EXTRATERRESTRIAL CAUSE FOR THE DEMISE OF BANDED IRON FORMATIONS AT 1.85 GA

In the Lake Superior region deposition of most banded iron formation (BIF) ended at 1.85 Ga, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. Major BIF deposition elsewhere, such as in Labrador, Western Australia, South Australia, and Botswana, ended ca. 1.88-1.84 Ga, broadly coeval within the limits of available radiometric dating. We propose here a new model in which impact of the Sudbury bolide produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, having only ~1 μM dissolved O₂ based on mass-balance calculations, greatly reduced Fe(II) solubility and prevented Fe transport from hydrothermal vents and plumes in the deep ocean to continental-margin settings, thereby ending a ~1.1 billion-year-long period of episodic BIF mineralization. Impact-induced mixing of the stratified global ocean may have been achieved by formation of a giant tsunami, interaction of the plume jet of vaporized seawater with the more oxygenated atmosphere, cavitation of the ocean and scouring of the seafloor, and formation of large submarine debris flows by collapse of continental margins. At 1.85 Ga, all major land masses were joined to the supercontinent Columbia, hence interference by land on such submarine processes was likely minimal. Deep-water exhalative chemical sediments also changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter, consistent with development of suboxic deep seawater at that time and continuing during the Mesoproterozoic. Large bolides that struck Earth in the Archean and early Paleoproterozoic did not end BIF deposition because they predated atmospheric oxidation during the Great Oxidation Event (GOE) at ca. 2.4 Ga and subsequent formation of a stratified global ocean. The 2.02 Ga Vredefort impact in South Africa could have affected the redox state of deep seawater, but it is unknown if this bolide struck the ocean. If not, the Sudbury impact at 1.85 Ga may have created the first megatsunami on Earth after the GOE and the first thorough mixing of the stratified global ocean.