91av

DID the oceans freeze 700 million years ago, turning the Earth into a huge
ball of ice? Controversy over the “Snowball Earth” theory is being stirred up by
new findings that suggest life went on as normal when the oceans were supposedly
covered with ice and almost lifeless.

Geochemist Martin Kennedy and sedimentologist Nicholas Christie-Blick studied
rocks from three continents and found no sign that life was affected at this
time. According to Christie-Blick, of the Lamont-Doherty Earth Observatory in
New York, their results prove that the oceans did not freeze over. “There was a
lot of sea ice,” he says, “but life went on [as normal].”

Harvard University’s Paul Hoffman and Daniel Schrag proposed their Snowball
Earth hypothesis in 1998. They suggested a runaway cooling mechanism that
covered the oceans with ice sheets and brought biological activity to a near
stop for millions of years during the Neoproterozoic period
(91av, 6 November 1999, p 28).

This striking idea appeared to solve several geological puzzles, including
signs that glaciers had existed in the tropics. There’s also a unique
sedimentary layer covering the debris from Neoproterozoic glaciation. It’s
called the “cap carbonate” layer, and it has a strange carbon signature. When
marine plants photosynthesise, they slightly prefer carbon-12 to carbon-13,
leaving a small excess of carbon-13 behind in seawater and the rocks that
precipitate from it. The more life there is, the more carbon-13. But cap
carbonates have a strikingly low ratio of carbon-13 to carbon-12, indicating
minimal biological activity.

Hoffman and Schrag had only studied rocks from before and after the glacial
period, however, so Kennedy and Christie-Blick set out to find rocks that formed
during the big freeze. If ice sheets sealed the oceans and brought life almost
to a standstill, sediments from that period should contain less carbon-13. But
Kennedy and Christie-Blick found the opposite. Samples from Africa, Australia
and North America had plenty of carbon-13, as did the oldest cap carbonates.
“The ecosystem was happy,” says Kennedy. “So was there really an ice sheet?”

Christie-Blick is even more pointed. “If the geochemical part doesn’t work,
all the rest of it doesn’t work either. The data makes the snowball hypothesis
incorrect, at least as initially published.” Their findings match the latest
results from computerised climate models, in which equatorial oceans stubbornly
resist freezing over
(91av, 9 June, p 12).

Schrag doesn’t necessarily reject Kennedy and Christie-Blick’s new data, but
his interpretation is different. He now thinks that patches of open water
persisted even during the global deep-freeze. That would have radically changed
the ocean’s carbon chemistry as it absorbed large amounts of carbon dioxide from
the atmosphere—a factor he says Kennedy and Christie-Blick ignored.
“They’re making simplistic predictions,” says Schrag, “so their analysis is just
plain wrong.”