theory (see here)
  1. without an oxygen rich atmosphere, eroded  iron enters oceans as ferrous ions (Fe2+).  Ferrous iron dissolves well in sea water.  Low oxygen content of oceans permits iron to remain in solution.  Meanwhile, silicate-rich sediments accumulate on ocean floor.
  2. as cyanobacteria (primitive photosynthesizers that produce oxygen but do not thrive in oxygen-rich environments) begin to prosper, oxygen level of oceans begins to increase.  In oxygen-richer environment, iron precipiates out via reactions like:

  3. 4Fe2+ + 3O2 --> 2Fe2O3. This forms iron-rich sediments and keeps oxygen from building up in atmosphere or ocean.
  4. Once iron is depleted in oceans, oxygen level builds up in oceans; but early photosynthesizers were poisoned by the overabundance of oxygen in oceans.
  5. With destruction of most of photosynthetic life, iron builds up in oceans, a silicate layer is deposited on the ocean floor, and the cycle repeats.
  6. This cycle largely ended when most of the iron (that was continually released through tectonic processes) was locked away into BIFs.
Questions and answers about BIFs


redbeds from when hematite (Fe2O3) is produced during weathering of iron-rich rocks in an oxidizing atmosphere

Questions and answers about redbeds

Questions and answers about uraninite: