Geochimica et Cosmochimica Acta最新文献

{"title":"Timescales of CM carbonaceous chondrite aqueous alteration determined by kinetic modelling","authors":"Robin L. Haller,&nbsp;Martin R. Lee,&nbsp;Mark E. Hodson","doi":"10.1016/j.gca.2026.04.022","DOIUrl":"10.1016/j.gca.2026.04.022","url":null,"abstract":"<div><div>Mighei-like (CM) meteorites are the most abundant group of hydrated carbonaceous chondrites. They display a wide range of degrees of aqueous alteration, from very mild to near complete, as demonstrated by the relative abundances of phyllosilicates and surviving anhydrous silicates. The reasons for differences in the extent of alteration are debated, and probably reflect variations within the CM parent body/bodies of one or more of water/rock (W/R) ratio, temperature, and timescale. The least understood is timescale, and so to constrain the duration of aqueous alteration and to investigate precipitation conditions of key CM minerals, we created kinetic models with the software PHREEQC. These models simulated a mineralogically detailed alteration of the CM3 proxy Dominion Range 08006 (CO3.00) by an aqueous fluid for different values of W/R ratio (by mass) and temperature over the timespan of 10⁻⁵–10⁵ years with incremental time steps. Additional laboratory experiments in which chips of Dominion Range 08006 were reacted with either pure water or a carbonated aqueous solution under reducing conditions were undertaken to validate parts of the models and to evaluate formation conditions of calcite, as the study of calcite in CM chondrites provides invaluable information on aqueous alteration timings and conditions. The models suggest that regardless of their petrologic type, CM chondrites could have been altered from a CM3 over timescales ranging from months to ∼10,000 years depending on temperature. The more highly altered CM1 chondrites that contain magnetite instead of cronstedtite would have needed to react at temperatures of at least 100°C but not necessarily different W/R ratios compared to less altered CM2s. Some of the modelled systems suggest that minimally altered CM chondrites like Asuka 12169 could have been exposed to aqueous solutions at ∼1°C and W/R ratios (by mass) of ∼3 or higher. Significant quantities of calcite only form in laboratory experiments with a carbonated solution, suggesting that at least for early formed calcite, aqueous carbonate was sourced from accreted C-bearing ice instead of indigenous organic carbon. According to the models, a typical CM2 chondrite could have been altered from a CM3 lithology within ∼1,000 years at 50°C and 0.8 W/R ratio (by mass).</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 389-405"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148553734","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Methane cycling and its roles in arsenic mobility in groundwater of floodplain aquifers","authors":"Song Feng,&nbsp;Zhipeng Gao,&nbsp;Yao Li,&nbsp;Yujie Liu,&nbsp;Xueda Hu,&nbsp;Huaming Guo","doi":"10.1016/j.gca.2026.04.021","DOIUrl":"10.1016/j.gca.2026.04.021","url":null,"abstract":"<div><div>The reductive dissolution of arsenic-bearing Fe(III) oxides is a key process driving groundwater arsenic mobility. In anoxic aquifers, both methane (CH₄) and organic matter are important electron donors for the Fe(III) oxide reduction. However, the relative contribution of these two pathways, i.e., Fe(III) oxide reduction coupled with CH₄ oxidation (AOM-Fe(III) reduction) and dissimilatory Fe(III) oxide reduction driven by organic matter degradation (DIR-Fe(III) reduction) to arsenic enrichment in groundwater remains poorly understood. Groundwater samples were taken along a groundwater flow path in the Hetao Basin (China) to determine δ¹³C_CH4 and δ⁵⁶Fe and geochemical characteristics. Groundwater CH₄, with concentrations from 0.002 to 29.9 mg/L, were mainly sourced from biogenic methanogenesis, as evidenced by δ¹³C_CH4 values &lt;−60‰ in around 88% of groundwater samples. The observed decrease in CH₄ concentrations, accompanied by a rapid increase in δ¹³C_CH4 values in reducing groundwater, indicated the occurrence of anaerobic oxidation of CH₄. Groundwater DOC, CH₄, Fe(II), and arsenic concentrations simultaneously increased along the groundwater flow path, indicating that both dissolved organic matter (DOM) and CH₄ were electron donors for Fe(III) oxide reduction. The more negative δ¹³C_CH4 and higher δ⁵⁶Fe values in the flat plain compared to the transition area suggested a decrease in the relative contribution of AOM-Fe(III) reduction from the transition area to the flat plain. The preferential use of CH₄ as electron donors in the transition area may be due to the limited biodegradable DOM. However, the DIR-Fe(III) reduction was stronger in the flat plain than that in the transition area, being supported by higher DOC concentrations and higher δ⁵⁶Fe values in the plain. Overall, the contribution of AOM-Fe(III) reduction to arsenic mobilization was more evident in the transition area, while arsenic enrichment in groundwater was primarily caused by the DIR-Fe(III) reduction in the flat plain. Our study highlights the importance of AOM-Fe(III) reduction in arsenic enrichment in groundwater, which provides a new perspective on the underlying mechanism of arsenic mobilization in floodplain aquifers.