华北地台中元古代串岭沟组页岩中的砂脉构造:17亿年前甲烷气逃逸的沉积标识?

华北地台中元古界串岭沟组暗色页岩中发育一种特殊的砂质脉状构造;层面上表现为不规则密集分布的细砂脊,垂向上由不连续薄砂层和近于直立的"肠状"砂脉体相交互组成.浅色砂脉由较纯的石英粉砂-细砂组成,含微量自生白云石、菱铁矿及微晶碳酸盐岩斑块,与黑色泥质围岩边界截然.研究表明,砂脉构造可能是在早期成岩阶段前由来自薄砂层的细-粉砂灌入甲烷缓慢逃逸通道而成,并由于压实缩短而褶皱成"肠状".薄砂层与黑色页岩形成的能量条件完全不同,可能系由风暴将海岸带或砂坝细砂带入低能环境而形成;甲烷源自沉积中埋藏的微生物席腐烂分解.围岩层面有微生物席成因微皱痕和气体逃逸形成的气泡构造,围岩中发现有细菌化石和草霉状黄铁矿.串岭沟组中密集发育的砂脉构造是目前地层中识别的最古老的甲烷排放证据,并有可能作为指示地质时期陆源碎屑沉积环境甲烷逃逸的沉积标识.大量高效温室气体进入大气圈可能是导致元古宙地球表层高温室气候和无冰川发育的重要原因.     

华北地台中元古代碳酸盐岩中的微生物成因构造及其生烃潜力

华北地台中元古界主要由环潮坪石英砂岩、浅海碳酸盐岩和浅海—泻湖相暗色页岩3种沉积相组合构成,以陆表海浅水碳酸盐岩占主导。碳酸盐岩中除含有丰富的微古植物、宏观藻类和微生物建隆外,还发育大量的微生物成因构造(MISS)和微生物诱发的碳酸盐沉淀(MMCP)。微生物席和MISS构造在高于庄组上部(约1.6 Ga)和雾迷山组下部(约1.45 Ga)碳酸盐岩中尤为发育,表明活跃的微生物活动和高有机质产量。在石化微生物席中,发现有丝状、球状细菌化石和草莓状黄铁矿;围岩中发现有针状文石、花瓣状重晶石、放射状菱铁矿、铁白云石和葡萄状碳酸盐胶结物等多种自生碳酸盐矿物,指示甲烷厌氧氧化(AOM)导致的自生碳酸盐沉淀。中元古代的温暖气候和海洋分层、缺氧、硫化条件有利于微生物的高生产量和高有机质埋藏率。气隆构造和核形石状碳酸盐结核反映浅埋藏条件下活跃的成烷作用和甲烷排放,围岩和MMCP中富沥青质。华北地台中元古界富微生物席碳酸盐岩有良好的生烃潜力,有可能形成重要的烃源岩。据微生物席、MISS构造及MMCP的研究,初步估算华北地台中元古代碳酸盐岩的概略生烃潜力约为10×108t石油当量。     

华北燕辽地区串岭沟组黑色页岩对中元古代早期海水氧化还原环境的制约

燕辽地区串岭沟组中上部保存有华北克拉通中元古代第一套大面积发育的黑色页岩,沉积于潮间带中下部,标志着该克拉通自形成以来最重要的海进过程,也是地球中年期早期阶段大气-海洋环境状态的特殊记录者。本文对蓟县八仙山剖面串岭沟组三段黑色页岩和顶部碳质白云岩开展了全岩主、微量元素和总有机碳(TOC)含量测定,认为其源区主要来自风化的长英质陆源碎屑物质。黑色页岩的TOC含量(平均值1.12%)、氧化还原敏感金属元素V (平均值82.6×10^-6)、Cr (平均值73.9×10^-6)、Ni (平均值29.8×10^-6)含量明显高于碳质白云岩(TOC、V、Cr、Ni含量均值分别为0.47%、29.6×10^-6、26.4×10^-6、11.7×10^-6)。此外,二者的氧化还原参数C_org/P分别为62.94和39.44,V/Cr值与沉积层位明显负相关,表明不同深度海水的氧化还原状态略有变化,但V/Cr-Th/U仍均落入富氧范围。研究结果表明,至少串岭沟组黑色页岩沉积时(不早于1.64Ga),较高的大气氧水平加剧了陆源物质风化及向海洋的输送量,并已经使得潮间带中下部静水进入到了相对氧化状态,从而可以为1.57~1.56Ga大型多细胞生物的出现提供持续含氧的发育环境。

