滇中易门地区早前寒武纪地层化石的发现及其意义

通过1：50 000区域地质调查,在滇中易门地区新厘定出一套古元古界地层易门群,可细分为6个组级单元。在易门群亮山组灰黑色碳泥质板岩中首次发现保存较好的宏体多细胞生物化石,在易门群永靖哨组深灰色泥质灰岩中发现数枚毫米级结构清晰的多细胞生物化石;在中元古界昆阳群美党组灰紫色泥质板岩中发现长约20cm的宏体多细胞化石。通过镜下分析、电子探针及电镜扫描的研究,化石成分主要为C、O、P元素。本次发现填补了云南境内早前寒纪地层无生物化石记录的空白,对地球早期环境演化与生命进化的研究具有重要意义。     

滇中易门地区古元古界易门群亮山组多细胞生物的年代学约束

滇中易门地区古元古界易门群亮山组中赋存有保存较好的多细胞生物化石,是大氧化事件,或广义的Lomagundi事件中生物与环境协同演化的产物。由于亮山组中普遍缺乏火山岩夹层及多期构造热事件的扰动,给精确测定这些多细胞生物的时代带来一定困难。以易门县铜厂地区罗洼垤组顶部灰白色凝灰岩1件、亮山组灰白色凝灰岩1件、粉砂质凝灰岩1件、灰黄色钙质粉砂质凝灰岩2件共5件样品260个锆石同位素测试结果为研究对象,结合区域构造热事件的分析,较准确地厘定了易门地区古元古界易门群亮山组中多细胞生物发育的时代为2.18～2.05Ga,与非洲加蓬Franceville地区发现的群居多细胞生物的时代一致,表明地球早期重大地质事件的同时性。     

Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ³³S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45–2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ³³S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ¹³C_org (<?30‰) and dampened Δ³³S (0.4‰ to ?0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ¹³C_org (?33.4‰ to ?19.2‰) and expanded Δ³³S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ¹³C_org (?30‰ to ?25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ³³S (?0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.     

Age constraints on the Palaeoproterozoic Lomagundi–Jatuli Event in Zimbabwe: Zircon geochronology of the Magondi Supergroup

The ca. 2.2–2.1 Ga Magondi Supergroup on the Zimbabwe Craton in Southern Africa is mainly composed of sedimentary rocks deposited in a rift basin/passive continental margin, which record a unique episode in carbon isotope perturbation called the Lomagundi–Jatuli Event (LJE). This study reports new U–Pb ages of detrital zircons from the Deweras and Lomagundi groups of the Magondi Supergroup, and of igneous zircons from underlying granitoids, to constrain the timing of the LJE and to identify the provenance of the Magondi Supergroup. Most analysed detrital zircon grains range in ages between ca. 2.9 and 2.6 Ga. Three ca. 2.3–2.2 Ga detrital zircons from sandstone of the Deweras Group, with the youngest ²⁰⁷Pb‐²⁰⁶Pb age of 2,216 ± 22 Ma, indicate the onset of LJE in the Zimbabwe Craton was almost simultaneous to that in Fennoscandia and the Superior Craton, supporting the global synchronicity of the LJE.     

扬子西南缘古元古界亮山组圆盘状化石的发现及其环境意义

通过1:5万区域地质调查研究,首次在滇中易门地区古元古界易门群亮山组发现圆盘状宏体化石,进一步丰富了滇中古元古代早期宏体化石群种类,为扬子西南缘前寒武纪地层大区域对比提供古生物依据.本次的初步报道为从不同角度探索距今2.8～0.8 Ga的地球早期发展、演化历史开启了新窗口;对地球早期的环境演化与生命进化的研究具有重要价...     

滇中易门地区古元古界易门群罗洼垤组火山岩锆石U-Pb年龄及其构造热事件

易门群是综合了元江撮科、易门铜厂、东川因民等地区的最新野外地质资料、同位素年代学资料后厘定的古元古界浅变质地层。其记录了地球发展演化早期的一系列重大地质事件,如古元古代早期的休伦冰期事件及紧随其后的地幔柱事件、简单多细胞生物的首现、狭义的Lomagundi碳同位素正漂移事件、富氧大气崩溃后的缺氧富硒沉积等。采用锆石激光剥蚀法（LA-ICP-MS）对易门地区的易门群罗洼垤组微晶凝灰岩、英安质凝灰岩进行了年龄测定,获得锆石U-Pb年龄加权平均值为2241±16 Ma、2252±14 Ma,2件样品的锆石Th/U值为0.2~1.0,均具清晰的振荡环带结构,为岩浆成因的锆石。结果表明,滇中易门地区罗洼垤组沉积时代为古元古代中期,可与元江撮科的地幔柱岩石构造组合及东川因民的罗洼垤组凝灰岩的时代对比。罗洼垤组沉积时代可能是Kenorland超大陆裂解作用在扬子陆块的响应。     

Chemostratigraphy of Paleoproterozoic carbonate successions of the Wyoming Craton: tectonic forcing of biogeochemical change?

