By Sid Perkins
The collision of a large extraterrestrial object with Earth almost 2 billion years ago may have stirred the seas worldwide and delivered a small but crucial serving of oxygen to the deep ocean.
The Sudbury impact, named after the Canadian city located near the center of what remains of the ancient crater, happened around 1.85 billion years ago (SN: 6/15/02, p. 378). Despite erosion since then, the impact structure —at least 200 kilometers across — is recognized to be the second-largest on the face of the planet, says William Cannon, a geologist with the U.S. Geological Survey in Reston, Va., and coauthor on a paper in the November Geology. The event fundamentally affected the concentrations of dissolved oxygen in the deep sea — enough to almost instantly shut down the accumulation of marine sediments known as banded iron formations, report Cannon and coauthor John F. Slack, also of the USGS in Reston.
Banded iron formations, massive deposits rich in iron oxides, have accumulated at several periods in Earth’s long-distant geological past, mostly when atmospheric concentrations of oxygen were low (SN: 6/20/09, p. 24).
One extended episode of banded iron formation (or BIF) buildup suddenly — and without an obvious explanation — ended about 1.85 billion years ago, says Cannon. Over a very short interval, he notes, “the environment shifted from one happily making banded iron to one that wasn’t.”