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 377-388"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147736826","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":"Experimental determination of uranium isotope fractionations during early diagenesis of green rust","authors":"Loren Tessier,&nbsp;Morten B. Andersen,&nbsp;Franck Poitrasson,&nbsp;Armel Descamps-Mandine,&nbsp;Camille Duquenoy,&nbsp;Ludovic Menjot,&nbsp;Pauline Méjean,&nbsp;Jérôme Chmeleff,&nbsp;Carole Causserand,&nbsp;Mathieu Leisen,&nbsp;Romain Guilbaud","doi":"10.1016/j.gca.2026.04.013","DOIUrl":"10.1016/j.gca.2026.04.013","url":null,"abstract":"<div><div>Uranium (U) isotope compositions of ancient carbonates and shales are increasingly used as a redox proxy to reconstruct the extent of past ocean oxygenation. In euxinic (anoxic and sulphide-rich) settings, it is well known that the reduction of labile U(VI) into solid U(IV) as reduced U oxides (<em>e.g.</em>, uraninite (UO₂)) is accompanied by a significant U isotope fractionation, with the heavy isotopes being preferentially incorporated into the reduced sediment. However, large uncertainty exists on the behaviour of U and its isotopes in ferruginous (anoxic and iron-rich) environments, which were prevalent for most of the Earth’s history, thus limiting our ability to interpret U isotope signatures in ancient sediments that deposited under those conditions. Here, we explore experimentally the fate of uranium during the diagenetic transformations of green rust, a predominant mineral in ferruginous environments. Sulphate and carbonate “Green Rust” minerals (GR(SO₄) and GR(CO₃), respectively), the most abundant green rust species in marine environments, were reacted with dissolved U(VI) at pH 8.4, and the effects of diagenesis were explored over a range of times and temperatures, from 1 h to 10 days at 25 °C and 90 °C. We find that the green rust species have different U reduction rates, and that U is immobilised in the sedimentary phase in the form of UO₂ during early diagenesis, suggesting that sedimentary U enrichments are a robust redox tracer even for ferruginous settings. We also report for the first time U isotope fractionation occurring during its abiotic reduction by GR(SO₄) and GR(CO₃). We show that reduced U is enriched in ²³⁸U over ²³⁵U with <span><math><mrow><msup><mrow><mi>Δ</mi></mrow><mn>238</mn></msup><msub><mi>U</mi><mrow><msub><mrow><mi>UO</mi></mrow><mn>2</mn></msub><mo>-</mo><mi>U</mi><mrow><mo>(</mo><mi>V</mi><mi>I</mi><mo>)</mo></mrow><mi>a</mi><mi>q</mi></mrow></msub></mrow></math></span> ranging from +0.92 to +1.68 ‰ for GR(SO₄) and from +0.40 to + 0.84 ‰ for GR(CO₃) at steady state. Our results imply that starting with modern seawater composition and under U removal at equilibrium between solid and fluid, sediments deposited under ferruginous conditions, and where GR(CO₃) is the predominant U-reducing mineral, should yield δ²³⁸U compositions ranging from ∼−0.2 to +0.4 ‰ (relative to CRM-145 standard), depending on the extent of reduction pathways and early diagenesis. By contrast, during the reduction by GR(SO₄), the nearly quantitative transfer of U to the solid phase suppresses any observable U isotope fractionation from the initial solution.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 271-287"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147681526","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hafnium and silicon isotope evidence for multiple unsilicified basaltic sources at the origin of Rhyacian TTG magmas in the Mineiro Belt (Brazil)","authors":"Syro Lacerda,&nbsp;Martin Guitreau,&nbsp;Leonardo Gonçalves,&nbsp;Hugo Moreira,&nbsp;Cristiane C. Gonçalves,&nbsp;Stéphany Rodrigues Lopes,&nbsp;Sandro Mauri,&nbsp;Marco Aurélio Pinheiro,&nbsp;Cassiano Costa e Castro,&nbsp;Régis Doucelance","doi":"10.1016/j.gca.2026.04.026","DOIUrl":"10.1016/j.gca.2026.04.026","url":null,"abstract":"<div><div>Tonalite-trondhjemite-granodiorite (TTGs) are silica-rich plutonic rocks that correspond to the dominant lithologies of Archean basements. Understanding their formation is essential to unravel the early Earth tectonic regime and constrain continental crust formation and evolution. Archean TTGs commonly exhibit heavy-silicon