[meteorite-list] Scientists Find New Evidence of 'Snowball Earth'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 7 19:38:56 2005
Message-ID: <200504072338.j37NcTh13543_at_zagami.jpl.nasa.gov>

http://www.knoxstudio.com/shns/story.cfm?pk=SNOWBALL-EARTH-04-07-05&cat=II
    
Scientists find new evidence of 'Snowball Earth'
By LEE BOWMAN
Scripps Howard News Service
April 07, 2005

Scientists have new evidence from interplanetary dust that Earth was
sheathed in ice at least twice during a period that began around
three-quarters of a billion years ago.

The theory that our planet - oceans and land alike - was encased in ice
several times from about 750 million to 580 million years ago has been
explored since the 1960s.

In the new study, geologists Bernd Bodiselitsch and Christian Koeberl
from the University of Vienna in Austria analyzed cores drilled by
copper miners in Zambia and the Congo, looking for signatures of ice.

What they found was a clear spike in iridium - a platinum-like metal
that's rare on Earth but fairly common in meteors and space dust - in
sediments from about 635 million years ago and slightly less conclusive
evidence from 710 million years ago.

The work, published Friday in the journal Science, is already spurring
other researchers to do similar research using other trace elements.

Iridium is a good signal for extreme events on Earth because, normally,
the material falls in a steady drizzle across the planet's surface. For
instance, a big spike of iridium from 66 million years ago signaled the
impact of a comet or meteor in what is now the Gulf of Mexico - a blast
that spelled doom for the dinosaurs.

The researchers suspected that dust falling on a so-called "Snowball
Earth" would have accumulated on ice sheets worldwide, including areas
near the equator. When the ice melted, the iridium would have been
carried by water into riverbeds and oceans, leaving unusually high
concentrations of the element.

Earlier estimates for the length of the snowball periods ranged from
several hundred thousand to 30 million years. Based on the amount of
iridium found at the bottom of a layer of glacial material, the Austrian
researchers determined that the glacial period that ended about 635
million years ago lasted at least 3 million but more probably about 12
million years.

Deposits of iridium from the earlier glacial period didn't land quite as
neatly among the glacial silt, which means, the researchers speculate,
that either the meltdown from that glacial period was spotty, or that
the ice didn't fully cover the oceans - a "Slushball Earth" situation.

Although there's still much debate about what caused the glaciers to
form in the first place - theories range from collisions with asteroids
to change in the tilt of Earth's orbit - the events are important
illustrations of how the planet regulates its temperature.

Scientists believe that the increasing ice cover over the globe bounced
more sunlight back into space, contributing to the freeze-up. However,
the tectonic plates continued to move under the ice, and volcanoes
continued to form and pump carbon dioxide into the atmosphere. With no
liquid water to absorb the carbon dioxide, eventually greenhouse
conditions developed, trapping heat and melting the ice rapidly.

Just what brought the global freeze-thaw cycle to an end is also
unclear. However, scientists do know that within 100 million years,
there was a dramatic change in life on the planet from primitive
single-celled forms living in the oceans to an abundance of complex,
multi-celled animals and plants in the sea.

Recent research around seafloor volcanoes and in modern icepacks makes
it clear that many exotic strains of bacteria could have easily survived
the "Snowball" episodes. Genetic evidence shows that the organisms that
lived through the glacial times were part of the oldest lines of life
forms on the planet.
Received on Thu 07 Apr 2005 07:38:28 PM PDT