The Archean Wyoming Craton is flanked on the south and east by belts of Paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop. However, carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy. The 10 km thick Paleoproterozoic Snowy Pass Supergroup in the Medicine Bow Mountains was deposited on the present-day southern flank of the Wyoming Craton; it contains three discrete levels of glacial diamictite correlative with those in the Huronian Supergroup, on the southern margin of the Superior Craton. The Nash Fork Formation of the upper Snowy Pass Supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the Paleoproterozoic glacial epoch. Carbonates at the base of the Nash Fork Formation record remarkable ¹³C-enrichment, up to +28‰ (V-PDB), whereas those from overlying members of the lower Nash Fork Formation have δ¹³C values between +6 and +8‰. Carbonates from the upper Nash Fork Formation above the carbonaceous shale have carbon isotope values ranging between 0 and +2.5‰. The transition from high carbon isotope values to those near 0‰ in the Nash Fork Formation is similar to that at the end of the ca. 2.2–2.1 Ga carbon isotope excursion in Fennoscandia. This chemostratigraphic trend and deposition of BIFs, Mn-rich lithologies, carbonaceous shales and phosphorites at the end of the global ca. 2.2–2.1 Ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the Kenorland supercontinent. Correlative carbonates from the Slaughterhouse Formation in the Sierra Madre, WY, and from the Whalen Group in the Rawhide Creek area in the Hartville Uplift, WY, have highly positive carbon isotope values. In contrast, carbonates from other exposures of the Whalen Group in the Hartville Uplift and all carbonate units in the Black Hills, SD, have carbon isotope values close to 0‰. Combined with existing geochronologic and stratigraphic constraints, these data suggest that the Slaughterhouse Formation and the succession exposed in the Rawhide Creek area of the Hartville Uplift are correlative with the lower and middle Nash Fork Formation and were deposited during the ca. 2.2–2.1 Ga carbon isotope excursion. The Estes and Roberts Draw formations in the Black Hills and carbonates from other exposures in the Hartville Uplift postdate the ca. 2.2–2.1 Ga positive carbon isotope excursion and are most likely correlative with the upper Nash Fork Formation. The passive margin, on which the carbonates with highly positive carbon isotope values were deposited, extended around the southern flank of the Wyoming Craton through the Sierra Madre, Medicine Bow Mountains and Hartville Uplift. The presence of carbonates with carbon isotope values close to 0‰ in the upper Nash Fork Formation and the Whalen Group indicates that the passive margin persisted on the southern flank of the Wyoming Craton after the carbon isotope excursion. Rifting in the Black Hills, likely related to the final breakup of the Kenorland, succeeded the carbon isotope excursion, since the Estes and Roberts Draw formations, deposited during rifting and ocean opening on the eastern flank of the Wyoming Craton, postdate the carbon isotope excursion.     

山西五台山滹沱群四集庄冰碛岩碎屑锆石年龄及其对大氧化事件研究意义

山西滹沱群四集庄组变质陆源碎屑混杂岩的砾石岩性复杂,分选和磨圆性差,砾石表面可见划痕和凹面,发育落石构造,表明其沉积于冰水过渡带环境,记录了华北克拉通古元古代冰期事件。四集庄组不同层位不同岩石的碎屑锆石U-Pb年龄集中于2.5 Ga左右,下部2件陆源碎屑混杂岩样品的碎屑锆石给出的最小年龄分别为(2 440±40) Ma和(2 400±23) Ma,而上部1件砂岩样品碎屑锆石的最小年龄为(2 121±22) Ma,说明冰川沉积作用发生在2.4 Ga之后, 2.1 Ga之前或2.1 Ga左右,与全球性休伦冰川事件(Huronian Glaciation Event,HGE)发生时间一致。砂岩相最小碎屑锆石年龄2.1 Ga左右,说明五台地区冰川事件之后沉积作用仍未结束,冰川沉积发生在陆缘广海盆地。四集庄组之上的大石岭组具有碳酸盐岩碳同位素正漂移,沉积于2.12 Ga之后,2.08 Ga之前,与全球碳酸盐岩碳同位素正漂移事件时间一致。青石村组的红层沉积于2.08 Ga左右,也与全球红层发育记录一致。华北克拉通滹沱群记录了相对完整的大氧化事件序列。     

