Pub Date : 2025-12-19DOI: 10.1016/j.gca.2025.12.026
Getachew Agmuas Adnew , Moritz Schroll , Thomas Röckmann , Frank Keppler , Sarah Elise Sapper , Christian Juncher Jørgensen , Thomas Blunier , Thomas Laemmel , Sönke Szidat , Carina van der Veen , Jesper Riis Christiansen
Subglacial meltwater from the western margin of the Greenland Ice Sheet (GrIS) is a net source of methane (CH4) and carbon dioxide (CO2). Estimating CH4 and CO2 emissions from GrIS meltwater and their relevance to the global carbon budget requires understanding their sources and controlling mechanisms. In this study, we measured the stable isotope composition and radiocarbon content of CH4 and CO2, as well as the Bernard ratio, for gas samples extracted from subglacial meltwater and air samples collected from an ice cave at the edge of Isunnguata Sermia glacier (ISG).
In June 2022, the concentration of CH4 in the subglacial meltwater at ISG was 100 times higher than in the supraglacial meltwater. Stable isotope data and Bernard ratios indicated that CH4 dynamics in subglacial meltwater were mainly controlled by microbial oxidation and hydrological conditions. In peak melt season, the bulk isotope composition of CH4 was enriched compared to early melt season, likely due to enhanced microbial CH4 oxidation.
The apparent 14C-age of subglacial CH4 at ISG is about 1.6 kyr BP, in good agreement with Neoglacial ice sheet advance. This implies that the source of CH4 is neither thermogenic nor from hydrates, as such sources would have a 14C-free signal. CO2 is older than CH4 (about 6 kyr BP). The different apparent ages of CH4 and CO2 indicate that the source of CH4 is not CO2 reduction. The most probably source of subglacial CH4 at ISG is acetoclastic methanogensis of organic matter buried under the ice sheet during the Neoglacial ice sheet advance. CO2 appears to originate from the remineralization of older carbon in sediments or soils, as well as from CH4 oxidation.
{"title":"Radiocarbon and bulk isotope composition of subglacial methane and carbon dioxide emitted at the western margin of the Greenland ice sheet","authors":"Getachew Agmuas Adnew , Moritz Schroll , Thomas Röckmann , Frank Keppler , Sarah Elise Sapper , Christian Juncher Jørgensen , Thomas Blunier , Thomas Laemmel , Sönke Szidat , Carina van der Veen , Jesper Riis Christiansen","doi":"10.1016/j.gca.2025.12.026","DOIUrl":"10.1016/j.gca.2025.12.026","url":null,"abstract":"<div><div>Subglacial meltwater from the western margin of the Greenland Ice Sheet (GrIS) is a net source of methane (CH<sub>4</sub>) and carbon dioxide (CO<sub>2</sub>). Estimating CH<sub>4</sub> and CO<sub>2</sub> emissions from GrIS meltwater and their relevance to the global carbon budget requires understanding their sources and controlling mechanisms. In this study, we measured the stable isotope composition and radiocarbon content of CH<sub>4</sub> and CO<sub>2</sub>, as well as the Bernard ratio, for gas samples extracted from subglacial meltwater and air samples collected from an ice cave at the edge of Isunnguata Sermia glacier (ISG).</div><div>In June 2022, the concentration of CH<sub>4</sub> in the subglacial meltwater at ISG was 100 times higher than in the supraglacial meltwater. Stable isotope data and Bernard ratios indicated that CH<sub>4</sub> dynamics in subglacial meltwater were mainly controlled by microbial oxidation and hydrological conditions. In peak melt season, the bulk isotope composition of CH<sub>4</sub> was enriched compared to early melt season, likely due to enhanced microbial CH<sub>4</sub> oxidation.</div><div>The apparent <sup>14</sup>C-age of subglacial CH<sub>4</sub> at ISG is about 1.6 kyr BP, in good agreement with Neoglacial ice sheet advance. This implies that the source of CH<sub>4</sub> is neither thermogenic nor from hydrates, as such sources would have a <sup>14</sup>C-free signal. CO<sub>2</sub> is older than CH<sub>4</sub> (about 6 kyr BP). The different apparent ages of CH<sub>4</sub> and CO<sub>2</sub> indicate that the source of CH<sub>4</sub> is not CO<sub>2</sub> reduction. The most probably source of subglacial CH<sub>4</sub> at ISG is acetoclastic methanogensis of organic matter buried under the ice sheet during the Neoglacial ice sheet advance. CO<sub>2</sub> appears to originate from the remineralization of older carbon in sediments or soils, as well as from CH<sub>4</sub> oxidation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"414 ","pages":"Pages 328-342"},"PeriodicalIF":5.0,"publicationDate":"2025-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785263","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}
Pub Date : 2025-12-19DOI: 10.1016/j.gca.2025.12.038
Liheng Sun , Yunying Zhang , Kan Li , Zhen Sun , Ruifang Huang , Long Li
