Understanding the marine environment of early Earth is crucial for understanding the evolution of climate and early life. However, the master variable of Archean and Proterozoic seawater, the pH, is poorly constrained, and published ideas about the pH range encompass ~7 pH units from mildly acidic to hyperalkaline. To better infer ancient seawater pH, we examine the possibility of a seawater pH proxy using rare earth elements (REEs) in marine carbonates. The principle is based on increasing concentrations of heavy rare earth elements in solution relative to the light REEs with decreasing pH due to REE complexation and scavenging. We calibrated such an REE pH proxy using pH variability in modern seawater and tested the proxy with ~100 REE measurements from 13 separate carbonate formations. We compared our pH estimates derived from the REE proxy to published pH estimates of Cenozoic and Neoproterozoic seawater that use the established pH proxy of boron isotopes (δ11B). REE-pH estimates agree with the Cenozoic and the Ediacaran δ11B-pH proxy based on the type of carbonate and boron isotopic composition at corresponding times. The uncertainty in our REE-pH proxy can probably be explained by model assumptions, noise from freshwater influence, siliciclastic input, and diagenesis. This proof-of-concept study demonstrates that the REE-pH method provides pH estimates comparable to boron isotope pH estimates within uncertainties, which potentially could constrain changes in Precambrian seawater pH to better understand the coevolution of life and early Earth’s environment.
{"title":"The Rare Earth Element Distribution in Marine Carbonates as a Potential Proxy for Seawater pH on Early Earth","authors":"Ping-Chun Lin, D. Catling","doi":"10.2475/001c.118215","DOIUrl":"https://doi.org/10.2475/001c.118215","url":null,"abstract":"Understanding the marine environment of early Earth is crucial for understanding the evolution of climate and early life. However, the master variable of Archean and Proterozoic seawater, the pH, is poorly constrained, and published ideas about the pH range encompass ~7 pH units from mildly acidic to hyperalkaline. To better infer ancient seawater pH, we examine the possibility of a seawater pH proxy using rare earth elements (REEs) in marine carbonates. The principle is based on increasing concentrations of heavy rare earth elements in solution relative to the light REEs with decreasing pH due to REE complexation and scavenging. We calibrated such an REE pH proxy using pH variability in modern seawater and tested the proxy with ~100 REE measurements from 13 separate carbonate formations. We compared our pH estimates derived from the REE proxy to published pH estimates of Cenozoic and Neoproterozoic seawater that use the established pH proxy of boron isotopes (δ11B). REE-pH estimates agree with the Cenozoic and the Ediacaran δ11B-pH proxy based on the type of carbonate and boron isotopic composition at corresponding times. The uncertainty in our REE-pH proxy can probably be explained by model assumptions, noise from freshwater influence, siliciclastic input, and diagenesis. This proof-of-concept study demonstrates that the REE-pH method provides pH estimates comparable to boron isotope pH estimates within uncertainties, which potentially could constrain changes in Precambrian seawater pH to better understand the coevolution of life and early Earth’s environment.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141355191","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Laura L. Barnett, Peter Copeland, Virginia B. Sisson, Steve Naruk
For over a century, the history of Grand Canyon has been of interest to many. In recent years, debates have centered around the hypothesis that Grand Canyon formed during the late Cretaceous, not the Miocene, as previously thought. In this study, fluid inclusions within carbonates from the Mauv, Redwall, Supai, and Kaibab Fms. from Grand Canyon yield entrapment temperatures between 135 and 60 °C. Comparison of these temperature to time-temperature histories based on thermochronology (U-Th/He and fission track) from nearby samples suggest that these carbonates had fluids trapped within them from 89 to 58 Ma and that major denudation of late Cretaceous strata occurred during this interval. Regionally derived burial histories and local thermochronology suggest that significant uplift of Grand Canyon and the adjacent Colorado Plateau occurred during the late Cretaceous. We interpret the timing of fluid entrapment, denudation of Cretaceous strata, and burial histories to be consistent with initial uplift associated with the early stages of formation of Grand Canyon during the late Cretaceous. Models of uplift of northern Arizona exclusively during the Cenozoic are inconsistent with these data.
