【作者】 郭华；

【导师】 杜远生；

【作者基本信息】 中国地质大学 ， 古生物学与地层学， 2013， 博士

【摘要】 中元古代记录了古海洋从太古代的完全厌氧、富铁向显生宙以来富氧转换的重要历程,同时它也是地球生物圈从最初的以微生物为主导的群落结构向宏体生物占统治地位转变的关键时期。因此,对这一地质历史时期古海洋化学状态以及海洋生物与环境之间的相互作用过程的研究极为重要,也是目前前寒武纪研究的重要命题之一。但迄今为止,由于地质时代久远,且在当时特殊的环境条件下,海洋微生物难以形成钙化的外壁,生物化石记录极不完整,对这一时期特殊的海洋化学条件下海洋主导的微生物功能类群及其与环境之间的相互作用关系的研究极为薄弱,目前还主要停留在一些概念性和轮廓性的认识上,严重阻碍了人们对这一时期生命与环境演化关系的深入认识。华北燕山地区中元古界特别是以碳酸盐岩沉积为主的蓟县群地层发育齐全、地层连续,且区域变质变形程度低,很好地保存了这一时期海洋生物与环境演化的原始信息。本文主要通过对华北地台中元古界蓟县群(1.6～1.4Ga)地层中保存的大量与微生物活动密切相关的生物沉积建造、典型的沉积微相(微组构)特征以及与生命代谢过程密切相关的碳、硫稳定同位素组成及演化,并结合典型层段氧化还原敏感元素的分布特征的深入分析,来探讨中元古代早期古海洋化学演化的基本规律以及在这一特殊的环境条件下海洋中主导的地球微生物学过程,获得了以下几点主要的认识：1、华北地台中元古界以碳酸盐岩为主的地层中发育了大量不同形态类型的微生物岩,包括叠层石、凝块石、纹层石、核形石(核形石状的碳酸盐岩结核)以及各类MISS构造,表明这一时期海洋微生物极为活跃。结合前人研究资料认为,叠层石、凝块石、纹层石以及核形石等浅水环境中产出的微生物岩类型主要代表了建礁的丝状或球状蓝细菌在不同的环境背景条件下的生物代谢作用过程,微生物岩的不同外部形态和内部微组构的差异受环境物理化学条件变化的影响相对较大；而在深水环境中产出的以核形石状的碳酸盐岩结核为代表的微生物岩的出现可能主要与厌氧型微生物如硫酸盐还原菌和食甲烷古菌等的代谢过程密切相关。这一时期海洋中不同的微生物功能类群及其主导的生物地球化学过程共同影响和改造着当时的大气和海洋环境。2、通过室内偏光显微镜下对大量岩石薄片观察发现了部分针状文石矿物,它们在地层中或以弥散状分布于深水相地层中,或以孔隙胶结物形式存在于浅水潮坪相受微生物席保护的局部“微环境”中。针状文石的出现表明这一时期深部海洋处于贫氧/缺氧状态,而在浅水环境一些受微生物席保护的局部微环境也可能贫氧；海水HCO3-离子过饱和,可能与当时较高的大气pC02有关。3、为了能够对中元古代早期地球表层系统包括大气圈、水圈以及生物圈层的演化有比较深入的了解,本文在华北燕山地区中元古界蓟县群三个剖面五个组地层(河北平泉高于庄组、杨庄组,辽宁凌源雾迷山组、铁岭组以及河北怀来洪水庄组)中连续采样,获得了623个δ13Ccarb和252个δ13Corg数据,这一研究代表了迄今为止中元古代盖层系(1.6～1.4Ga)时期国际上最完整、最高分辨率的碳同位素数据资源,从而也使得我们对这一时期的全球碳循环过程及其机理有了一个前所未有的认识与理解。从整个地层序列来看,δ13Ccarb主要集中在0±2‰范围内波动,具有明显的旋回性特征,与国际国内同期地层剖面能够很好的对比。相对于同一时期国际其他地层剖面而言,燕山地区具有地层连续、区域变质变形程度低、有良好的锆石年代学约束等优势,因此,在中元古界蓟县群地层序列中,本文工作获得的如此高分辨率的碳同位素数据为我们提供了一条较为可靠的中元古代盖层系碳同位素参考曲线,也为今后更好的进行国际国内地层剖面的对比、地层界线的划分以及地质事件的厘定等方面的研究提供了良好的基础资料。4、在整个蓟县群地层序列中,δ13Ccarb在平均同位素组成及其偏移幅度上表现为随时间演变而逐渐增加的趋势。高于庄组除中部层位～50m的层段外,δ13Ccarb平均值-0.3‰,变化范围在-1‰～1‰之间；至雾迷山组δ13Ccarb平均值增加到-0.1‰,变化范围在-1.8‰～+1.8‰。然而高于庄期和雾迷山期有机碳的埋藏速率变化范围不大,forg始终保持在0.15～0.25较小的范围内波动,因此记录在这一段地层序列中δ13Ccarb组成和偏移幅度的增加很可能与这一时期大气pC02的降低以及海水DIC库缓冲能力的下降有关。由此推测,中元古代早期,并司能在整个中元古代时期(1.6～1.0Ga),大气pCO2、海水DIC库经历了长期的、逐渐降低减小的演化过程。5、基于对蓟县群δ13Ccarb-δ13Corg的综合分析,认识到δ13Corg组成具有明显的深度梯度效应。在高于庄组顶部和雾迷山组,δ13Corg变化范围主要集中在-269--30%o之间,平均值约-28‰,δ13Corg与δ13Ccarb耦合性较好,表明在浅水潮坪环境中,以自养型微生物(菌藻类为主)对碳的代谢过程居于统治地位,有机碳的埋藏和氧化分解是影响碳同位素组成的重要区素；而在高于庄组中部和下部层位,δ13Corg明显偏负,波动范围在-26.3‰～34.4‰,平均值-31.1‰,多数样品碳同位素的分馏(AC)可以达到32%o以上,由此可以推断在中元古代淘洋深水环境中,异养型或化能自养型微生物对有机碳的再矿化可能是当时深部海洋中非常重要的碳的代谢途径。微生物及其碳代谢途径的空间差异性同时也暗示当时较低的大气p02,这一时期海洋处于一种表层适度氧化、而深部厌氧硫化的分层的状态,这与上述通过地层沥积相和沉积微相特征获得的初步认识是一致的。6、为了能够对中元古代早期古海洋氧化还原条件有更准确的认识,本文选取平泉剖面高于庄组中部-80m厚的一段地层序列进行了氧化还原敏感元素的测试分析,由于该段地层记录了古海洋从深潮下带静水环境向浅潮下带至潮间带下部逐渐变浅的演化过程,为古海洋氧化还原状态的研究提供了良好的载体。分析结果显示,中元古代早期海洋氧化还原界面较浅,可能在潮下带下部既已缺氧甚至硫化。7、在高分辨率碳同位素研究的基础上,本文选取碳酸盐含量高且重结晶程度较低的高于庄组和雾迷山组共110件样品进行了碳酸盐晶格硫酸盐(CAS)硫同位素的测试分析,经果显示这一时期海洋硫酸盐δ34S组成极不稳定,表现为快速、大幅度的波动特征,变化范围在+3.7‰～38.6‰,表明当时海洋硫库规模较小,极易受到外界环境和生物因素的影响,海洋硫酸盐水平较低,可能仅～1mM或者更低,这与中元古代早期较低的pO2分压以及贫氧条件下较低的陆地风化作用强度有着密不可分的联系。两次明显的834SCAs正偏(δ34ScAs>+35%o)分别出现于高于庄组中部和雾迷山组顶部,表明中元古代早期至少存在两次(以上)全球性或区域性环境厌氧程度的加剧；综合的C-S同位素组成特征还显示,在氧化还原化变层之下存在着巨大的DOC库,这一时期深部海洋中活跃的微生物类群除SRB以外,还包括如甲烷菌等厌氧型微生物,并导致了广泛的有机碳的矿化。8、中元古代早期较低的海水硫酸盐浓度严重阻碍了甲烷厌氧氧化过程(AOM)的发生,极大的促进了甲烷古菌类微生物的发展、繁衍,从而使得大部分的甲烷气体得以上升至上部水柱或逃逸到大气层中。作为一类非常重要的温室气体,大量CH4的释放也必然导致这一时期强烈的温室效应,从而维持了地质历史上长达16亿年的漫长的高温时期。迄今为止,在中元古界地层中也发现了大量与CH4气体逃逸密切相关的沉积相标志,进一步印证和支持了甲烷逃逸的发生。更多还原