Nitrogen (N) enrichment during alteration of the upper oceanic crust is a key regulator of N fluxes across the hydrosphere and lithosphere. However, the mechanisms that govern N uptake in altered oceanic crust (AOC) and its contribution to subduction budgets remain inadequately constrained, especially in marginal seas. Here, we present N concentration and isotopic data of altered basalts recovered from IODP Holes U1500B, U1503A, U1431E, U1433B, and U1434A in the South China Sea (SCS), a representative marginal sea. Our data show that the SCS altered basalts exhibit significantly higher N enrichment (7.0 to 72.6 ppm) than altered basalts from open ocean floors (1.3 to 48.4 ppm), while their δ15N values are broadly comparable (−6.6 ‰ to + 5.8 ‰ for the SCS basalts versus − 20.1 ‰ to + 8.3 ‰ for open ocean basalts). Two-component mixing modeling indicates that the enriched N in SCS altered basalts was mostly derived from sediments/seawater with a minor contribution from abiotic N2 reduction. The pronounced N enrichment in the upper AOC of the SCS verifies that high N availability from the overlying clay-rich sediments (N = 227.0 to 453.8 ppm) is a main controlling factor. Meanwhile, our results also reveal that the alteration degree of basalt emerges as an important modulator of the magnitude of N uptake under high N-availability conditions. Integrating our new findings, we refine the global N input flux of the upper AOC section into subduction zones to 6.8 ± 0.7 × 109 mol·yr−1, nearly doubled previous estimates. Although marginal seas constitute only ∼ 15 % of global trench lengths, they contribute 25 % of global N input flux from the upper AOC, owing to their elevated unit N input flux (25.9 ± 8.1 × 104 mol yr−1∙km−1) relative to open ocean floors (15.6 ± 1.6 × 104 mol∙yr−1∙km−1). Together, these results highlight the pivotal role of marginal seas AOC in the deep N cycle and motivate a refined understanding of global N recycling that accounts for tectonic setting and alteration history.
{"title":"Significant nitrogen enrichment in altered upper oceanic crust in marginal seas: New insights into global subducting nitrogen budget and deep nitrogen recycling","authors":"Liheng Sun , Yunying Zhang , Kan Li , Zhen Sun , Ruifang Huang , Long Li","doi":"10.1016/j.gca.2025.12.038","DOIUrl":"10.1016/j.gca.2025.12.038","url":null,"abstract":"<div><div>Nitrogen (N) enrichment during alteration of the upper oceanic crust is a key regulator of N fluxes across the hydrosphere and lithosphere. However, the mechanisms that govern N uptake in altered oceanic crust (AOC) and its contribution to subduction budgets remain inadequately constrained, especially in marginal seas. Here, we present N concentration and isotopic data of altered basalts recovered from IODP Holes U1500B, U1503A, U1431E, U1433B, and U1434A in the South China Sea (SCS), a representative marginal sea. Our data show that the SCS altered basalts exhibit significantly higher N enrichment (7.0 to 72.6 ppm) than altered basalts from open ocean floors (1.3 to 48.4 ppm), while their δ<sup>15</sup>N values are broadly comparable (−6.6 ‰ to + 5.8 ‰ for the SCS basalts versus − 20.1 ‰ to + 8.3 ‰ for open ocean basalts). Two-component mixing modeling indicates that the enriched N in SCS altered basalts was mostly derived from sediments/seawater with a minor contribution from abiotic N<sub>2</sub> reduction. The pronounced N enrichment in the upper AOC of the SCS verifies that high N availability from the overlying clay-rich sediments (N = 227.0 to 453.8 ppm) is a main controlling factor. Meanwhile, our results also reveal that the alteration degree of basalt emerges as an important modulator of the magnitude of N uptake under high N-availability conditions. Integrating our new findings, we refine the global N input flux of the upper AOC section into subduction zones to 6.8 ± 0.7 × 10<sup>9</sup> mol·yr<sup>−1</sup>, nearly doubled previous estimates. Although marginal seas constitute only ∼ 15 % of global trench lengths, they contribute 25 % of global N input flux from the upper AOC, owing to their elevated unit N input flux (25.9 ± 8.1 × 10<sup>4</sup> mol yr<sup>−1</sup>∙km<sup>−1</sup>) relative to open ocean floors (15.6 ± 1.6 × 10<sup>4</sup> mol∙yr<sup>−1</sup>∙km<sup>−1</sup>). Together, these results highlight the pivotal role of marginal seas AOC in the deep N cycle and motivate a refined understanding of global N recycling that accounts for tectonic setting and alteration history.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"415 ","pages":"Pages 166-180"},"PeriodicalIF":5.0,"publicationDate":"2025-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785259","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}
Analysis of carbonate minerals in ungrouped carbonaceous chondrites offer valuable insights into the geological activity on a diverse range of early-formed, hydrated planetesimals in the outer Solar System. Essebi is a C2-ung chondrite, which originated from a water-rich asteroid with close affinities to the CM chondrites group. We performed a detailed geochemical, petrographic and isotopic study of Essebi. Modal mineralogy demonstrates that Essebi is dominated by a poorly crystalline, fine-grained phyllosilicate matrix (mostly a mix of saponite and serpentine ∼63 vol%) with a modest quantity of anhydrous silicates (20 vol%) and accessory magnetite (7.5 vol%), Fe-sulphides (5.5 vol%) and carbonates (2 vol%). Its bulk O-isotope composition (2.71 ‰ δ17O (± 0.018 1σ), 8.11 ‰ δ18O (± 0.002 1σ) and −1.53 ‰ Δ17O (± 0.017 1σ) and 2.56 ‰ δ17O (± 0.040 1σ), 7.65 ‰ δ18O (± 0.009 1σ) and −1.42 ‰ Δ17O (± 0.039 1σ)) places Essebi as part of the “CM field”, although overlapping with the “CR field”. Petrographic observations reveal multiple generations of carbonate that formed both before and after brecciation, exhibiting distinct characteristics that differ from the carbonates found in established groups (CMs). Essebi’s carbonate generations have distinct morphologies and C- and O- isotope compositions and, based on these data, are interpreted as two main generations and a series of other localised carbonate expressions.
