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Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon
Earth-Science Reviews ( IF 12.1 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.earscirev.2020.103296
Kevin Lepot

Abstract The Archean era (4 to 2.5 billion years ago, Ga) yielded rocks that include the oldest conclusive traces of life as well as many controversial occurrences. Carbonaceous matter is found in rocks as old as 3.95 Ga, but the oldest (graphitic) forms may be abiogenic. Due to the metamorphism that altered the molecular composition of all Archean organic matter, non-biological carbonaceous compounds such as those that could have formed in seafloor hydrothermal systems are difficult to rule out. Benthic microbial mats as old as 3.47 Ga are supported by the record of organic laminae in stromatolitic (layered) carbonates, in some stromatolitic siliceous sinters, and in some siliciclastic sediments. In these deposits, organic matter rarely preserved fossil cellular structures (e.g. cell walls) or ultrastructures (e.g. external sheaths) and its simple textures are difficult to attribute to either microfossils or coatings of cell-mimicking mineral templates. This distinction will require future nanoscale studies. Filamentous-sheath microfossils occur in 2.52 Ga rocks, and may have altered counterparts as old as 3.47 Ga. Surprisingly large spheres and complex organic lenses occur in rocks as old as 3.22 Ga and ~ 3.4 Ga, respectively, and represent the best candidates for the oldest microfossils. Titaniferous microtubes in volcanic or volcanoclastic rocks inferred as microbial trace fossils have been reevaluated as metamorphic or magmatic textures. Microbially-induced mineralization is supported by CaCO3 nanostructures in 2.72 Ga stromatolites. Sulfides 3.48 Ga and younger bear S-isotope ratios indicative of microbial sulfate reduction. Ferruginous conditions may have fueled primary production via anoxygenic photosynthesis–as suggested by Fe-isotope ratios–possibly as early as 3.77 Ga. Microbial methanogenesis and (likely anaerobic) methane oxidation are indicated by C-isotope ratios as early as 3.0 Ga and ~ 2.72 Ga, respectively. Photosynthetic production of O2 most likely started between 3.2 and 2.8 Ga, i.e. well before the Great Oxidation Event (2.45–2.31 Ga), as indicated by various inorganic tracers of oxidation reactions and consistent with morphology of benthic deposits and evidence for aerobic N metabolism in N-isotope ratios at ~ 2.7 Ga. This picture of a wide diversification of the microbial biosphere during the Archean has largely been derived of bulk-rock geochemistry and petrography, supported by a recent increase in studied sample numbers and in constraints on their environments of deposition. Use of high-resolution microscopy and micro- to nanoscale analyses opens avenues to (re)assess and decipher the most ancient traces of life.

中文翻译:

太古宙(4 至 2.5 Ga)时代早期微生物生命的特征

摘要 太古代时代(4 至 25 亿年前,Ga)出产的岩石包括最古老的确定性生命痕迹以及许多有争议的事件。在 3.95 Ga 的岩石中发现了碳质物质,但最古老的(石墨)形式可能是非生物成因的。由于变质作用改变了所有太古代有机物的分子组成,因此很难排除非生物碳质化合物,例如可能在海底热液系统中形成的那些。叠层石(层状)碳酸盐、一些叠层硅质烧结矿和一些硅质碎屑沉积物中有机层的记录支持了底栖微生物垫的历史,记录年龄为 3.47 Ga。在这些沉积物中,有机质很少保存化石细胞结构(例如细胞壁)或超微结构(例如 外鞘)及其简单的质地很难归因于微化石或细胞模拟矿物模板的涂层。这种区别将需要未来的纳米级研究。丝状鞘微化石出现在 2.52 Ga 的岩石中,可能有 3.47 Ga 的蚀变对应物。令人惊讶的是,大球体和复杂的有机透镜分别出现在 3.22 Ga 和 ~ 3.4 Ga 的岩石中,代表了最好的候选者最古老的微化石。推断为微生物痕迹化石的火山或火山碎屑岩中的钛微管已被重新评估为变质或岩浆结构。2.72 Ga 叠层石中的 CaCO3 纳米结构支持微生物诱导的矿化。硫化物 3.48 Ga 和更年轻的 S-同位素比率表明微生物硫酸盐还原。铁质条件可能通过无氧光合作用促进初级生产——如 Fe 同位素比率所表明的那样——可能早在 3.77 Ga。微生物产甲烷作用和(可能是厌氧的)甲烷氧化由 C 同位素比率表明,早在 3.0 Ga 和 ~ 2.72分别是嘎。O2 的光合作用生产最有可能在 3.2 至 2.8 Ga 之间开始,即远在大氧化事件(2.45-2.31 Ga)之前,如氧化反应的各种无机示踪剂所示,并与底栖沉积物的形态和有氧氮代谢的证据一致N 同位素比率约为 2.7 Ga。这张太古宙时期微生物生物圈广泛多样化的图片主要来自大块岩石地球化学和岩石学,最近增加的研究样本数量和对其沉积环境的限制提供了支持。高分辨率显微镜和微米到纳米级分析的使用为(重新)评估和破译最古老的生命痕迹开辟了道路。
更新日期:2020-10-01
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