A widespread positive δ13Ccarb anomaly at around 2.33–2.06 Ga on the Fennoscandian Shield: a paradox?

Abstract

ABSTRACTCarbon isotope measurements carried out on 201 carbonate samples from the early Proterozoic of the Kola Peninsula, N. Karelia and Norway yield δ13C (PDB) spanning ‐ 20.5% to + 11%. A general δ13C secular trend shows that prior to 2.33 Ga values are typically ‘normal’ marine, averaging around ‐ 3%0. Between 2.33 and 2.06 Ga, in Jatulian time, there follows a rapid excursion to positive δ13C of around + 6%. Post‐Jatulian time is characterized by δ13C of sedimentary carbonates fluctuating between ‐ 5% and +3%; also it is remarkable for the first pronounced development of diagenetic carbonates, which have δ13C between ‐ 14% and ‐ 6%. The c. 6% positive δ13C shift with a duration of about 270 Myr coincides with a maximum in the diversity and abundance of stromatolites, and with widespread development of ‘red beds’, but does not coincide with the maximum of buried Corg mass. The Fennoscandian Shield represents the largest isotoically anomalous carbonate province yet reported, and the positive δ13C excursion together with a series of major global palaeoenviromental changes seems to be more intense than the Precambrian/Cambrian transition events. However, it is still not clear what kind of mechanism this phenomenon could be attributed to. An increase of the ‘Ronov ratio’, and/or ‘Broecker ratio’ and other possible models are discussed as the target for future investigations.