Geochimica et Cosmochimica Acta最新文献

{"title":"Controls on the hydrogen isotope composition of tetraether lipids in an autotrophic ammonia-oxidizing marine archaeon","authors":"W.D. Leavitt , S.H. Kopf , Y. Weber , B. Chiu , J.M. McFarlin , F.J. Elling , S. Hoeft-McCann , A. Pearson","doi":"10.1016/j.gca.2023.04.033","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.033","url":null,"abstract":"<div><p>The stable hydrogen isotope composition of persistent biomolecules is used as a palaeohydrological proxy. While much previous work has focused on plant leaf wax-derived <em>n</em>-alkanes, the potential of prokaryotic lipid biomarkers as carriers of H isotope signatures remains underexplored, particularly in the Archaea. Here we investigated H isotope distributions in the membrane lipids of the ammonia-oxidizing chemoautotroph <em>Nitrosopumilus maritimus</em> strain SCM1. Hydrogen isotope ratios were measured on the cleaved biphytane chains of tetraether membrane lipids extracted from steady-state continuous cultures cultivated at slow, medium, and fast growth rates. In contrast to recent work on bacterial fatty acids, where the direction and magnitude of isotopic fractionation varies widely (<em>ca.</em> 600‰ range) as a function of central C and energy metabolism, archaeal biphytane data in the present work are relatively invariant. The weighted average <sup>2</sup>H/<sup>1</sup>H fractionation values relative to growth water (<sup>2</sup><em>ε</em><sub>L/W</sub>) ranged from –272 to –260‰, despite a three-fold difference in doubling times (30.8–92.5 hr), yielding an average growth-rate effect <0.2‰ hr<sup>−1</sup>. These <sup>2</sup><em>ε</em><sub>L/W</sub> values are more negative than most heterotrophic microbial lipid H isotope measurements in the literature, and are on par with those from other autotrophic archaea, as well as with phytol from photoautotrophic algae. <em>N. maritimus</em> values of <em><sup>2</sup>ε</em><sub>L/W</sub> also varied systematically with the number of internal rings (cyclopentyl + cyclohexyl), increasing for each additional ring by 6.4 ± 2.7‰<em>.</em> Using an isotope flux-balance model in tandem with a comprehensive analysis of the sources of H in archaeal lipid biosynthesis, we use this observation to estimate the kinetic isotope effects (KIEs) of H incorporation from water; from reducing cofactors such as flavins and NADPH, and for the transhydrogenation reaction(s) that convert the electron-donor derived NADH into these cofactors. Consistent with prior studies on bacteria and plants, our results indicate the KIEs of reducing cofactors in archaea are highly fractionating, while those involving exchange of water protons are less so. When combined with the observation of minimal growth-rate sensitivity, our results suggest biphytanes of autotrophic 3HP/4HB utilizing Nitrososphaerota (a.k.a. Thaumarchaeota) may be offset from their growth waters by a nearly constant <em><sup>2</sup>ε</em><sub>L/W</sub> value. Together with the ring effect, this implies that all biphytanes originating from a common source should have a predictable ordering of their isotope ratios with respect to biphytane ring number, allowing precise reconstruction of the original δ<sup>2</sup>H value of the environmental water. Collectively, these patterns indicate archaeal biphytanes have potential as paleo-hydrological proxies, e","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 194-210"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"1608311","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":"Species and biosynthetic effects cause uncorrelated variation in oxygen and hydrogen isotope compositions of plant organic compounds","authors":"Jochem Baan,&nbsp;Meisha Holloway-Phillips,&nbsp;Daniel B. Nelson,&nbsp;Ansgar Kahmen","doi":"10.1016/j.gca.2023.04.013","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.013","url":null,"abstract":"<div><p>Strong variation in plant organic compound δ²H and δ¹⁸O values such as leaf waxes or cellulose among species is commonly observed; however, the extent to which this isotopic variation is driven by leaf water or biochemical isotope effects is relatively unknown. Therefore, we compared variation in leaf water and organic compound δ²H and δ¹⁸O values (cellulose - δ²H and δ¹⁸O, and <em>n</em>-alkanes – δ²H) across 192 species grown in a botanical garden to assess covariation of isotope values (1) between elements in a given compound, (2) between different (organic) compounds of a given element, and (3) across different growing seasons. Our results suggest that variation in leaf water δ²H values are