Fluid-present melting and fluid-absent melting are two primary mechanisms for the chemical differentiation of continental crust. However, it is still challenging to decode these processes with conventional geochemical methods. In this study, we present systematic Fe isotope data of anatectic migmatites and gneisses from the Dabie orogen, China, which were formed by different mechanisms of crustal anatexis. Fluid-present melting of biotite generates migmatites with restricted Fe3+/ΣFe (0.31−0.44) and homogeneous δ56Fe values (0.06‰−0.17‰). In contrast, fluid-absent melting of phengite produces migmatites and migmatitic gneisses with dramatic Fe3+/ΣFe (0.26−0.94) and δ56Fe (0.04‰−0.61‰) variations. Quantitative modeling of Fe distribution during partial melting reveals that Fe isotope fractionation is governed by source mineral assemblages under varying melting regimes. During fluid-absent melting, the reactant phengite has much higher Fe3+/ΣFe and δ56Fe values than the peritectic biotite, resulting in high and heterogeneous δ56Fe values in the complementary melt. In contrast, during fluid-present melting, the reactant biotite and peritectic amphibole have similarly low Fe3+/ΣFe and δ56Fe values, leading to low and homogeneous δ56Fe values in the complementary melt. This establishes Fe isotopes as a novel tracer for crustal anatexis, critical for understanding continental reworking and intracrustal differentiation.
{"title":"Fe isotope decoding of fluid-absent versus fluid-present melting of deeply subducted continental crust","authors":"Er-Lin Zhu, Qiong-Xia Xia, Yi-Xiang Chen, Ren-Xu Chen, Hao-Hong Shu, Zhao-Ya Li, Yong-Fei Zheng","doi":"10.1130/g52997.1","DOIUrl":"https://doi.org/10.1130/g52997.1","url":null,"abstract":"Fluid-present melting and fluid-absent melting are two primary mechanisms for the chemical differentiation of continental crust. However, it is still challenging to decode these processes with conventional geochemical methods. In this study, we present systematic Fe isotope data of anatectic migmatites and gneisses from the Dabie orogen, China, which were formed by different mechanisms of crustal anatexis. Fluid-present melting of biotite generates migmatites with restricted Fe3+/ΣFe (0.31−0.44) and homogeneous δ56Fe values (0.06‰−0.17‰). In contrast, fluid-absent melting of phengite produces migmatites and migmatitic gneisses with dramatic Fe3+/ΣFe (0.26−0.94) and δ56Fe (0.04‰−0.61‰) variations. Quantitative modeling of Fe distribution during partial melting reveals that Fe isotope fractionation is governed by source mineral assemblages under varying melting regimes. During fluid-absent melting, the reactant phengite has much higher Fe3+/ΣFe and δ56Fe values than the peritectic biotite, resulting in high and heterogeneous δ56Fe values in the complementary melt. In contrast, during fluid-present melting, the reactant biotite and peritectic amphibole have similarly low Fe3+/ΣFe and δ56Fe values, leading to low and homogeneous δ56Fe values in the complementary melt. This establishes Fe isotopes as a novel tracer for crustal anatexis, critical for understanding continental reworking and intracrustal differentiation.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"27 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900811","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}
Maija J. Raudsepp, Sasha Wilson, Benjamin M. Tutolo
The silicate−carbonate cycle controls atmospheric CO2 concentrations and moderates Earth’s climate over geologic time scales. Chemical weathering of silicate minerals by CO2 results in the release of cations and the neutralization of CO2 to HCO3− or CO32−. The precipitation of Ca- and Mg-carbonate minerals is expected once waters are supersaturated. However, quantifying the magnitude of supersaturation required, particularly for Mg-carbonates, has remained challenging. Here we present a database of 854 water samples from the Central Plateau, British Columbia, Canada, representing a wide range of salinities, including both Na−(SO4)−HCO3−CO3 and Mg−Na−SO4 hypersaline lakes, to determine the geochemical thresholds for Ca- and Mg-carbonate formation. For HCO3-dominated waters, the data indicate maximum alkalinity thresholds of ∼5 mEq/kg for Ca-carbonates and ∼40 mEq/kg for Mg-carbonates. Activity plots of Ca−CO32− and Mg−CO32− suggest that maximum saturation thresholds for both Ca-carbonates and Mg-carbonates are applicable to HCO3-dominated, SO4-dominated, and CO2-rich waters. These geochemical thresholds may be used to optimize geochemical carbon dioxide removal (geoCDR) technologies, such as enhanced rock weathering, as HCO3− has up to double the CDR efficiency of carbonate minerals.
