Carbon isotopes and the rise of atmospheric oxygen

New data for the isotopic composition of carbon in carbonate sediments deposited between 2.6 and 1.6 Ga indicate that the value of δ¹³C in these sediments underwent a very large positive excursion between 2.22 and 2.06 Ga. A reassessment of the earlier δ¹³C data for carbonate sediments shows that this excursion was probably worldwide, and that it was preceded and followed by several hundred million years during which the δ¹³C of carbonate sediments differed little from that of modern carbonates. The large δ¹³C excursion between 2.22 and 2.06 Ga was probably related to an abnormally high rate of organic carbon deposition, which generated an abnormally high rate of O₂ production. We estimate that the total excess O₂ produced during the excursion was between 12 and 22 times the present atmospheric O₂ inventory. The δ¹³C data therefore suggest that the O₂ content of the atmosphere increased very significantly between 2.22 and 2.06 Ga. This inference is supported strongly by several other lines of evidence.