SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions
SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions
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DOI：
10.1073/pnas.1718063115
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发表时间：
2018-01-02
影响因子：
11.1
通讯作者：
Valley, John W.
中科院分区：
文献类型：
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作者：
Schopf, J. William;Kitajima, Kouki;Valley, John W.
用二次离子质谱仪(SIMS)对在西澳大利亚西北部3465 Ma尖端硅质岩岩相薄片中发现的5个原核丝状角化细胞微体化石的11个标本进行了二次离子质谱(SIMS)分析，结果表明，它们的Delta C-13组成从-31‰到-39‰之间有系统地变化，这些岩石样本是20多年前描述这种最早的细胞化石组合的岩石样本。这些与形态种相关的碳同位素组成证实了Apex化石的生物起源，并验证了它们基于形态的分类归属。也许最值得注意的是，这五个分类群中每一个的增量C-13值都低于Apex干酪根的大样本(-27‰)、SIMS测量的化石伴生分散颗粒干酪根的Delta C-13值(-27.6‰)以及现代原核光养生物的典型值(-25‰+/-10‰)。增量C-13顶端最高的两个分类群的SIMS数据与现存的光养细菌的SIMS数据一致；增量C-13分类群稍低的SIMS数据与非细菌产甲烷的古生菌一致；而增量C-13最低的两个分类群的SIMS数据与现存的光养细菌一致，后者具有甲烷代谢的伽马蛋白细菌。尽管从2,500 Ma前干酪根的碳同位素组成的整体分析中推断出甲烷菌和甲烷营养菌的存在，但这些对单个微化石的原位SIMS分析提供了可解释的数据，证明了此类微生物的细胞保存，并与古生代在rRNA系统发育中的近基底位置一致。
Analyses by secondary ion mass spectroscopy (SIMS) of 11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the similar to 3,465Ma Apex chert of northwestern Western Australia, prepared from the same rock sample from which this earliest known assemblage of cellular fossils was described more than two decades ago, show their delta C-13 compositions to vary systematically taxon to taxon from -31 parts per thousand to -39 parts per thousand. These morphospecies-correlated carbon isotope compositions confirm the biogenicity of the Apex fossils and validate their morphology-based taxonomic assignments. Perhaps most significantly, the delta C-13 values of each of the five taxa are lower than those of bulk samples of Apex kerogen (-27 parts per thousand), those of SIMS-measured fossil-associated dispersed particulate kerogen (-27.6 parts per thousand), and those typical of modern prokaryotic phototrophs (-25 +/- 10 parts per thousand). The SIMS data for the two highest delta C-13 Apex taxa are consistent with those of extant phototrophic bacteria; those for a somewhat lower delta C-13 taxon, with nonbacterial methane-producing Archaea; and those for the two lowest delta C-13 taxa, with methane-metabolizing gamma-proteobacteria. Although the existence of both methanogens and methanotrophs has been inferred from bulk analyses of the carbon isotopic compositions of pre-2,500 Ma kerogens, these in situ SIMS analyses of individual microfossils present data interpretable as evidencing the cellular preservation of such microorganisms and are consistent with the near-basal position of the Archaea in rRNA phylogenies.