2.0 Ga orogenic graphite deposits and associated 13C-enriched meta-carbonate rocks from South China Craton: Implications for global Lomagundi event

The Lomagundi (-Jatuli) event, characterized by extremely high positive global inorganic carbon isotope excursion at about 2.2 billion years ago, is pivotal in investigating the causes and consequences of great oxygenation event, inventory and sequestration of carbon on the Earth’s surface, evolution of life, and more profoundly tectonic control on Earth’s environment. However, the reasons that caused the isotopic excursion are not resolved yet. Herein, we report the discovery of meta-carbonate rocks with distinct positive carbon isotopic excursion from the Paleoproterozoic continental collision zone of the Kongling Complex, South China Craton. The δ¹³C_V-PDB values for meta-carbonate rocks show positive values in the range from +5.5‰ to +11.6‰, whereas the δ¹³C_V-PDB values of associated graphite deposits range from ?25.8‰ to ?9.5‰. Zircon U-Pb-Hf isotopes from zircon-bearing meta-carbonate sample yielded weighted average ²⁰⁷Pb/²⁰⁶Pb age of 2001.3 ± 9.5 Ma, with corresponding ε_Hf(t) range from ?7.05 to ?3.16, comparable to the values of local 2.9–2.6 Ga basement rocks. Geochemical characteristics of meta-carbonate rocks, such as their rare earth element patterns and the trace element parameters of La, Ce, Eu, and Gd anomalies and Y/Ho ratio, suggest that the carbonate deposition took place in passive continental margin in association with large volumes of organic carbon. The extensive graphite deposits from Kongling Complex in South China Craton, their equivalents in the North China Craton and elsewhere across the globe prove that the burial of ¹²C-enriched organic carbon has eventually resulted in the global enrichment of ¹³C in the atmospheric CO₂, which is recorded in the marine carbonate rocks. Isotopic mass balance estimates indicate that more than half of the organic carbon was buried during the oceanic closure. Hence, the observed global shift could be directly related to the continent collision event in greater China, thus resolving the long-standing paradox of the Lomagundi global positive carbon isotope excursion. Moreover, the present results suggest that orogenesis play a significant role in sequestration of carbon into the continental crust.     

Mo isotopic variations of a Cambrian sedimentary profile in the Huangling area,South China: Evidence for redox environment corresponding to the Cambrian Explosion

The radiation of macroscopic animals in the Early Cambrian, commonly be called as ‘Cambrian explosion’, had a close relationship with Earth oxygenation. These macrofossils are widely preserved within the Early Cambrian in the northern and western Yangtze Craton. In order to show the casual relationship between animal evolution and Earth redox environment, Mo content and isotopic composition analyses have been conducted on dolomites of the Tianzhushan Formation and the black shales of the Shuijingtuo Formation from the lowermost Cambrian profile in the eastern Huangling dome, northern Yangtze Craton. The calcitic dolostones from the Tianzhushan Formation display lower concentrations of Mo contents (0.055–0.666 ppm) than that of the black silty shales from the Shuijingtuo Formation (6.0–172 ppm). This magnitude difference is most likely related to the different lithologies due to Mo enrichment in shales. The Mo isotope values (δ^98/95Mo) of the calcitic dolostones for the Tianzhushan Formation vary from −0.73‰ to 0.41‰. In comparison, the black sandy shales from the Shuijingtuo Formation exhibit δ^98/95Mo of −1.04–1.84‰, including 5 samples from the earliest Shuijingtuo Formation displaying systematically higher δ^98/95Mo values (0.09–1.84‰) than those of the other 5 samples from upper layers (δ^98/95Mo = −1.04 to −0.19‰). These geochemical data suggest that the redox environment of the early Cambrian is fluctuating, and the peak oxygenation of the ocean had already reached a level similar to modern environments. According to the Mo concentrations and δ^98/95Mo data, the sedimentary succession can be divided into five intervals, which are closely correlated to the episodic evolution of early life in the Cambrian. Together with the available published data from other profiles in the Yangtze Craton, our new data reveal that the higher oxidation environment had a positive effect on the evolution of early life.     