{"title":"The fate of CO2, Ca, and Mg after terrestrial rock weathering","authors":"Maija J. Raudsepp, Sasha Wilson, Benjamin M. Tutolo","doi":"10.1130/g53354.1","DOIUrl":"https://doi.org/10.1130/g53354.1","url":null,"abstract":"The silicate−carbonate cycle controls atmospheric CO2 concentrations and moderates Earth’s climate over geologic time scales. Chemical weathering of silicate minerals by CO2 results in the release of cations and the neutralization of CO2 to HCO3− or CO32−. The precipitation of Ca- and Mg-carbonate minerals is expected once waters are supersaturated. However, quantifying the magnitude of supersaturation required, particularly for Mg-carbonates, has remained challenging. Here we present a database of 854 water samples from the Central Plateau, British Columbia, Canada, representing a wide range of salinities, including both Na−(SO4)−HCO3−CO3 and Mg−Na−SO4 hypersaline lakes, to determine the geochemical thresholds for Ca- and Mg-carbonate formation. For HCO3-dominated waters, the data indicate maximum alkalinity thresholds of ∼5 mEq/kg for Ca-carbonates and ∼40 mEq/kg for Mg-carbonates. Activity plots of Ca−CO32− and Mg−CO32− suggest that maximum saturation thresholds for both Ca-carbonates and Mg-carbonates are applicable to HCO3-dominated, SO4-dominated, and CO2-rich waters. These geochemical thresholds may be used to optimize geochemical carbon dioxide removal (geoCDR) technologies, such as enhanced rock weathering, as HCO3− has up to double the CDR efficiency of carbonate minerals.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"7 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900813","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}
Nicholas L. Swanson-Hysell, Yiming Zhang, Francis A. Macdonald, Isabel Koran, Adrian R. Tasistro-Hart, Annabel F. Jay
The closure of the Mozambique Ocean defines the final assembly of the megacontinent Gondwana and is associated with a vast region of crustal growth in the Arabian-Nubian Shield. Despite this central paleogeographic position, there are few constraints on the position of terranes within and bounding the Mozambique Ocean. We report paleomagnetic data from ca. 726 Ma dikes exposed in southern Oman. Well-resolved magnetite magnetization is constrained to be primary by a conglomerate test on mafic clasts within overlying Cryogenian diamictite. The resulting paleomagnetic pole indicates that Oman was at a paleolatitude of 37 ± 2.5°N and was rotated ∼80° counterclockwise from its present-day orientation. This position is consistent with Oman forming a contiguous plate with the India and South China cratons on the northern margin of the Mozambique Ocean in a distinct tectonic domain from Arabian-Nubian arcs to the south. This position reveals an ∼5500-km-wide oceanic realm prior to subsequent closure that resulted in a major zone of Neoproterozoic crustal growth.