likely not a strong driver for the observed variation in organic compound δ²H values across species, and that this may also be true for δ¹⁸O values. Furthermore, even though correlation between leaf water δ²H and δ¹⁸O values appears to be transferred to organic compounds, the explanatory power of this correlation is strongly diminished (R² &lt; 0.04). This indicates that additional biochemical isotope fractionation leads to substantial variation in organic compound δ²H and possibly also δ¹⁸O values across species. Moreover, the low explanatory power of the correlation between cellulose and <em>n</em>-alkane δ²H values (R² = 0.06) suggests that the biochemical pathways associated with the different compounds are accompanied by different isotope effects. Lastly, cellulose δ²H and δ¹⁸O values appeared sensitive to environmental differences between growing seasons, while differences in model-predicted source water δ¹⁸O and δ²H values and also climate were negligible between years. By contrast, the species pattern in <em>n</em>-alkane δ²H values was highly conserved between the two years. This indicates that the environmental forcing effects on isotope values were not equal between compounds. Therefore, we conclude that variation in organic compound δ²H (and possibly also δ¹⁸O) values among species and growing seasons was more strongly driven by biochemical isotope fractionation rather than by isotope values of plant water. This should be considered in the application of organic compound δ²H and δ¹⁸O values to reconstruct past climate, where invariable biochemical isotope fractionation is often assumed. Alternatively, organic compound δ²H and δ¹⁸O values could be further developed into a tool to extract plant metabolic information.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 1-13"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3207530","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":"Mechanism of late diagenetic alteration of glauconite and implications for geochronology","authors":"Jan Środoń ,&nbsp;Lynda Williams ,&nbsp;Marek Szczerba ,&nbsp;Tatiana Zaitseva ,&nbsp;Maciej J. Bojanowski ,&nbsp;Beata Marciniak-Maliszewska ,&nbsp;Artur Kuligiewicz ,&nbsp;Krzysztof Starzec ,&nbsp;Zuzanna Ciesielska ,&nbsp;Mariusz Paszkowski","doi":"10.1016/j.gca.2023.05.010","DOIUrl":"https://doi.org/10.1016/j.gca.2023.05.010","url":null,"abstract":"<div><p>Glauconites – the most common authigenic minerals of sedimentary rocks suitable for radiometric stratigraphic dating – are known to give sometimes strongly rejuvenated ages (i.e. younger than stratigraphic ages of their host sediments). The glauconites separated from the Ediacaran/Cambrian sediments of the western part of the East European Craton (Baltica) were used to investigate the mechanism of age rejuvenation. All pure samples gave strongly rejuvenated (Paleozoic) ages, using both K-Ar and Rb-Sr dating, older than ages of Proterozoic illite-smectites from the same area. Their diverse characteristics were compared to younger glauconite samples of the same radiometric age as the stratigraphic age of their host sediment, and to younger glauconite samples with rejuvenated ages. It was established that the rejuvenated samples differ from the primary samples in their X-ray diffraction characteristics. Their 060 reflections are broader and displaced towards higher angles, an indication of chemical heterogeneity (layers with lower, variable Fe contents). This finding was confirmed by electron probe micro-analysis (less Fe and Mg, more Al in rejuvenated glauconites), and by Fourier transform infrared spectroscopy in the OH stretching region. Average boron contents are lower in the rejuvenated glauconites, and the boron isotope composition is quite variable, with no systematic trends observed. Modeling indicates that ancient seawater δ¹¹B was not much different from the present-day value, and that input of organic-derived boron to glauconite began at the sedimentary stage. All data combined are indicative of the mechanism of burial diagenetic alteration of glauconite: the growth of glauconitic 10 Å layers with lower iron contents at the expense of Fe-smectite layers present in the original glauconitic mineral crystallized during sedimentation, while the primary high-Fe glauconitic 10 Å layers are preserved during burial diagenesis. This mechanism is responsible for the observed chemical heterogeneity of many glauconitic samples, and for their mixed isotopic ages, intermediate between the ages of primary and diagenetic growth of 10 Å layers.