SNOWBRAWLS - Snowball Earth - page 80

J UST OVER 600 million years ago, colonies of bacteria floated invisibly in the sea that would one day
become northwest Namibia. They hung in the water perhaps a hundred feet below the surface. Undis-
turbed by wind or waves, they busied themselves with the endless operation of their internal chemical
factories. Make food. Consume food. Make food. Consume food. The sea around them turned hazy
with the accumulation of their minuscule efforts. As the chemical balance of the water changed in
response to their factory effluents, tiny flakes of carbonate sprang out of solution and floated softly
down to the seafloor.
Though the Snowball had already gripped the outside world, there was little sign, this deep in the
water, of the icebergs passing overhead. Just the occasional dropstone, a boulder released from the ice
that would appear suddenly from above, pass gently by the clouds of bacteria, and sink into the flakes
of mud that were slowly, steadily accumulating on the seafloor beneath.
Today this carbonate mud has transformed into layers of rock perhaps six inches thick. Its layers
stand out clearly among the ice rocks of the Namibian outcrops. They're the colour of rancid butter,
occasionally enlivened with a white or tan dropstone. Above and below them the rocks are grey with
shattered carbonates, silicates and sand, landslides of debris brought suddenly in from the shore. But
the serene yellow layers speak of gentler moments in the life of the Snowball ocean. Made of micro-
scopic carbonate flakes, balled up into tiny spheres called peloids, they are extremely delicate. Intact
peloids are a signal that the rock hasn't moved since they formed. If it had tumbled, the peloids would
have been smashed to pieces.
The peloidal mud also has occasional cracks, filled with crystals of carbonate cement that jut out-
wards from the walls of the cracks like the spears of a tiny white picket fence. They too must have
formed in the Snowball ocean.
Martin Kennedy had collected samples of these rocks from Namibia back in 1996. He'd also col-
lected similar carbonates from his beloved Australian Outback in the early 1990s. The Australian
samples were stromatolites, those strange domed structures created during the Slimeworld, when bac-
terial mats sat on the growing surface of a rocky edifice, and sediment accumulated beneath them.
They were tall, thin columns of carbonate rock, perhaps six inches wide, embedded in yellow dolo-
mite, and they provided another direct window into the Snowball ocean.
Now Martin looked back at these rocks and wondered. Could they help him test the Snowball idea?
In early 2001 he returned to Death Valley to collect samples from yet another set of Snowball carbon-
ates, which he'd spotted mixed in with the ice rocks there. These carbonates were called oolites, and
were strange rock forms with a caviar-like texture. You find them today in places like the Bahamas.
They grow as grains that roll backwards and forwards in the waves, coating themselves with carbon-
ate that precipitates from the seawater. Like the peloids, they're extremely delicate. If you find them
intact, they can't have come from some other place, or some other time slice. Since they are mixed in
with Death Valley's ice rocks, they too must have formed in the Snowball ocean.
Martin realized that all these carbonate samples gave him a direct window into the Snowball ocean
and, with it, a way to test the Snowball hypothesis. Remember that according to Paul and Dan, the
Snowball ocean was essentially lifeless. That was Paul's first idea, the one that set the Snowball
rolling. Paul had worked it out by looking at the ratio of light and heavy isotopes in carbonate rocks
from just before the Snowball time. Heavy carbon equals life. Light carbon equals no life. And im-
mediately before the Snowball, Paul had found unusually light carbon in his carbonate rocks. He con-
cluded that many living things must have died off as the ice advanced, leaving only a few small groups
to huddle together and wait for the thaw.