The first generation (GA) carbonates formed prior to phyllosilicate growth, and have inferred maximum formation temperatures of +45 °C. They formed under high water-to-rock (W/R) ratios. The second generation (GB) carbonates show lower W/R ratios and at higher, although unquantified temperatures. They formed near the end of the alteration sequence from a residual fluid containing abundant dissolved cations. In addition to the two main generations, we identified a third population of vein carbonates (GC) that partially infilled fractures generated by brecciation. We also identified dolomite (GD) grains found exclusively within an xenolithic clast. This clast displays a more advanced stage of alteration (C1-ung) and shows evidence of fluid leaching after being embedded, resulting in the formation of a localized ring of calcites, referred to as GE, that remain distinct from all other carbonates in this sample.
Despite textural differences, the isotopic trends observed in these Essebi carbonates closely resemble the sequence described by T1 and T2 calcites in CM chondrites, suggesting that multiple distinct episodes of carbonate precipitation, aqueous alteration along a prograde metasomatic sequence, and isotopic evolution from 16O-poor to 16O-rich trajectories were common across several water-rich planetesimals that formed in the outer Solar System.
{"title":"Fluid history in the ungrouped C2 Essebi meteorite parent body","authors":"L.J. Riches , M.D. Suttle , I.A. Franchi , X. Zhao , M.M. Grady","doi":"10.1016/j.gca.2025.12.035","DOIUrl":"10.1016/j.gca.2025.12.035","url":null,"abstract":"<div><div>Analysis of carbonate minerals in ungrouped carbonaceous chondrites offer valuable insights into the geological activity on a diverse range of early-formed, hydrated planetesimals in the outer Solar System. Essebi is a C2-ung chondrite, which originated from a water-rich asteroid with close affinities to the CM chondrites group. We performed a detailed geochemical, petrographic and isotopic study of Essebi. Modal mineralogy demonstrates that Essebi is dominated by a poorly crystalline, fine-grained phyllosilicate matrix (mostly a mix of saponite and serpentine ∼63 vol%) with a modest quantity of anhydrous silicates (20 vol%) and accessory magnetite (7.5 vol%), Fe-sulphides (5.5 vol%) and carbonates (2 vol%). Its bulk O-isotope composition (2.71 ‰ δ<sup>17</sup>O (± 0.018 1σ), 8.11 ‰ δ<sup>18</sup>O (± 0.002 1σ) and −1.53 ‰ Δ<sup>17</sup>O (± 0.017 1σ) and 2.56 ‰ δ<sup>17</sup>O (± 0.040 1σ), 7.65 ‰ δ<sup>18</sup>O (± 0.009 1σ) and −1.42 ‰ Δ<sup>17</sup>O (± 0.039 1σ)) places Essebi as part of the “CM field”, although overlapping with the “CR field”. Petrographic observations reveal multiple generations of carbonate that formed both before and after brecciation, exhibiting distinct characteristics that differ from the carbonates found in established groups (CMs). Essebi’s carbonate generations have distinct morphologies and C- and O- isotope compositions and, based on these data, are interpreted as two main generations and a series of other localised carbonate expressions.</div><div>The first generation (GA) carbonates formed prior to phyllosilicate growth, and have inferred maximum formation temperatures of +45 °C. They formed under high water-to-rock (W/R) ratios. The second generation (GB) carbonates show lower W/R ratios and at higher, although unquantified temperatures. They formed near the end of the alteration sequence from a residual fluid containing abundant dissolved cations. In addition to the two main generations, we identified a third population of vein carbonates (GC) that partially infilled fractures generated by brecciation. We also identified dolomite (GD) grains found exclusively within an xenolithic clast. This clast displays a more advanced stage of alteration (C1-ung) and shows evidence of fluid leaching after being embedded, resulting in the formation of a localized ring of calcites, referred to as GE, that remain distinct from all other carbonates in this sample.</div><div>Despite textural differences, the isotopic trends observed in these Essebi carbonates closely resemble the sequence described by T1 and T2 calcites in CM chondrites, suggesting that multiple distinct episodes of carbonate precipitation, aqueous alteration along a prograde metasomatic sequence, and isotopic evolution from <sup>16</sup>O-poor to <sup>16</sup>O-rich trajectories were common across several water-rich planetesimals that formed in the outer Solar System.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"415 ","pages":"Pages 146-165"},"PeriodicalIF":5.0,"publicationDate":"2025-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785265","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}
Pub Date : 2025-12-18DOI: 10.1016/j.gca.2025.12.014
William F. Heck, Achim D. Herrmann, Colin R.M. Jackson