一个多世纪以来，许多人都对大峡谷的历史感兴趣。近年来，围绕大峡谷形成于白垩纪晚期而非之前认为的中新世这一假设展开了争论。在这项研究中，来自大峡谷毛夫、红墙、苏派和凯巴布地层的碳酸盐岩中的流体包裹体产生了 135 至 60 ° C 的夹层温度。将这些温度与附近样本的热年代学（U-Th/He 和裂变轨迹）所得出的时间-温度历史进行比较后发现，这些碳酸盐岩在 89 Ma 到 58 Ma 之间有流体夹杂，晚白垩世地层的大规模剥蚀就发生在这一时期。根据地区推断的埋藏历史和当地的热时学表明，在白垩纪晚期，大峡谷和邻近的科罗拉多高原发生了明显的隆起。我们对流体裹挟、白垩纪地层剥蚀和埋藏历史的时间解释与白垩纪晚期大峡谷形成初期的初始隆升相一致。亚利桑那州北部仅在新生代期间隆升的模型与这些数据不一致。
{"title":"Late Cretaceous Uplift of Grand Canyon: Evidence From Fluid Inclusions","authors":"Laura L. Barnett, Peter Copeland, Virginia B. Sisson, Steve Naruk","doi":"10.2475/001c.116737","DOIUrl":"https://doi.org/10.2475/001c.116737","url":null,"abstract":"For over a century, the history of Grand Canyon has been of interest to many. In recent years, debates have centered around the hypothesis that Grand Canyon formed during the late Cretaceous, not the Miocene, as previously thought. In this study, fluid inclusions within carbonates from the Mauv, Redwall, Supai, and Kaibab Fms. from Grand Canyon yield entrapment temperatures between 135 and 60 °C. Comparison of these temperature to time-temperature histories based on thermochronology (U-Th/He and fission track) from nearby samples suggest that these carbonates had fluids trapped within them from 89 to 58 Ma and that major denudation of late Cretaceous strata occurred during this interval. Regionally derived burial histories and local thermochronology suggest that significant uplift of Grand Canyon and the adjacent Colorado Plateau occurred during the late Cretaceous. We interpret the timing of fluid entrapment, denudation of Cretaceous strata, and burial histories to be consistent with initial uplift associated with the early stages of formation of Grand Canyon during the late Cretaceous. Models of uplift of northern Arizona exclusively during the Cenozoic are inconsistent with these data.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141102371","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Zeichner, Woodward W. Fischer, Noam Lotem, Kelsey R. Moore, Joshua E. Goldford, J. Eiler
The Archean rock record is limited and there is minimal organic matter available to understand the origin and evolution of life on early Earth. Low carbon isotope ratios have been measured in organic and reduced carbon phases in Archean rocks and have been invoked as biosignatures. However, it can be challenging to distinguish whether these low values reflect biotic formation, abiotic reactions, or post-depositional processes. To re-address this long-standing question, we compiled a comprehensive dataset of carbon isotope ratio measurements from organic carbon phases from Archean units that were analyzed using a variety of geochemical techniques. Our compilation also includes available descriptions and measurements of the stratigraphy, mineralogy, elemental ratios, and metamorphic grade for each data point. Our statistical analyses re-enforce a result that has been noted by prior compilations, that the carbon isotopic composition of Archean organic matter (OM) is broadly more 13C-deplete than the composition of Phanerozoic OM: The median δ13C values ( ±SD) of Archean total organic carbon and kerogen were −30.5±8‰ (n=2421) and −33.7±11.3‰ (n=556; Phanerozoic OM δ13C ±SD = −26.7±4.6‰ with n=449 from a prior compilation). Our study also identifies a previously unrecognized bimodality within the δ13C values of Archean OM that is observed even with subsampling of the data to account for geographic and stratigraphic sampling bias. We describe and model the isotopic and structural changes associated with the transformation of marine Type II kerogen from formation through diagenesis, catagenesis and metagenesis, and metamorphism, as described by trends on a van Krevelin diagram. Empirically, early maturation of organic matter during diagenesis results in shifts up to a few per-mille, which can occur in either direction depending on selective preservation and degradation of compounds. Thermal cracking that occurs during catagenesis can drive increases in δ13C of 5–12‰. At temperatures above greenschist metamorphism, carbon atoms exchange with other reactive carbon pools, driving increases in δ13C of up to 20‰. Together, our analyses suggest that the most metamorphosed graphitic samples from the earliest Archean are likely signatures of alteration, while low and multimodal ranges of δ13C values may preserve records of Archean ecology.
