Geochimica et Cosmochimica Acta最新文献

{"title":"Influence of anions on the formation of a passivating Fe-rich interfacial layer during olivine dissolution","authors":"Vincenzo Ettore Alagia, Shilpa Mohanakumar, Michael Hendrikus Gerardus Duits, Frieder Mugele","doi":"10.1016/j.gca.2025.06.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.012","url":null,"abstract":"To better understand the rate and mechanism of olivine dissolution under acidic conditions, we exposed mm-sized ‘Norwegian olivine’ pebbles to hydrochloric, nitric and sulfuric acid at 65 °C and varying pH, for up to 50 days and monitored the evolution of Mg, Fe, and Si concentrations as well as pH. The evolution of the surface of the pebbles was examined ex-situ with Confocal Raman microscopy, Scanning Electron Microscopy (SEM), energy-dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). For initial pH values of 1 and 2, we observed distinct anion effects. Most notably, initial olivine dissolution was fastest in sulfuric acid. When the pH of the solution gradually increased to values beyond 3, a visible surface alteration layer was formed and dissolution stopped. Raman spectra, SEM-EDX, and XPS consistently demonstrate that the surface layer, which is most prominent in sulfuric and least prominent in hydrochloric acid, is enriched in iron oxides. XPS data show a co-existence of Fe<ce:sup loc=\"post\">2+</ce:sup> and Fe<ce:sup loc=\"post\">3+</ce:sup> species, presumably facilitated by the oxidative power of sulfuric acid. Raman spectra suggest initial precipitation of goethite and magnetite followed by hematite at later stages of dissolution. We propose that the dissolution process is governed by an initial dissolution of Fe<ce:sup loc=\"post\">2+</ce:sup> followed by (partial) oxidation in solution to Fe<ce:sup loc=\"post\">3+</ce:sup> and subsequent re-precipitation of Fe-(oxy)hydroxides that slow down further dissolution","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"142 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304949","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":"Kinetics of methanethiol oxidation by oxygen under aqueous conditions","authors":"Irina Zweig, Alexey Kamyshny Jr.","doi":"10.1016/j.gca.2025.06.009","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.009","url":null,"abstract":"Methanethiol is one of the most abundant volatile organic sulfur compounds in natural aquatic systems and one of the main products of dimethylsulfoniopropionate decomposition. This study focuses on the kinetic parameters of the reaction of methanethiol and its deprotonated form, methanethiolate, with dissolved oxygen in aqueous solutions at various reactant concentrations, pH, and temperatures. The reaction proceeds through two distinct pathways: a slow reaction between protonated methanethiol and oxygen under acidic to neutral conditions, and a fast reaction between methanethiolate and oxygen under basic conditions. At the environmentally relevant pH and concentrations, the reaction order with respect to methanethiol is 2.2 ± 0.4 for the protonated form and 1.6 ± 0.2 for the deprotonated form, while in both cases it is 1.0 ± 0.3 with respect to oxygen. Dimethyl disulfide was the only product detected in both reaction pathways. The ratio between the consumption rates of oxygen and methanethiolate was approximately 1:4, while the ratio of oxygen to methanethiol consumption rates was close to 1:2. This implies that dimethyl disulfide is not the only product of methanethiol oxidation. The half-life of methanethiol in oxic water column at 25 °C and typical marine methanethiol concentrations of 0.02–2 nM was estimated to be 80 to 1200 years. Rates of chemical oxidation of MT in the surface waters are lower than the rates of its photooxidation and degassing. Thus, the contribution of the chemical oxidation of MT in marine systems to its budget is negligible but should be considered for the MT-rich aphotic hydrothermal and limnic waters. In systems with methanethiol concentrations exceeding 1 mM, such as bioreactors, chemical oxidation rather than microbial decomposition is likely the primary mechanism for methanethiol removal, even at low oxygen levels. Under fully oxic conditions, the rate of chemical oxidation of methanethiol is expected to surpass the rate of microbial degradation observed under anoxic conditions typically utilized in these reactors.