Postdepositional degradation of biologically produced organic carbon (d13C = -27) may be assigned to a number of processes which are marked by different isotopic compositions of CO2 (e.g. Irwin et al. 1977): (i) microbial oxidation (-27), (ii) bacterial sulphate reduction (-27), (iii) microbial fermentation (up to +15), (iv) abiotic decarboxylation (-15 to -20). CH4 produced in the microbial fermentation can be oxidised to CO2 (-50). Diagenetically derived CO2 may be bound in carbonates with isotopic signatures which differ distinctly from those of the 'normal' marine carbonate, 0±2.
If samples from 'red beds' (2.3-2.0 Ga) and banded iron formations (BIF) are excluded, d13C values for sedimentary and diagenetic carbonates in the time interval from 3.7 to 1.6 Ga are near 0±2. Diagenetic shifts of carbon-isotopic abundances are systematically lower for Archaean (e.g. Schidlowski et al., 1979; Veizer et al., 1989, 1990) and Proterozoic carbonates (Schidlowski et al., 1983; Strauss et al., 1992) and fall within the 0.1-3.0 (e.g. Tucker, 1982; Knoll et al., 1986; Fairchild & Spiro, 1987; Aharon et al., 1987; Kaufman et al., 1990; Veizer et al., 1992a, 1992b; Mirota & Veizer, 1994). Reliable Archaean diagenetic d13C values (lower than -5) have been reported only from successions which are marked by the intensive development of BIF formed at 3.7, 2.7, 2.5 Ga (e.g. Becker & Clayton, 1972; Baur et al., 1985). The first Palaeoproterozoic carbonates with diagenetic d13C values (-5 to -20.5) were formed immediately after the development of world-wide 'red beds', including BIF (2.1 Ga), and the Jatulian 'heavy carbonate phenomenon' (2.3-2.06 Ga), and coincide in time (ca. 2 Ga) with the first abundant development of diagenetic carbonate concretions (Melezhik, 1992).
Early Precambrian carbonates with diagenetically contributed CO2 were very rare though they became abundant at 3.7, 2.7, 2.5, 2.0 Ga ago where they were associated with short-term episodes which were marked by highly oxygenated environments. Abundant diagenetic carbonates with low d13C can be taken as evidence of redox gradients. One can assume that in the Early Precambrian, low d13C (diagenetic) CO2-bicarbonate-carbonate become widely available only episodically, within a limited time, namely at 3.7, 2.7, 2.5, and particularly at 2.0 Ga ago. The d13C of these carbonates, -5 to -21, reflects an input of CO2 generated by the processes (i), (ii) or (iv). At least (i) requires oxygenic environments and (ii) depends on sulphate availability. If currently available records on the early Precambrian diagenetic d13C are correct, then organic carbon recycling processes during much of Early Precambrian history differed from those observed in the later periods. The available d13C values of Early Precambrian carbonates do not reflect a significant contribution from CO2 related to any of postdepositional degradation processes which are specified in (i), (ii), (iii) or (iv). This may be partly attributed to the oxygen and sulphate lows which could suppress microbial oxidation and bacterial sulphate reduction, and consequently be reflected in a low production of CO2 with d13C=-27.
For the period of time 3.7 to 1.6 Ga: (I) the d13C diagenetic records demonstrate an irregular cyclic development through Early Precambrian time; (II) at 3.7, 2.7, 2.5 and 2.0 Ga the mode of organic carbon recycling, as reflected in d13C, can be ascribed to microbial oxidation, bacterial sulphate reduction, and abiotic decarboxylation; (III) the general trend of d13Ccarb indicates that organic carbon was very little recycled, or was recycled under anoxic environments; (IV) it is unlikely that 'Ronov ratio' remained constant.
The general lack of isotopically light carbonates raises the question: was it the minimal oxygenation of the environment, or generally different carbon pathways within the global ecosystems, or physiological differences in microbial communities that was responsible for the low diagenetic shifts of carbon isotopes between 3.7 and 1.6 Ga ago?