Stromatolites are domes, columns, or nearly flat crusts of laminated sedimentary rocks, usually consisting of Ca-Mg carbonates. Stromatolites result from lithification of microbial mats, which are benthic microbial ecosystems where microorganisms arrange themselves in layers according to their physiology. Despite a century of research, the hypothesis of stromatolite genesis by lithification of microbial mats remains controversial, and a convincing explanation for how stromatolites arise from microbial mats is still lacking. In this work, we analyze in detail a stromatolite from Lagoa Vermelha, a coastal hypersaline lagoon in Rio de Janeiro State, Brazil. The stromatolite presents a laminated core and thrombolitic regions at the periphery. Both thrombolitic and laminated facies consist of fine-grained authigenic minerals with minor contributions of bioclasts and quartz grains. X-ray diffraction shows aragonite, high-magnesium calcite (HMC) containing about 17% MgCO3, a very-high-Mg calcite (VHMC) containing 29–46% MgCO3, and small amounts of quartz and pyrite. Scanning electron microscopy of polished samples coupled to energy-dispersive X-ray analysis (EDS) showed that each lamina was composed of 1–4 distinct mineral phases embedded within each other, indicating sequential steps of precipitation of Ca-Mg carbonates under distinct biogeochemical conditions. The coexistence of different phases in a single lamina suggests that several processes contribute to mineral deposition as the incipient stromatolite laminae are left behind by microorganisms from the lower layers of the microbial mat when they grow and/or move upwards.

中文翻译：

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巴西高盐泻湖Lagoa Vermelha的全新世叠层岩的微观结构和矿物组成：对层状成因的认识

层积岩是圆顶状，柱状或几乎平坦的夹层沉积岩地壳，通常由钙镁碳酸盐组成。基质石是由于微生物垫的石化作用而形成的。微生物垫是底栖微生物生态系统，微生物根据其生理学将自身分层排列。尽管进行了一个世纪的研究，但通过微生物垫的石化作用来生成叠层石的假设仍然存在争议，并且仍然缺乏关于叠层石如何从微生物垫中产生的令人信服的解释。在这项工作中，我们详细分析了来自巴西里约热内卢州沿海高盐泻湖Lagoa Vermelha的叠层岩。叠层岩在外围具有叠层的岩心和血栓形成区域。血栓相和层状相均由细粒的自生矿物组成，而碎屑和石英颗粒的贡献较小。X射线衍射显示文石，含有约17％MgCO3的高镁方解石（HMC），含有29-46％MgCO3的极高镁方解石（VHMC）以及少量石英和黄铁矿。抛光样品的扫描电子显微镜与能量色散X射线分析（EDS）结合显示，每个薄层由1-4种相互嵌入的不同矿物相组成，表明在不同生物地球化学条件下钙镁碳酸盐的连续沉淀步骤。