Lomagundi Carbon Isotope Excursion

Abstract

Definition The Lomagundi carbon isotope excursion (LCIE), sensu stricto (s.s.), refers to highly positive carbon isotope values (on the average +8 ‰, but values as high as +16 ‰ are not uncommon with the highest values observed at +28 ‰) in Paleoproterozoic sedimentary carbonates deposited between ~2.22 and 2.06 Ga (Karhu and Holland 1996; Bekker et al. 2003), likely reflecting seawater composition. The beginning and the end of the excursion have not so far been recognized in the sedimentary record, and there is also uncertainty about the exact age of these transitions. Positive (at ~2.32 Ga and between ~2.45 and ~2.32 Ga; Bekker et al. 2001; Rasmussen et al. 2013) and negative (between ~2.45 and 2.32 Ga; Bekker et al. 2005) carbon isotope excursions have been found leading to the LCIE s.s. The older positive carbon isotope excursions, in particular the ~2.32 Ga one, are considered to be an early expression of the LCIE sensu lato (s.l.). Although originally considered to be a plausible cause for the Great Oxygenation Event (GOE) (e.g., Karhu and Holland 1996), it is now widely agreed that the LCIE s.s. followed the GOE and potentially occurred in response to it (e.g., Konhauser et al. 2011; Bekker and Holland 2012). The LCIE s.l. is associated with the Huronian ice ages and the final stage in the assembly of the supercontinent, while the LCIE s.s. followed them and ended during the sea level rise associated with the supercontinent breakup. The excursion ended up with the ocean deoxygenation event and in association with deposition of Mn-and P-rich sediments. This is by far the longest-lived and largest perturbation to the biogeochem-ical carbon cycle in the Earth's history, and yet, in contrast to Neoproterozoic events, it followed rather than led to the first series of global-scale ice ages. History Paleoproterozoic sedimentary carbonates with highly positive carbon isotope values were first discovered by Galimov et al. (1968) on the Fennoscandian Shield (Kola Peninsula and Karelia, Russia). At that time, authors did not recognize global significance of this phenomenon nor related it to a high relative burial rate of organic carbon. Seven years later, Galimov et al. (1975) reproduced earlier observations on a larger set of samples from Karelia and Kola Peninsula and Schidlowski et al. (1975) found similar values in