Minnesota Geology Pictures - Stromatolites in the Biwabik Iron Formation

In northern Minnesota, the Biwabik Iron Formation was deposited 1,900 to 1,885 years old along the shores of the Animikie Sea.  At this time, surface water was busy eroding the continent located to the north bringing fine materials like silt and clay into this shallow sea that would later be incorporated into the iron formation.  Before the time of iron deposition though, reduced iron was easily dissolved and concentrated into the sea water because there was not free oxygen molecules in the atmosphere or seas at the time.

This was a unique time period in Earth's history, as photosynthetic organisms begin to appear in the rock record worldwide.  These organisms, called stromatolites, formed algal reefs along the shoreline of these ancient seas and as a waste product of photosynthesis, produced oxygen.  It is this oxygen that caused the iron to precipitate out of the seawater and be deposited as iron.  This time period is called the Great Oxidation Event (GOE) and is responsible for most of the iron deposition throughout the world.

Each domed structure above would have been an algal mound located at or near the surface of the water 1,900 years ago, busy precipitating iron to the seafloor as a consequence of photosynthesis.  This particular field of stromatolite domes is found within an inactive iron mine and is being preserved, there are no future plans to mine the iron in this location.