{"title":"Erratum to “Experimental degassing of moderately volatile chalcophile elements from silicate melt with and without sulfur present” [Geochim. Cosmochim. Acta 402 (2025) 153–172]","authors":"William F. Heck, Achim D. Herrmann, Colin R.M. Jackson","doi":"10.1016/j.gca.2025.12.014","DOIUrl":"10.1016/j.gca.2025.12.014","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"413 ","pages":"Page 267"},"PeriodicalIF":5.0,"publicationDate":"2025-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145786147","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}
Pub Date : 2025-12-18DOI: 10.1016/j.gca.2025.12.013
Zhewen Xu , Patrick J. Frings , Cong-Qiang Liu , Gaojun Li
The Li isotopic composition of seawater (δ7LiSW) has been recognized as a promising tracer for deep-time geochemical cycles, reflecting water–rock reactions linked to the interactions between lithosphere, hydrosphere, biosphere and atmosphere. A major challenge in interpreting δ7LiSW is the significantly lower δ7LiSW values observed during many intervals of the geological history compared to the present-day level. Specifically, a δ7LiSW lower than ∼22 ‰ generally requires a globally riverine input with an average δ7Li much lower than that of the hydrothermal input (∼8.3 ‰) while keeping the net Li isotopic fractionation between seawater and Li sinks unchanged. However, riverine δ7Li values lower than ∼8.3 ‰ are relatively uncommon in modern Earth surface environments, whose spatial variability likely resembles a wide range of environments in the geological past. This study suggests that the low δ7LiSW of the Precambrian to early Paleozoic ocean can be attributed to the low efficiency of continental silicate weathering, driven by the low partial pressure of atmospheric oxygen (pO2) that results in shallow weathering fronts and thus thin regolith. A numerical model shows that a two-order-of-magnitude increase in pO2 from the Neoproterozoic to the Paleozoic could induce a ∼22 ‰ rise in δ7LiSW, fully accounting for the observed rise of δ7LiSW during this period. This is because enhanced continental weathering not only increases riverine Li input to the ocean with high δ7Li, but also provides more degraded weathering products that serve as precursors for authigenic clay formation in marine sediments, thus leading to a higher net Li isotopic fractionation between seawater and Li sinks.
{"title":"Linking lithium isotopic composition of seawater to atmospheric oxygenation","authors":"Zhewen Xu , Patrick J. Frings , Cong-Qiang Liu , Gaojun Li","doi":"10.1016/j.gca.2025.12.013","DOIUrl":"10.1016/j.gca.2025.12.013","url":null,"abstract":"<div><div>The Li isotopic composition of seawater (δ<sup>7</sup>Li<sub>SW</sub>) has been recognized as a promising tracer for deep-time geochemical cycles, reflecting water–rock reactions linked to the interactions between lithosphere, hydrosphere, biosphere and atmosphere. A major challenge in interpreting δ<sup>7</sup>Li<sub>SW</sub> is the significantly lower δ<sup>7</sup>Li<sub>SW</sub> values observed during many intervals of the geological history compared to the present-day level. Specifically, a δ<sup>7</sup>Li<sub>SW</sub> lower than ∼22 ‰ generally requires a globally riverine input with an average δ<sup>7</sup>Li much lower than that of the hydrothermal input (∼8.3 ‰) while keeping the net Li isotopic fractionation between seawater and Li sinks unchanged. However, riverine δ<sup>7</sup>Li values lower than ∼8.3 ‰ are relatively uncommon in modern Earth surface environments, whose spatial variability likely resembles a wide range of environments in the geological past. This study suggests that the low δ<sup>7</sup>Li<sub>SW</sub> of the Precambrian to early Paleozoic ocean can be attributed to the low efficiency of continental silicate weathering, driven by the low partial pressure of atmospheric oxygen (<em>p</em>O<sub>2</sub>) that results in shallow weathering fronts and thus thin regolith. A numerical model shows that a two-order-of-magnitude increase in <em>p</em>O<sub>2</sub> from the Neoproterozoic to the Paleozoic could induce a ∼22 ‰ rise in δ<sup>7</sup>Li<sub>SW</sub>, fully accounting for the observed rise of δ<sup>7</sup>Li<sub>SW</sub> during this period. This is because enhanced continental weathering not only increases riverine Li input to the ocean with high δ<sup>7</sup>Li, but also provides more degraded weathering products that serve as precursors for authigenic clay formation in marine sediments, thus leading to a higher net Li isotopic fractionation between seawater and Li sinks.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"414 ","pages":"Pages 259-273"},"PeriodicalIF":5.0,"publicationDate":"2025-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145786146","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}
Pub Date : 2025-12-18DOI: 10.1016/j.gca.2025.12.041