isotope compositions attributed to ocean-derived silica in their sources, thereby indicating an interaction with the surface. The sedimentary record shows that the dissolved silica in seawater (DSi) has decreased secularly from the Archean onward. Despite the general disappearance of TTGs after the Archean, some granitoids of TTG affinity can still be found in Paleoproterozoic crustal domains that are tectonically well-constrained. Comparing the geochemical characteristics of Archean and Proterozoic TTGs, as well as their sources, therefore appears important for feeding debates on the evolution of continental crust and tectonic settings. To shed new light on these questions, the present study examines the magmatic record of three Paleoproterozoic TTG units in the Mineiro Belt (Brazil) by focusing on whole-rock petrography and geochemistry alongside zircon U-Pb geochronology, elemental, and isotopic (Hf and Si) geochemistry. These new data indicate episodic formation of TTG magmas during the Rhyacian (2.15–2.10 Ga), mainly driven by melting of Rb- and Ba-enriched low-K mafic sources with variably long crustal residence times (∼0.2–0.8 Ga). Studied Paleoproterozoic TTGs exhibit lower silicon isotopic signatures than their Archean counterparts. This difference is likely linked to variations in source pre-melting histories, such that they may not have been hydrothermally silicified, seawater may have had a distinct Si isotope signature, or the mechanisms of Si enrichment may not have been the same as for Archean TTG sources. Finally, a limited, though detectable, influence of pre-existing crust is nevertheless visible in these magmas through zircon inheritance, which could also account for some of the low Si isotope signatures observed. Our study also provides an alternative explanation for the apparent homogenization of Si isotope signatures in the upper-continental crust over time, since Proterozoic TTGs can exhibit relatively low Si isotope signatures compared to their Archean equivalents.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 445-461"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148553769","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sector-zoned zircon: differential trace element incorporation and implications for petrogenetic indices","authors":"John T. Caulfield,&nbsp;Teresa Ubide,&nbsp;Charlotte M Allen,&nbsp;Tony Wang","doi":"10.1016/j.gca.2026.04.024","DOIUrl":"10.1016/j.gca.2026.04.024","url":null,"abstract":"<div><div>Zircon is a chemically and physically resistant accessory mineral that is commonly used to date and characterise intermediate to felsic rocks and associated metal ore deposits. The development of robust petrogenetic vectors, however, relies on understanding the distribution of trace elements in zircon, which often show complexity such as sector zoning. Sector-zoned grains comprise cathodoluminescence (CL)-dark and CL-light prism sectors growing perpendicular to the c-axis, making the sides of the crystal, and CL-intermediate pyramid sectors growing along the c-axis, building crystal tips.</div><div>To assess the chemical impact of sector zoning in zircon, here we investigate texturally constrained Andean detrital zircon data (n = 585), demonstrating that: (1) the REE + Y (except Ce) and Th are enriched in dark prisms relative to light prisms and pyramids; (2) the extent of dark prism enrichment increases from heavy to light REE (up to &gt;300 %) as ionic potential decreases, reflecting the preferential substitution of low charge density cations in protosites accompanied by in-plane oxygen atoms; (3) Al is enriched in light prisms, where increased bond spacing within the zircon atomic lattice is interpreted to more easily accommodate the large Al cation in the Si site; (4) Ti, Zr, Hf, U and Ce are sector-independent; and (5) the highly charged cations P⁺⁵, Nb⁺⁵ and Ta⁺⁵ show limited inter-sector dependence, but show clear sector trends versus ΣREE + Y (molar). These trends suggest sector-specific incorporation: light prisms follow the xenotime substitution mechanism whereas dark prisms and pyramids require additional charge balancing configurations. We show that distinct atomic arrangements on discrete zircon sector growth surfaces result in differential uptake of cations from the melt, leading to sector specific partition coefficients.