扬子地台晚震旦至早寒武世沉积碳同位素研究

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	扬子地台晚震旦至早寒武世沉积碳同位素研究
其他题名	Carbon isotope investigation of Late Neoproterozoic and Early Cambrian sediments on the Yangtze Platform, South China
	郭庆军
	2003
学位授予单位	中国科学院地球化学研究所
学位授予地点	中国科学院地球化学研究所
学位名称	博士
学位专业	地球化学
关键词	晚震旦世早寒武世沉积岩碳酸盐和有机碳同位素组成生物爆发事件古环境变化扬子地台
摘要	震旦-寒武交变期是地史上一个重大转折期，从隐生宙向显生宙过渡，海、陆、空发生了显著不同的变化，是一个具有特殊意义的过渡时期。中国扬子地区广泛发育的海相沉积层序有效地记录了震旦一寒武交变期重要的地质事件，因此为研究该时期大气圈、生物圈、岩石圈和水圈的相互联系提供了独一无二的场所。在前寒武纪一寒武纪地质研究中，由于缺少标志性生物化石，行之有效的生物地层学方法在上前寒武系的划分和对比中受到很大限制，沉积有机，质干酪根和相应碳酸盐的稳定碳同位素分析已经成为全球对比和划分的一个极为重要的的研究方法。尽管一些学者对扬子地台进行了多年的地球化学研究，使用碳酸盐和与之共生的有机质碳同位素组成对广泛的扬子地台变化的沉积环境进行研究还很欠缺、对分析和探讨该时期的生命演化过程和环境变化的关系研究方面还不足。本研究是以中国扬子地区为研究范围，用沉积碳酸盐和与之共生的有机质碳同位素组成对广泛的扬子地台变化的沉积环境（台地相、盆地相、过渡地带）进行分析，初步建立了一个地球化学模型，用于解释震旦一寒武交变期沉积地球化学记录，分析和探讨区域扬子地台碳循环和环境变化与地质事件之间的内在和外在联系：（1）南沱期：有机碳同位素组成（瓮安剖面平均值在-35.0‰左右）表现为较强负异常。地球被称为雪球（Snow-ball）或部分冰雪覆盖球体（Slush-ball），水体滞留和水动力不强，原始产率较低，物源以深源为主；生物不发育，主要是细菌和低等的真核细胞生物；空气和海水的气体通过冰裂缝进行交换，促进了碳酸盐的溶解；有机碳循环主要通过厌氧过程，比如细菌硫酸盐还原作用进行。（2）陡山沱期和灯影期：南沱晚期一陡山沱早期，海水的碳酸盐碳同位素组成短期仍然较负（瓮安剖面的δl3Ccarb-avg为-2.8‰ 松桃剖面1、2的δ13Ccarb-avg分别是-3.5％0和-8.6％。）；有机质的碳同位素组成总体呈现正漂移（瓮安护3Corg－avg：-26.3‰；南明剖面的δ13Corg-avg高达-26.7‰），这正是全球分布的“帽”碳酸盐出现的时期：接近地表的火山去气作用释放出较之现代350倍的CO2，导致地球迅速变暖，冰雪融化，大陆风化作用加强，海平面上升，“雪球，，转化为“温室，大气中大量的CO2快速转化为碳酸钙沉入海水中。全球可能处于一个异常高的海洋沉积速率时期。随后陡山沱组的护3Ccarb值显著上升，暗示了这一时期生物作用加强，有机碳埋藏速率明显提高，有机碳和还原硫埋藏的增加，导致上层海水345的富集，硫同位素组成较高。热液作用和上升洋流作用促成了瓮安磷矿的形成和瓮安生物群的繁盛。在南沱冰期后的陡山沱期和灯影期，高的别3c的出现主要是由于进行光合作用的海洋植物群体产率的迅速增加、海洋沉积速率的升高、海洋深部水柱中缺氧层的存在、热液活动、上升洋流作用、海水分层结构引起的，而短期同位素组成的负漂移和生物产率的变化则可能是区域事件所造成。明显的碳同位素组成负漂移出现在前寒武／寒武界线附近，这反映了碳短期变化的翻覆，与震旦纪末生物绝灭、环境变化的地质事件相符合。（3）牛蹄塘期：本研究结果发现，在牛蹄塘组／郭家坝组底部黑色岩系中，有机碳、无机碳、有机硫、黄铁矿硫同位素组成值相对较低，有机碳和黄铁矿含量相对较高。δ3Corg-avg和δ3Ccarb-avg分别是-33.9士0.7‰和-2.5＝0.4‰；TOC＞0.5；黄铁矿平均含量为0.96％间变化；黄铁矿（δ4ScRs）和有机硫同位素组成（δ34SOBS）平均值分别为0.3士7.5‰和3.4土7.1‰。由于牛蹄塘初期的环境变化频繁和不稳定，扬子区处于一段特殊的时期，碳、硫同位素组成延续上震旦统的负漂移现象，海侵事件、还原环境、缺氧事件、裂谷作用火山喷发、、气液喷溢、热水作用等造成海水相刘较深，有机碳埋藏量增大，多金属富集成矿。在牛蹄塘中晚期碳同位素组成趋于稳定，碳同位素组成重化，有机碳和黄铁矿含量降低：碳酸盐和有机质的碳同位素组成平均值分别是0.31±1.0‰和-31．41．3％。（沙滩），呈现稳定的正漂移；TOC平均值是0.8％；沙滩剖面郭家坝组中上部样品的黄铁矿平均含量0.5％；δ34SCRS-avg和δ34SOBS-avg为17.8士2.0‰和16.9±1.8‰。在牛蹄塘中期，随着大气圈和水圈中CO2含量降低、环境稳定，促使寒武纪生物繁盛，可能与增加的寒武系生物产量和微生物作用有关。对牛蹄塘期的环境情况有如下分析：随着全球变暖、海平面迅速上升，上升洋流活跃，由于分层海水的存在，海水在氧化带附近及其上部具有较高的有机物生成率，使寒武纪初期成为形成植物繁衍和带壳动物爆发的重要时期。碳同位素组成由震旦一寒武交变期的不稳定负漂移变化到稳定正漂移，这与世界其它地区的变化相一致。下寒武统富有机碳和黄铁矿的黑色页岩沉积，暗示了早寒武世缺氧环境的存在。（4）凯里期：早中寒武世交变期有机碳和无机碳同位素组成规律的变化，出现在扬子地台台江剖面上。有机质埋藏的变化，与生物从下寒武到中寒武统的变化相联系。碳酸盐和有机碳同位素组成的变化规律，反映了震旦一寒武交变期沉积环境的多变和震旦一寒武交变期碳循环的波动，这与变化的古环境背景、环境条件和生物演化的变化相互联系。碳酸盐碳同位素组成反映了海水最初的同位素信息；海底热液作用和上升一洋流作用可能成为影响碳同位素组成的重要因素。然而，各一地区在同一时期存在相似性，也有很大的不同，所以针对区域和局部事件，还需要进一步研究和探讨。
其他摘要	The Precambrian - Cambrian transition is one of the critical time intervals in Earth history. Profound geotectonic, climatic and biological changes occur during the terminal Neoproterozoic and its transition into the early Cambrian. China's Yangtze Platform documents a sedimentary succession of terminal Neoproterozoic and early Cambrian age, which offers a unique opportunity to study the interaction between atmosphere, hydrosphere, biosphere and lithosphere during this critical