【Abstract】 The Mesoproterozoic has recorded a critical evolution of the palaeoceanic chemistry from completely anoxic, iron-rich for the Archean to fully oxic for most of the Phanerozoic eon, meantime, it is also a critical geological stage when the Earth’s biosphere evolution from dominated by microbes into macro-organisms. Therefore, it is very important to study the palaeoceanic chemisty and the interaction between the organisms and environments during this time interval, and also is one of important topics of the Precambrian researches. But so far, due to so long-time geological courses, and special environmental conditions, microbes are very hard to preserve their calcified outerwalls, leading to incomplete fossil records. Therefore, the study on the eary Mesoproterozoic oceanic chemistry, dominant microbial functional groups and interactions with natural environments has been extremely weak, that strongly hampered further understanding for the organisms and environments.The Mesoproteozoic, especially the Jixian Group dominated by carbonate strata developed very well at the Yanshan Basin, North China, they were relatively complete, continuous and have resisted a high degree of regional metamorphisms and deformations, holding the potential to record their primary information about marine organisms and environments. This study mainly focuses on the bio-sedimentary construction, typical microfacies (micro-fabrics), and the carbon and sulfur stable isotopic compositions, combined with distribution of the redox-sensitive trace elements from a～80m thick sequence of the Gaoyuzhuang Formation, to disscuss the evolutionary patterns of the early Mesoproterozoic oceanic chemistry and dominant biogeochemistry processes under the special environmental conditions. The main results related to this study are given as follows.1, Microbialites are much more abundant in the well-preserved Mesproterozoic Jixian successions, including stromatolites, thrombolites, biolaminites, oncolites (oncolite-like carbonate concretions) and various types of microbially induced sedimentary structures (MISS), indicating diversified microbial activities in the early Mesoproterozoic ocean. On the basis of this study and previous achievements, microbialites that mainly occurred in shallw water environments, such as stromatolites, thrombolites, biolaminites and oncolites are believed to be probably related with fiamentous and/or spherical cyanobacteria metabolizing processes under distinct environmental conditions. The differences in external morphology and internal microfabrics for these shallow-water microbialites are mainly controlled by changes of environment physical/chemical conditions. However, the oncolite-like carbonate concretions, mainly appeared in relatively deeper water environments, are closely associated with anaerobic microbes such as sulfate-reducing bacteria (SRB), and methanogenic archaea (ANME) et al. The various microbial fuctional groups and their biogeochemical processes might have put significant influences on the early Mesoproterozoic climate and oceanic environments.2, The abundance of acicular aragonites are recognized in the thin-sections under microscopes, these argonites generally occur as fibrous cladding at organic-rich micro-pellets edges or as cements filling in some "mini-environments" protected by intensive microbial mats layers, suggesting dysoxic/anoxic conditions are prevalent in the Mesoproterozoic deep ocean, and also, some special shallow water environments protected by microbial mat layers are probably dysoxic due to oxygen consumption