S. Bhattacharya , M. Paul , R.N. Sahoo , R. Purtschert , H.F.R. Hoffmann , M. Pichotta , K. Zuber , D. Bemmerer , T. Döring , R. Schwengner , M.L. Avila , E. Lopez-Saavedra , J.C. Dickerson , C. Fougères , J. McLain , R.C. Pardo , K.E. Rehm , R. Scott , I. Tolstukhin , R. Vondrasek , S. Vaintraub
<div><div>The cosmogenic <sup>39</sup>Ar(t<span><math><msub><mrow></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub></math></span> = 268 years) isotope of argon is used for geophysical dating and tracing of underground and ocean water, as well as ice owing to its appropriate half-life and chemical inertness as a noble gas; <sup>39</sup>Ar serves also in nuclear weapon test monitoring. We measured for the first time the total cross section of the main <sup>39</sup>Ar cosmogenic production reaction in the atmosphere, namely <sup>40</sup>Ar<span><math><msup><mrow><mrow><mo>(</mo><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi><mo>)</mo></mrow></mrow><mrow><mn>39</mn></mrow></msup></math></span>Ar, using 14.8 ± 0.3 <span><math><mi>MeV</mi></math></span> neutrons. The neutrons, produced by a deuterium-tritium generator, impinged on a stainless steel sphere filled with Ar gas highly enriched in the <sup>40</sup>Ar isotope and were monitored by a stack of fast-neutron activation foils. The reaction yield was measured by atom counting of long-lived <sup>39</sup>Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon (<sup>39</sup>Ar/Ar= <span><math><mrow><mn>8</mn><mo>.</mo><mn>12</mn><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mo>−</mo><mn>16</mn></mrow></msup></mrow></math></span>). A total <sup>40</sup>Ar<span><math><msup><mrow><mrow><mo>(</mo><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi><mo>)</mo></mrow></mrow><mrow><mn>39</mn></mrow></msup></math></span>Ar cross section of 610 ± 100 mb was determined at 14.8 ± 0.3 <span><math><mi>MeV</mi></math></span> incident neutron energy. This result serves as a benchmark for recent theoretical calculations and evaluations, found to reproduce well the experimental total cross section. We use these energy-dependent theoretical cross sections together with experimental spectra of cosmogenic neutrons at different altitudes to calculate the global average rate of neutron-induced <sup>39</sup>Ar atmospheric production, resulting in <span><math><mrow><mn>770</mn><mo>±</mo><mn>240</mn></mrow></math></span> <sup>39</sup>Ar atoms/cm<span><math><msup><mrow></mrow><mrow><mn>2</mn></mrow></msup></math></span>/day. The secular equilibrium between the <sup>39</sup>Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance <sup>39</sup>Ar/Ar<span><math><mrow><mspace></mspace><mo>=</mo><mrow><mo>(</mo><mn>5</mn><mo>.</mo><mn>9</mn><mo>±</mo><mn>1</mn><mo>.</mo><mn>8</mn><mo>)</mo></mrow><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mo>−</mo><mn>16</mn></mrow></msup></mrow></math></span>, showing that <span><math><mo>≈</mo></math></span>73% of atmospheric <sup>39</sup>Ar is produced by cosmogenic neutrons, the remaining part believed to be induced by muons and high-energy <span><math><mi>γ</mi></math></span> rays. The <sup>40</sup>Ar(<span><math><mrow><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi></mrow></math></span>)<sup>
{"title":"First experimental determination of the 40Ar(n,2n)39Ar reaction cross section and 39Ar production in Earth’s atmosphere","authors":"S. Bhattacharya , M. Paul , R.N. Sahoo , R. Purtschert , H.F.R. Hoffmann , M. Pichotta , K. Zuber , D. Bemmerer , T. Döring , R. Schwengner , M.L. Avila , E. Lopez-Saavedra , J.C. Dickerson , C. Fougères , J. McLain , R.C. Pardo , K.E. Rehm , R. Scott , I. Tolstukhin , R. Vondrasek , S. Vaintraub","doi":"10.1016/j.gca.2025.12.041","DOIUrl":"10.1016/j.gca.2025.12.041","url":null,"abstract":"<div><div>The cosmogenic <sup>39</sup>Ar(t<span><math><msub><mrow></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub></math></span> = 268 years) isotope of argon is used for geophysical dating and tracing of underground and ocean water, as well as ice owing to its appropriate half-life and chemical inertness as a noble gas; <sup>39</sup>Ar serves also in nuclear weapon test monitoring. We measured for the first time the total cross section of the main <sup>39</sup>Ar cosmogenic production reaction in the atmosphere, namely <sup>40</sup>Ar<span><math><msup><mrow><mrow><mo>(</mo><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi><mo>)</mo></mrow></mrow><mrow><mn>39</mn></mrow></msup></math></span>Ar, using 14.8 ± 0.3 <span><math><mi>MeV</mi></math></span> neutrons. The neutrons, produced by a deuterium-tritium generator, impinged on a stainless steel sphere filled with Ar gas highly enriched in the <sup>40</sup>Ar isotope and were monitored by a stack of fast-neutron activation foils. The reaction yield was measured by atom counting of long-lived <sup>39</sup>Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon (<sup>39</sup>Ar/Ar= <span><math><mrow><mn>8</mn><mo>.