Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: evolution of a concept
Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: evolution of a concept
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DOI：
10.1016/s0301-9268(00)00128-5
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发表时间：
2001-02
影响因子：
3.8
通讯作者：
M. Schidlowski
中科院分区：
文献类型：
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作者：
M. Schidlowski
自从20世纪30年代后期的开创性研究表明无机碳转化为生物源物质需要稳定碳同位素的大量重新分布以来，生物介导的13 C/12 C分馏已被认为是生物化学反应的常见必然结果。与此同时，它是坚定地建立在有利于12 C表征生物材料的普遍偏见主要来自于动力学同位素效应，这是强加在第一个碳固定酶羧化反应在初级代谢的CO2固定（自养）生物体。这种对12 C的偏好已经被证明是生物前体物质的“有序状态”的最持久的遗迹之一，这些生物前体物质可能保存在化石有机物中数十亿年。根据目前已知的沉积记录，可以有把握地说，生物碳同位素分馏在有记录的地球历史中持续了3.8 Ga，表明微生物（原核生物和古原核生物）生态系统已经在太古代地球上多产。虽然对于<3.5 Ga的时间跨度，同位素证据是明确的，但先前记录中编码的信息通常因变质叠加而变得模糊。这尤其适用于3.8 Ga老Isua表壳带西格陵兰的沉积物，除了广泛的交代作用，遭受角闪岩级变质作用。众所周知，在角闪岩和麻粒岩相变质作用期间，如果存在第二碳伙伴（流体或碳酸盐），有机（干酪根）岩石组分中会发生13 C/12 C交换，由于交换反应的动力学缓慢，同位素再平衡往往只能部分实现。然而，热力学平衡预测，干酪根和石墨中的13 C/12 C比率在此过程中增加。因此，高温交换平衡总是将δ 13 C值推向正方向，因此遇到的最低值是交换最少和最原始的。在早期的伊苏阿研究中获得的还原（石墨）碳的最低值落在-22到-28 ‰的范围内[PDB]，我们从20世纪70年代末开始就有直接的证据表明，具有生物物质同位素组成的碳组分确实存在于变质前的伊苏阿岩套中。因此，毫不奇怪，最近利用先进的仪器微量分析技术对Isua带状铁地层中磷灰石托管的碳质微区进行的同位素工作的结果也得到了类似的结论。因此，沉积物碳同位素记录的主流可以最好地解释为生物碳同化中主要CO2固定反应的同位素区分性质的地球化学表现，表明自养碳固定生物化学中极端程度的进化保守主义。因此，地球化学碳循环的生物调节至少在3.8 Ga前就已经建立，在地球最古老的沉积物形成时已经完全运作。
Ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes, biologically mediated13C/12C fractionations have come to be recognized as a common corollary of biochemical reactions. Meanwhile, it is firmly established that the universal bias in favour of12C characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon-fixing enzymatic carboxylation reaction in the primary metabolism of CO2-fixing (autotrophic) organisms. This preference for12C has turned out to be one of the most enduring relics of the ‘ordered state’ of the biological precursor substances that may be preserved in fossil organics over billions of years. With the currently known sedimentary record at hand, it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3.8 Ga of recorded Earth history, indicating that microbial (prokaryotic and archaeoprokaryotic) ecosystems had been prolific already on the Archean Earth. While for the time span<3.5 Ga the isotopic evidence is unequivocal, the information encoded in the preceding record is commonly blurred by a metamorphic overprint. This holds particularly for the metasediments of the 3.8 Ga old Isua Supracrustal Belt of West Greenland which, apart from widespread metasomatism, have suffered amphibolite-grade metamorphism. It is known that13C/12C exchange occurs in organic (kerogenous) rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available (as either fluids or carbonate), with isotopic re-equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction. Thermodynamic equilibria predict, however, that13C/12C ratios in kerogen and graphite increase during this process. Hence, high-T exchange equilibria are always bound to drive δ13C values in positive direction, the lowermost values encountered being consequently the least exchanged and most pristine. With the lowest values of reduced (graphitic) carbon obtained in early Isua studies falling into the range −22 to −28‰ [PDB], we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre-metamorphic Isua suite. It was, therefore, by no means surprising that the results of recent isotope work performed on apatite-hosted carbonaceous microdomains in Isua banded iron-formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions. Hence, the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope-discriminating properties of the principal CO2-fixing reactions(s) in biological carbon assimilation, suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation. As a consequence, biological modulation of the geochemical carbon cycle had been established at least 3.8 Ga ago, having been fully operative by the time of formation of the Earth's oldest sediments.