Abstract
Redox sensitive elements such Fe have been used to evaluate the oxidation state of paleosols in the Precambrian, and thus, atmospheric oxygen evolution. The fractionation of light REE occurs during oxic weathering because Ce(3+) is oxidized to form cerianite, CeO2, and La and Nd are accommodated in rhabdophane. In contrast, La, Ce and Nd behave similarly and form rhabdophane during anoxic weathering. Paleosols of Mt. Roe (2.8 Ga), Pronto (2.5 Ga), Cooper Lake (2.5 Ga), Ville Marie (2.2 Ga), and Sub-Thelon (1.8 Ga) were examined by SEM-EDS. The ratios of Ce to La (wt %) were in the range of 2.1 to 2.3 for rhabdophane grains of Mt. Roe, Pronto and Cooper Lake, suggesting anoxic weathering. On the other hand, the ratios were in the range of 0.2 to 1.6 for Ville Marie and Sub-Thelon, suggesting oxic to slightly oxic weathering. Our results are consistent with the hypothesis that atmospheric oxygen rose in 2.6 - 1.6 Ga gradually.