interval in Earth history. The seawater carbonate carbon isotopic composition represents an important tool for developing a global correlation scheme, particularly for those sections lacking age-diagnostic biostratigraphic markers and/or precise radiometric age determinations. Paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time, but changes in atmospheric pCO2 as well. Although the carbon isotopic composition has been previously studied in sediments from the Yangtze platform, carbonate carbon and organic carbon isotopes typically have been analyzed in isolation on the carbonate platform, transition belt or basin belt. Especially organic carbon chemostratigraphy, and its paleoceanographic significance remains poorly understood. This study has researched on paired carbonate and organic carbon isotope determinations from different belts of the Yangtze Platform, and provided a preliminary geochemical explanation for environmental variations and bio-events observed on the Yangtze Platform during the Precambrian - Cambrian transitional interval and their causal relationship. The following conclusions have been drawn: (1) Nantuo stage: Negative carbon isotope anomalies in carbonate and organic matter (Weng'an section: δ13Corg-avg=-35.0‰) bracketing Neoproterozoic glacial deposits in the world. It has been proposed that biological productivity in the surface ocean collapsed for millions of years due to complete coverage of the sea by ice, creating a snowball earth. Primary productivity was much lower than present or non-existent. Organic carbon cycling was largely via anaerobic processes, such as bacterial sulfate reduction. Input of carbon dioxide into the ocean resulted from hydrothermal sources. (2) Doushantuo and Dengying stage: 5I3Ccarb (Weng'an section: 5 Ccarb.avg = -2.8‰; Songtao section(1): δ13Ccarb.aVg= 2.8‰; Songtao section(2): δ13CCarb-avg:= -8.6‰) are low and quite variable during the deposition of the cap carbonate: Global glaciation ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times modem level. A full hydrological cycle developed including increased continent weathering. Extreme greenhouse conditions resulted in global warming and a rapid termination of the snowball Earth. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally. Precipitation would be strongly localized on warm shallow-water platform, where CaCO3 solubility is minimized, which is in agreement with the regional variation in cap carbonate thickness and the observed increase in stratigraphic thickness of the negative 813C excursion from the slope to the platform. In oxic surface waters, abundant marine algae rely on dissolved nutrient such as phosphate for primary production. When marine algae die under sub-oxic and anoxic conditions, decomposition by microbial processes begins. Vertical mixing of the water column transferred P-rich bottom waters to the surface layer (above redox front), and shelf sediments contain phosphorite deposits. Heavier 813Ccarb and 813Corg values have been obtained for the Doushantuo and Dengying stages indicate an increase