Methanotrophy in a P aleoproterozoic oil field ecosystem, Z aonega F ormation, K arelia, R ussia

Organic carbon rich rocks in the c. 2.0 Ga Z aonega F ormation ( ZF ), K arelia, R ussia, preserve isotopic characteristics of a P aleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ 13 C org from c. −25‰ in the lower part of the succession to c. −40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward 13 C ‐depleted organic matter (δ 13 C org  < −25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH 4 triggered the activity of methanotrophic organisms, resulting in the production of anomalously 13 C ‐depleted biomass. The stratigraphic shift in δ 13 C org records the change from CO 2 ‐fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ 13 C org reflects global forcing and progressive oxidation of the E arth. However, the lithologic indication for local thermogenic CH 4 , sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ 13 C org negative isotopic shift observed in the ZF .