Direct evidence of late Archean to early Proterozoic anoxic atmosphere from a product of 2.5 Ga old weathering

Because Precambrian paleosols (ancient soils formed by weathering) are usually subjected to later alteration, the evidence gleaned from chemical studies has provided inconclusive evidence on the atmospheric O2 evolution. In a 2.6–2.45 Ga paleosol developed on Archean granite near Pronto mine, Canada, we found that Ce-rich rhabdophane formed directly during weathering, replaced primary apatite, and has survived for about 2.5 Ga because of its low solubility and high resistance to heat. Our data show that La, Ce, and Nd behaved similarly in both rocks and rhabdophane, i.e., most Ce existed as Ce3+ in the weathering solution, unlike the younger weathering profiles where Ce3+ oxidizes and forms cerianite, CeO2. The presence of rhabdophane with Ce3+ throughout the Pronto paleosol provides compelling evidence of an anoxic atmosphere 2.6–2.45 Ga ago. Because apatite is a common accessory mineral in granitic rocks, Ce content of the replaced rhabdophane can be a useful indicator for tracing O2 evolution in the Precambrian.