</div><div>Assessment of the role of sector zoning on zircon petrogenetic indices shows that indicators dependent on Ti, Zr, Hf, U and Ce only (e.g. Ce/√(U × Ti)) remain robust as they are sector-independent. Similarly, indices that ratio neighbouring REE with similar ionic potential show minimal sector zoning effects (e.g. Eu/Eu*). In contrast, we caution the use of petrogenetic indicators incorporating elements that show variable sensitivity to sector zoning (e.g. Ce/Ce*, Dy/Yb, (10³ × Ce/Nd)/Y, (10⁴ × Eu/Eu*)/Y), which may generate misleading trends. Intriguingly, the utility of sector zoning combined with key geochemical characteristics of zircon (e.g. low P) shows promise as a recorder of the dynamic crystallisation conditions associated with the development of porphyry copper mineralisation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 406-418"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148566646","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Carbon and phosphorus regeneration during organic matter decomposition in aquatic ecosystems: characteristics, mechanisms, and environmental effects","authors":"Chao Yin,&nbsp;Yan Zeng,&nbsp;Jingan Chen,&nbsp;Peng Liao,&nbsp;Jia Yu,&nbsp;Guangrong Ran,&nbsp;Baohua Xiao,&nbsp;Xudong Guo,&nbsp;Ziyan Zhang,&nbsp;Jingfu Wang,&nbsp;Haiquan Yang,&nbsp;Quan Yuan","doi":"10.1016/j.gca.2026.04.018","DOIUrl":"10.1016/j.gca.2026.04.018","url":null,"abstract":"<div><div>Carbon (C) and phosphorus (P) are essential biogenic elements on Earth, significantly influencing ecosystems and climate change through their cycles within the Earth’s spheres. During organic matter (OM) formation, C and P are assimilated in specific stoichiometric ratios. However, it has long been debated whether C and P regeneration follows the same proportional relationship during OM decomposition. In this study, typical terrestrial plants and aquatic algae were selected for decomposition simulation experiments. The content of organic carbon (C_org) and organic phosphorus (P_org), along with their speciation, was systematically investigated to uncover differences in their regeneration behaviors. The results showed that the (C:P)_org ratios (in mol) of OM residues generally increased during the early decomposition stage, then decreased as decomposition continued. These ratios were higher under anoxic conditions compared to oxic conditions and remained consistently above their initial values throughout the process. This indicated that the preferential regeneration of P over C mainly occurred in the early decomposition stage and was more pronounced under anoxic conditions. Moreover, the preferential regeneration of P was more pronounced in more labile OM. Fourier transform infrared spectroscopy (FTIR) results indicated that the low bond energies of P_org-functional groups were the primary factor underlying preferential regeneration of P relative to C. Consistently, ³¹P-Nuclear magnetic resonance spectroscopy (³¹P NMR) and ¹³C-Nuclear magnetic resonance spectroscopy (¹³C NMR) data also indicated that P_org was regenerated more rapidly than C_org during the early stage of OM decomposition. Global warming and enhanced aquatic hypoxia are expected to intensify the preferential regeneration of P, thereby promoting eutrophication and increasing carbon sequestration. This study provides new insights into mechanisms sustaining eutrophication under global warming and for accurately assessing carbon sink-source dynamics in aquatic ecosystems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 331-346"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147736828","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":"Towards understanding archaeal lipid biomarker (GDGT) distributions in central Arctic Ocean sediments","authors":"Jannik Martens,&nbsp;Bärbel Hönisch,&nbsp;Helen K. Coxall,&nbsp;Matt O’Regan,&nbsp;Flor Vermassen,&nbsp;Martin Jakobsson,&nbsp;Beizhan Yan,&nbsp;Sarah Hurley","doi":"10.1016/j.gca.2026.04.019","DOIUrl":"10.1016/j.gca.2026.04.019","url":null,"abstract":"<div><div>The Arctic is undergoing rapid climate change, warming nearly four times faster than the global average, leading to increasingly ice-free conditions and significant changes in biogeochemical feedbacks in the Arctic Ocean. Understanding past climate variability in this region is crucial for constraining the effects of ongoing climate change. Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are microbial membrane lipids widely used to reconstruct past ocean temperatures from sediment archives. However, proxy-temperature relationships based on GDGTs in the Arctic Ocean differ from those observed in global calibration datasets. In high-latitude environments, GDGT-based proxies display reduced sensitivity and increased variability, potentially limiting their reliability for temperature reconstructions. These inconsistencies may be due to distinct processes governing GDGT distributions in these regions, highlighting the need for further investigation of their spatial and temporal variability.