap mass spectrometer. While ketones completely exchanged oxygen with water within hours, carboxylates achieved only up to 8 % oxygen exchange after days of incubation. The time evolution of their ¹⁸O/¹⁶O were consistent with first order kinetics, with exchange rate constants of 1.99 (±0.1) × 10⁻⁴ s⁻¹ (acetone), 6.55 (±0.23) × 10⁻⁵ s⁻¹ (cyclopentanone), 3.9 (±0.1) × 10⁻⁸ s⁻¹ (acetate), 3.9 (±0.2) × 10⁻⁸ s⁻¹ (butyrate) at 298 ± 1 K and activation energies of 14.47 ± 0.43 kcal/mol (acetone), 20.35 ± 1.47 kcal/mol (cyclopentanone), 3.53 ± 0.26 kcal/mol (acetate), 2.81 ± 0.43 kcal/mol (butyrate). Rapid exchange for ketones is explained by their hydration in aqueous media, and implies that ketones extracted from meteorites should have completely re-equilibrated with the last water they were in contact with over periods of hours or more (possibly at asteroidal parent bodies, or by exposure to terrestrial meteoric or laboratory waters). Carboxylic acids are resistant to exchange due to electrostatic repulsion between the hydroxyl ion and carboxylate ion in basic media. Furthermore, we predict that significant oxygen isotopic exchange between meteoritic insoluble organic matter (IOM) and aqueous fluid is likely during aqueous alteration, based on rate constants reported in this study and previous studies and constraints on the bonding environments of oxygen in IOM.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 257-273"},"PeriodicalIF":5.0,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890550","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":"Extreme negative Eu anomalies in peraluminous granites of South China: implications for extensive fluid extraction","authors":"Zi Ye, Ming Tang, Xu Chu","doi":"10.1016/j.gca.2025.07.028","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.028","url":null,"abstract":"Mesozoic peraluminous granites in South China (aluminum saturation index &gt; 1.0) often exhibit strong Eu depletions. These extreme Eu anomalies (Eu/Eu* down to &lt;0.01) are observed primarily in the early Yanshanian period, coeval to extensive W-Sn mineralization in the area (162–141 Ma), but the origin of such extremely low Eu/Eu* remains elusive. Here, we analyzed representative Mesozoic granite samples from South China for whole rock major and trace elements, zircon U-Pb ages, trace elements, and oxygen isotopes. Our results suggest that amount of sediment incorporation is insufficient in generating the extremely low Eu/Eu*. If plagioclase retention played a major role in achieving such low Eu/Eu*, oxygen fugacities of &lt;FMQ–5 would be required, even under conditions of extreme feldspar retention and fractionation; such reducing conditions do not align with zircon redox indicators (FMQ–2 to FMQ + 1). On the other hand, modeling results show that fluid extraction can substantially decrease Eu/Eu* in granitic melts, but the efficiency strongly depends on fluid salinity. Achieving Eu/Eu* &lt; 0.01 in residual melts requires a fluid-to-melt mass ratio of &gt;0.2 for brine systems and &gt;0.8 for low-salinity fluids. Correlations between the Eu/Eu* and Ba/Th and Ba/Nb ratios further support this fluid-driven mechanism. We thus propose that extensive halogen-rich fluids exsolution and extraction were critical to generating extremely low Eu/Eu* in the peraluminous granites and likely contributed to W-Sn mineralization in South China. As much of South China existed as an isolated inland sea at low latitudes in the Indosinian period, intense evaporation combined with high productivity may have formed massive halogen- and organic-rich sedimentary strata. These strata were buried to deep crustal level during the Early Jurassic in response to the low-angle subduction of the Paleo-Pacific Plate. We further propose that reworking of these sedimentary strata provided halogens and other volatiles essential for the magmatism and W-Sn mineralization during the early Yanshanian period.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"20 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900907","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":"Si isotope composition of Earth’s mantle and oceanic basalts","authors":"Xiao-Ning Liu (刘效宁) ,&nbsp;Martijn Klaver ,&nbsp;Remco C. Hin ,&nbsp;Christopher D. Coath ,&nbsp;Michael Bizimis ,&nbsp;Dmitri A. Ionov ,&nbsp;Tim Elliott","doi":"10.1016/j.gca.2025.07.025","DOIUrl":"10.1016/j.gca.2025.07.025","url":null,"abstract":"<div><div>We have re-examined mass dependent silicon isotope variations in the terrestrial mantle by exploiting the capability of critical mixture double spiked analyses to measure Si isotope ratios with a reproducibility close to