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 157-174"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"2376535","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":"Phosphorous-solubility in carbonatite melts: Apatite crystallization modeled via its solubility product","authors":"Gino Sartori ,&nbsp;Max W. Schmidt","doi":"10.1016/j.gca.2023.04.034","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.034","url":null,"abstract":"<div><p>We model apatite-saturation in carbonatite melts based on a compilation of experimental data ranging from 650 to 1430 °C and 1 to 60 kbar. The data show a very strong correlation of inverse temperature with the apatite solubility product, a relation expressed by the equation.</p><p><span><math><mrow><mi>ln</mi><mo>(</mo><msup><mrow><mfenced><mrow><mi>C</mi><mi>a</mi><mi>O</mi></mrow></mfenced></mrow><mn>5</mn></msup><mo>·</mo><msup><mrow><mfenced><mrow><mi>P</mi><msub><mi>O</mi><mrow><mn>2.5</mn></mrow></msub></mrow></mfenced></mrow><mn>3</mn></msup><mo>)</mo><mo>=</mo><mo>-</mo><mn>27450</mn><mo>/</mo><mi>T</mi><mo>+</mo><mn>5.79</mn></mrow></math></span></p><p>(T in Kelvin). Within the available dataset, F and Cl do not play a discernable role. Application of the solubility product to natural Ca-carbonatites indicates that a few rocks with &gt;8 wt% P₂O₅ have cumulative apatite while most Ca-carbonatites (with typically &lt;5 wt% P₂O₅) are apatite undersaturated at their liquidus temperatures, defined by calcite crystallization. To address true carbonatite liquids, we model calcite fractionation and melt evolution for natural rock compositions with 5, 10 and 20 mol% H₂O and/or (Na,K)₂CO₃ added, 5% representing the lower bound for any carbonatite formation model. Both H₂O or (Na,K)₂CO₃ cause very similar liquidus depressions of ∼10 °C/mol%. The model result is that saturation of apatite occurs in most natural carbonatite melts only after &gt;45, 30–55, and 10–30 mol% calcite-fractionation for 5, 10, and 20 mol% fluxing components added, respectively. We further estimate the melt fractions necessary to dissolve all apatite in carbonatite melts generated from carbonated MORB and pelites, opening the discussion on an unlikely restitic nature of subducted apatites. In both the crystallization and forward melting cases, apatite crystallization or dissolution is mostly governed by temperature, surprisingly, carbonatite melt evolution through calcite-fractionation has a minor influence on the solubility product.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 122-132"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"2375798","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":"Preservation of stable isotope signatures of amino acids in diagenetically altered Middle to Late Holocene archaeological mollusc shells","authors":"N.L. Vokhshoori ,&nbsp;T.C. Rick ,&nbsp;T.J. Braje ,&nbsp;M.D. McCarthy","doi":"10.1016/j.gca.2023.05.005","DOIUrl":"https://doi.org/10.1016/j.gca.2023.05.005","url":null,"abstract":"&lt;div&gt;&lt;p&gt;Stable isotope proxies measured in the proteinaceous fraction of archaeological mollusc shell represents an increasingly important archive for reconstructing past ecological and biogeochemical conditions of nearshore environments. A major issue, however, is understanding the impact of diagenetic alteration in sub-fossil shell isotope values. “Bulk” stable isotope values of nitrogen (δ&lt;sup&gt;15&lt;/sup&gt;N), and especially carbon (δ&lt;sup&gt;13&lt;/sup&gt;C) often shift strongly with increasing C/N ratios in degraded shell, resulting in unreliable data. Here, we examine preservation of an entirely new set of shell paleo-proxies, compound-specific isotopes of amino acids (CSI-AA). We examine carbon (δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;AA&lt;/sub&gt;) and nitrogen (δ&lt;sup&gt;15&lt;/sup&gt;N&lt;sub&gt;AA&lt;/sub&gt;) patterns and values from the organic fraction of California mussel (&lt;em&gt;Mytilus californianu&lt;/em&gt;s) shells from the California Channel Islands. Archaeological shell samples ranging in age from ca. 6,100 to 250 cal BP exhibiting a wide range of degradation states were collected from varied depositional environments (e.g., exposed coastal bluff, buried strata, etc.), and were directly compared to modern shells of the same species and region.