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"43 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304964","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":"Seawater intrusion causes substantial release of dissolved organic carbon in coastal paddy soils","authors":"Wenting Chi, Yang Yang, Pei Wang, Chao Guo, Zebin Hong, Shiwen Hu, Kuan Cheng, Shan Wang, Han Li, Qi Wang, Ying Li, Yang Wu, Yibing Ma, Tongxu Liu","doi":"10.1016/j.gca.2025.06.007","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.007","url":null,"abstract":"Paddy soils located in coastal areas are readily exposed to seawater intrusion; however, the impact of salt ions on the adsorption of dissolved organic carbon (OC<ce:inf loc=\"post\">Dissolved</ce:inf>) at the soil–water interface and the transformation of solid-phase soil organic carbon (SOC) fractions remains quantitatively underexplored. Therefore, we investigated the dynamics of SOC, greenhouse gases (CH<ce:inf loc=\"post\">4</ce:inf> and CO<ce:inf loc=\"post\">2</ce:inf>), and soil chemical properties under anoxic (40 d) and oxic (55 d) conditions at different salinities. A significant release of soil OC<ce:inf loc=\"post\">Dissolved</ce:inf> was observed in response to high salinity, which were mainly attributed to the competitive processes as the presence of large amounts of ions (e.g., Na<ce:sup loc=\"post\">+</ce:sup>, Cl<ce:sup loc=\"post\">–</ce:sup> and so on). High salinity inhibited anaerobic Fe(III) reduction by affecting the activity of iron-reducing bacteria (FeRB), leading to a decrease in the decomposition of Fe/Al mineral-bound organic carbon (OC<ce:inf loc=\"post\">Fe/Al mineral-bound</ce:inf>). Moreover, high salinity notably inhibited the expression and activity of soil microbial carbon cycling functional genes, thereby reducing the production of CH<ce:inf loc=\"post\">4</ce:inf> and CO<ce:inf loc=\"post\">2</ce:inf>. Further kinetic modeling showed that under high salinity, OC<ce:inf loc=\"post\">Dissolved</ce:inf> contributed 2.54 % of the SOC after 40 days of anoxic incubation, which was significantly higher than that of the control (1.35 %) and low salinity treatments (1.42 %). After 15 days of oxic incubation, the OC<ce:inf loc=\"post\">Dissolved</ce:inf> content also showed relatively higher contribution in the high salinity treatment (1.91 %) compared to the control (0.75 %) and low (0.72 %) treatments. These results found that the release of labile organic carbon increased with increasing salinity levels, indicating a potential risk of organic carbon loss. These findings provide a fundamental understanding of SOC dynamics in coastal paddy fields.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"7 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304965","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":"Copper isotopes in mare basalts reveal metal-silicate equilibration in the lunar magma ocean","authors":"G. Florin, P. Gleißner, H. Becker","doi":"10.1016/j.gca.2025.06.006","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.006","url":null,"abstract":"In the last decade, several studies have reported enrichments of the heavy isotopes of moderately volatile elements in lunar mare basalts. However, the mechanisms controlling the isotope fractionation are still debated and may differ for elements with variable geochemical behaviour. Here, we present a new comprehensive dataset of mass-dependent copper isotope compositions (δ<ce:sup loc=\"post\">65</ce:sup>Cu) of 30 mare basalts sampled during the Apollo missions. The new δ<ce:sup loc=\"post\">65</ce:sup>Cu data range from +0.14 ‰ to +1.28 ‰ (with the exception of two samples at 0.01 ‰ and –1.42 ‰), significantly heavier than chondrites and the bulk silicate Earth. A comparison with mass fractions of major and trace elements and thermodynamic constraints reveals that Cu isotopic variations within different mare basalt suites are mostly unrelated to fractional crystallisation of silicates or oxides and late-stage magmatic degassing. Instead, we propose that the δ<ce:sup loc=\"post\">65</ce:sup>Cu average of each suite is representative of the composition of its respective mantle source. The observed differences across geographically and temporally distinct mare basalt suites, suggest that this variation relates to large-scale processes that formed isotopically distinct mantle sources. Based on a Cu isotope fractionation model during metal melt saturation in crystal mush zones of the lunar magma ocean, we propose that distinct δ<ce:sup loc=\"post\">65</ce:sup>Cu compositions and Cu abundances of mare basalt mantle sources reflect local metal melt–silicate equilibration and trapping of metal in mantle cumulates during lunar magma ocean solidification. Differences in δ<ce:sup loc=\"post\">65</ce:sup>Cu and mass fractions and ratios of siderophile elements between low- and high-Ti mare basalt sources reflect the evolving compositions of both metal and silicate melt during the late cooling stages of the lunar magma ocean.