in organic carbon burial, possibly related to increasing productivity (including Ediacaran biota, Weng'an biota, Miaohe biota, Gaojiashan biota - radiation). Sharp negative shifts of the carbon isotopic composition have been found at the boundaries between the Doushantuo and Dengying Fm. and across the Precambrian - Cambrian transition. This is in agreement with previous reports and consistent with biomass extinction at these stratigraphic boundaries. It reflects a greatly reduced demand for dissolved inorganic carbon due to a lower primary productivity. (3) Niutitang stage: In the lower part of the Niutitang/Guojiaba Fm., total organic carbon content and pyrite content are high, carbon and sulfate isotopic compositions are lower( 513Ccaib.avg: -2.5±0.4‰, δ13Corg-avg: -33.9±0.7‰, δ34SCRs-avg: 0.3±7.5‰, δ34SOBS-avg: 3.4±7.1‰, TOG: 0.5-11%, Total Pyriteavg: 0.96‰). Fluctuations in carbon and sulfur content and isotopic compositions in the lower part of the Niutitang/Guojiaba Fm. suggest rapidly changing environmental conditions during the early Niutitang stage, which could reflect local perturbations. It also reflects temporal variations in global carbon turnover, i.e. an initial decrease in photosynthetic carbon fixation followed by an increasing demand for carbon dioxide during primary production and subsequent fractional organic carbon burial. In the middle-upper part of the Niutitang/Guojiaba Fm., total organic carbon content and pyrite content become lov/er and carbon isotopic compositions are higher in comparison with those of the lower part ( δ13Ccarb-avg: 0.3±1.0‰, δ13Corg.avg:-31.4±1.3‰, δ34SCRS-avg: 17.8±2.O960, 834SOBs-avg: 16.9±1.8‰, TOC: 0.2-1.6‰, Total Pyriteavg: 0.5‰). An evolution to lower total organic carbon content, pyrite content and heavier δ3Ccarb, δ13Corg, δ34SCRS and 834SOBS values indicates an increase in organic carbon burial, possibly related to increasing productivity (such as Cambrian bio- radiation) and biological sulfur cycling under sulfate-limited conditions. The increase of δ13C values reflects the increased productivity in biomass of the new fauna. Through the above research results, the following preliminary conclusions with respect to the environmental conditions during the Niutitang stage have been drawn: Following global wanning, sea level rise occurred. A pronounced bloom of microbial life characterizes the Niutitang stage. The evolution towards more negative carbon isotope values across the Precambrian-Cambrian transition followed by a gradual increase in 13C appears to be a prominent feature which has previously been reported from other equivalent successions world-wide. The transition into the Cambrian is marked by the deposition of black shales (Guojiaba/Niutitang Fm.), suggesting the onset of anoxic conditions. The wide geographic deposition of organic and pyrite-rich black shales across the Yangtze Platform suggest the existence of a large stratified ocean, or at least the existence of numerous isolated anoxic basins in the region. The paleogeographic setting of the Lower Cambrian continental