{"title":"Oman was on the northern margin of a wide late Tonian Mozambique Ocean","authors":"Nicholas L. Swanson-Hysell, Yiming Zhang, Francis A. Macdonald, Isabel Koran, Adrian R. Tasistro-Hart, Annabel F. Jay","doi":"10.1130/g53450.1","DOIUrl":"https://doi.org/10.1130/g53450.1","url":null,"abstract":"The closure of the Mozambique Ocean defines the final assembly of the megacontinent Gondwana and is associated with a vast region of crustal growth in the Arabian-Nubian Shield. Despite this central paleogeographic position, there are few constraints on the position of terranes within and bounding the Mozambique Ocean. We report paleomagnetic data from ca. 726 Ma dikes exposed in southern Oman. Well-resolved magnetite magnetization is constrained to be primary by a conglomerate test on mafic clasts within overlying Cryogenian diamictite. The resulting paleomagnetic pole indicates that Oman was at a paleolatitude of 37 ± 2.5°N and was rotated ∼80° counterclockwise from its present-day orientation. This position is consistent with Oman forming a contiguous plate with the India and South China cratons on the northern margin of the Mozambique Ocean in a distinct tectonic domain from Arabian-Nubian arcs to the south. This position reveals an ∼5500-km-wide oceanic realm prior to subsequent closure that resulted in a major zone of Neoproterozoic crustal growth.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"24 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144901768","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}
James G. Saulsbury, Anna Piwoni-Piórewicz, Piotr Kukliński, Emanuela Di Martino, Lee Hsiang Liow
Under the calcite-aragonite seas hypothesis, the evolutionary history of calcifying marine organisms reflects changes in global seawater chemistry, alternately favoring precipitation of skeletons made of calcite or aragonite. Most calcifying groups including corals, coccolithophores, and stromatoporoids do not switch mineralogies in response to these changes; instead, they expand and dwindle as their favored seawater conditions come and go. Cheilostome bryozoans have been suggested as an exception to this rule, apparently transitioning between aragonite and calcite many times. These changes have never been surveyed in detail, but have important consequences for material properties, fossil preservation, and the capacity of marine organisms to adapt to environmental change. We used new large-scale phylogenetic, paleontological, and mineralogical data sets to analyze the evolution of skeletal mineralogy in cheilostomes as they diversified across the early Cenozoic calcite-aragonite seas transition. Ancestral state reconstructions and stochastic character maps indicate at least 50 independent acquisitions of partly or fully aragonitic skeletons from calcitic ancestors, with many more transitions toward the aragonitic state than away from it. Fossil faunas are dominated almost entirely by calcitic species in the Cretaceous, but bimineralic species become common by the Oligocene, and aragonitic species by the Pliocene−Pleistocene. Phylogenetic and fossil analyses reveal a coherent timeline consistent with the shift to aragonite seas. Cheilostome skeletal development may be predisposed to mineralogical flexibility, with adaptive consequences for colony construction and modularity.
{"title":"Evolution of skeletal mineralogy in cheilostome bryozoans from calcite to aragonite seas","authors":"James G. Saulsbury, Anna Piwoni-Piórewicz, Piotr Kukliński, Emanuela Di Martino, Lee Hsiang Liow","doi":"10.1130/g53795.1","DOIUrl":"https://doi.org/10.1130/g53795.1","url":null,"abstract":"Under the calcite-aragonite seas hypothesis, the evolutionary history of calcifying marine organisms reflects changes in global seawater chemistry, alternately favoring precipitation of skeletons made of calcite or aragonite. Most calcifying groups including corals, coccolithophores, and stromatoporoids do not switch mineralogies in response to these changes; instead, they expand and dwindle as their favored seawater conditions come and go. Cheilostome bryozoans have been suggested as an exception to this rule, apparently transitioning between aragonite and calcite many times. These changes have never been surveyed in detail, but have important consequences for material properties, fossil preservation, and the capacity of marine organisms to adapt to environmental change. We used new large-scale phylogenetic, paleontological, and mineralogical data sets to analyze the evolution of skeletal mineralogy in cheilostomes as they diversified across the early Cenozoic calcite-aragonite seas transition. Ancestral state reconstructions and stochastic character maps indicate at least 50 independent acquisitions of partly or fully aragonitic skeletons from calcitic ancestors, with many more transitions toward the aragonitic state than away from it. Fossil faunas are dominated almost entirely by calcitic species in the Cretaceous, but bimineralic species become common by the Oligocene, and aragonitic species by the Pliocene−Pleistocene. Phylogenetic and fossil analyses reveal a coherent timeline consistent with the shift to aragonite seas. Cheilostome skeletal development may be predisposed to mineralogical flexibility, with adaptive consequences for colony construction and modularity.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"26 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900958","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}
Sneha Suresh, Simon J. Barker, Paul W. Williams, Colin J.N. Wilson, Trevor H. Worthy, Jeffrey Lang, John Hellstrom, Travis Cross, Shane J. Cronin, Joel A. Baker
Limestone caves are commonly located close to volcanic regions and can preserve signals of past eruptions, providing crucial chronostratigraphic constraints within and beyond U-Th dating limits for karst development and cave evolution. Here we document five caves in the Waitomo karst region of New Zealand that contain volcanic ash (tephra) from the Taupō Volcanic Zone, a highly active region of silicic volcanism. The cave-hosted deposits are glassy due to their protection from weathering, with one example being poorly sorted and locally indurated where pyroclastic flows filled the cave. Other deposits are bedded and inferred to have been water remobilized into the caves from surficial ash deposits. Glass compositions indicate that tephras located in cave floors and roof cavities and on cave walls were sourced from four caldera-forming eruptions, the 1.55 Ma Ngaroma, 1 Ma Kidnappers, 349 ka Whakamaru, and ca. 50 ka Rotoiti events, plus a smaller-volume event from Taupō volcano at ca. 40 ka, highlighting the repeated impact of explosive eruptions on this region. Tephra studies in caves thus provide crucial information that can be used to constrain cave sediment and volcanic histories, vertebrate fossil chronologies, and cave system and landscape evolution.