</mo><mn>12</mn><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mo>−</mo><mn>16</mn></mrow></msup></mrow></math></span>). A total <sup>40</sup>Ar<span><math><msup><mrow><mrow><mo>(</mo><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi><mo>)</mo></mrow></mrow><mrow><mn>39</mn></mrow></msup></math></span>Ar cross section of 610 ± 100 mb was determined at 14.8 ± 0.3 <span><math><mi>MeV</mi></math></span> incident neutron energy. This result serves as a benchmark for recent theoretical calculations and evaluations, found to reproduce well the experimental total cross section. We use these energy-dependent theoretical cross sections together with experimental spectra of cosmogenic neutrons at different altitudes to calculate the global average rate of neutron-induced <sup>39</sup>Ar atmospheric production, resulting in <span><math><mrow><mn>770</mn><mo>±</mo><mn>240</mn></mrow></math></span> <sup>39</sup>Ar atoms/cm<span><math><msup><mrow></mrow><mrow><mn>2</mn></mrow></msup></math></span>/day. The secular equilibrium between the <sup>39</sup>Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance <sup>39</sup>Ar/Ar<span><math><mrow><mspace></mspace><mo>=</mo><mrow><mo>(</mo><mn>5</mn><mo>.</mo><mn>9</mn><mo>±</mo><mn>1</mn><mo>.</mo><mn>8</mn><mo>)</mo></mrow><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mo>−</mo><mn>16</mn></mrow></msup></mrow></math></span>, showing that <span><math><mo>≈</mo></math></span>73% of atmospheric <sup>39</sup>Ar is produced by cosmogenic neutrons, the remaining part believed to be induced by muons and high-energy <span><math><mi>γ</mi></math></span> rays. The <sup>40</sup>Ar(<span><math><mrow><mi>n</mi><mo>,</mo><mn>2</mn><mi>n</mi></mrow></math></span>)<sup>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"415 ","pages":"Pages 196-203"},"PeriodicalIF":5.0,"publicationDate":"2025-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785264","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}
Pub Date : 2025-12-17DOI: 10.1016/j.gca.2025.12.030
Haiyang Liu, Ying-Yu Xue, Shichao An, Hai-Ou Gu, He Sun, Kun Wang, Shun Guo, Maoqiang Yan, Wei-Dong Sun
{"title":"Extremely light potassium isotopic compositions of the continental eclogites and high-pressure metamorphic veins reveal the fluid-rock interactions in subduction zones","authors":"Haiyang Liu, Ying-Yu Xue, Shichao An, Hai-Ou Gu, He Sun, Kun Wang, Shun Guo, Maoqiang Yan, Wei-Dong Sun","doi":"10.1016/j.gca.2025.12.030","DOIUrl":"https://doi.org/10.1016/j.gca.2025.12.030","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"173 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785756","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}
Pub Date : 2025-12-17DOI: 10.1016/j.gca.2025.12.027
Penglei Liu , Wanjun Lu , Chao Wang
Whether carbonate melting can take place and its role in recycling carbon in deep subduction zones remain ambiguous in many aspects. Studies on ultrahigh-pressure (UHP) rocks exhumed back from great depths can help resolve these ambiguities, but evidence for hydrous carbonatitic liquids has been rarely documented in such rocks. Here, we report primary polyphase solid inclusions (PSIs) of calcite + chlorite in garnet from an UHP diopside-garnet rock enclosed in impure marbles from the Dabieshan, China. These PSIs display typical fluid-mediated microstructures, such as inclusion-garnet interfaces that reflect the control of the crystallographic structure of garnet. In addition, garnet was partially re-equilibrated around the PSIs to form new patches with higher Ca, Fe3+, and Mn contents. In combination with phase equilibria modelling and analogue melting experiments on the calcite-chlorite system, the PSIs are inferred to have resulted from reactions between UHP hydrous carbonatitic liquid inclusions and host garnet during exhumation. The PSIs contain higher contents of light rare earth elements (LREE) and large ion lithophile elements (e.g., Pb and Sr) than garnet but display a LREE-depleted distribution pattern. The latter feature is comparable to the bulk-rock composition, implying that the carbonatitic liquid resulted from partial melting of the host diopside-garnet rock. In addition, our experiments reveal that carbonated chlorite-rich rocks, as a common metasomatic rock in subduction channels, have a solidus temperature between 800 and 850 °C at 4.5 GPa and can thus partially melt to form hydrous carbonatitic liquids at sub-arc depths in warm and hot subduction zones. To sum up, both our natural and experimental studies suggest that the formation of carbonatitic liquid could be more common than expected in subducted slabs. Carbonate melting, being an efficient mechanism for carbon release, thus deserves more attention in deep carbon recycling in subduction zones.