阿新世岩石记录有限，可用于了解早期地球生命起源和演化的有机物极少。在阿基坦岩石中的有机碳和还原碳相中测量到了低碳同位素比值，并将其作为生物特征。然而，要区分这些低值是反映了生物形成、非生物反应还是沉积后过程，却很有难度。为了重新解决这个长期存在的问题，我们汇编了一个全面的碳同位素比测量数据集，该数据集来自使用多种地球化学技术分析的Archean单元的有机碳相。我们的汇编还包括对每个数据点的地层学、矿物学、元素比率和变质等级的描述和测量。我们的统计分析再次证实了之前汇编中提到的一个结果，即与新生代有机质（OM）的组成相比，阿尔川有机质（OM）的碳同位素组成在很大程度上更缺乏 13C：阿尔川总有机碳和角质的δ13C 中位值（±SD）分别为-30.5±8‰（n=2421）和-33.7±11.3‰（n=556；新生代OM δ13C ±SD = -26.7±4.6‰，n=449来自先前的汇编）。我们的研究还发现了一种以前未曾认识到的双峰现象，即使对数据进行子取样以考虑地理和地层取样的偏差，也能观察到这种现象。我们描述并模拟了海洋 II 型角质从形成到成岩作用、成因和变质作用以及变质作用转变过程中相关的同位素和结构变化，如 van Krevelin 图上的趋势所描述的那样。根据经验，成岩过程中有机物的早期成熟会导致高达几百万分之一的转变，这种转变可以朝任一方向发生，取决于化合物的选择性保存和降解。成岩过程中发生的热裂解可使 δ13C 上升 5-12‰。在高于绿泥石变质作用的温度下，碳原子与其他活性碳池发生交换，导致δ13C升高达20‰。总之，我们的分析表明，最早阿寒纪变质程度最高的石墨样品可能是蚀变的标志，而δ13C值的低值和多模态范围可能保留了阿寒纪生态学的记录。
{"title":"The Carbon Isotopic Composition of Archean Kerogen and Its Resilience Through the Rock Cycle","authors":"S. Zeichner, Woodward W. Fischer, Noam Lotem, Kelsey R. Moore, Joshua E. Goldford, J. Eiler","doi":"10.2475/001c.116058","DOIUrl":"https://doi.org/10.2475/001c.116058","url":null,"abstract":"The Archean rock record is limited and there is minimal organic matter available to understand the origin and evolution of life on early Earth. Low carbon isotope ratios have been measured in organic and reduced carbon phases in Archean rocks and have been invoked as biosignatures. However, it can be challenging to distinguish whether these low values reflect biotic formation, abiotic reactions, or post-depositional processes. To re-address this long-standing question, we compiled a comprehensive dataset of carbon isotope ratio measurements from organic carbon phases from Archean units that were analyzed using a variety of geochemical techniques. Our compilation also includes available descriptions and measurements of the stratigraphy, mineralogy, elemental ratios, and metamorphic grade for each data point. Our statistical analyses re-enforce a result that has been noted by prior compilations, that the carbon isotopic composition of Archean organic matter (OM) is broadly more 13C-deplete than the composition of Phanerozoic OM: The median δ13C values ( ±SD) of Archean total organic carbon and kerogen were −30.5±8‰ (n=2421) and −33.7±11.3‰ (n=556; Phanerozoic OM δ13C ±SD = −26.7±4.6‰ with n=449 from a prior compilation). Our study also identifies a previously unrecognized bimodality within the δ13C values of Archean OM that is observed even with subsampling of the data to account for geographic and stratigraphic sampling bias. We describe and model the isotopic and structural changes associated with the transformation of marine Type II kerogen from formation through diagenesis, catagenesis and metagenesis, and metamorphism, as described by trends on a van Krevelin diagram. Empirically, early maturation of organic matter during diagenesis results in shifts up to a few per-mille, which can occur in either direction depending on selective preservation and degradation of compounds. Thermal cracking that occurs during catagenesis can drive increases in δ13C of 5–12‰. At temperatures above greenschist metamorphism, carbon atoms exchange with other reactive carbon pools, driving increases in δ13C of up to 20‰. Together, our analyses suggest that the most metamorphosed graphitic samples from the earliest Archean are likely signatures of alteration, while low and multimodal ranges of δ13C values may preserve records of Archean ecology.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-04-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140654321","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Detrital zircon grains in the ∼1740–1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470–2490 Ma. These zircon grains are distributed over ∼40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ∼2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400–3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730–1900 Ma zircons. From this relationship we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ∼2450–2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.