石灰岩洞穴通常位于火山区域附近，可以保存过去火山喷发的信号，为岩溶发育和洞穴演化提供了重要的年代地层限制。在这里，我们记录了新西兰怀托莫喀斯特地区的五个洞穴，这些洞穴含有来自陶普火山带的火山灰（tephra），陶普火山带是一个高度活跃的硅火山活动区域。岩洞型矿床由于不受风化作用而呈玻璃状，其中一个例子是分选不良，在火山碎屑流充满洞穴的地方局部硬化。其他沉积物是层状的，据推测是水从地表火山灰沉积物中重新注入洞穴。玻璃成分表明，洞底、洞顶洞洞和洞壁上的有机质来自四次火山口形成的喷发，分别是1.55 Ma Ngaroma、1 Ma Kidnappers、349 ka Whakamaru和约50 ka Rotoiti事件，以及约40 ka tauphi火山的一次较小体积的喷发，突出了爆炸喷发对该地区的反复影响。因此，洞穴中的洞穴研究提供了重要的信息，可以用来限制洞穴沉积物和火山历史，脊椎动物化石年代学，洞穴系统和景观演化。
{"title":"Nowhere to hide: Volcanic ash invasion of limestone caves in New Zealand","authors":"Sneha Suresh, Simon J. Barker, Paul W. Williams, Colin J.N. Wilson, Trevor H. Worthy, Jeffrey Lang, John Hellstrom, Travis Cross, Shane J. Cronin, Joel A. Baker","doi":"10.1130/g53695.1","DOIUrl":"https://doi.org/10.1130/g53695.1","url":null,"abstract":"Limestone caves are commonly located close to volcanic regions and can preserve signals of past eruptions, providing crucial chronostratigraphic constraints within and beyond U-Th dating limits for karst development and cave evolution. Here we document five caves in the Waitomo karst region of New Zealand that contain volcanic ash (tephra) from the Taupō Volcanic Zone, a highly active region of silicic volcanism. The cave-hosted deposits are glassy due to their protection from weathering, with one example being poorly sorted and locally indurated where pyroclastic flows filled the cave. Other deposits are bedded and inferred to have been water remobilized into the caves from surficial ash deposits. Glass compositions indicate that tephras located in cave floors and roof cavities and on cave walls were sourced from four caldera-forming eruptions, the 1.55 Ma Ngaroma, 1 Ma Kidnappers, 349 ka Whakamaru, and ca. 50 ka Rotoiti events, plus a smaller-volume event from Taupō volcano at ca. 40 ka, highlighting the repeated impact of explosive eruptions on this region. Tephra studies in caves thus provide crucial information that can be used to constrain cave sediment and volcanic histories, vertebrate fossil chronologies, and cave system and landscape evolution.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"9 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144900634","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}
Xiaocong Luan, Colin D. Sproat, Jisuo Jin, Peir K. Pufahl, Rongchang Wu, Renbin Zhan
Cool-water settings characterize Ordovician occurrences of Phanerozoic ooidal ironstones (POIs), contrasting sharply with the warm and humid climate typical of other POIs. This geological puzzle was deciphered in this study based on a complex suite of Middle−Late Ordovician POIs in South China hosted in tropical sediments of a warm-water origin, and coeval ferruginous ooid-bearing sediments and ferruginous microbialites that accumulated in a relatively cool-water setting. Here, we demonstrate that, despite their different depositional settings, all Ordovician ferruginous deposits of South China share similar petrographic ultrastructures and δ56Fe isotopic signatures. This suggests that the formation of POIs was not likely controlled by water temperature but instead is attributable to a microbial iron factory, associated with active upwelling generated from frequent cool-water incursions from Gondwana. This model also explains the predominant occurrence of Ordovician ferruginous deposits in cool-water realms in peri-Gondwana. The Darriwilian peak of their abundance corresponds to the onset of an icehouse episode and a paleotropical cold-water tongue that created complex depositional environments across the South China plate.