Isotopic compositions of C,O, Sr,and S and problem of ages of the Katera and Uakit Groups,western Transbaikal region

The age of the Katera Group, which occupies a large area in the western North Muya Range and occurs 100–150 km east of the Uakit Group, is a debatable issue. Based on geological correlations with reference sections of the Baikal Group and Patom Complex, the Katera and Uakit groups were previously considered nearly coeval units and assigned to Late Precambrian (Khomentovskii and Postnikov, 2002; Salop, 1964). This was supported partly by the Sm–Nd model datings (Rytsk et al., 2007, 2009, 2011). Finds of the Paleozoic flora substantiated the revision of age of the Uakit Group and its assignment to the Late Devonian–Early Carboniferous (Gordienko et al., 2010; Minina, 2003, 2012, 2014). We have established that Sr and C isotopic compositions in carbonates of these groups differ drastically, as suggested by their different ages. Sediments of the Nyandoni Formation (Katera Group), which contains carbonates characterized by minimum values of ⁸⁷Sr/⁸⁶Sr = 0.7056 and maximum values of δ¹³C = 4.9‰, were accumulated in the first half of Late Riphean (800–850 Ma ago), whereas the overlying Barguzin Formation (⁸⁷Sr/⁸⁶Sr_min = 0.70715, δ¹³C_max= 10.5‰) was deposited at the end of Late Riphean (700–750 Ma). Judging from the isotope data, the Nerunda Formation (Uakit Group), which contains carbonates with characteristics matching the most rigorous criteria of fitness for the chemostratigraphic correlation (Sr content up to 4390 μg/g, Mn/Sr < 0.1, δ¹⁸O = 23.0 ± 1.8‰), was deposited at the end of Vendian ~550–540 Ma ago). The sequence includes thick typical carbonate horizons with very contrast carbon isotopic compositions: the lower unit has anomalous high δ¹³C values (5.8 ± 1.0‰); the upper unit, by anomalous low δ¹³C values (–5.2 ± 0.5‰]). Their Sr isotopic composition is relatively homogeneous (⁸⁷Sr/⁸⁶Sr = 0.7084 ± 0.0001) that is typical of the Late Vendian ocean. The S isotopic composition of pyrites from the Nyandoni Formation (Katera Group) (δ³⁴S = 14.1 ± 6.8‰) and pyrites from the Mukhtunny Formation (Uakit Group) (δ³⁴S = 0.7 ± 1.4‰) does not contradict the C and Sr isotopic stratigraphic data.     

浙江长兴二叠系和三叠系界限地层的碳同位素

研究海相碳酸盐岩的碳和氧同位素已有三十多年,积累了数千个数据,其目的在于研究古海洋碳和氧同位素的演变。在此期间,一部分研究者认为,海相碳酸盐岩的δ¹³C值在0±2范围内变化,未表现出与地质时代相关的变化趋势(Clayton和Degens,1959;Degens和Epstein,1962;Keith和Weber,1964;Galimov,1965;Becker和Clayton,1972;Schidlowski等,1975)。但是,另一些学者,如Jeffery等(1955),Baertschi(1975),Compston(1960),Weber(1967),Garrels和Parry(1974)却认为,海相碳酸盐岩的δ¹³C值随地质时代而有规律地变化。     

A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?

Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time,as long as they are acquired from waters where the dissolved inorganic carbon(DIC)is in isotope equilibrium with the atmospheric CO2.However,in shallow water platforms and epeiric settings,the influence of local to regional parameters on carbon cycling may lead to DIG isotope variations unrelated to the global carbon cycle.This may be especially true for the terminal Neoproterozoic,when Gondwana assembly isolated waters masses from the global ocean,and extreme positive and negative carbon isotope excursions are recorded,potentially decoupled from global signals.To improve our understanding on the type of information recorded by these excursions,we investigate the pairedδ^13Ccarb andδ^13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior:the basal Bambui Group.This succession represents a 1~(st)-order sedimentary sequence and records two majorδ^13Ccarb excursions in its two lowermost lower-rank sequences.The basal cap carbonate interval at the base of the first sequence,deposited when the basin was connected to the ocean,hosts antithetical negative and positive excursions forδ^13Ccarb andδ^13Corg,respectively,resulting inΔ^13C values lower than 25‰.From the top of the basal sequence upwards,an extremely positiveδ^13Ccarb excursion is coupled toδ^13Corg,reaching values of+14‰and-14‰,respectively.This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambui Group that occurs with only minor changes inΔ^13C values,suggesting change in the DIC isotope composition.We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles.This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis,favored by the basin restriction.The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere,resulting in a^13C-enriched DIC influenced by methanogenic CO2.Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere,potentially affecting both the global carbon cycle and the climate.     