亚利桑那州西北部∼1740-1750 Ma Vishnu片岩及类似岩石单元中的碎屑锆石颗粒中，有多达30%的颗粒通过U-Pb同位素分析被测定为2470-2490 Ma。这些锆石颗粒分布在 ∼ 40,000 平方公里的范围内，其年龄峰值为 2480.0 ± 27.3 Ma (2SE)。与其他年龄的锆石相比，这些锆石的 207Pb/206Pb 年龄异常一致，分析的不确定性较小，与理想的 U-Pb 演化更为一致。该锆石群的加权平均年龄为 2480 ± 0.9 Ma（2SE），反映了一致的分析结果和较高的分析精度，但并不反映年龄的准确性。这些锆石的来源尚未确定。为了更好地确定不明来源的特征，我们用激光烧蚀质谱仪分析了其中 45 个晶粒的痕量元素和稀土元素，并用电子显微镜扫描了 16 个晶粒，以确定矿物包裹体。质谱仪对 Sc/Yb 和 Nb/Sc 的测定结果表明，这些矿石来源于大洋岛屿火成岩。电子微探针扫描显示，16 个晶粒中有 5 个含有石英，表明其来源于长英岩。207Pb/206Pb 日期的低变异性以及 U 和 Th 之间的线性关系支持锆石来源于单一火成岩单元或无异质锆石的一组密切相关的单元。通过文献检索，我们在蒙大拿州西南部的中新生代（？）元古代砂岩和砾岩中发现了具有相似年龄和U/Th比的其他锆石群，其中有2480 Ma锆石。这种砂岩沉积于怀俄明克拉通边缘附近，几乎全部含有2400-3600 Ma锆石，这与毗湿奴片岩中的锆石颗粒不同，后者含有大量1730-1900 Ma锆石。根据这种关系，我们推断这两个地区的 2480 Ma 锆石都来自怀俄明克拉通。我们的结论是，2480 Ma Vishnu锆石来源于在2450-2480 Ma Matachewan大火成岩省相关热点岩浆之上形成的长英岩，该岩床是由地幔衍生的热点岩浆与同化的阿切安大陆地壳混合形成的，而源岩则是在怀俄明陨石坑与苏必利尔省之间的初始断裂过程中形成的。
{"title":"Abundant 2480 Ma Detrital Zircons in ∼1740 Ma Vishnu Schist in Northwestern Arizona: Derivation From the Matachewan Large Igneous Province?","authors":"Jon E. Spencer, M. Doe, K. Domanik","doi":"10.2475/001c.115633","DOIUrl":"https://doi.org/10.2475/001c.115633","url":null,"abstract":"Detrital zircon grains in the ∼1740–1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470–2490 Ma. These zircon grains are distributed over ∼40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ∼2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400–3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730–1900 Ma zircons. From this relationship we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ∼2450–2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140700501","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Encinas, E. Rosselot, L. Sagripanti, A. Folguera, B. Horton, D. Orts, Victor A. Valencia, Gabriel Arriagada, Paz Butikofer, Andrés Solórzano
The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene.