{"title":"Upwelling-related ferruginous ooids, microbialites, and the Darriwilian tipping point of Ordovician climate","authors":"Xiaocong Luan, Colin D. Sproat, Jisuo Jin, Peir K. Pufahl, Rongchang Wu, Renbin Zhan","doi":"10.1130/g53374.1","DOIUrl":"https://doi.org/10.1130/g53374.1","url":null,"abstract":"Cool-water settings characterize Ordovician occurrences of Phanerozoic ooidal ironstones (POIs), contrasting sharply with the warm and humid climate typical of other POIs. This geological puzzle was deciphered in this study based on a complex suite of Middle−Late Ordovician POIs in South China hosted in tropical sediments of a warm-water origin, and coeval ferruginous ooid-bearing sediments and ferruginous microbialites that accumulated in a relatively cool-water setting. Here, we demonstrate that, despite their different depositional settings, all Ordovician ferruginous deposits of South China share similar petrographic ultrastructures and δ56Fe isotopic signatures. This suggests that the formation of POIs was not likely controlled by water temperature but instead is attributable to a microbial iron factory, associated with active upwelling generated from frequent cool-water incursions from Gondwana. This model also explains the predominant occurrence of Ordovician ferruginous deposits in cool-water realms in peri-Gondwana. The Darriwilian peak of their abundance corresponds to the onset of an icehouse episode and a paleotropical cold-water tongue that created complex depositional environments across the South China plate.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"183 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144850889","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}
Slab break-off and polarity reversal are tectonic events that often occur in relation to the subduction process. For example, the Western Alps and Ontong Java are high profile cases of slab break-off and polarity reversal. A sufficient number of high-quality geological examples indicate a very short time frame (an average of <6 m.y.) for slab break-off and polarity reversal after a collisional event. On the other hand, previous numerical modeling studies propose the timing of these collision break-off events to be on an average time scale between 7.5 and 23.2 m.y. This study reconciles the geological observations from 17 ancient and contemporary subduction zones with a suite of numerical experiments. By synthesizing geological evidence and our geodynamic models, we propose that the majority of slab break-off and polarity reversal processes can be geologically fast (average <6 m.y.), contrary to current thinking.
{"title":"Slab break-off and subduction polarity reversal after collision can be very fast","authors":"Erkan Gün, Philip J. Heron, Russell N. Pysklywec","doi":"10.1130/g52507.1","DOIUrl":"https://doi.org/10.1130/g52507.1","url":null,"abstract":"Slab break-off and polarity reversal are tectonic events that often occur in relation to the subduction process. For example, the Western Alps and Ontong Java are high profile cases of slab break-off and polarity reversal. A sufficient number of high-quality geological examples indicate a very short time frame (an average of &lt;6 m.y.) for slab break-off and polarity reversal after a collisional event. On the other hand, previous numerical modeling studies propose the timing of these collision break-off events to be on an average time scale between 7.5 and 23.2 m.y. This study reconciles the geological observations from 17 ancient and contemporary subduction zones with a suite of numerical experiments. By synthesizing geological evidence and our geodynamic models, we propose that the majority of slab break-off and polarity reversal processes can be geologically fast (average &lt;6 m.y.), contrary to current thinking.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"79 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144850893","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}
Juliana Olsen-Valdez, Cedric J. Hagen, Sarah J. Widlansky, Elizabeth J. Trower, Kathryn E. Snell, William C. Clyde
Thrombolites—clotted organo-sedimentary deposits—forming today preserve carbonate carbon isotope (δ13Ccarb) values that suggest that photosynthesis shifts the δ13C value of the local dissolved inorganic carbon pool, resulting in carbonate minerals with δ13Ccarb values 1‰−6‰ higher than that expected for carbonate minerals precipitated in equilibrium with ambient lake water. To test whether these signals are preserved in the geologic record, we analyzed the δ13Ccarb values of thrombolites from the Cretaceous to Eocene Sheep Pass Formation (Nevada, USA). We performed fabric-specific analyses of both the clot component (interpreted to reflect photosynthetically influenced precipitation) and adjacent matrix carbonate (interpreted to reflect abiotic precipitation) and find that the δ13Ccarb values of clot components are consistently offset to higher values than adjacent matrix. The mean offset (Δ13C) between the clots and matrix was +2.77‰ (±0.94, 2 s.e.). These Δ13C values are consistent with predictions by a model of diurnal carbon cycling driven by photosynthesis. Modeled Δ13C values also match the documented Δ13C values in modern lakes with thrombolites. We interpret that Sheep Pass Formation Δ13C values preserve signals related to both local and external influences on the dissolved inorganic carbon pool. Fabric-informed sampling can disentangle these two signals, allowing for more robust chemostratigraphy from microbialite archives in addition to identification of a biosignature of photosynthesis.