五台地区滹沱群时代与地层划分新认识:地质学与锆石年代学证据

本文从五台地区滹沱群豆村亚群四集庄组、东冶亚群纹山组和郭家寨亚群西河里组地层中共采集了5件浅变质砂岩样品,并对其进行了La-MC-ICPMS锆石U-Pb年龄测定。分析结果显示,四集庄组2件砂岩样品碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄主要集中于~2.5Ga和2.1~2.2Ga两个峰值,其中~2.5Ga碎屑锆石来自新太古代五台群和五台地区花岗质杂岩;2.1~2.2Ga碎屑锆石获得²⁰⁷Pb/²⁰⁶Pb加权平均年龄2134±5Ma,限定了四集庄组砂岩沉积下限为2134Ma。结合四集庄组火山岩形成时代(2140±10Ma)和四集庄组底部发育厚层砾岩,我们认为滹沱群初始形成时代为~2.2Ga,即早元古代中期。东冶亚群纹山组底部砂岩中碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄主要集中于2050~2122Ma之间,其中64粒相对年轻的锆石获得²⁰⁷Pb/²⁰⁶Pb加权平均年龄2068±3Ma,代表了东冶亚群形成时代下限为2070Ma左右。综合豆村亚群青石村组火山岩形成时代2087±9Ma,我们认为东冶亚群初始形成于2070Ma左右。郭家寨亚群中最年轻碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄为1958±10Ma,表明郭家寨亚群开始沉积时代小于1.95Ga,为早元古代晚期/末期。区域上,早元古代末期是华北最终克拉通阶段,而郭家寨亚群与东冶亚群呈明显的角度不整合接触关系,两者记录了明显不同的地质过程。因此,我们建议郭家寨亚群应从滹沱群中解体出来并独立命名为郭家寨群,且郭家寨群可能沉积于华北克拉通化过程中/之后,开始沉积的时代为1.9~1.8Ga。     

Stable carbon isotopes of alkane gases from the Xujiahe coal measures and implication for gas-source correlation in the Sichuan Basin,SW China

The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C₁–C₄ n-alkanes, with the highest δ¹³C₂ values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ¹³C₁ values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ¹³C₂ values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.     

Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents

Stable Cl and C isotope ratio results for 3 selected chlorinated solvents, perchloroethylene (PCE), trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) provided by 4 different manufacturers are presented. The isotope ratio for all compounds range between −3.5 and +6.0‰ forδ³⁷Cl and from −37.2 to −23.3%. forδ¹³C. The greatest³⁷Cl difference between manufacturers is observed in the TCE samples which showδ³⁷CI values of −2.5%o for PPG, +2.43‰ for ICI and +4.4‰ for DOW. TCAs show a smaller range (−2.4 to +2.0‰), while the TCEs have slightly different³⁷Cl contents. The¹³C data show the most distinctδ¹³C values for PCEs (−23.3 for DOW, −24.1 for Vulcan, −33.8 for PPG and −37.2‰ for ICI) while both TCEs and TCAs show a smallerδ¹³C range, but still distinct differences. These preliminary data suggest that each manufacturer and solvent type may have distinctiveδ⁶³⁷Cl andδ¹³C values. These results show that by using a combination of³⁷Cl and¹³C, there is a potential to indicate a specific source of chlorinated solvents, as well as an ability to delineate contamination episodes caused by these compounds in groundwaters.     

Revised estimate of δ34S for marine sulfates from the Upper Ordovician: data from the Williston Basin,North Dakota,U.S.A.