analytical precision (±0.03 ‰ on <em>δ</em>^30/28Si). Our measurements are sufficiently precise to determine Si isotopic differences between mantle phases (Δ^30/28Si_Ol/Cpx = 0.075 ± 0.037 ‰, Δ^30/28Si_Ol/Opx = 0.056 ± 0.028 ‰, and Δ^30/28Si_Ol/Gar of 0.123 ± 0.075 ‰) and crucially between melt and olivine (Δ^30/28Si_Ol/Melt of −0.001 ± 0.029 ‰). Applying these fractionation factors, we predict insignificant (&lt;0.03 ‰) Si isotope variations during basaltic melt differentiation, but detectable fractionation during mantle partial melting; typical melts of a peridotite are ∼0.03 ‰ isotopically heavier than their source, and melts of pyroxenite are ∼0.04 to ∼0.1 ‰ isotopically heavier than their sources. These calculations give context for our <em>δ</em>^30/28Si analyses of various samples of the mantle, provided by mantle peridotites, mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). Eight peridotite measurements, chosen to cover a range of compositions and settings, have a uniform Si isotope composition of −0.308 ± 0.013 ‰ (2 s.e., <em>δ</em>^30/28Si). Twenty-one MORB analyses, from fast to ultra-slow spreading ridges from three ocean basins, and seventeen OIB samples, that span much of the range of their global radiogenic isotope ratios, yield similar mean values of −0.280 ± 0.009 ‰ (2 s.e., <em>δ</em>^30/28Si) and −0.271 ± 0.013 ‰ (2 s.e., <em>δ</em>^30/28Si), respectively. The absence of systematic <em>δ</em>^30/28Si variability in MORB from ridges with variable spreading rates, in contrast to recent observations of mass dependent Mg and Fe isotope fractionation, argues against kinetic fractionation of Si isotopes during melt transport. Moreover, our combined dataset shows that the differences between mantle and mantle derived melts are in good agreement with our calculated values for equilibrium melting, implying <em>δ</em>^30/28Si homogeneity in diverse samples of the accessible mantle at the level of ∼±0.015 ‰. The modelled difference in Si isotope composition of peridotite melt and pyroxenite melt furthermore allows us to quantify pyroxenite melt contribution to oceanic basalt. The modelled difference in <em>δ</em>^30/28Si between peridotite melt and pyroxenite melt combined with the statistically similar mean <em>δ</em>^30/28Si values for MORB and OIB indicate an upper limit of pyroxenite melt contribution of ∼18 % to MORB samples and ∼23 % to OIB samples on average.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 223-233"},"PeriodicalIF":5.0,"publicationDate":"2025-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890551","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":"Metal-silicate partitioning behaviors of Ga and Ge at high pressures and implications for the Earth’s volatile accretion","authors":"Zhengyang Wu, Chang Pu, Xiujin Gao, Jinfeng Li, Zhixue Du, Zhicheng Jing","doi":"10.1016/j.gca.2025.07.024","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.024","url":null,"abstract":"Gallium (Ga) and germanium (Ge) are moderately siderophile and volatile elements whose metal-silicate partitioning behaviors are valuable to understand both core-formation and volatile accretion processes. In this study, we performed metal-silicate partitioning experiments at pressures of 22–70 GPa and temperatures of 3728–4740 K, using laser-heated diamond anvil cells, to explore the effects of pressure, temperature, and metal composition on Ga and Ge partitioning. Thermodynamic modeling using our experimental data and those from the literature reveals that the metal affinities of both Ga and Ge decrease as pressure and temperature increase, with Ga being less siderophile than Ge. Our fitting results confirm that the presence of S and Si in metal can reduce the siderophility of both Ga and Ge, consistent with previous findings at relatively low pressures and temperatures. Our results also demonstrate that O in metal has opposing effects on the metal-silicate partitioning of Ga and Ge. It increases the metal affinity of Ga, contrary to previous thought, but decreases that of Ge. Incorporating these partitioning behaviors, we performed multi-stage core formation modeling to search for accretion scenarios and factors that can reproduce the bulk silicate Earth abundances of Ga, Ge, and S. Our results suggest that Ga and Ge were likely accreted throughout the entire stages of Earth’s accretion rather than accreted solely in the late stage for the final 10–20 % of Earth’s mass growth.