METHANOTROPHY IN 2.7 GA SOUTH AFRICAN LAKES

Organic carbon isotopic compositions from ~2.7 Ga strata commonly record values below -30 ‰ and many below -50 ‰. The negative organic carbon values constitute an excursion to much lighter values in the global organic carbon isotopic record and have been interpreted as the first evidence of methanotrophic metabolisms, which convert methane into organic carbon, retaining the light carbon isotopes of methane. Methanotrophy requires oxygen or oxidized species such as nitrate or sulfate to function, and the 2.7 Ga excursion has been hypothesized as evidence for oxygenic photosynthesis before the Great Oxidation Event 2.4 Ga.

This study approaches the global isotopic record of Archean organic carbon with two questions: 1) Is the 2.7 Ga excursion global in nature? and 2) If current data show a localized excursion, will examination of under-sampled regions provide evidence for a more widespread event? When parsed into separate regions, the only Archean organic carbon isotope values lighter than -45 ‰ come from Australian lake deposits of the Fortescue Group. After removing Fortescue data from global isotope curves, there is still an excursion circa-2.7 Ga, though much less pronounced. The Ventersdorp Supergroup in South Africa is a contemporaneous suite of lacustrine deposits, and provides an excellent comparison with Fortescue environments. Far fewer studies of organic carbon isotopes have been performed on Ventersdorp samples, but the data produced show lighter values than most South African marine deposits (-37 to -43 ‰). This study provides the first organic carbon isotopes from the Ventersdorp Supergroup lighter than -43 ‰, as well as the first values from Ventersdorp stromatolites. The values confirm extremely light organic carbon in multiple lake environments 2.7 Ga, most likely produced by methanotrophs. Further investigation into contemporaneous marine deposits is needed to determine whether the excursion is uniquely lacustrine.