during the processes of organic matter decompositions. In addition, the appearence of abundant acicular aragonites indicates HCO3-supersaturated seawater conditions, which is probably related with much higher PCO2during the early Mesoproteorozic.3, To improve our knowledge about the evolutional processes of the early Mesoproterozoic surface system including atmosphere, hydrosphere and biosphere, the Mesoproterozoic Jixian depositional successions composed of five formations from three study sections (Gaoyuzhuang and Yangzhuang formations from the Pingquan Section, Wumishan and Tieling formations from the Lingyuan Section and Hongshuizhuang formation from the Huailai Section) are systematically ananlyzed on high-resolution carbonate and organic carbon isotopic compositions, including623δ13Ccarb and252δ13Corg data. The dataset presented here currently reprsents the highest resolution chemostratigraphic dataset from the early Mesoproterozoic Calymmian period (1.6～1.4Ga) and, as such, permits unprecedented exploration of the isotopic patterns and origin of isotopic variation in the early Mesoproterozoic carbon cycle. In the Jixian succession,623carbonate carbon isotope data points reveal values within a narrow range from-2‰to+2‰, oscillating in a repeated succession of positive and negative excursions around values near0%o. Data presented here show strong isotopic similarity to the previously reported that from other stratigraphic successions elsewhere, however, compared to them, strata within the Yanshan area have a series of superiority in strata continuity, integrity, low degree of regional metamorphisms and deformations and superb age constraints, thus, so high-resolution carbon isotope dataset presented here have the potcntical to serve as a reference for the entire early Mesoproterozoic, and provide important data materials for stratigraphic comparision, subdivision and definition of geological events in the further.4, The carbon isotope data from the early Mesoproterozoic Jixian succession record an increase in both the average isotopic composition and an increase in the magnitude of isotopic excursions. Data from the Gaoyuzhuang Formation, which record-with the exception of a single50 m thick interval-only minimal isotopic variation from-1‰to+1‰, with an average isotope composition of-0.3‰. While data shift to more variable from-1.8‰to+1.8‰, with an increased average value at-0.1‰in the Wumishan Formation. Despite relatively differences in carbon isotopes for the two formations, the degree of organic carbon burial (forg) calculated here remained within a fairly narrow range (0.15～0.25), which suggests that the increased isotopic variation observed in marine carbonate between the two successions must have been driven, instead by a decrease in the buffering capacity of the marine DIC system, probably related to a long-term decrease in PCO2through the early Mesoproterozoic and perhaps until the whole Mesoproteorzoic.5, A vertical depth-gradient in the isotopic composition of organic carbon is identified from the studied Jixian succession. δ13Corg vary from-26‰to-30‰, with values averaging-28‰, in the upper Gaoyuzhuang Formation and the entire Wumishan Formation, and the δ13Corg is coupled with the coeval δ13Ccarb, reflecting a microbial community dominated by autotrophic organisms in shallow water peritidal environments, and