</div><div>This study determines concentrations and relative abundances of relevant GDGT groups (isoprenoid, hydroxylated, and branched) in sediment cores and surface sediments from the Arctic Ocean. We observe greater-than-expected variability in downcore records of the temperature proxies TEX₈₆ and those based on hydroxylated GDGTs (e.g., TEX₈₆^OH or RI-OH’), exceeding spatial variability observed in Arctic Ocean surface sediments or as expected due to past climate change. GDGT concentrations in downcore sediments are an order of magnitude lower than at the sediment surface and more than two orders of magnitude lower than in surface sediments from more productive outer shelf and slope environments. These differences in concentrations suggest that GDGT records in central Arctic sediments may be highly influenced by shifts in terrestrial input and lateral transport of allochthonous GDGTs, as well as possible in situ production of GDGTs. Substantial and consistent subsurface temperature excursions in downcore records are potentially linked to in situ GDGT production by methanotrophic archaea. Furthermore, the abundance of hydroxylated GDGTs may depend on microbial community changes, potentially signifying past changes in biological productivity alongside calcareous microfossils. These complexities contribute to large variability and systematic temperature misrepresentation in GDGT-based temperature estimates. Our findings emphasize the need for a refined understanding of GDGT sources when interpreting GDGT-derived climate records in complex environments such as the Arctic Ocean.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 347-360"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148553735","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Laboratory-grown crust in planktic foraminifera Neogloboquadrina pachyderma; insights into resolving inaccuracies in polar palaeotemperature estimates","authors":"Adele Westgård,&nbsp;Mohamed M. Ezat,&nbsp;Freya E. Sykes,&nbsp;Gavin L. Foster,&nbsp;Julie Meilland,&nbsp;Thomas B. Chalk,&nbsp;J. Andy Milton","doi":"10.1016/j.gca.2026.04.027","DOIUrl":"10.1016/j.gca.2026.04.027","url":null,"abstract":"<div><div>Proxy based records of past changes in the polar ocean-cryosphere-climate system can provide invaluable constraints on ongoing and future climate change. However, the dominant planktic foraminiferal species in polar regions, <em>Neogloboquadrina pachyderma</em>, exhibits considerable intra- and inter-test heterogeneity in Mg/Ca and potentially other elemental ratios, compromising their utility as palaeothermometers and tracers of other environmental variables such as carbonate chemistry. In particular, <em>N. pachyderma</em> often exhibits a thick “crust” characterised by low Mg/Ca calcite outside its higher Mg/Ca internal “lamellar calcite”. The cause of this heterogeneity is currently uncertain, but it is variably ascribed to environmental change via depth migration or to a “vital effect” related to variations in biomineralisation mechanism and/or foraminiferal physiology.</div><div>To help resolve this issue, we performed large scale culturing experiments of <em>N. pachyderma</em> under controlled environmental conditions and used Laser Ablation ICP-MS to quantify the intra-shell variability in trace element ratios. By developing an open-access script we can distinguish between the crust and lamellar calcite using the trace element profiles themselves and as a consequence we document, for the first time, different Na/Ca, Li/Ca, Mg/Ca, and B/Ca in the calcite crust of <em>N. pachyderma</em> compared to the inner lamellar calcite, even when both components are grown in equivalent environmental conditions. This implies that the crust and lamellar calcite of <em>N. pachyderma</em> are formed through either contrasting biomineralisation strategies and/or varied precipitation rates, leading not only to distinct geochemical compositions across multiple elements, but also to varying sensitivities to changing environmental conditions. Given the crust constitutes a large, but variable percentage of a specimen’s final shell weight, this complicates the application of proxies using this and other similar species. It is therefore recommended that future studies either apply in situ techniques, such as applied here, to analytically separate the crust from lamellar calcite, or target specimens with only one component to avoid the complicating influence of this “vital effect” on environmental reconstruction.