Analyses for δ³⁴S of 13 bedded, marine anhydrite samples from the “C” anhydrite member of the Red River Formation (Upper Ordovician) in the North Dakota portion of the Williston basin represent an addition of δ³⁴S data to a portion of the S isotope age curve with few data. Previously published estimates of δ³⁴S for Upper Ordovician marine sulfates apparently are limited to 4 samples from the Saskatchewan portion of the same basin. An adjusted mean value of +25.5‰ was calculated for all known Upper Ordovician δ³⁴S determinations. This value is approximately 2 to 3‰ lighter than the previous estimate, which suggests that δ³⁴S of the world ocean during the Upper Ordovician may have been lighter than previously thought. However, because all δ³⁴S data are from one sedimentary basin, additional S isotopic data from several globally-distributed evaporite basins are needed to evaluate this hypothesis and further constrain δ³⁴S for the Upper Ordovician. Similar re-examination of other portions of the S isotope age curve with limited amounts of data may increase our understanding of the secular variation in δ³⁴S.     

Discovery of rhyolitic tuffaceous slate in the southwestern margin of Yangtze Craton: Zircon U-Pb ages (2491 Ma) and tectonic-thermal events

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area, central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks, which are closely related to the early tectonic evolution of the Earth. However, its formation era, sedimentary filling sequence, and geotectonic characteristics have always been in dispute. In this study, several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone. This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village, Tongchang Township, Yimen County. LA-ICP-MS zircon dating was conducted, achieving the crystallization age of magma of 2491 ± 15 Ma and the metamorphic ages of about 2.3 Ga, 2.0 Ga, and 1.8 Ga for the first time. Meanwhile, according to in-situ Hf isotope analysis, the zircon ε_Hf(t) values were determined to range from −3.0 to 7.6, with an average of 2.7. Furthermore, the first-stage Hf model age (T_DM1) was determined to be 2513−2916 Ma, indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean. Therefore, it is believed that the strata of the slate were deposited in the Late Neoarchean, instead of the Mesoproterozoic as determined by previous researchers. Accordingly, it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group. Instead, they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information. Furthermore, the magma ages of 2491 ± 15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton, and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic. The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland. Meanwhile, the metamorphic ages of 2.3 Ga, 2.0 Ga, and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga, 1.95 Ga, and 1.85 Ga of the northern margin of the Yangtze Craton, indicating that the strata experienced Paleoproterozoic tectonic-thermal events. The study area is located on the eastern margin of Qinghai-Tibet Plateau, and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous. The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.©2021 China Geology Editorial Office.     

Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks

Comprehensive nitrogen biogeochemical cycle has been reconstructed for representative lacustrine organic-rich sedimentary rock in China, namely the Triassic Yanchang Formation (YF, 199–230 Ma) in Ordos and the Cretaceous Qingshankou Formation (QF, 86–92 Ma) in Songliao basins, by evaluating the organic and inorganic nitrogen isotopic compositions rather than only organic or bulk nitrogen isotopic compositions. The results indicate that the nitrogen isotope values of bulk rock (δ¹⁵N_bulk) in the non-metamorphic stage are significantly different from that of kerogen, which challenge the conceptual framework of sedimentary nitrogen isotope interpretation. The δ¹⁵N_bulk from the YF and QF were lower than their respective the nitrogen isotope values of kerogen (δ¹⁵N_ker), with offsets up to ～5.1‰, which have the inverse relationship for the metamorphosed rock. Thermal evolution did not significantly modify the δ¹⁵N of bulk rock and kerogen. The δ¹⁵N of sediments from the YF (δ¹⁵N_bulk, 1.6‰–5.6‰) were lower than that of rock from the QF (δ¹⁵N_bulk, 10.2‰–15.3‰). The nitrogen isotope values of silicate incorporated nitrogen (δ¹⁵N_sil) were slightly lower than those of the δ¹⁵N_ker in the YF and obviously lower for the QF. The fact that different nitrogen cycles occur in the YF and QF due to the different depositional redox conditions leads to different isotopic results. The YF water environment dominated by oxic conditions is not conducive to the occurrence of denitrification and anammox, and no abundant N₂ loss leads to the relatively light δ¹⁵N_bulk. In the stratified water for the QF, redox transition zone promotes denitrification and anammox, resulting in the heavy δ¹⁵N_bulk of rock and promotes the DNRA, resulting in heavy δ¹⁵N_ker and low δ¹⁵N_sil.     

Crassulacean acid metabolism influences D/H ratio of leaf wax in succulent plants

This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ¹³C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C₃ photosynthesis and indicate a wide range of CAM use, with δ¹³C values ranging from −33.01‰ to −18.54‰ (C₂₇–C₃₃ n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ¹³C_bulk and δD_C31 values with R² = 0.60 (δ¹³C_C31 and δD_C31 values with R² = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.