| Abstract: | Detailed isotopic study (δ13C, δ18O) of Lower Proterozoic sedimentary carbonates from biostratigraphically dated successions in Karelia (Russia) revealed a large positive δ13C anomaly (average δ13C-value constitutes approximately +10‰ PDB) at 2.3 Byr ago and a huge drop of δ13C-values in the subsequent ‘black shale period’(2.2 Byr ago). These data are interpreted to be a result of large-scale changes in the oxygen content of the atmosphere. According to this model the positive carbon isotopic anomaly of sedimentary carbonates at about 2.3 Byr ago reflects a very high O2-content of the atmosphere at this time; the subsequent drop in δ13C-values equates with a sharp decrease of the O2-content of the atmosphere. |
