Evidence for cavity-dwelling microbial life in 3.22 Ga tidal deposits
Evidence for cavity-dwelling microbial life in 3.22 Ga tidal deposits
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DOI：
10.1130/g37272.1
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发表时间：
2016
期刊：
影响因子：
5.8
通讯作者：
M. Homann;C. Heubeck;T. Bontognali;A. Bouvier;L. Baumgartner;A. Airo
中科院分区：
文献类型：
--
作者：
M. Homann;C. Heubeck;T. Bontognali;A. Bouvier;L. Baumgartner;A. Airo
洞穴被认为是早期地球上生命的合理和有利的栖息地。在这样的微环境中，生物可能已经找到了足够的保护，以抵御太古宙无臭氧大气的强烈紫外线辐射。然而，虽然有明确的证据表明底栖生物存在于古太古代，但在新太古代的岩石中发现了最古老的穴居微生物（腔虫）的痕迹。本文介绍了对南非巴伯顿绿岩带3.22 Ga modies群中最古老的保存完好的硅屑潮沉积中早期硅化空洞的详细研究结果。由干酪根纹层组成的向下生长的微叠层石柱通常存在于潮汐带沉积物中形成的平面、层理平行、现在充满硅质的孔洞中。干酪根原位δ13CPDB （PDB-Peedee蛭石）测量值在-32.3‰~ -21.3‰之间，符合生物成因特征。对硅化腔的扫描电镜分析显示，保存完好的细胞大小的霉菌链被解释为丝状微生物化石。地质环境、微叠层石的形态、干酪根的δ13C组成以及微化石的存在都表明，洞穴中存在微生物群落。这些结果将腔体的地质记录延长了约500英里，支持了腔体是早期微生物占据的第一个生态位的观点。
Cavities are considered plausible and favorable habitats for life on early Earth. In such microenvironments, organisms may have found an adequate protection against the intense ultraviolet radiation that characterized the Archean ozone-free atmosphere. However, while there is clear evidence that benthic life existed in the Paleoarchean, the oldest traces of cavity-dwelling microbes (coelobionts) have been found in Neoarchean rocks. Here we present the results of a detailed investigation of early silicified cavities occurring in the oldest well-preserved siliciclastic tidal deposits, the 3.22 Ga Moodies Group of the Barberton Greenstone Belt (South Africa). Downward-growing microstromatolitic columns composed of kerogenous laminae are commonly present in planar, bedding-parallel, now silica-filled cavities that formed in sediments of the peritidal zone. In-situ δ13CPDB (PDB—Peedee belemnite) measurements of the kerogen range from –32.3‰ to –21.3‰ and are consistent with a biogenic origin. Scanning electron microscopy analysis of the silicified cavities shows well-preserved chains of cell-sized molds that are interpreted as fossil filamentous microorganisms. The geological context, the morphology of the microstromatolites, the δ13C composition of the kerogen, and the presence of microfossils all suggest that a microbial community inhabited the cavities. These results extend the geological record of coelobionts by ∼500 m.y., supporting the view that cavities were among the first ecological niches to have been occupied by early microorganisms.