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"29 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304966","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":"Does greater molecular diversity in soil organic matter imply greater persistence: Insights from molecular and multi-property analyses in Western China","authors":"Shanyi Tian ,&nbsp;Sihua Zhu ,&nbsp;Joeri Kaal ,&nbsp;Jitao Lv ,&nbsp;Rong Huang ,&nbsp;Tongwen Zhang ,&nbsp;Xiaolei Su ,&nbsp;Yongguang Yin ,&nbsp;Tao Jiang ,&nbsp;Dingyong Wang","doi":"10.1016/j.gca.2025.06.008","DOIUrl":"10.1016/j.gca.2025.06.008","url":null,"abstract":"<div><div>Chemodiversity of soil organic matter (SOM) is vital for its formation and eco-functions. However, the link between SOM molecular diversity and its persistence across various ecosystems remains unclear. This study investigates the correlation between SOM molecular diversity and persistence in cropland, grassland, and forest ecosystems in western China. We analyzed soil samples and identified 42 parameters related to SOM persistence, which were grouped into six property categories: SOM carbon pools, carbon and nitrogen element properties, optical properties of dissolved organic matter (DOM), DOM molecular size, soil iron oxides, and soil amino acid properties. The persistence index (PI) based on multi-property was calculated using the Hill-number multifunctionality method to describe SOM persistence within each property category. The SOM molecular diversity (i.e., richness index, Shannon index, and evenness index) and complexity derived from topological metrics of molecular network analysis were assessed through pyrolysis products from pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) and thermally assisted hydrolysis and methylation gas chromatography–mass spectrometry (THM–GC–MS) using tetramethylammonium hydroxide (TMAH) as the methylating agent. The correlations between the PI for each SOM property category, SOM molecular diversity, and network complexity were evaluated. Results demonstrated that forest soils had the highest levels of SOM persistence, chemodiversity, and complexity, with positive correlations between SOM molecular diversity/complexity and its persistence across all ecosystems, most pronounced in forests. This research emphasizes the ecosystem-specific interplay between SOM molecular diversity and persistence, enhancing our understanding of the role of SOM in carbon sequestration and informing strategies for carbon storage and soil health management. In addition, the combined use of Py–GC–MS and THM–GC–MS allowed for a more comprehensive picture of SOM molecular characteristics.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 136-148"},"PeriodicalIF":4.5,"publicationDate":"2025-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304967","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Corrigendum to “Benthic deposition and burial of total mercury and methylmercury estimated using thorium isotopes in the high-latitude North Atlantic” [Geochim. et Cosmochimica Acta, 399 (2025) 191–204]","authors":"Xiangming Shi, Amber L. Annett, Rhiannon L. Jones, Rob Middag, Robert P. Mason","doi":"10.1016/j.gca.2025.06.002","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.002","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"33 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304968","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":"Effects of mineralogy on Δ47 and Δ48 of carbonate-derived CO2 below analytical resolution","authors":"Miguel Bernecker ,&nbsp;Magali Bonifacie ,&nbsp;Philip Staudigel ,&nbsp;Niels Meijer ,&nbsp;Julien Siebert ,&nbsp;Nicolas Wehr ,&nbsp;Eiken Haussühl ,&nbsp;Stefano M. Bernasconi ,&nbsp;Daniel A. Petrash ,&nbsp;Martin Dietzel ,&nbsp;Jens Fiebig","doi":"10.1016/j.gca.2025.06.004","DOIUrl":"10.1016/j.gca.2025.06.004","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Due to the lack of direct methods capable of determining the abundance of isotopologues containing multiple heavy isotopes within the crystal lattice, carbonates are typically reacted with phosphoric acid to produce CO&lt;sub&gt;2&lt;/sub&gt; analyte for clumped isotope analyses. This reaction is associated with fractionations of both bulk oxygen and clumped isotopes. Accurate knowledge of the effect of cation substitution on the degree of isotopic clumping in the carbonate phase (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;63&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;64&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;) and on acid fractionation factors (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/mrow&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/mrow&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;) is crucial for accurate temperature reconstructions based on clumped isotope measurements (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;) of CO&lt;sub&gt;2&lt;/sub&gt; extracted from various carbonate mineralogies. Previous studies have yielded contradicting results on the effect of carbonate mineralogy on both &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/mrow&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; acid fractionation factors and the validity of a universal &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;−T relationship, and, so far, a systematic investigation of mineralogy-specific effects on &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/mrow&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; is lacking.