GLOBAL RECORDS OF THE PALEOPROTEROZOIC CARBON ISOTOPE EXCURSION

The isotopic composition of carbon in Paleoproterozoic carbonate sediments indicates that the isotopic composition of marine dissolved carbon experienced a large positive excursion between ~2.2 and 2.1 Ga. First indications for the excursion were obtained from Fennoscandian supracrustal successions, but, afterwards, the existence of a global event affecting the Paleoproterozoic carbon cycle has been confirmed by numerous studies from different continents. The minimum duration of the excursion is 100 Ma, from 2.21 Ga to 2.11 Ga. The start of the excursion is poorly constrained between 2.32 and 2.21 Ga, based on Fennoscandian and South African data, but the end of the event is well defined between 2.11 and 2.06 Ga. New data from the Väystäjä Formation, northern Finland, confirm the end of the excursion to > 2.05 Ga. Recently published carbon isotope records from the Duitschland Formation, South Africa, give indications of a separate positive carbon isotope shift of undetermined duration and of unknown global extent, at ~2.32 Ga. In contrast to the major excursion, the Duitschland carbonate sediments are closely associated with glacial sediments, analogously with the relationship observed between Neoproterozoic ¹³C-enriched carbonate sediments and glaciations.

The long-term record of the carbon isotope composition of sedimentary carbonates is related to the operation of the geochemical carbon cycle. Carbon cycle mass balance considerations indicate that the excursion was associated with an increase in the fractional burial rate of organic carbon relative to carbonate carbon and a subsequent release of oxygen as a by-product. Geological evidence for excessive organic carbon sedimentation seems to be largely missing from successions deposited between 2.2 and 2.1 Ga, and the site of organic carbon burial is not known. Sediment subduction is a potential mechanism removing organic carbon from the surfacial Earth, resulting in a permanent change in the oxidation state of the Earth’s surface.