在渐新世-中新世早期，智利中南部和阿根廷边缘经历了一个区域性的伸展构造阶段，形成了横跨前弧、安第斯科迪勒拉山系和后弧地区的多个盆地。这些盆地积聚了厚厚的火山岩和沉积岩。随后，新近纪收缩构造导致了目前安第斯科迪勒拉山系的形成，并在前弧盆地沉积了同生碎屑岩沉积。传统上，库拉马林地层由下火山岩单元（CMV）和上沉积岩单元（CMS）组成，被解释为形成于渐新世-中新世早期的伸展阶段。不过，也有研究提出，CMS沉积于早中新世-晚中新世的前陆盆地。为了揭示智利和阿根廷中南部安第斯山脉从伸展构造到收缩构造的转变过程，我们进行了新的地质年代分析（U-Pb、LA-ICP-MS），并将这些结果与智利-阿根廷边境（37°-38°S）主科迪勒拉山系CMS地层及其上覆Trapa-Trapa地层主要地段的构造、地层学和沉积学观察结果相结合。我们的研究结果表明，只有CMS的下部是在伸展环境中沉积的，其证据是存在一个倒置的伸展楔，其年代为20 Ma∼。CMS中上部（19-9Ma）和东部的同期单元显示出同步收缩变形的证据，表明沉积在西部主科迪勒拉山系缩短后形成的前陆盆地中。大约 9 Ma 时，主科迪勒拉山系以东的阿格里奥和乔斯马拉尔褶皱与推力带的抬升导致了前陆盆地的分割。特拉帕地层沉积于腹地盆地，沉积物来自东部。6.5 Ma以后，主要的收缩变形向西转移，导致CMS和Trapa Trapa地层强烈褶皱，随后西部主科迪勒拉山系在中央凹陷上发生推移。我们的研究表明，在早中新世至晚中新世期间，变形向东部前陆发展，然后在晚中新世至更新世期间向西部前弧转移。
{"title":"Cenozoic Basin Evolution During Alternating Extension and Shortening in the Southern Central Andes Along the Chile-Argentina Border, 37–38°S","authors":"A. Encinas, E. Rosselot, L. Sagripanti, A. Folguera, B. Horton, D. Orts, Victor A. Valencia, Gabriel Arriagada, Paz Butikofer, Andrés Solórzano","doi":"10.2475/001c.115328","DOIUrl":"https://doi.org/10.2475/001c.115328","url":null,"abstract":"The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140736357","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Shellnutt, Tung-Yi Lee, Yoshiyuki Iizuka, Hao-Yang Lee, Chi Thi Pham, Kenshi Suga
The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.
{"title":"Petrogenesis of the Early Paleogene North Island Syenite Complex, Seychelles","authors":"J. Shellnutt, Tung-Yi Lee, Yoshiyuki Iizuka, Hao-Yang Lee, Chi Thi Pham, Kenshi Suga","doi":"10.2475/001c.94773","DOIUrl":"https://doi.org/10.2475/001c.94773","url":null,"abstract":"The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-03-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140222492","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Soukup, L. Beranek, Stefanie Lode, D. Goudie, David Grant
Sediment provenance studies were conducted to constrain the establishment of the eastern Laurentian or Humber passive margin in Newfoundland, Canada, and examine models for the opening of the Iapetus Ocean and Humber Seaway. Ediacaran to Cambrian Series 2 strata of the lower Labrador and Curling groups contain garnet, muscovite, and feldspar, and yield 1000–1500 Ma detrital zircon grains that reflect local derivation from Grenville Province basement rocks during regional extensional deformation. Cambrian Series 2 to early Miaolingian units of the upper Labrador and Curling groups are quartz-rich and characterized by 556–586 Ma and 1000–2700 Ma detrital zircon grains that instead reflect continental-scale drainage and transition to passive margin deposition along eastern Laurentia. The geological relationships along the Humber margin are compared with modern analogues in the Newfoundland-west Iberia rift system to propose a magma-poor rift model that includes two breakup sequences which formed in response to isostatic adjustment after the rupture of crust and mantle, respectively. Crustal breakup resulted in an Ediacaran to Cambrian Series 2 breakup sequence that was connected to hyperextension, mantle exhumation, and bimodal magmatism. Mantle breakup likely occurred >20 Myr after first mantle exhumation and resulted in a breakup sequence that is best characterized by Cambrian Series 2 to early Miaolingian strata. The mantle breakup sequence consists of regressive-transgressive cycles that record the transition from breakup to thermal subsidence and was probably driven by the separation of the Dashwoods microcontinent from eastern Laurentia and outboard opening of west Iapetus. The Humber Seaway opened between the Humber margin and Dashwoods and was at least partially underlain by exhumed continental mantle. Our scenarios support hypotheses for equivalent magma-poor rift elements elsewhere in the Caledonian-Appalachian orogen, and we predict that crustal and mantle breakup sequences are exposed in the Scotland-Ireland and Quebec-New England segments of the eastern Laurentian margin.
