Results of a study of Proterozoic weathering crusts in Karelia

Resemblances between Precambrian and post-Precambrian weathered profiles over granites are taken as evidence of very little change in physicochemical conditions of regional weathering in Proterozoic-Cenozoic time. Presence of substantial quantities of oxygen in the Middle Proterozoic atmosphere may account for the general increase in ferric (and corresponding decrease in ferrous) iron, from the bottom up, in the profiles examined here. -- V.P. Sokoloff