&lt;/p&gt;&lt;p&gt;Our results indicate organic matter C/N ratios as the best bulk diagnostic indicator of the relative degradation state of shell organic fraction, including changes at the molecular level. Modern shell organic C/N ratios ranged from 2.8 to 3.5, while those in archaeological shell were substantially elevated (3.4–9.5), exhibiting strong and significant negative correlations with bulk δ&lt;sup&gt;13&lt;/sup&gt;C values, weight %C, and weight %N, and a significant but weaker correlation with δ&lt;sup&gt;15&lt;/sup&gt; N values. An additional “cleaning” step using weak NaOH helped to remove possible exogenous contaminants and improved bulk values of some samples. However, relative molar AA abundances revealed that some AAs, especially the two most abundant, Glycine and Alanine, progressively decreased with increasing C/N ratio. The loss of these amino acids permanently alters bulk isotope values regardless of removal of contaminants. Modeling the bulk isotope change expected due to amino acid molar composition showed major and predictable shifts in bulk δ&lt;sup&gt;13&lt;/sup&gt;C values from selected AA loss, and similarly large but far more variable impacts from exogenous contaminants.&lt;/p&gt;&lt;p&gt;In contrast to bulk data, key CSI-AA values and patterns remained almost entirely unaltered, even in the most degraded shell samples, closely matching expected biosynthetic isotope patterns in modern mussel shell. AA isotope proxies for “baseline” (δ&lt;sup&gt;15&lt;/sup&gt;N-Phenylalanine and average δ&lt;sup&gt;13&lt;/sup&gt;C-Essential AAs) and planktonic trophic structure (δ&lt;sup&gt;15&lt;/sup&gt;N-Glutamic Acid and δ&lt;sup&gt;15&lt;/sup&gt;N-Phenylalanine) were not statistically altered with degradation in any sample. Overall, we conclude that while bulk isotopes, particularly δ&lt;sup&gt;13&lt;/sup&gt;C, are very likely to be unreliable in archaeolog","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 36-50"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"2952784","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":"Heavy magnesium isotopic compositions of basalts erupted during arc inception: Implications for the mantle source underlying the nascent Izu-Bonin-Mariana arc","authors":"Shuai Yuan ,&nbsp;He Li ,&nbsp;Richard J. Arculus ,&nbsp;Yongsheng He ,&nbsp;Shan Ke ,&nbsp;Weidong Sun","doi":"10.1016/j.gca.2023.04.017","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.017","url":null,"abstract":"<div><p>Basalts formed during the early development of an arc are usually buried, but they record critical information relating to the mantle source of the overriding plate prior to the addition of subducting slab components. Basalts recovered at Site U1438 of International Ocean Discovery Program (IODP) Expedition 351, in the Amami Sankaku Basin (ASB), formed during the transition from forearc basalt (FAB) and boninite eruptions to stratovolcano developments in the Izu-Bonin-Mariana (IBM) arc. In this study, we present magnesium (Mg) isotopic data (δ²⁶Mg) for the ASB basalt core samples, formed at arc inception in the absence of down-going slab components, and report the implications of these data for the characteristics of the mantle sources underlying the proto-IBM arc. The δ²⁶Mg values of the ASB basalts range from −0.21‰ to +0.08‰, with an average value of −0.13 ± 0.07‰. These values are systematically higher than those of mid-ocean ridge basalts (MORBs) (−0.25 ± 0.06‰). No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or subduction component addition can be identified in the Mg isotopic compositions either of the ASB basalts or their mantle source, given the absence of correlations between the δ²⁶Mg values and the proxies of these processes (e.g., K/Nb, Nb/Zr, Ba/La, Nb/Y, etc.). We conclude that the high δ²⁶Mg values of the ASB basalts are inherited from their mantle source that was enriched in heavy Mg isotopes. Melting of an ultra-depleted, refractory spinel peridotite source containing talc-bearing serpentinite components can explain the origin of the heavy Mg isotopic compositions in the ASB basalts. The presence of water-rich serpentinite components in the mantle provides a new perspective on how a refractory mantle source can undergo partial melting at temperatures and pressures similar to the formation of MORBs without the influence of subducted materials, which has important implications for the initiation of plate subduction. Furthermore, the Mg isotopic contributions of subducted additions and mantle source materials should be carefully discussed in future studies due to the heterogeneity of Mg isotopes in the mantle.