EUREKA - Snowball Earth - page 49

was astonished. They had a bizarre carbon isotope signature, one that he had never seen before, with
much less carbon-13 than he had anticipated.
Usually, ocean water and the carbonates that it produces are both rich in carbon-13. The ratio gets
skewed to the heavier side because of the activity of living creatures. Bacteria in the ocean need carbon
to grow, and carbon-12 is their favourite flavour. They grab carbon-12, and leave carbon-13 behind.
Think of a box of red and green jelly beans. As you gradually pick out the red ones, the rest of the
box will start to look more and more green. The same thing happens with carbon isotopes in seawater.
When bacteria grow and grab carbon-12, the seawater ends up with proportionately more carbon-13.
This seawater carbon is then bound up in carbonate rock.
So, when life is flourishing, the carbonate rocks formed at the same time have the skewed heavy
seawater ratio. That's what was so strange about Paul's rocks. For carbonates, they were extraordinar-
ily light. Before the Snowball, and for what looked like a long time afterwards, life apparently wasn't
active at all.
Paul felt this was important, but he couldn't figure out what it meant. He was more baffled still
by the “cap” carbonates that came after the ice. These are the same rocks that show up all around the
world. Brian Harland had seen them in Svalbard. They stretch for miles in Australia, Canada, almost
everywhere that the glacial rocks appear. And that's peculiar. One of the first things you learn in geo-
logy is that the Earth is emphatically not one big layer cake. Sure, individual regions might end up
with layers of different rock types, cut through by rivers the way a knife cuts through a cake. But the
rock layers are still different in different places. Take a snapshot of the rocks forming today on Earth,
and here's what you might see. One place might have sandy seafloors or beaches that eventually solid-
ify to produce sandstone. Somewhere else might be in the act of producing mudstone. Perhaps some
volcanoes spew out their lava to cover another region with black basaltic rocks, and elsewhere you
might find rocks that have been pummelled and transformed by the inner churnings of a mountain belt.
The Earth is a very big, very patchy place. You simply don't get single events that blanket the entire
planet with one type of rock. Period.
So where did these cap carbonates come from? Everywhere the ice rocks appeared, the caps
seemed to be. And the ice rocks showed up on every single continent. Why? Why?
The caps also contained strange textures. The strangest were in the rock outcrops around the Huab
River. There, set in a fawncoloured cliff of carbonate, Paul found brown tubes resembling burn marks
made by long, thin pokers. From a distance they were dark vertical stripes marring the cliff face for
hundreds of feet. They weren't just on the surface of the cliff, either. Where chunks of rock had broken
off, you could see more tubes marching on into the interior. In some places the breaks had sliced ho-
rizontally through the tubes, exposing them as a neat array of dark circles, each the size of a penny.
The tubes looked like the regimented burrows of a highly organized worm colony, but there were no
worms in the Precambrian. Paul was baffled by them.
Across the valley from the tubes, a student of Paul's found something just as strange. He had
climbed up a steep ridge to inspect the carbonate outcrop. At the top was a plethora of huge rose-col-
oured crystals. They stood out against the pale carbonate rock around them, looking like giant splayed
paw prints set into the vertical rock face. Or like the kind of feathered fans that Victorian ladies carried
to the opera, though some of them were as tall as Paul himself. When Paul first saw these fans, he
was astonished. He thought at first that they were fossils of some kind. They looked almost like giant
clamshells. But no clams existed in the Precambrian, nor did any other creature that could make shells
like these. The fans had to come from some bizarre physical process. But what?