{"title":"Carbon mobilization via hydrous carbonatitic liquid in deep subduction zones: evidence from natural and experimental observations","authors":"Penglei Liu , Wanjun Lu , Chao Wang","doi":"10.1016/j.gca.2025.12.027","DOIUrl":"10.1016/j.gca.2025.12.027","url":null,"abstract":"<div><div>Whether carbonate melting can take place and its role in recycling carbon in deep subduction zones remain ambiguous in many aspects. Studies on ultrahigh-pressure (UHP) rocks exhumed back from great depths can help resolve these ambiguities, but evidence for hydrous carbonatitic liquids has been rarely documented in such rocks. Here, we report primary polyphase solid inclusions (PSIs) of calcite + chlorite in garnet from an UHP diopside-garnet rock enclosed in impure marbles from the Dabieshan, China. These PSIs display typical fluid-mediated microstructures, such as inclusion-garnet interfaces that reflect the control of the crystallographic structure of garnet. In addition, garnet was partially re-equilibrated around the PSIs to form new patches with higher Ca, Fe<sup>3+</sup>, and Mn contents. In combination with phase equilibria modelling and analogue melting experiments on the calcite-chlorite system, the PSIs are inferred to have resulted from reactions between UHP hydrous carbonatitic liquid inclusions and host garnet during exhumation. The PSIs contain higher contents of light rare earth elements (LREE) and large ion lithophile elements (e.g., Pb and Sr) than garnet but display a LREE-depleted distribution pattern. The latter feature is comparable to the bulk-rock composition, implying that the carbonatitic liquid resulted from partial melting of the host diopside-garnet rock. In addition, our experiments reveal that carbonated chlorite-rich rocks, as a common metasomatic rock in subduction channels, have a solidus temperature between 800 and 850 °C at 4.5 GPa and can thus partially melt to form hydrous carbonatitic liquids at sub-arc depths in warm and hot subduction zones. To sum up, both our natural and experimental studies suggest that the formation of carbonatitic liquid could be more common than expected in subducted slabs. Carbonate melting, being an efficient mechanism for carbon release, thus deserves more attention in deep carbon recycling in subduction zones.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"414 ","pages":"Pages 75-88"},"PeriodicalIF":5.0,"publicationDate":"2025-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785269","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}
Pub Date : 2025-12-17DOI: 10.1016/j.gca.2025.12.033
Katherine L. French , Paul C. Hackley , Erik A. Sperling
Green sulfur bacteria biomarkers that indicate euxinia within the photic zone sometimes co-occur with evidence of contradictory depositional redox conditions, such as oxygen-requiring fossils or bioturbation. Intermittent euxinia may explain this apparent contradiction, and recent studies of modern environments show that green sulfur bacteria dwell in transiently euxinic settings. As a result, new approaches are needed to distinguish green sulfur bacteria biomarkers indicative of persistent versus intermittent euxina in ancient sedimentary samples, which this study addresses by investigating how the distribution of isorenieratene derivatives relates to depositional redox conditions. To accomplish this objective, this study focuses on two drill cores through the Upper Cretaceous Mowry Shale and the Eagle Ford Group. These drill cores are comparably thermally immature according to vitrinite reflectance data, and these formations in these cores capture a spectrum of depositional redox conditions according to multiple organic and inorganic proxies, including newly acquired iron speciation and kerogen organic sulfur data. The results presented here reveal that higher molecular weight diagenetic products of isorenieratene are preferentially preserved under persistent euxinia compared to intermittently euxinic intervals that contain isorenieratene derivatives that are shifted to lower molecular weights. Further, the total inventory of aromatic carotenoid diagenetic products contains features that can be used to identify green sulfur bacteria biomarkers from reworked petrogenic sources. Accordingly, the diagenetic fate of isorenieratene and the distribution of its diagenetic products distinguish persistent versus intermittent euxinia, which can be used to sharpen our evaluation of euxinia in the geologic record.
