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 delta(13)C in these sediments underwent a very large positive excursion between 2.22 and 2.06 Ga. A reassessment of the earlier delta(13)C 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 delta(13)C of carbonate sediments differed little from that of modern carbonates. The large delta(13)C 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 O-2 production, We estimate that the total excess O-2 produced during the excursion was between 12 and 22 times the present atmospheric O-2 inventory. The delta(13)C data therefore suggest that the O-2 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.