Organic carbon isotopically negative

Figure 10 Isotopic composition of carbonates and organic carbon in sedimentary and metasedimentary rocks through geologic time. The negative excursions in organic matter at 2.7 Ga and 2.0 Ga may relate to extensive methanogenesis as a mechanism of carhon fixation (Schidlowski, 2001) (reproduced by permission of Elsevier from...

A striking point which can be drawn from the histogram of all the carbonate carbon isotope data compiled in Fig. 4 is that isotopically negative carbon derived from organic material has contributed significantly to the majority of the diagenetic carbonates represented. Less than 8% of the carbonate cements could be interpreted as having wholly marine bicarbonate components with 6 C of around 0%o, but over 80% have 6 C of -5%o and lower, and approximately 40% of the data have 6 C of between -9 and -1 l%o. 

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