Possible reasons for the δ¹³C anomaly of Lower Proterozoic sedimentary carbonates

Detailed isotopic study (δ 13 C, δ 18 O) of Lower Proterozoic sedimentary carbonates from biostratigraphically dated successions in Karelia (Russia) revealed a large positive δ 13 C anomaly (average δ 13 C‐value constitutes approximately +10‰ PDB) at 2.3 Byr ago and a huge drop of δ 13 C‐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 O 2 ‐content of the atmosphere at this time; the subsequent drop in δ 13 C‐values equates with a sharp decrease of the O 2 ‐content of the atmosphere.