对沉积物出处进行了研究，以确定加拿大纽芬兰的东劳伦伦或亨伯被动边缘的形成，并研究伊佩图斯洋和亨伯海道的开辟模式。下拉布拉多群和卷曲群的埃迪卡拉纪至寒武纪系列 2 地层含有石榴石、黝帘石和长石，并产生了 1000-1500 Ma 的铁锆石颗粒，反映了当地在区域延伸变形过程中从格伦维尔省基底岩石衍生而来。上拉布拉多组和 Curling 组的寒武系 2 至早期庙岭组单元富含石英，其特征是含有 556-586 Ma 和 1000-2700 Ma 的锆石颗粒，反映了大陆尺度的排水和沿劳伦提亚东部向被动边缘沉积的过渡。将亨伯边缘的地质关系与纽芬兰-西伊比利亚裂谷系统的现代类似物进行了比较，提出了一个贫岩浆裂谷模型，其中包括两个断裂序列，它们分别是在地壳和地幔断裂后因等静力调整而形成的。地壳断裂导致了埃迪卡拉纪至寒武纪的第二系列断裂序列，该序列与过伸展、地幔排空和双峰岩浆活动有关。地幔断裂很可能发生在第一次地幔掘起之后 20 多亿年，其断裂序列的最佳特征是寒武系 2 至早期庙岭期地层。地幔断裂序列由退行-横行循环组成，记录了从断裂到热沉降的过渡，其驱动力可能是达什伍兹微大陆与东劳伦提亚的分离以及西伊佩托斯的向外张开。亨伯海道开辟于亨伯边缘和达什伍兹之间，至少有部分地层是由被掘出的大陆地幔所覆盖。我们的假设支持喀里多尼亚-阿巴拉契亚造山带其他地方的等效贫岩浆裂谷要素的假设，我们预测地壳和地幔破裂序列暴露在劳伦提亚东缘的苏格兰-爱尔兰和魁北克-新英格兰段。
{"title":"Late Ediacaran to Early Cambrian Breakup Sequences and Establishment of the Eastern Laurentian Passive Margin, Newfoundland, Canada","authors":"M. Soukup, L. Beranek, Stefanie Lode, D. Goudie, David Grant","doi":"10.2475/001c.93038","DOIUrl":"https://doi.org/10.2475/001c.93038","url":null,"abstract":"Sediment provenance studies were conducted to constrain the establishment of the eastern Laurentian or Humber passive margin in Newfoundland, Canada, and examine models for the opening of the Iapetus Ocean and Humber Seaway. Ediacaran to Cambrian Series 2 strata of the lower Labrador and Curling groups contain garnet, muscovite, and feldspar, and yield 1000–1500 Ma detrital zircon grains that reflect local derivation from Grenville Province basement rocks during regional extensional deformation. Cambrian Series 2 to early Miaolingian units of the upper Labrador and Curling groups are quartz-rich and characterized by 556–586 Ma and 1000–2700 Ma detrital zircon grains that instead reflect continental-scale drainage and transition to passive margin deposition along eastern Laurentia. The geological relationships along the Humber margin are compared with modern analogues in the Newfoundland-west Iberia rift system to propose a magma-poor rift model that includes two breakup sequences which formed in response to isostatic adjustment after the rupture of crust and mantle, respectively. Crustal breakup resulted in an Ediacaran to Cambrian Series 2 breakup sequence that was connected to hyperextension, mantle exhumation, and bimodal magmatism. Mantle breakup likely occurred >20 Myr after first mantle exhumation and resulted in a breakup sequence that is best characterized by Cambrian Series 2 to early Miaolingian strata. The mantle breakup sequence consists of regressive-transgressive cycles that record the transition from breakup to thermal subsidence and was probably driven by the separation of the Dashwoods microcontinent from eastern Laurentia and outboard opening of west Iapetus. The Humber Seaway opened between the Humber margin and Dashwoods and was at least partially underlain by exhumed continental mantle. Our scenarios support hypotheses for equivalent magma-poor rift elements elsewhere in the Caledonian-Appalachian orogen, and we predict that crustal and mantle breakup sequences are exposed in the Scotland-Ireland and Quebec-New England segments of the eastern Laurentian margin.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140430296","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The interpretation of phase equilibria and reactions in geological materials is based on standard thermodynamics that assumes hydrostatic and homogeneous stress conditions. However, rocks and minerals in the lithosphere can support stress gradients and nonhydrostatic stresses. Currently, there is still not an accepted macroscopic thermodynamic theory to include the effect of nonhydrostatic stress on mineral reactions, and the use of several thermodynamic potentials in stressed geological system remains under debate. In experiments under nonhydrostatic stress, it is often difficult to resolve the direct effect of differential stress on phase equilibria because pressure gradients may be developed. Such gradients can affect the metamorphic equilibria at the local scale. Here, we investigate the direct effect of a homogeneous, nonhydrostatic stress field on the solid-fluid equilibrium using molecular dynamics simulations at non-zero pressure and elevated temperature conditions. Our results show that, for simple single-component systems at constant temperature, the equilibrium fluid pressure of a stressed system is always larger than the value of fluid pressure at hydrostatic stress conditions. The displacement of the equilibrium value of the fluid pressure is about an order of magnitude smaller compared to the level of differential stress in the solid crystal. Thus, phase equilibria can be accurately predicted by taking the fluid pressure as a proxy of the equilibration pressure. On the contrary, the mean stress of the solid can deviate substantially from the pressure of the fluid in stressed systems at thermodynamic equilibrium. This has implications on the use of thermodynamic pressure in geodynamic models since the fluid pressure is a more accurate proxy for predicting the location of metamorphic reactions, while the equilibrium density of the solid has to be determined from its mean stress.