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 462-478"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148553736","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Carbon and oxygen isotope compositions in calcite from the CM chondrites Aguas Zarcas and MIL 13005: Implications for O and C sources and alteration conditions","authors":"S.P. Goudy,&nbsp;M. Telus,&nbsp;K. Nagashima,&nbsp;G.R. Huss","doi":"10.1016/j.gca.2026.04.014","DOIUrl":"10.1016/j.gca.2026.04.014","url":null,"abstract":"<div><div>Here we present petrographic, O isotope, and C isotope data on calcite and petrographic and O isotope data on magnetite in Aguas Zarcas (CM2) and Miller Range (MIL) 13005 (CM1/2) in an effort to test CM aqueous alteration models. Our O and C isotope data for Aguas Zarcas and MIL 13005 are within the ranges reported in previous work for calcite in CMs. Using O isotope data from Δ¹⁷O-matched calcite and magnetite grains in our samples, we calculated equilibrium-model formation temperatures for the analyzed calcites in each meteorite. Combining our isotopic and temperature data with literature data, we sort the data into less altered (CM2) and more altered (CM1/2 and CM1) categories, and examine that data for differences between the categories by analyzing the δ¹⁸O-δ¹⁷O, δ¹⁸O-δ¹³C, and model formation temperature data of the categories. We find potential differing cluster patterns between δ¹⁸O and δ¹³C in calcites between our two CM alteration categories, and find that sparse extant temperature data imply that more altered CMs (types 1 and 1/2) may have undergone alteration at a lower average temperature than CM2s. We also find that the O isotopic compositions between CM1s, CM2s, and CM1/2s do not differ significantly. Through use of a novel mass-balance model, we infer a pre-alteration ice Δ¹⁷O within the range of 5.3–13.3‰. We find two generations of calcite formation in MIL 13005 with different proportions of their O being sourced from ices and anhydrous silicates, and one generation of calcites within Aguas Zarcas. We created concentration mixing models through an original machine-learning-based analytical approach, which show that the calcite data can be readily explained using three optimally determined C- and O-bearing sources.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 288-299"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147681530","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Reversible anaerobic oxidation of methane, carbon assimilation, and mineral authigenesis in pockmark sediments over the past 40 kyr: Insights from C–S–O isotopes and diagenetic modelling","authors":"Jurjen Rooze,&nbsp;Pierre Sansjofre,&nbsp;Christof Meile,&nbsp;Michael Ernst Böttcher,&nbsp;Gwenael Jouet,&nbsp;Lucie Pastor","doi":"10.1016/j.gca.2026.04.020","DOIUrl":"10.1016/j.gca.2026.04.020","url":null,"abstract":"<div><div>The long-term effects of deeply sourced methane (CH₄) seepage on sediment geochemistry, authigenic mineral formation, and stable isotope signatures were investigated by comparing sediment cores retrieved from a seep and a reference non-seep site on the northwestern continental margin of Madagascar. To constrain the underlying diagenetic processes, measurements of C, S, and Fe in solid and dissolved phases, stable isotope (δ¹³C, δ³⁴S, δ¹⁸O) signatures, and ¹⁴C sediment dating were combined with reaction-transport modeling. This approach enabled distinction between effects related to seepage and anaerobic oxidation of methane (AOM) and those associated with organic matter mineralization.</div><div>Assimilation of CH₄-derived carbon can form ¹³C-depleted organic carbon (OC), which results in a substantially more negative δ¹³C-OC at the seep than at the control site throughout the 0‒10 m core. Measurements showed a correlation between the concentration of authigenic mineral phases and the isotopic depletion of bulk OC and also stronger depletion in OC isolated from carbonate concretions. Modeling further suggests that the carbonates provide physical protection and that this mechanism is responsible for the preservation of the overall depleted bulk OC signal during burial. The δ¹³C and δ¹⁸O in carbonate minerals vary substantially with depth, but the C and O isotopic signatures are strongly negatively correlated. This isotopic signature is interpreted to reflect a mixture between a depositional end-member formed in tropical surface water and an authigenic overprint at lower bottom-water temperatures driven by AOM. Additionally, the modeling demonstrates that reversible AOM and gas dissolution effectively shape the dissolved δ¹³C-CH₄ profile, yielding an excellent model fit when the rate of AOM is simulated with a Monod-dependency on both the electron donor (CH₄) and terminal acceptor (SO₄²⁻). Finally, our analyses document the effects of millennial-scale seepage on the bulk geochemistry, the dithionite extractable iron pool, the degree of pyritization, and δ³⁴S-FeS₂ profiles.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"424 ","pages":"Pages 361-376"},"PeriodicalIF":5.2,"publicationDate":"2026-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147736827","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}