     

Microbial Mats in the Mesoproterozoic Carbonates of the North China Platform and Their Potential for Hydrocarbon Generation

The well-preserved Mesoproterozoic succession in the North China platform consists mainly of three llthological associations including peritidal quartz sandstone, shallow marine and lagoonai dark to black shales, and shallow epeiric carbonates, with a total thickness of up to 8 000 m. In addition to well-documented microplants, macroalgae, and microbial buildups, abundant microbially induced sedimentary structures (MISS) and mat-related sediments have been recognized in these rocks. Intensive microbial mat layers and MISS are especially well preserved in the carbonates of the upper Gaoyuzhuang (高于庄) (ca. 1.5 Ga) and lower Wumishan (雾迷山) (ca. 1.45 Ga) formations, Indicating diversified microbial activities and a high organic production. In these petrified blomats, putative microbial fossils (both coccoidal and filamentous) and framboidal pyrites have been identified. The abundance of authigenic carbonate minerals in the host rocks, such as, acicular aragnnites, rosette barites, radial siderites, ankerites, and botryoidai carbonate cements, suggests authigenlc carbonate precipitation from anaerobic oxidation of methane (AOM) under anoxic/euxinic conditions. Warm climate and anoxic/euxinic conditions in the Mesoproterozoic oceans may have facilitated high microbial productivity and organic burial in sediments. Although authigenic carbonate cements may record carbonate precipitation from anaerobic methane oxidation, gas blister (or dome) structures may indicate gas release from active methanogenesls during shallow burial Bituminous fragments in mat-related carbonates also provide evidence for hydrocarbon generation. Under proper conditions, the Mesoproterozoic mat-rich carbonates will have the potential for hydrocarbon generation and serve as source rocks. On the basis of petrified biomats, a rough estimation suggests that the Mesoproterozoic carbonates of the North China platform might have a hydrocarbon production potential in theorder of 10×108t.     

南海东北部HD196A岩心的自生条状黃铁矿

自生黄铁矿是海洋沉积物缺氧硫酸盐还原过程的主要产物。南海东北部的HD196A岩心中发现大量条状的自生黄铁矿,以中空或实心为主。含量分析表明,岩心500cm附近为黄铁矿富集带,与沉积物有机碳(Corg)、硫酸盐[SO4]2-、甲烷(CH4)以及碳酸钙(CaCO3)的地球化学特征分界一致,是岩心的沉积边界,反映了沉积物所处的缺氧环境。岩心沉积物地球化学剖面表明,有机质参与的硫酸盐还原过程和甲烷厌氧氧化作用是黄铁矿形成的主要因素。黄铁矿异常可以作为指示沉积物甲烷异常的标志之一。     

Large earthquake‐triggered liquefaction mounds and a carbonate sand volcano in the Mesoproterozoic Wumishan Formation,Beijing, North China

Ten well‐preserved, earthquake‐triggered liquefaction mounds and a carbonate sand volcano have been found in the Mesoproterozoic Wumishan Formation (1550–1400 Ma) in the Beijing area, North China. These features crop out in a roadcut near Zhuanghuwa Village. All ten mounds occur in the same sedimentary layer and have rounded shapes with some concentric and radial fissures arising from the centre. They range from 1.5 to 4 m in diameter and from 10 cm to 30 cm in height. The carbonate sand volcano has a diameter of 110 cm and the ‘crater’ at the top has a depth of about 30 cm. Associated with these mounds and the sand volcano are many ‘normal’ sedimentary structures and numerous soft‐sediment deformation structures. The former include ripple marks, cross‐bedding, stromatolites and desiccation cracks, indicating deposition in a stable shallow‐water peritidal platform environment. The latter include intrastratal faults and folds, seismically formed breccias and carbonate clastic dykes. The morphological features and the genesis of these liquefaction mounds are very similar to mounds formed recently by the great Wenchuan Earthquake of China (2008). Detailed thin‐section study of the mounds found no signs of any kind of biological constructional process; instead it reveals some obvious fluidification and liquefaction characteristics. Comparative studies have shown that these features are probably the products of Mesoproterozoic earthquake activity. Copyright © 2013 John Wiley & Sons, Ltd.     