&lt;/div&gt;&lt;div&gt;In this study, we have analyzed the dual clumped isotope composition of stochastic and non-stochastic calcites, aragonites, dolomites, witherites‚ and siderites with unprecedented long-term repeatabilities (1SDs) of 8.6 and 28.1 ppm for &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, respectively. In order to facilitate complete acid digestion of dolomite and siderite in a reasonable timeframe, an acid digestion temperature of 110 °C was used for these minerals instead of the 90 °C applied to calcite, aragonite and witherite. A set of calcite samples was reacted at both temperatures to determine the calcite-specific difference in acid digestion-related fractionation factors between 90 and 110 °C, yielding &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;47&lt;/mn&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mn&gt;110&lt;/mn&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;msup&gt;&lt;mn&gt;90&lt;/mn&gt;&lt;mo&gt;°&lt;/mo&gt;&lt;/msup&gt;&lt;mi&gt;C&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/mrow&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; = -0.0147 ± 0.002 % and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;48&lt;/mn&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mn&gt;110&lt;/mn&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;msup&gt;&lt;mn&gt;90&lt;/mn","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 89-103"},"PeriodicalIF":4.5,"publicationDate":"2025-06-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144279589","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":"The mechanism and kinetics of the replacement of chalcopyrite by covellite under mild-hydrothermal conditions (120–200 °C)","authors":"Zhen He ,&nbsp;Gujie Qian ,&nbsp;Allan Pring ,&nbsp;Sarah Harmer","doi":"10.1016/j.gca.2025.05.041","DOIUrl":"10.1016/j.gca.2025.05.041","url":null,"abstract":"<div><div>In this work, the replacement of chalcopyrite by covellite was investigated under hydrothermal conditions and the kinetic data was extrapolated to lower temperatures. Chalcopyrite was successfully replaced by covellite in the temperature range of 140–200 °C under acidic conditions (pH 0.7–4) via a dissolution-reprecipitation reaction mechanism. Covellite was examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The replacement reaction was controlled by solution chemistry (pH), temperature and dissolved oxygen (provided by H₂O₂). At 120 °C, covellite formed prior to 18 days, and then completely transformed into elemental sulfur. X-ray photoelectron spectroscopy (XPS) analysis was carried out to help understand the reaction mechanism of the chalcopyrite dissolution step. The XPS results confirmed that the formation of elemental sulfur as an intermediate reaction product occurred in the early stage of chalcopyrite dissolution. Hydrothermal experimental results showed that chalcopyrite was replaced by covellite in the presence of H₂O₂ at pH 0.7, 1, 1.5 and 2 and in the temperature range of 140–200 °C. The overall activation energies (<em>E_a</em>) of the chalcopyrite replacement reactions were calculated, using the <em>Avrami-Arrhenius</em> method, to be 90 ± 21 kJ/mol, 86 ± 17 kJ/mol and 27 ± 7 kJ/mol at pH 0.7, 1 and 1.5, respectively. The increasing <em>E_a</em> with decreasing pH suggests that the reaction mechanism may change from interface-controlled to diffusion-controlled reactions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 122-135"},"PeriodicalIF":4.5,"publicationDate":"2025-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144306418","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":"HERFD-XAS evidence for an octahedrally coordinated CoSn-polysulfide precursor as a probe for the mechanism of pyrite formation","authors":"Qianyu Deng ,&nbsp;Pierre Le Pape ,&nbsp;Julie Aufort ,&nbsp;Marc Blanchard ,&nbsp;Benoit Baptiste ,&nbsp;Ludovic Delbes ,&nbsp;Camille Baya ,&nbsp;Farid Juillot ,&nbsp;George Ona-Nguema ,&nbsp;Nicolas Menguy ,&nbsp;Jean-Michel Guigner ,&nbsp;Olivier Proux ,&nbsp;Camille Duquenoy ,&nbsp;Romain Guilbaud ,&nbsp;Franck Poitrasson ,&nbsp;Guillaume Morin","doi":"10.1016/j.gca.2025.06.003","DOIUrl":"10.1016/j.gca.2025.06.003","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Trace element contents in authigenic pyrite (FeS&lt;sub&gt;2&lt;/sub&gt;) are often considered as a reliable geochemical archive of past marine conditions. For instance, cobalt (Co) abundance in marine sedimentary pyrite may track back the extent of past ocean anoxia and is considered as a reverse proxy for the rise of atmospheric oxygen. However, the molecular-scale route of Co incorporation in pyrite at low temperature is not well documented. Thus, any insight on this aspect are expected to help better assess the actual role of pyrite in Co cycling in modern and past subsurface environments. In this study, a series of pyrites were synthesized in solution via the polysulfide pathway under