the burial or oxidative decomposition of benthic microbial material put much influence on carbon isotopic compositions. By constrast, the lower and middle Gaoyuzhuang Formation representing relatively deeper water environments shows substantially lighter isotopic compositions for organic carbon (varying from-26.3‰～-34.4‰, and averaging-31.1‰) and correspoding higher AC values (≥32‰), which suggests a substantially enhanced heterotrophic remineralization of benthic microbial mats. These differences in organic and inorganic carbon isotopes under different depositional environment most likely reflect generally low oxygen conditions and a dynamically maintained stratified ocean, where anoxic conditions likely occurred close to oxygenated, well-mixed surface oceans, this conclusion is consistent with that derived from sedimentary features.6, The analyses on redox-sensitive trace elements, such U, V, Mo, Cr and Co et al., are performed in a～80m thick carbonate succession from the middle Gaoyuzhuang Formation. This succession spans an interval from relatively queit and deeper subtidal zone to high energic shallower subtidal/intertidal environments and therefore is particularly well-suited to explore the early Mesoproterozoic ocean redox states. The distributions of redox-sensitive trace elements suggest a relatively shallow chemocline in the ocean, transitions from anoxic/euxinic to oxic seawater conditions may occur in lower subtidal zone.7, Based on high-resolution carbon isotope records,110carbonate samples are selected from the Gaoyuzhuang and Wumishan formations for CAS sulfur isotope analyses. Unlike the δ13Ccarb, CAS-sulfur isotopes show significant stratigraphic variations, with values ranging from+3.7‰～+38.6%o, indicating a very small sulfate reservoir size, which are more easily affected by environmental and biological changes. The oceanic sulfate concentration is speculated to be very low during the early Mesoproterozoic, probably near or less than1mM. The extremely low sulfate level is compatible with low atmospheric oxygen concentrations and low weathering ratios in that time period. More than two episodes of enrichments of534SCAS exceeding+35‰were present in the middle Gaoyuzhuang and top Wumishan formations, indicating multiple-episodes of ocean anoxic aggravation on a local or global scale during the early Mesoproterozoic. A combined study of C-S cycles indicates a huge DOC reservoir below the chemocline, heterotrophic and secondary chemoautotrophic microbes (eg. methanogen) except for sulfate-reducing bacteria probably exists and contributes widespread organic carbon remineralization in the defined anoxic deep ocean.8, The declined oceanic sulfate concentration would also have reduced anaerobic oxidation of methane (AOM) dominated by a consortia of methanogenic archaea and sulfate-reducing bacteria, but in turn facilitated the development of methanogenic archaea and enhanced fluxes of CH4release into the surface ocean and the atmosphere. Being an important greenhouse gas, the release of abundant CH4would have intensified the global warming at that time period, and maintained as long as1.6Ga "no ice" ages in the geological period. The abundance of well preserved sedimentary signatures, such as gas blister structures et al., provide evidences for CH4generation and release under some proper conditions.更多还原