Anaerobic oxidation of methane in coastal sediment from Guishan Island (Pearl River Estuary), South China Sea

The concentrations of CH₄, SO₄²⁻, σCO₂ and the carbon isotope compositions of ΣCO₂ and CH₄ in the pore-water of the GS sedimentary core collected from Guishan Island (Pearl River Estuary), South China Sea, were determined. The methane concentration in the pore-water shows dramatic changes and sulfate concentration gradients are linear at the base of the sulfate reduction zone for the station. The carbon isotope of methane becomes heavier at the sulfate-methane transition (SMT) likely because of the Raleigh distillation effect; ¹²CH₄ was oxidized faster than ¹³CH₄, and this caused the enrichment of residual methane δ ¹³C and δ ¹³C-ΣCO₂ minimum. The geochemical profiles of the pore-water support the existence of anaerobic oxidation of methane (AOM), which is mainly controlled by the quality and quantity of the sedimentary organic matter. As inferred from the index of δ ¹³C-TOC value and TOC/TN ratio, the organic matter is a mix of mainly refractory terrestrial component plus some labile alga marine-derived in the study area. A large amount of labile organic matter (mainly labile alga marine-derived) is consumed via the process of sedimentary organic matter diagenesis, and this reduces the amount of labile organic matter incorporated into the base of the sulfate reduction zone. Due to the scarcity of labile organic matter, the sulfate will in turn be consumed by its reaction with methane and therefore AOM takes place. Based on a diffussion model, the portion of pore-water sulfate reduction via AOM is 58.6%, and the percentage of ΣCO₂ in the pore-water derived from AOM is 41.4%. Thus, AOM plays an important role in the carbon and sulfur cycling in the marine sediments of Pearl River Estuary.     

Morphology and formation mechanism of pyrite induced by the anaerobic oxidation of methane from the continental slope of the NE South China Sea

In order to understand the response of authigenic pyrite to gas hydrate geo-systems, pyrite tubes or rods at the sulfate–methane transition (SMT) zone of core GC10 from the northern continental slope of the South China Sea (SCS) were investigated. In situ X-ray diffraction (XRD) results show that the pyrite tube consists of pyrite micro-crystals with trace amount of graphite in the inner tube. Scanning electron microscope (SEM) observations of pyrite tubes indicate various aggregations in the form of framboidal, euhedral, and colloidal pyrite microcrystals. Typical framboidal pyrite is considered as packing of octahedral microcrystals. Interestingly, many framboids in the tubes consist of round or irregular microcrystals and have an outer crust that consists of secondary pyrite. The size of the framboids in the inner wall of the tube is larger than that in the middle wall or foraminifer-filled pyrite. High-resolution transmission electron microscopic (HRTEM) images show marcasite lamellae defects in the spherulitic pyrite crystals, which reveal different solution conditions during the pyrite precipitation. Nano-foil-like graphitic carbon was observed to be closely associated with the pyrite spherules. The occurrence of both marcasite layers and nano-foil-like graphitic carbon suggest that the migration of methane from deep sediment. It is suggested that the formation of pyrite serves as a catalyst during the reaction from methane to elemental carbon under the anaerobic oxidation of methane. Meanwhile, this reaction results in local acidification of the solution inside the pyrite tubes, which favors marcasite lamellae growth on the host pyrite substrate.     

华北中元古界古地磁测试新结果与Columbia超级大陆研究

华北地块中元古代杨庄组的古地磁研究提供了一个同时通过褶皱检验和倒转检验的古地磁极,极位置为经度190.4°、纬度2.4°。通过与Laurentia、Baltica和Siberia可靠的古地磁数据对比,支持了古—中元古代可能存在Columbia超级大陆的假说。华北地块与Laurentia、Baltica和Siberia视极移曲线的对比表明华北地块与Baltica在古—中元古代可能长期相连。     

华北串岭沟组凝灰岩锆石SHRIMP年龄及其地层学意义

华北克拉通燕辽地区是中国中新元古代沉积地层的标准剖面区。蓟县群和青白口群已经取得了很多重要的高精度定年数据,但是长城群除了顶部大红峪组有了高精度定年以外,其他组还缺乏可靠的高精度定年数据。为了准确标定华北克拉通中元古界长城群串岭沟组的年龄,对采自河北宽城地区神仙岭剖面串岭沟组顶部火山岩凝灰岩锆石进行了SHRIMP U-Pb定年。结果表明所测锆石207Pb/206Pb表面年龄加权平均值为(1 634.8±6.9) Ma(95% 置信度,MSWD=1.2,n=23)。可见长城群串岭沟组顶部的准确年龄为(1 634.8±6.9) Ma,表明长城群串岭沟组到大红峪组整体上属于中元古代地层。