91av

TINY shelled sea creatures could be the only thing keeping our planet from plunging into the next ice age.

According to a new theory, it was the evolution of these organisms that ended a 200-million-year period of Earth’s history known as Snowball Earth, when the planet was almost completely frozen over. And they have since prevented the same thing from happening again.

Andy Ridgwell and Martin Kennedy at the University of California, Riverside, and Ken Caldeira at Lawrence Livermore National Laboratories, also in California, argue that shelled plankton have helped to stabilise the planet’s carbon cycle by dumping calcium carbonate into deep water when they die.

Chemical processes in the sea that dissolve calcium carbonate deposits alter the acidity of the water. This affects the amount of atmospheric carbon dioxide that can dissolve in the seawater, which in turn helps the planet to regulate its temperature. When calcium carbonate dissolves, for example, it makes the ocean more acidic, and more able to dissolve the greenhouse gas CO₂. Calcium carbonate deposits have the opposite effect, reducing acidity and the amount of CO₂ that dissolves from the atmosphere. Creatures like coccolithophoridae and foraminifera help with this process. They make protective shells out of calcium carbonate in seawater and when they die, their bodies fall to the bottom of the sea, carrying the calcium carbonate to the deep ocean floor. But during the Neoproterozoic period 540 million years ago, such organisms had yet to evolve. Instead, most calcium carbonate was deposited in shallow coastal regions, through a gradual process of burial.

Ridgwell and colleagues think this is why Earth’s climate used to undergo such dramatic swings. They created a computer model of the climate, and found that if the plankton weren’t around, shallow-water deposits of calcium carbonate were able to regulate CO₂ levels, serving as a kind of global thermostat. But this mechanism was not very robust, particularly if sea levels started to drop as falling temperatures caused the ice caps to grow.

With fewer shallow regions to bury the calcium carbonate, the thermostat would stop working, says Ridgwell. The ice caps themselves would add to the cooling effect by reflecting more of the sun’s warmth, and the combined effect would be enough to plunge the Earth into a severe ice age (Science, vol 302, p 859).

Ridgwell says that the arrival of shelled plankton, which carried carbonate down to the deep oceans, provided a new buffer that was not so sensitive to sea levels. He believes this could be why the planet has not suffered any catastrophic ice ages since the plankton evolved.

“What’s new is the idea that having calcium carbonate deposition in the deep sea makes the system more stable than it would be with only shallow-water deposition,” says David Archer, an expert on the marine carbon cycle from the University of Chicago, Illinois. “It’s an intriguing idea.”