A paleoweathering profile from Griqualand West, South Africa: evidence for a dramatic rise in atmospheric oxygen between 2.2 and 1.9 bybp.

A core drilled near Wolhaarkop in Griqualand West, South Africa, intersected highly oxidized Kuruman Iron Formation below red beds of the Gamagara Formation. The lateral equivalents of the Kuruman Iron Formation in this drill hole consist largely of siderite, ankerite, magnetite, greenalite, and quartz. The oxidation of the Kuruman Iron Formation in WOL 2 occurred almost certainly during weathering prior to the deposition of the Gamagara Formation. The date of this weathering episode is bracketed between about 2.2 and 1.9 bybp by the age of the Ongeluk lavas in the Transvaal sequence below the unconformity and by the age of the Hartley lavas in the Olifantshoek Group above the unconformity. The ratio of iron to SiO2 in the several facies of the weathered Kuruman Iron Formation in WOL 2 is nearly the same as that in their unweathered equivalents. Since SiO2 loss during weathering was almost certainly minor, the similarity of the Fe/SiO2 ratio in the weathered and unweathered BIF indicates that nearly all the "FeO" in the Kuruman Iron Formation was oxidized and retained as FeO3 during weathering. Such a high degree of iron retention is best explained by an O2 content of the atmosphere > or = 0.03 atm at the time of weathering. Such an O2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt > or = 2.2 bybp but is consistent with the highly oxidized nature of the 1.85 by Flin Flon paleosol. The new data suggest that PO2 rose dramatically from about 1 percent PAL (present atmospheric level) to > or = 15 percent PAL between 2.2 and 1.9 bybp.