Ice-Age Shell Game

Categories: Geology

A trio of scientists including a researcher from the Lawrence Livermore National Laboratory has found that humans may owe the relatively mild climate in which their ancestors evolved to tiny marine organisms with shells and skeletons made out of calcium carbonate.

A layered stromatolite, produced by the activity of ancient cyanobacteria.
Credit: UC Berkeley

In a paper titled "Carbonate Deposition, Climate Stability and Neoproterozoic Ice Ages" in the Oct. 31 edition of Science, UC Riverside researchers Andy Ridgwell and Martin Kennedy along with LLNL climate scientist Ken Caldeira, discovered that the increased stability in modern climate may be due in part to the evolution of marine plankton living in the open ocean with shells and skeletal material made out of calcium carbonate.

Carbonates are minerals that form when negatively charged carbonate ions (a carbon atom and three oxygen atoms) combine with a positive ions such as calcium, magnesium or iron. The conventional reaction occurs in solution, and carbonate crystallizes out of the liquid – sometimes with help from marine organisms that incorporate the carbonates into their shells.

The most common carbonates on Earth are calcite – also known as limestone or calcium carbonate – and dolomite, which is made of carbonates of calcium and magnesium.

The climate modellers conclude that these marine organisms helped prevent the ice ages of the past few hundred thousand years from turning into a severe global deep freeze.

Terrestrial options for early climate. Early earth, snowball, cauldron or temperate?Credit: NASA

"The most recent ice ages were mild enough to allow and possibly even promote the evolution of modern humans," Caldeira said. "Without these tiny marine organisms, the ice sheets may have grown to cover the earth, like in the snowball glaciations of the ancient past, and our ancestors might not have survived."

The researchers used a computer model describing the ocean, atmosphere and land surface to look at how atmospheric carbon dioxide would change as a result of glacier growth. They found that, in the distant past, as glaciers started to grow, the oceans would suck the greenhouse gas — carbon dioxide out of the atmosphere — making the Earth colder, promoting an even deeper ice age. When marine plankton with carbonate shells and skeletons are added to the model, ocean chemistry is buffered and glacial growth does not cause the ocean to absorb large amounts of carbon dioxide from the atmosphere.

But in Precambrian times (which lasted up until 544 million years ago), marine organisms in the open ocean did not produce carbonate skeletons — and ancient rocks from the end of the Precambrian geological age indicate that huge glaciers deposited layers of crushed rock debris thousands of meters thick near the equator. If the land was frozen near the equator, then most of the surface of the planet was likely covered in ice, making Earth look like a giant snowball, the researchers said.

What the world looked like 250 million years ago. Plate tectonics pushed the continents together to form the super-continent Pangea and the super-ocean Panthalassa. Weather patterns and ocean currents shifted, many coastlines and their shallow marine ecosystems vanished, sea levels dropped. Credit: Chris Scotese. [more]

Around 200 million years ago, calcium carbonate organisms became critical to helping prevent the earth from freezing over. When the organisms die, their carbonate shells and skeletons settle to the ocean floor, where some dissolve and some are buried in sediments. These deposits help regulate the chemistry of the ocean and the amount of carbon dioxide in the atmosphere. However, in a related study published in Nature on Sept. 25, 2003, Caldeira and LLNL physicist Michael Wickett found that unrestrained release of fossil-fuel carbon dioxide to the atmosphere could threaten extinction for these climate-stabilizing marine organisms.

What’s Next

Carbon is a fundamental constituent of biogenic materials, but its lifecycle–how it is created, modified or destroyed–has taken on increasing importance to interpreting perplexing problems in predicting climate. In addition to the importance of calcium carbonate to our own climatic history, its presence or absence has often been suggested as a promising indicator of water itself.

Liquid water often is considered a requirement for life, on Earth or beyond. And until recently, the presence of extraterrestrial carbonate chemicals – believed to form only in water – was thought to be a reliable indicator of the past or current presence of water. Whether this mineral can appear without water has focused a debate not only on habitability of other solar systems, but even in our own neighborhood, may point to whether Mars once had surface water or not. Recent global surveys of Mars have found no strong carbonate signatures anywhere on the planet, despite clear evidence of geological processes that have exposed ancient rocks. Carbonate deposits in dust could be partially responsible for Mars’ atmosphere growing even colder, to become as cold, thin and dry as it is today.