The evolution of tiny, shelled sea creatures ended a 200 million year era of extreme ice ages and has protected the Earth from any repeat ever since, suggest the results of a new modelling study.
During the frozen period, known as 鈥渟nowball Earth鈥� the polar ice caps extended far down into low latitudes, covering much of the planet.
The emergence of the plankton, which incorporate carbon dioxide into calcium carbonate shells, created a new stability in the planet鈥檚 carbon cycle, argue Andy Ridgwell, at the University of Riverside, California, and his colleagues. The minute organisms did this by providing for the first time a way to dump calcium carbonate into the deep waters below the open oceans.
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Chemical processes in the sea that dissolve calcium carbonate deposits alter the acidity of the water. This helps regulate the amount of atmospheric carbon dioxide that can dissolve in seawater. And this in turn helps the planet to regulate its temperature.
Drift down
Today, plankton with carbonate shells, like coccolithophoridae and foraminifera, live throughout large parts of the ocean. When they die, most drift down and settle deep on the ocean floor, where their shells dissolve.
But during the Neoproterozoic period, from 1000 to 540 million years ago, such organisms had not yet evolved. As a result, the dissolution of most calcium carbonate deposits occurred in shallow coastal regions.
Ridgwell and his colleagues think that while these shallow water processes served well as a global thermostat for most of the time, they could be overwhelmed relatively easily. This was particularly the case when sea level dropped, reducing the area of the shallow oceans.
Feedback effect
A snowball Earth glaciation would start with a slight cooling of the climate, causing the ice caps to grow. This would have led to sea level dropping, enhancing the cooling. The thermostat would stop working, Ridgwell says.
The ice caps themselves would also cause a feedback effect by reflecting more of the Sun鈥檚 warmth than ocean. This, and the broken thermostat, would plunge the Earth into a severe ice age, which would only be broken by the eventual build up of carbon dioxide emitted from volcanoes.
But the arrival of the plankton, which could carry carbonate to the deep oceans, provided a new buffer. Unlike the shallow oceans, the deep oceans never become saturated with carbonate, making them a much more reliable and sensitive thermostat. Ridgwell told 快猫短视频 this may well be why the Earth has not suffered any catastrophic ice ages since the plankton evolved.
鈥淚t鈥檚 an intriguing idea,鈥� says David Archer, in the Department of Geophysical Sciences at the University of Chicago. 鈥淏ut it will certainly be contentious.鈥�
It has been know for some time that plankton play a role in burying organic carbon, he says. 鈥淲hat鈥檚 new here 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.鈥�
Journal reference: Science (vol 302, p 859)