Thursday, November 12, 2009

Meteorite Impact Ended Banded Iron Formation Deposition

From the November issue of Geology:

Extraterrestrial demise of banded iron formations 1.85 billion years ago; John F. Slack and William F. Cannon

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact 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, characterized by only small concentrations of dissolved O2 (~1 μM), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ~1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter.

I don't have access to the full paper but if this holds up here is another example of how extraterrestrial matter has shaped the geological and biological evolution of earth.  A chance meteorite impact ends a prolonged process of iron oxide deposition and enables greater amounts of oxygen to accumulate in the oceans and eventually the atmosphere. That was a precursor for the evolution of more complex cell types like the eukaryotes.

Coincidentally in the October 29 issue of  Nature is another paper which hypothesizes that the earth was dry very early in its history and much of the earth's water and hence the ultimate source of the oceans has been derived from ice rich asteroid bombardment about 100 million years after the formation of the solar system. Science Daily has the summary.


  1. Suvrat,
    I have a copy of the article which I can send you. Just shoot me an e-mail and I'll reply with it.