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 14-23"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3339796","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":"Titanium transport and isotopic fractionation in the Critical Zone","authors":"Sarah M. Aarons ,&nbsp;Nicolas Dauphas ,&nbsp;Nicolas D. Greber ,&nbsp;Mathieu Roskosz ,&nbsp;Julien Bouchez ,&nbsp;Tamara Carley ,&nbsp;Xiao-Ming Liu ,&nbsp;Roberta L. Rudnick ,&nbsp;Jérôme Gaillardet","doi":"10.1016/j.gca.2023.05.008","DOIUrl":"https://doi.org/10.1016/j.gca.2023.05.008","url":null,"abstract":"<div><p>Stable Ti isotopes have been applied in the detrital sediment record to reconstruct the bulk composition of Earth’s continental crust due to the relationship between magmatic differentiation and Ti isotopic compositions. However, no study has systematically evaluated the influence of provenance, physical, and chemical weathering on the composition of sediments relative to the protolith they originated from. To test the influence of these processes on Ti isotopic compositions we investigate the Ti isotope composition of 82 surface samples including loess, volcaniclastic rocks, river sediment, and two separate weathering profiles through igneous rocks, collected from a broad geographical area and a range of environmental conditions. Limited but significant Ti isotope fractionation exists in samples subjected to extreme chemical weathering processes, potentially as a result of elemental mobilization. For example, the δ⁴⁹Ti isotopic composition of bauxites developed on Columbia River basalt varies by up to 0.1‰, becoming isotopically heavier with increasing weathering intensity. However, negligible variation in δ⁴⁹Ti was found in a second profile of saprolites developed on weathered diabase. Titanium isotope variations in loess do not correlate with chemical weathering intensity or size sorting, but may instead be related to the provenance of the sediment. We find that the δ⁴⁹Ti of Amazon River sediments is correlated with the Al/Zr ratio, indicating that δ⁴⁹Ti is impacted by sediment sorting. At our study sites, the river averaged offset between the isotopic composition of the bedload and the suspended sediment fraction is 0.051‰, with the largest offset being + 0.116‰. Our data suggest that during chemical weathering, heavy Ti isotopes are preferentially incorporated into secondary minerals producing higher δ⁴⁹Ti in intensely weathered soils. During fluvial transport, the Ti isotopic composition of fine-grained sediment is heavier than that of its coarser counterpart. Crustal protolith composition and sorting during transport and sedimentation have a stronger effect on the Ti isotopic composition than chemical weathering. Our results have implications for studies that utilize the Ti elemental concentration to calculate relative enrichment or depletion during chemical weathering and physical transport processes in the Critical Zone and for studies using Ti isotopes in terrigenous sediments to infer the composition of their provenance.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 175-193"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3019048","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":"Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis","authors":"Shuai Yang ,&nbsp;Xinze Lu ,&nbsp;Xinming Chen ,&nbsp;Wang Zheng ,&nbsp;Jeremy D. Owens ,&nbsp;Seth A. Young ,&nbsp;Brian Kendall","doi":"10.1016/j.gca.2023.04.027","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.027","url":null,"abstract":"&lt;div&gt;&lt;p&gt;The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;org&lt;/sub&gt; values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ&lt;sup&gt;98&lt;/sup&gt;Mo and δ&lt;sup&gt;238&lt;/sup&gt;U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ&lt;sup&gt;98&lt;/sup&gt;Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ&lt;sup&gt;238&lt;/sup&gt;U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ&lt;sup&gt;238&lt;/sup&gt;U values (&gt;0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ&lt;sup&gt;98&lt;/sup&gt;Mo and δ&lt;sup&gt;238&lt;/sup&gt;U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an impor","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 133-156"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3019047","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":"Mantle plume plays an important role in modern seafloor hydrothermal mineralization system","authors":"Xia Zhang ,&nbsp;Zhilei Sun ,&nbsp;Nengyou Wu ,&nbsp;Hong Cao ,&nbsp;Kun Guo","doi":"10.1016/j.gca.2023.05.012","DOIUrl":"https://doi.org/10.1016/j.gca.2023.05.012","url":null,"abstract":"<div><p>Deyin-1 hydrothermal field is located on the Southern Mid-Atlantic Ridge (SMAR) 15°S. The geochemical characteristics of substrate basalts record the incorporation of Saint Helena plume components in the magma source, which implies that the hydrothermal mineralization characteristics in Deyin-1 might be influenced by the mantle