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."
In northern Minnesota and other areas nearby, the formations lie directly underneath a thick layer of material only recently recognized as ejecta from the Sudbury impact. Mark Jirsa, a geologist with the Minnesota Geological Survey in St. Paul, was a member of the team that identified the ejecta layer. "We intuitively connected the Sudbury impact with the shutdown of BIF accumulation," he says. "But now [Cannon and Slack] have come up with a model for how that might have happened."
About 1.85 billion years ago, Earth's now separate landmasses were joined in a single supercontinent. That also means there was one large ocean, says Cannon. Many scientists suggest that the object that slammed into Earth then - probably an asteroid abut 10 kilometers across - splashed down in that ocean, in waters about 1 kilometer deep on the shallow shelf surrounding the supercontinent. Models hint that the tsunami spawned by the event would have been 1 kilometer tall at the impact site and remained at least 100 meters tall about 3,000 kilometers away, Cannon adds.
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, the researchers propose. Sediments deposited on the seafloor before the impact, including BIFs, contained little if any iron in its Fe(III) form but were high in Fe(II), a sign that most parts of the ocean were oxygen-free. But marine sediments deposited after the impact included substantial amounts of Fe(III) but very little Fe(II) - and, therefore, sizable amounts of dissolved oxygen. The team's analyses suggest that after the impact, dissolved iron spewed into the deepest parts of the ocean by hydrothermal vents would have reacted with oxygen within a day or so, thereby choking off most of the supply of Fe(II) to shallower waters where BIFs typically accumulated.
While Cannon and Slack's model explains how BIF accumulation might have suddenly ceased 1.85 billion years ago, it doesn't prove that's how it happened, Jirsa warns. Nevertheless, he notes, "scientists are closer to an explanation than we previously were." The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact, he adds, "and the role that the impact played, if any, in shutting down BIF accumulation isn't well understood."