{"title":"Photosynthetic carbon cycling signal preserved in carbonate δ13C values of ancient thrombolites","authors":"Juliana Olsen-Valdez, Cedric J. Hagen, Sarah J. Widlansky, Elizabeth J. Trower, Kathryn E. Snell, William C. Clyde","doi":"10.1130/g53716.1","DOIUrl":"https://doi.org/10.1130/g53716.1","url":null,"abstract":"Thrombolites—clotted organo-sedimentary deposits—forming today preserve carbonate carbon isotope (δ13Ccarb) values that suggest that photosynthesis shifts the δ13C value of the local dissolved inorganic carbon pool, resulting in carbonate minerals with δ13Ccarb values 1‰−6‰ higher than that expected for carbonate minerals precipitated in equilibrium with ambient lake water. To test whether these signals are preserved in the geologic record, we analyzed the δ13Ccarb values of thrombolites from the Cretaceous to Eocene Sheep Pass Formation (Nevada, USA). We performed fabric-specific analyses of both the clot component (interpreted to reflect photosynthetically influenced precipitation) and adjacent matrix carbonate (interpreted to reflect abiotic precipitation) and find that the δ13Ccarb values of clot components are consistently offset to higher values than adjacent matrix. The mean offset (Δ13C) between the clots and matrix was +2.77‰ (±0.94, 2 s.e.). These Δ13C values are consistent with predictions by a model of diurnal carbon cycling driven by photosynthesis. Modeled Δ13C values also match the documented Δ13C values in modern lakes with thrombolites. We interpret that Sheep Pass Formation Δ13C values preserve signals related to both local and external influences on the dissolved inorganic carbon pool. Fabric-informed sampling can disentangle these two signals, allowing for more robust chemostratigraphy from microbialite archives in addition to identification of a biosignature of photosynthesis.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"15 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144792717","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}
Paleoelevation data from continental arcs provide a record of tectonic processes and reveal feedbacks between surface uplift and paleoclimate. Here we reconstruct the Oligocene–Miocene paleoelevation of the central Cascade arc (northwestern USA) using δD values of paleoprecipitation preserved within hydrated volcanic glass. From the early Oligocene to early Miocene (ca. 33–17 Ma), we observe a 30‰ decrease in δD values of samples collected east of the range. Interpreted using a Rayleigh distillation model, these data indicate arc elevations increased by 1.1 ± 0.4 km during this interval, coeval with geochemical evidence for a 12 km increase in Moho depth. In the context of existing geochemical, thermochronologic, and structural data, Oligocene to early Miocene surface uplift was predominantly driven by magmatic additions to the crust during a period of high magmatic production rates. Surface uplift was synchronous with leeward drying, suggesting that an orographic rain shadow was established by the early Miocene. These results highlight the role of crustal thickness and paleoelevation changes in controlling regional climate in magmatic arcs.