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"47 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900909","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":"Impact of silica on the identity of minerals formed in Archean oceans during Fe cycling by cyanobacteria and iron(III)-reducing bacteria","authors":"Carolin L. Dreher ,&nbsp;Manuel Schad ,&nbsp;Jan-Peter Duda ,&nbsp;Stefan Fischer ,&nbsp;Jeremiah Shuster ,&nbsp;Yuhao Li ,&nbsp;Kurt O. Konhauser ,&nbsp;Andreas Kappler","doi":"10.1016/j.gca.2025.07.023","DOIUrl":"10.1016/j.gca.2025.07.023","url":null,"abstract":"<div><div>Banded Iron Formations (BIF) are iron- and silica-rich marine sediments deposited between 3.8 and 1.85 Ga. With the evolution of cyanobacteria, iron cycling via abiotic Fe(II) oxidation with O₂ produced by cyanobacteria and Fe(III) reduction by Fe(III)-reducing microorganisms is hypothesized to be responsible for BIF mineral deposition and transformation both in the water column and in the resulting sedimentary deposits. However, the impact of silica on iron mineralogy during microbially influenced iron cycling has not been determined experimentally under relevant ancient conditions. Hence, we set up batch incubation experiments with different concentrations of silica (0–2.2 mM) in the presence of varying concentrations of Fe(II) (0.5–5 mM), a marine cyanobacterium (<em>Synechococcus</em> sp. PCC 7002) and a marine dissimilatory Fe(III)-reducer (<em>Shewanella colwelliana</em>). We found that the fluctuation between microbial production of O₂ (cyanobacteria) and the activity of Fe(III)-reducing bacteria led to alternating Fe(II) oxidation and Fe(III) reduction and the precipitation of various Fe(II)- and Fe(III)-bearing minerals. Using a combination of X-ray diffraction and ⁵⁷Fe Moessbauer spectroscopy we identified poorly crystalline Fe(III)-bearing minerals (e.g., ferrihydrite), and the well crystalline ones (e.g., goethite, and lepidocrocite) in the absence of silica. Fe minerals precipitated in the presence of silica showed more pronounced reflections in µXRD, indicating higher crystallinity, while ⁵⁷Fe Moessbauer spectroscopy suggested the formation of Fe(II) silicate minerals and Fe(III) (oxyhydr)oxide minerals associated with silica. Furthermore, the presence of silica led to higher oxidation and reduction rates but more incomplete Fe(II) oxidation, while the reduction extent was higher in the presence of silica. In summary, our experiments showed that the presence of silica clearly affects Fe cycling and Fe mineral (trans)formation by cyanobacteria and Fe(III)-reducing bacteria, with relevance for the deposition of Precambrian Banded Iron Formations.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 202-222"},"PeriodicalIF":5.0,"publicationDate":"2025-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890414","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":"Diversity among fossil micrometeorites in the late Devonian","authors":"Lisa Krämer Ruggiu ,&nbsp;Johan Villeneuve ,&nbsp;Anne-Christine Da Silva ,&nbsp;Vinciane Debaille ,&nbsp;Sophie Decrée ,&nbsp;Lutz Hecht ,&nbsp;Felix E.D. Kaufmann ,&nbsp;Steven Goderis","doi":"10.1016/j.gca.2025.07.016","DOIUrl":"10.1016/j.gca.2025.07.016","url":null,"abstract":"<div><div>A total of 1222 Micrometeorites (MMs) from the late Devonian period were extracted from 26 kg of carbonates host rock fragments from the Chanxhe section in Belgium, from the Latest Famennian around 360 Myr, through magnetic separation and optical picking following dissolution with mild HCl, making it one of the largest fossil MMs collection, the largest from the late Devonian. The collection shows a wide diversity of texture, comparable to modern day collection but with different distribution. The majority of the MMs were I-type (90 %), with G-type particles constituting 6 % and S-type particles at 1 %. Some of the S-types spherules are amongst the first silicate-type spherules, and amongst the most well-preserved in terms of texture and composition, to be described in fossil MMs collections. Additionally, intermediate type G/I representing &lt;1 % of the sample are introduced for future fossil MMs classification.</div><div>Distinguishing extraterrestrial (ET) MMs from terrestrial spherules is challenging due to weathering effects that modify both texture and composition during long residency time on Earth. The Na₂O + K₂O versus Fe/Si ratio plot is used for distinguishing ET from terrestrial spherules. Using textural and compositional data in combination creates a reliable ET spherule identification.</div><div>I-type spherules show significant terrestrial alteration with notable loss of Ni and Cr, also observed in S-type spherules, with their silicate phases recrystallized in palagonite. G-type spherules display a mix of characteristics from I-type and S-type MMs. The study also highlights the presence of smaller spherules (&lt;125 µm) compared to modern micrometeorites (210–330 µm), attributed to the predominance of I- and G-type spherules and long-term dissolution effects.