{"title":"Fluid-mineral Equilibrium Under Nonhydrostatic Stress: Insight From Molecular Dynamics","authors":"M. Mazzucchelli, E. Moulas, B. Kaus, Thomas Speck","doi":"10.2475/001c.92881","DOIUrl":"https://doi.org/10.2475/001c.92881","url":null,"abstract":"The interpretation of phase equilibria and reactions in geological materials is based on standard thermodynamics that assumes hydrostatic and homogeneous stress conditions. However, rocks and minerals in the lithosphere can support stress gradients and nonhydrostatic stresses. Currently, there is still not an accepted macroscopic thermodynamic theory to include the effect of nonhydrostatic stress on mineral reactions, and the use of several thermodynamic potentials in stressed geological system remains under debate. In experiments under nonhydrostatic stress, it is often difficult to resolve the direct effect of differential stress on phase equilibria because pressure gradients may be developed. Such gradients can affect the metamorphic equilibria at the local scale. Here, we investigate the direct effect of a homogeneous, nonhydrostatic stress field on the solid-fluid equilibrium using molecular dynamics simulations at non-zero pressure and elevated temperature conditions. Our results show that, for simple single-component systems at constant temperature, the equilibrium fluid pressure of a stressed system is always larger than the value of fluid pressure at hydrostatic stress conditions. The displacement of the equilibrium value of the fluid pressure is about an order of magnitude smaller compared to the level of differential stress in the solid crystal. Thus, phase equilibria can be accurately predicted by taking the fluid pressure as a proxy of the equilibration pressure. On the contrary, the mean stress of the solid can deviate substantially from the pressure of the fluid in stressed systems at thermodynamic equilibrium. This has implications on the use of thermodynamic pressure in geodynamic models since the fluid pressure is a more accurate proxy for predicting the location of metamorphic reactions, while the equilibrium density of the solid has to be determined from its mean stress.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140438731","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Adam G. G. Smith, Matthew Fox, Jeffrey R. Moore, Scott W. Miller, L. Goren, Matthew C. Morriss, Andrew Carter
Displacement along the Wasatch Fault, Utah, has created the Wasatch Range. Owing to its topographic prominence, location on the eastern boundary of the Basin and Range, presently active fault slip, and proximity to Utah’s largest cities, the range and fault have garnered much attention. On the 102–103 year timescale, the behavior, displacement and seismic history of the Wasatch Fault has been well categorized in order to assess seismic hazard. On the 107 year timescale, the rock uplift rate history of the Wasatch range has also been resolved using thermochronometric data, owing to its importance in inferring the history of extension in the western US. However, little data exists that bridges the gap between these two timescales. Here, we infer an approximately 1 Ma rock uplift rate history from analysis of three river networks located in the center of the range. Our recovered rock uplift rate histories evidence periodic changes to rock uplift on the Wasatch Fault, that coincide with climate driven filling and unfilling of lakes in the Bonnneville Basin. To ensure our rock uplift rate histories are robust, we use field data and previously published cosmogenic 10Be erosion rate data to tightly constrain the erodibility parameter, and investigate an appropriate value for the slope exponent of the stream power model, n. We use our river network inversion to reconcile estimates of erodibility from a number of methodologies and show that the contrast between bedrock and bedload strength is an important factor that determines erodibility.
