Abstract The sedimentary fabrics of Precambrian mat-related structures (MRS) represent some of the oldest convincing evidence for early life on Earth. The ca. 2.1 billion-year (Ga) old MRS in the FB2 Member of the Francevillian basin in Gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated, shallow-marine settings, but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source. Despite these insights, what remains lacking is a full characterization of the geochemical composition of the MRS to test whether the bulk composition of fossilized MRS is distinct from the host sediments (sandstones and shales). Here, we show that the trace element (TE) content of microbial textures belonging to pyritized MRS, poorly pyritized MRS, and “elephant-skin” textures (EST) is highly variable and differs from that of the host sediments. The poorly pyritized MRS contain a unique matrix with embedded Ti- and Zr-rich minerals and syngenetically enriched in TE. The EST, some of which are developed along the same stratigraphic horizon as the poorly pyritized MRS, display a distinct distribution of TE-bearing heavy minerals, suggesting a local difference in physical conditions during sedimentation. Similarly, high chalcophile-element (CE) content in pyritized MRS relative to the host sediments of the FB2 Member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis. The geochemical relationship between the MRS and the Francevillian sediments (e.g., FB, FC, and FD formations) indicates that specific biological pathways for CE enrichments (i.e., microbially controlled accumulation) are not apparent. Our findings highlight bulk-rock TE distinction between the 2.1-billion-year-old MRS and their host sediments, but also indicate that environmental conditions, such as hydrodynamic regime and water-column redox chemistry, may simply overwhelm any potential biological signal. Our data suggest that the microbial impact may have only passively influenced TE enrichment in the studied sediments, implying that TE concentrations in MRS are a poor biosignature. Importantly, this work indicates that bulk TE geochemistry does not unveil specific microbiological processes in the rock record, which is consistent with the observed patterns in modern analogues.

中文翻译：

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微量元素透视成ca。来自加蓬 Francevillian Group 的 21 亿年前的浅海微生物垫

摘要 前寒武纪垫层相关结构 (MRS) 的沉积结构代表了地球上早期生命的一些最古老的令人信服的证据。约。加蓬 Francevillian 盆地 FB2 成员中 21 亿年 (Ga) 历史的 MRS 受到了相当大的关注，不仅因为它们含有微生物垫的残余物，这些微生物垫在含氧的浅海环境中大面积定植，而且它们还包含证据古老的多细胞生物在这些微生物垫上茁壮成长，将它们用作食物来源。尽管有这些见解，但仍然缺乏对 MRS 地球化学组成的完整表征，以测试化石 MRS 的整体组成是否与宿主沉积物（砂岩和页岩）不同。这里，我们表明，属于黄铁矿化 MRS、弱黄铁矿化 MRS 和“象皮”纹理 (EST) 的微生物质地的微量元素 (TE) 含量变化很大，与寄主沉积物的含量不同。黄铁矿化程度低的 MRS 包含一个独特的基质，其中嵌入了富含 Ti 和 Zr 的矿物质，并同时富含 TE。EST，其中一些沿与弱黄铁矿 MRS 相同的地层层位发育，显示出含 TE 的重矿物的明显分布，表明沉积过程中物理条件的局部差异。类似地，相对于 FB2 成员的宿主沉积物，黄铁矿化 MRS 中的高亲硫元素 (CE) 含量进一步表明，在早期成岩过程中，局部受细菌影响的富集与微生物硫酸盐的高还原率。MRS 和 Francevillian 沉积物（例如，FB、FC 和 FD 地层）之间的地球化学关系表明 CE 富集（即微生物控制的积累）的特定生物途径并不明显。我们的研究结果突出了 21 亿年前的 MRS 与其宿主沉积物之间的大块岩石 TE 区别，但也表明环境条件，如流体动力学状态和水柱氧化还原化学，可能会压倒任何潜在的生物信号。我们的数据表明，微生物的影响可能只是被动地影响了研究沉积物中 TE 的富集，这意味着 MRS 中的 TE 浓度是一种较差的生物特征。重要的是，这项工作表明大量 TE 地球化学并未揭示岩石记录中的特定微生物过程，