Banded Iron Formation - Effect of Asteroid Impact

Effect of Asteroid Impact

Northern Minnesota's banded iron formations lie directly underneath a thick layer of material only recently recognized as ejecta from the Sudbury Basin impact. At the time of formation the earth had a single supercontinent with substantial continental shelves.

An asteroid (estimated at 10 km across) slammed into waters about 1,000 m deep some 1.85 billion years ago. Computer models suggest that the tsunami would have been at least 1,000 m at the epicentre, and 100 m high about 3,000 km away. Those immense waves and large underwater landslides triggered by the impact stirred the ocean, bringing oxygenated waters from the surface down to the ocean floor.

Sediments deposited on the seafloor before the impact, including BIFs contained little if any oxidized iron (Fe(III)), but were high in reduced iron (Fe(II)). This Fe(III) to Fe(II) ratio suggests that most parts of the ocean were relatively devoid of oxygen.

Marine sediments deposited after the impact included substantial amounts of Fe(III) but very little Fe(II). This suggests that sizeable amounts of dissolved oxygen were available to form sediments rich in Fe(III). Following the impact dissolved iron was mixed into the deepest parts of the ocean. This would have choked off most of the supply of Fe(II) to shallower waters where BIFs typically accumulated.

The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact. The role the Sudbury Basin impact played in temporarily shutting down BIF accumulation is not fully understood.