犹他州瓦萨奇断层的位移造就了瓦萨奇山脉。由于其地形突出、位于盆地和山脉的东部边界、目前活跃的断层滑动以及靠近犹他州最大的城市，该山脉和断层受到了广泛关注。在 102-103 年的时间尺度上，瓦萨奇断层的行为、位移和地震历史已被很好地归类，以评估地震危害。在 107 年的时间尺度上，瓦萨奇山脉的岩石隆起率历史也已通过热时计数据得到解决，因为这对推断美国西部的延伸历史非常重要。然而，很少有数据可以弥补这两个时间尺度之间的差距。在这里，我们通过对位于山脉中心的三条河流网络的分析，推断出了大约 1 Ma 的岩石隆起率历史。我们复原的岩石隆起率历史证明，瓦萨奇断层上的岩石隆起发生了周期性变化，这与邦纳维尔盆地中受气候影响的湖泊的填充和解填充相吻合。为了确保岩石隆起率历史数据的可靠性，我们利用野外数据和之前公布的宇宙成因 10Be 侵蚀率数据来严格限制侵蚀性参数，并研究了河流动力模型斜率指数 n 的适当值。我们利用河网反演来协调多种方法对侵蚀性的估计，并表明基岩和基载强度之间的对比是决定侵蚀性的一个重要因素。
{"title":"One Million Years of Climate-Driven Rock Uplift Rate Variation on the Wasatch Fault Revealed by Fluvial Topography","authors":"Adam G. G. Smith, Matthew Fox, Jeffrey R. Moore, Scott W. Miller, L. Goren, Matthew C. Morriss, Andrew Carter","doi":"10.2475/001c.92194","DOIUrl":"https://doi.org/10.2475/001c.92194","url":null,"abstract":"Displacement along the Wasatch Fault, Utah, has created the Wasatch Range. Owing to its topographic prominence, location on the eastern boundary of the Basin and Range, presently active fault slip, and proximity to Utah’s largest cities, the range and fault have garnered much attention. On the 102–103 year timescale, the behavior, displacement and seismic history of the Wasatch Fault has been well categorized in order to assess seismic hazard. On the 107 year timescale, the rock uplift rate history of the Wasatch range has also been resolved using thermochronometric data, owing to its importance in inferring the history of extension in the western US. However, little data exists that bridges the gap between these two timescales. Here, we infer an approximately 1 Ma rock uplift rate history from analysis of three river networks located in the center of the range. Our recovered rock uplift rate histories evidence periodic changes to rock uplift on the Wasatch Fault, that coincide with climate driven filling and unfilling of lakes in the Bonnneville Basin. To ensure our rock uplift rate histories are robust, we use field data and previously published cosmogenic 10Be erosion rate data to tightly constrain the erodibility parameter, and investigate an appropriate value for the slope exponent of the stream power model, n. We use our river network inversion to reconcile estimates of erodibility from a number of methodologies and show that the contrast between bedrock and bedload strength is an important factor that determines erodibility.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139598182","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The boron isotopic composition (δ11B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of δ11B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (δ11B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone marine burial diagenesis. Our results reveal significant decreases in both δ11B (~13 ‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (δ11B ~20 ‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (δ11B ~-1 ‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides an estimate of average neomorphism rate ( k0) in the range of 1×10-6 to 5×10-6 yr-1, which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting δ11B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and pCO2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy.
{"title":"Resetting of Shallow-Water Carbonate Boron Isotope Values During Marine Burial Diagenesis","authors":"Mingyu Zhao, B. Beaty, L. Tarhan, Noah Planavsky","doi":"10.2475/001c.91398","DOIUrl":"https://doi.org/10.2475/001c.91398","url":null,"abstract":"The boron isotopic composition (δ11B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of δ11B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (δ11B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone marine burial diagenesis. Our results reveal significant decreases in both δ11B (~13 ‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (δ11B ~20 ‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (δ11B ~-1 ‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides an estimate of average neomorphism rate ( k0) in the range of 1×10-6 to 5×10-6 yr-1, which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting δ11B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and pCO2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139444745","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：