METHANOTROPHY IN 2.7 GA SOUTH AFRICAN LAKES
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.