anoxic conditions and at ambient temperature, with various initial aqueous Co concentrations (Co:Fe = 0.13–5). Rietveld refinement analysis of the powder X-ray diffraction (XRD) patterns shows that pyrite is the principal component (69(5) wt%) of the final solid products, with small fractions of marcasite (17(4) wt%) and FeS (10(2) wt%). High Energy Resolution Fluorescence Detected (HERFD) Co &lt;em&gt;K&lt;/em&gt;-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray absorption fine structure (EXAFS) analysis indicate that a minor fraction (20–36 %) of Co substitutes for Fe in the pyrite structure as compared with a theoretical spectrum of a Co-substituted pyrite supercell calculated using Density Functional Theory (DFT). Besides, in the final products, the major part (64–80 %) of Co persists in the form of an amorphous CoS&lt;sub&gt;n&lt;/sub&gt;-polysulfide precursor phase that represents the whole Co speciation before pyrite nucleation. In this precursor observed at the monosulfide FeS pre-pyrite stage, Co early adopts an octahedral coordination as attested by Co pre-edge data, whereas Co is in tetrahedral coordination in our Co-doped mackinawite FeS reference compound. The local structure of the CoS&lt;sub&gt;n&lt;/sub&gt;-polysulfide precursor is further elucidated by EXAFS shell-by-shell analysis that points to monomeric units (&lt; 1 nm), where Co is octahedrally coordinated to first neighboring S atoms with at least three of these S neighbors belonging to a polysulfide chain of undetermined length. Aggregation of such monomeric units into an amorphous CoS&lt;sub&gt;n&lt;/sub&gt;-polysulfide phase is supported by X-ray scattering-pair distribution function analysis (XRD-PDF) of an analogous amorphous CoS&lt;sub&gt;n&lt;/sub&gt; compound. These results could have important implications on our understanding of pyrite nucleation mechanisms via the polysulfide pathway since the observed octahedral CoS&lt;sub&gt;n&lt;/sub&gt;-polysulfide precursor differs from the generally proposed models of tetrahedral FeS precursors. In addition, the persistence of these peculiar species and the observed delay of Co incorporation in pyrite highlights the importance of trace elements in pyrite formation kinetics at low temperature. Lastly, our results illustrate the high affinity of Co for polysulfides and raise questions on the p","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 104-121"},"PeriodicalIF":4.5,"publicationDate":"2025-06-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144290724","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":"Fast CO2 uptake by intense weathering of volcanic islands during interglacial stages","authors":"Francisco Hevia-Cruz ,&nbsp;Anthony Hildenbrand ,&nbsp;Nathan D. Sheldon ,&nbsp;François Chabaux ,&nbsp;Fernando O. Marques ,&nbsp;Julie Carlut","doi":"10.1016/j.gca.2025.05.035","DOIUrl":"10.1016/j.gca.2025.05.035","url":null,"abstract":"<div><div>The weathering of basaltic rocks, especially on volcanic islands, plays a crucial role in global carbon cycling. Intense precipitation and warm atmospheric conditions accelerate weathering processes in these environments. While most estimates of weathering rates derive from river chemistry, soils and paleosols remain underexplored. In this study, we investigated the geochemistry of paleosols developed from volcanic rocks in the Azores Archipelago over the past 1 Myr. Precise geochronology of volcanic units bracketing paleosols indicates high soil formation rates (3–180 mm kyr⁻¹), similar to modern soil formation rates in tropical volcanic islands. Geochronological evidence suggests a logarithmic decrease of soil formation rates over time, with high initial values reaching near zero in less than 35 kyr. This might be attributed to a combination of cation depletion and precipitation of stable minerals. Paleosols have generally developed faster on pyroclastic deposits than on lava flows. However, those formed on lava flows required less vertical development to sustain high cation exports due to their higher density. Based on the geochemistry of paleosols and their parental materials, we estimated cation exports (0–2600 t km⁻² yr⁻¹) and associated CO₂ uptake (0–35 × 10⁶ Mol km⁻² yr⁻¹). These estimates generally exceed previous estimates based on the geochemistry of modern rivers in the Eastern Azores, by a factor of up to tenfold. Our data highlight the transient character of weathering processes and the criticality of precise geochronological control to constrain past weathering and soil formation rates. They further imply that atmospheric CO₂ may have experienced short episodes of intense consumption during interglacial stages, possibly contributing to subsequent cooling events over the past 1 Myr.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 62-76"},"PeriodicalIF":4.5,"publicationDate":"2025-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144253806","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}