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 usua lly subjected to later alteration, the evidence gleaned from chemical studi es has provided inconclusive evidence on the atmospheric O-2 evolution. In a 2.6-2.45 Ga paleosol developed on Archean granite near Pronto mine, Canad a, we found that Ce-rich rhabdophane formed directly during weathering, rep laced 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 C e3+ in the weathering solution, unlike the younger weathering profiles wher e Ce3+ oxidizes and forms cerianite, CeO2. The presence of rhabdophane with Ce3+ throughout the Pronto paleosol provides compelling evidence of an ano xic atmosphere 2.6-2.45 Ga ago. Because apatite is a common accessory miner al in granitic rocks, Ce content of the replaced rhabdophane can be a usefu l indicator for tracing O-2 evolution in the Precambrian. (C) 2001 Elsevier Science B.V. All rights reserved.