plume indirectly. In this study, the pyrite trace element contents and sulfide in situ Pb and He-Ar isotope compositions of a chimney sample from Deyin-1 field are determined systematically in order to clarify the mantle plume influence on local hydrothermal mineralization process. The higher Ba/Co ratios in Deyin-1 pyrites, compared with those from other mid-ocean ridge hydrothermal fields, may be related to the Saint Helena mantle plume influence. Besides, Deyin-1 pyrites possess lower Pb/As, Ag/As, Ni/As ratios and higher As contents than ultramafic-hosted hydrothermal fields, indicating the Deyin-1 is a mafic-hosted hydrothermal system. The higher Se, Co and lower Zn, As, Pb contents in pyrites from interior layers of the chimney demonstrate that the fluid temperature increases during the chimney growth. The in situ Pb isotope compositions of Deyin-1 sulfides are similar to those of mantle plume-influenced basalts in SMAR 15°S and more radiogenic than that of the plume-free SMAR basalt, which demonstrates that the Saint Helena mantle plume has contributed parts of Pb (and other metal elements) to the Deyin-1 sulfides. The He isotope compositions of Deyin-1 sulfides also prove the mantle plume contribution to the hydrothermal system. End-member mixing calculation of Pb-He isotopes indicates that the Saint Helena plume components account for about 3% of the mineralization material sources in Deyin-1 sulfides. Many hydrothermal fields have been discovered near the hot spots besides Deyin-1. Our study emphasizes that mantle plume influence on hydrothermal mineralization might be ubiquitous and needs to be revaluation. This study might shed some light on modern seafloor hydrothermal mineralization process and lithosphere-seawater mass exchange.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 211-221"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3019049","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 indium isotopic composition of the bulk silicate Earth","authors":"Deze Liu ,&nbsp;Frederic Moynier ,&nbsp;Paolo A. Sossi ,&nbsp;Raphael Pik ,&nbsp;Sæmundur Ari Halldórsson ,&nbsp;Edward Inglis ,&nbsp;James M.D. Day ,&nbsp;Julien Siebert","doi":"10.1016/j.gca.2023.04.018","DOIUrl":"https://doi.org/10.1016/j.gca.2023.04.018","url":null,"abstract":"<div><p>Indium (In) behaves as a moderately volatile metal during nebular and planetary processes, and its volatility depends strongly on oxygen fugacity. The In isotopic composition of the bulk silicate Earth (BSE) could provide a critical constraint on the nature of Earth’s building blocks and mechanisms that lead to its volatile depletion. However, accurately and precisely determining the isotopic composition of In of the silicate Earth is challenging due to its low abundance in igneous rocks and the presence of significant isobaric interferences on its isotopes (e.g., ¹¹³Cd⁺ on ¹¹³In⁺ and ¹¹⁵Sn⁺ on ¹¹⁵In⁺). Here, we present a purification procedure for In from rock matrices and report the first dataset of In isotopic compositions of 30 terrestrial igneous rocks, one biotite geostandard, and one carbonaceous chondrite (Allende) measured on a Nu Sapphire collision-cell equipped multi-collector inductively-coupled-plasma mass-spectrometer (CC-MC-ICP-MS) with an external reproducibility of 0.11‰ (2SD). At this level of precision, we find no statistically significant difference in the In isotopic compositions of mid-ocean-ridge basalts (MORB), oceanic island basalts (OIB), and continental flood basalts (CFB). Furthermore, Canary Islands, Iceland and Afar lavas display no analytically resolvable In isotopic variations from basalts to rhyolites. Therefore, In isotope fractionation during igneous processes is smaller than our analytical uncertainty and the In isotopic compositions of basalts are likely to be representative samples of their mantle sources. The twenty-one terrestrial basalts from diverse geological settings have an average δ¹¹⁵In of 0.35 ± 0.07 ‰ (2SD). This value represents the current best estimate of the In isotopic composition of the mantle as well as of the bulk silicate Earth, assuming limited In isotope fractionation during mantle partial melting, and due to the small contribution of the continental crust to the In budget (&lt;5%). This isotopic composition provides a baseline with which to compare with chondrites and differentiated planetary bodies in future studies.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"352 ","pages":"Pages 24-35"},"PeriodicalIF":5.0,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3339797","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}