{"title":"Surface uplift of the central Cascade Range, northwestern USA, via Oligocene to early Miocene crustal thickening","authors":"Luke C. Basler, Elizabeth J. Cassel","doi":"10.1130/g53421.1","DOIUrl":"https://doi.org/10.1130/g53421.1","url":null,"abstract":"Paleoelevation data from continental arcs provide a record of tectonic processes and reveal feedbacks between surface uplift and paleoclimate. Here we reconstruct the Oligocene–Miocene paleoelevation of the central Cascade arc (northwestern USA) using δD values of paleoprecipitation preserved within hydrated volcanic glass. From the early Oligocene to early Miocene (ca. 33–17 Ma), we observe a 30‰ decrease in δD values of samples collected east of the range. Interpreted using a Rayleigh distillation model, these data indicate arc elevations increased by 1.1 ± 0.4 km during this interval, coeval with geochemical evidence for a 12 km increase in Moho depth. In the context of existing geochemical, thermochronologic, and structural data, Oligocene to early Miocene surface uplift was predominantly driven by magmatic additions to the crust during a period of high magmatic production rates. Surface uplift was synchronous with leeward drying, suggesting that an orographic rain shadow was established by the early Miocene. These results highlight the role of crustal thickness and paleoelevation changes in controlling regional climate in magmatic arcs.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"15 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144786517","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}
Yuhan Li, Ingo Grevemeyer, Adam H. Robinson, Timothy J. Henstock, Milena Marjanović, Anke Dannowski, Norbert Kaul, Ingo Klaucke, Paola Vannucchi, Helene-Sophie Hilbert, Damon A.H. Teagle, Jason Phipps Morgan
The subduction of seamounts greatly affects arc volcanism, earthquakes, and tectonic deformation of the overriding plate, but the role of seamounts during bending and hydration of the incoming plate at subduction zones is poorly understood. We present seismic tomographic results along three profiles from the Middle America Trench offshore northern Costa Rica. The crustal and upper mantle P-wave velocities decrease toward the trench, with the onset of velocity reduction at ∼70 km from the trench axis, indicating bend-faulting, alteration, and hydration of the incoming plate. The most prominent low-velocity anomaly of 7.6−7.8 km/s in the upper mantle occurs beneath a seamount at the outer rise, indicating enhanced hydration with ∼2.4 wt% water content, compared to ∼1.1−1.2 wt% in the subducting plate away from the seamount. Near the seamount, extremely low heat flow (<10 mW/m2) supports vigorous hydrothermal recharge of seawater. Our results reveal that subducting seamounts efficiently increase the permeability of the oceanic crust prior to subduction, facilitate the transport of seawater into the mantle, exert control on widespread serpentinization, and potentially promote water recycling back into Earth’s interior.
{"title":"Impact of seamounts on the hydration of subducting plates","authors":"Yuhan Li, Ingo Grevemeyer, Adam H. Robinson, Timothy J. Henstock, Milena Marjanović, Anke Dannowski, Norbert Kaul, Ingo Klaucke, Paola Vannucchi, Helene-Sophie Hilbert, Damon A.H. Teagle, Jason Phipps Morgan","doi":"10.1130/g53355.1","DOIUrl":"https://doi.org/10.1130/g53355.1","url":null,"abstract":"The subduction of seamounts greatly affects arc volcanism, earthquakes, and tectonic deformation of the overriding plate, but the role of seamounts during bending and hydration of the incoming plate at subduction zones is poorly understood. We present seismic tomographic results along three profiles from the Middle America Trench offshore northern Costa Rica. The crustal and upper mantle P-wave velocities decrease toward the trench, with the onset of velocity reduction at ∼70 km from the trench axis, indicating bend-faulting, alteration, and hydration of the incoming plate. The most prominent low-velocity anomaly of 7.6−7.8 km/s in the upper mantle occurs beneath a seamount at the outer rise, indicating enhanced hydration with ∼2.4 wt% water content, compared to ∼1.1−1.2 wt% in the subducting plate away from the seamount. Near the seamount, extremely low heat flow (&lt;10 mW/m2) supports vigorous hydrothermal recharge of seawater. Our results reveal that subducting seamounts efficiently increase the permeability of the oceanic crust prior to subduction, facilitate the transport of seawater into the mantle, exert control on widespread serpentinization, and potentially promote water recycling back into Earth’s interior.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"5 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144763515","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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