margin of the Yangtze craton indicates local basins controlled by syn-sedimentary rifting, stagnant water episodically replenished by upwelling oxidized seawater is thought to be responsible for the formation of the poly-metallic sulfide layers and of phosphorite, barite, and sapropelitic "stone coal" (combustible black shale) beds. Below the redox front, bacterial sulfate reduction affects the isotopic composition of sedimentary sulfides and organic matte]-. Sulfides form at the seabed through bacterial sulfate reduction and organic matter is oxidized. Still, anoxic conditions result in widespread preservation of organic rich sediments on the Yangtze Platform. (4) Kaili stage: Secular variations in the carbon isotopic composition of organic and carbonate carbon characterize the lower to middle Cambrian transition that is exposed on the Yangtze Platform at Taijiang, Guizhou Province. 513C values for organic matter vary between -31.0 and -26.9%o (513Corg.avg: -28.5±1.0‰). The carbon isotopic composition for carbonate carbon displays variations between -2.7 and +1.9‰ (δ13Ccarb-avg: -0.8±0.9‰). Small variations in the isotopic difference (A5) between both isotope records reflect some secondary reworking of sedimentary organic matter during diagenesis. In general, the observed changes are interpreted to reflect primary depositional values, notably variations in the burial of organic matter. These, in turn, are linked to biological changes across the lower to middle Cambrian transition. The geobiodynamic model explains many features of both the geochemical record and the sedimentology of the period. The classical upwelling model and hyclrothermal activity were principally important mechanisms to effect the carbon and sulfur isotopic compositions. This study presents new data for the organic and carbonate carbon isotopic compositions from marine sedimentary rocks deposited between 600 and 500Ma. The parallel fluctuation of δ13Corg and δ13Coarb strongly suggests that the negative or positive excursions are a primary isotopic signature of seawater. No other mechanism could cause such a co-variation. They reflect differences in the sedimentary facies as well as show variations in the global carbon cycle. Hence, sediments from the Yangtze platform and their temporal carbon isotope trends can be compared to the global record of δ13C. Secular variations in the carbon isotopic compositions of carbonate and organic carbon are interpreted to reflect perturbations of the global carbon cycle, specifically changes in the fractional burial of organic carbon. These are coupled to changing environmental conditions as well as the biological evolution during the terminal Neoproterozoic and its transition into the early Cambrian. These secular variations are interpreted to reflect perturbations of the global carbon cycle.
页数	105
语种	中文
文献类型	学位论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/3670
专题	研究生_研究生_学位论文环境地球化学国家重点实验室
推荐引用方式 GB/T 7714	郭庆军. 扬子地台晚震旦至早寒武世沉积碳同位素研究[D]. 中国科学院地球化学研究所. 中国科学院地球化学研究所,2003.

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