[meteorite-list] Add asteroid, stir vigorously

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Mon, 16 Nov 2009 18:43:12 -0500
Message-ID: <cpo3g5d4p904lrrg0h2abi3jdlf6rq9urk_at_4ax.com>

http://www.wired.com/wiredscience/2009/11/giant-asteroid-impact-could-have-stirred-entire-ocean/

Giant Asteroid Impact Could Have Stirred Entire Ocean

The collision of a large extraterrestrial object with Earth almost 2 billion
years ago may have stirred the seas worldwide and delivered a huge 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.?

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.?
Received on Mon 16 Nov 2009 06:43:12 PM PST