华北地台串岭沟组砂脉中自生碳酸盐沉淀和自生黄铁矿——中元古代甲烷厌氧氧化的沉积证据

华北中元古界串岭沟组暗色页岩中普遍发育的砂脉构造被解释为气体逃逸形成的沉积构造。在砂脉和相邻围岩中发现有自生碳酸盐沉淀和自生黄铁矿。自生碳酸盐主要由白云石组成，表现为微晶质条带或隐晶质斑块状胶结物。微晶白云石条带多出现在有机质纹层密集发育的部位，沿微生物膜两侧发育；而隐晶质斑块多以胶结物形式充填粒间孔隙或沿砂脉外缘密集产出。围岩中共生草莓状黄铁矿，砂脉中有具环带黄铁矿。这种特殊的矿物组构以及富有机质沉积和丰富的微生物成因构造(MISS)均表明串岭沟组形成于缺氧环境。层面发育的气泡构造为砂脉的气体逃逸成因提供了支持，而自生碳酸盐和黄铁矿则是沉积浅层硫酸盐-甲烷转换带(SMTZ)甲烷厌氧氧化(AOM)和细菌硫酸盐还原(BSR)的产物；气体源于有机质的厌氧分解和甲烷菌活动。砂脉中发现有疑似微生物化石，围岩含古菌和硫细菌生物标志物，表明有发生AOM和BSR作用的条件。串岭沟组砂脉中识别的自生碳酸盐沉淀及其AOM成因对进一步认识中元古代古海洋与古气候条件具有重要意义。由于海洋的低硫酸盐浓度，SMTZ带很浅，海底沉积层的甲烷仅少量被AOM消耗；而大量甲烷进入大气必然导致中元古代强烈的温室气候效应。

Authigenic carbonate precipitation and pyrites from sand veins in the Chuanlinggou Formation of the North China Platform-Evidence for anaerobic oxidation of methane in the Mesoproterozoic

The Chuanlinggou Formation(ca 17 Ga)is characterized by dark shale and is widespread in the North China Platform， with a thickness up to 900 m. In this formation abundant centimeter scaled pygmatic sand veins are well developed. They have been interpreted as deriving from the sand filled fissures created by biogenic gas release. In the sand veins minor amounts of authigenic carbonate precipitation(ACP)and pyrites have been identified . The ACP is mainly of crypto-to micro crystalline dolomites， occurring either as horizontally continuous seams or as discrete spots， often accompanied with organic rich lamina. The ACP seams generally occur at the places where biofilms densely concentrated， and tend to extend along the biofilms. In contrast, the ACP spots prefer to exist in inter grain pores as cryptocrystalline cements or along the outer margins of sand veins as microcrystalline dolomites. These two types of ACP have been recognized both in the sand veins and host sediments. The associated  framboidal pyrites are more abundant in mud sediments. while rimmed pyrite crystals in the sand veins. Siderite crystals also exist in the sand veins. These minerals are often associated with ACP forming characteristic fabrics， along with the organic rich shale and well developed microbially induced sedimentary structures(MISS)， suggesting an anoxic environment for the Chuanlinggou deposition. Analyses of the morphology and rheology of the sand veins and their distribution in relation to the dark shale suggest that they were derived from the fissures created by gas release as passages and subsequently filled by storm transported sands. The gas release origin is also supported by the existence of gas blisters(MISS)on sedimentary surfaces. The co existence of ACP and authigenic pyrites and their preferential occurrence in the sand veins indicate that they resulted from the anaerobic oxidation of methane(AOM)and bacterial sulfate reduction(BSR)happened in a sulfate methane transition zone(SMTZ)few centimeters below the sedimentary surface. The recognition of putative microbial fossils in the sand veins and the biomarkers indicative of archaea and sulfur bacteria from the dark shale may also provide evidence for AOM. Methane might derive from anaerobic degradation of buried organic matter， with part from the possible methanogenesis as well. The recognition of ACP and AOM in the Chuanlinggou Formation is of significant implications for the Mesoproterozoic plaeoclimate and paleoceanographic analyses. With a low seawater sulfate concentration and therefore a shallow SMTZ during the Mesoproterozoic ocean， the consumption of methane by AOM would be proportionally lower than that in the modern marine environments. As a result， the methane fluxed into the atmosphere would inevitably increase the relative importance of methane in maintaining the Mesoproterozoic greenhouse climate.