{"title":"Differentiating persistent and intermittent euxinia from the molecular derivatives of green sulfur bacteria carotenoids","authors":"Katherine L. French , Paul C. Hackley , Erik A. Sperling","doi":"10.1016/j.gca.2025.12.033","DOIUrl":"10.1016/j.gca.2025.12.033","url":null,"abstract":"<div><div>Green sulfur bacteria biomarkers that indicate euxinia within the photic zone sometimes co-occur with evidence of contradictory depositional redox conditions, such as oxygen-requiring fossils or bioturbation. Intermittent euxinia may explain this apparent contradiction, and recent studies of modern environments show that green sulfur bacteria dwell in transiently euxinic settings. As a result, new approaches are needed to distinguish green sulfur bacteria biomarkers indicative of persistent versus intermittent euxina in ancient sedimentary samples, which this study addresses by investigating how the distribution of isorenieratene derivatives relates to depositional redox conditions. To accomplish this objective, this study focuses on two drill cores through the Upper Cretaceous Mowry Shale and the Eagle Ford Group. These drill cores are comparably thermally immature according to vitrinite reflectance data, and these formations in these cores capture a spectrum of depositional redox conditions according to multiple organic and inorganic proxies, including newly acquired iron speciation and kerogen organic sulfur data. The results presented here reveal that higher molecular weight diagenetic products of isorenieratene are preferentially preserved under persistent euxinia compared to intermittently euxinic intervals that contain isorenieratene derivatives that are shifted to lower molecular weights. Further, the total inventory of aromatic carotenoid diagenetic products contains features that can be used to identify green sulfur bacteria biomarkers from reworked petrogenic sources. Accordingly, the diagenetic fate of isorenieratene and the distribution of its diagenetic products distinguish persistent versus intermittent euxinia, which can be used to sharpen our evaluation of euxinia in the geologic record.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"415 ","pages":"Pages 130-145"},"PeriodicalIF":5.0,"publicationDate":"2025-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785266","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}
The recent discovery in ordinary chondrites of a population of small, sub-TFL (i.e., negative Δ17O; TFL = Terrestrial Fractionation Line) chondrules isotopically similar to relict grains in larger chondrules offered a glimpse into the sub-TFL precursors of inner disk chondrules. This prompts a search for similar relicts and small chondrules in enstatite chondrites (EC; here the EH3 Sahara 97096, Qingzhen, Caleta el Cobre 025, NWA 14425, and the EL3-an NWA 8785). Our SIMS oxygen isotopic analyses reveal that most olivine grains in E3 chondrites are isotopically indistinguishable from enstatite and must have crystallized during the same thermal event. Yet one analyzed chondrule contains relict olivine grains, one isotopically similar to refractory inclusions (a first in EC) and others with Δ17O values ranging from −9.5 to −7.7 ‰. The latter, and similar discoveries in the literature, must have derived from preexisting chondrules. Small porphyritic chondrules or isolated pyroxene grains tend to be 16O-enriched (and more reduced), as in ordinary chondrites, but their Δ17O never goes below –2 ‰. Cryptocrystalline chondrules found in Sahara 97096 do extend to −7.3 ‰, but are olivine-free. This thus provides evidence for sub-TFL reservoirs ancestral to the E reservoir, although more than one may have existed.
最近在普通球粒陨石中发现了一群小的，亚TFL（即负Δ17O； TFL =陆地分异线）的球粒，同位素上与较大球粒中的残余颗粒相似，这为内盘球粒的亚TFL前体提供了一瞥。这促使人们在辉化辉石球粒陨石（EC， EH3 Sahara 97096、Qingzhen、Caleta el Cobre 025、NWA 14425和el3 - and NWA 8785）中寻找类似的遗迹和小球粒。我们的SIMS氧同位素分析显示，E3球粒陨石中的大多数橄榄石颗粒在同位素上与顽辉石难以区分，并且必须在同一热事件中结晶。然而，一个分析的球粒含有残余橄榄石颗粒，一个同位素与难熔包裹体相似（EC中第一次），其他球粒的Δ17O值在−9.5 ~−7.7‰之间。后者，以及文献中类似的发现，一定是源自先前存在的球粒。与普通球粒陨石一样，小的斑状球粒或孤立的辉石颗粒往往富含16o（并且更还原），但它们的Δ17O从不低于-2‰。在撒哈拉97096中发现的隐晶球粒延伸至- 7.3‰，但不含橄榄石。因此，这为次tfl水库的祖先E水库提供了证据，尽管可能存在多个。
{"title":"Oxygen isotope compositions of chondrules in enstatite chondrites: insights from relict olivine, chondrule size and redox state","authors":"Yves Marrocchi , Laurette Piani , Dorian Thomassin , Emmanuel Jacquet","doi":"10.1016/j.gca.2025.12.036","DOIUrl":"10.1016/j.gca.2025.12.036","url":null,"abstract":"<div><div>The recent discovery in ordinary chondrites of a population of small, sub-TFL (i.e., negative Δ<sup>17</sup>O; TFL = Terrestrial Fractionation Line) chondrules isotopically similar to relict grains in larger chondrules offered a glimpse into the sub-TFL precursors of inner disk chondrules. This prompts a search for similar relicts and small chondrules in enstatite chondrites (EC; here the EH3 Sahara 97096, Qingzhen, Caleta el Cobre 025, NWA 14425, and the EL3-an NWA 8785). Our SIMS oxygen isotopic analyses reveal that most olivine grains in E3 chondrites are isotopically indistinguishable from enstatite and must have crystallized during the same thermal event. Yet one analyzed chondrule contains relict olivine grains, one isotopically similar to refractory inclusions (a first in EC) and others with Δ<sup>17</sup>O values ranging from −9.5 to −7.7 ‰. The latter, and similar discoveries in the literature, must have derived from preexisting chondrules. Small porphyritic chondrules or isolated pyroxene grains tend to be <sup>16</sup>O-enriched (and more reduced), as in ordinary chondrites, but their Δ<sup>17</sup>O never goes below –2 ‰. Cryptocrystalline chondrules found in Sahara 97096 do extend to −7.3 ‰, but are olivine-free. This thus provides evidence for sub-TFL reservoirs ancestral to the E reservoir, although more than one may have existed.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"414 ","pages":"Pages 62-74"},"PeriodicalIF":5.0,"publicationDate":"2025-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145785267","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}
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