</div><div>Despite some alteration for some spherules, due diagenesis of the sedimentary host rocks, the collection shows extremely well-preserved spherules, with even some oxygen isotopes signature being preserved. Indeed, triple oxygen isotope analysis reveals that 5.8 % of the particles are related to ordinary chondrites (OC) and 33 % to carbonaceous chondrites (CCs), yielding a CC/OC ratio of approximately 5.6, with comparable distribution for all major types. Also, 9 % of I- and G/I-types are OC-related. Most I-type spherules likely originate from CM, CR, or H chondrites, with some possibly from iron meteorites. The findings suggest that the source materials of the ET flux have remained relatively consistent over the past 360 Myr, providing insights into historical Solar System events and Earth’s environmental changes and extends the study of ET flux to Earth to CC compared to meteorites. In addition, combined with chemical and isotopic proxies and chrome spinel, the fossil MMs could assess the complete flux of cosmic dust to Earth. Finally, the use of fossil MMs could represent potential proxies for paleo-atmospheric oxygen levels and CO<s","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"405 ","pages":"Pages 114-131"},"PeriodicalIF":5.0,"publicationDate":"2025-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144901765","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":"Extremely high alkalinity due to dissolution of Mg-rich phyllosilicate in the hemipelagic sediments of the Ulleung Basin (East/Japan Sea): stable Si isotopic evidence and reactive transport modeling","authors":"Tzu-Hao Huang ,&nbsp;Xiaole Sun ,&nbsp;Ji-Hoon Kim ,&nbsp;Chris Mark ,&nbsp;Wei-Li Hong","doi":"10.1016/j.gca.2025.07.022","DOIUrl":"10.1016/j.gca.2025.07.022","url":null,"abstract":"<div><div>Marine silicate alteration includes the processes of lithogenic silicate (LSi) dissolution, clay formation, and biogenic silica dissolution. LSi dissolution consumes CO₂ and results in marine silicate weathering. Formation of cation-rich clay minerals produces CO₂, which is known as reverse weathering. The net effects on carbon cycling of both processes are poorly constrained as the responsible silicate phases and controlling factors are unclear. We investigate the coupling between LSi dissolution and clay formation by analyzing stable Si isotopic signatures (δ³⁰Si) of porewater and solid Si phases (reactive LSi, biogenic silica, and amorphous secondary Si phases) in two drill cores from the Ulleung Basin, East/Japan Sea. High porewater total alkalinity (up to 131 meq L⁻¹) was measured, indicating net marine silicate weathering. Based on the elemental composition (Si, K, and Al) as well as δ³⁰Si of the reactive LSi phase in sediments, phyllosilicates that are potentially mica group silicates are identified as the primary silicate group that sustains marine silicate weathering in the Ulleung Basin. Our reactive transport modeling supports such an inference and further reveals how early diagenetic reactions could affect the downcore occurrence and rates of LSi dissolution and clay formation. Predominant clay formation/reverse weathering in sulfate-reducing sediments is evident from the high δ³⁰Si values in porewater. In the shallow methanogenesis zone, net marine silicate weathering due to phyllosilicate dissolution explains the observed high total alkalinity and low δ³⁰Si in porewater. Nonetheless, we show that the rates of clay formation primarily control the level of porewater total alkalinity, even in the condition of net marine silicate weathering. Clay formation is strongly suppressed by the low porewater pH as a result of the active fermentation in the site with the highest rate of organic matter degradation. Deep in the methanogenesis zone, enrichment of dissolved aluminum from LSi dissolution counteracts the influence of pH on silicate alteration processes, thereby further limiting the extent of LSi dissolution, as supported by the downcore increasing of porewater δ³⁰Si. Our results demonstrate that δ³⁰Si in porewater reflects downcore variations in marine silicate alteration, with microbial processes and dissolved aluminum accumulation regulating the rates of LSi dissolution and clay formation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"405 ","pages":"Pages 132-147"},"PeriodicalIF":5.0,"publicationDate":"2025-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144920031","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}