Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: evolution of a concept

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 mediated C-13/C-12 fractionations have come to be recognized as a common corollary of biochemical reactions. Meanwhile, it is firmly established that the universal bias in favour of C-12 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 for C-12 has turned out to be one of the most enduring relies 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 that C-13/C-12 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, that C-13/C-12 ratios in kerogen and graphite increase during this process. Hence, high-T exchange equilibria are always bound to drive C-13 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(0)/(00) [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. (C) 2001 Elsevier Science B.V. All rights reserved.