Pre-Cambrian atmosphere:evidence from the Pre-Cambrian of Sweden

Sedimentary haematite ores of central Sweden give evidence of the existence of an oxidizing atmosphere about 2000 × 10⁶ years ago. This is not consistent with Rankama's conclusions from the unoxidized condition of a diorite conglomerate in Finland. Therefore a (later) Pre-Cambrian conglomerate in northern Sweden has been examined for comparison. Where it is made up of dioritic material no proofs of oxidation can be found, but where the material is mainly iron-ore pebbles, magnetite ore is found to have been completely oxidized to haematite (martite) before being embedded as pebbles in the conglomerate.