Methanotrophy in a Paleoproterozoic Oil Field Ecosystem, Zaonega Formation, Karelia, Russia

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic 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 Earth. 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.