Ancient ocean acidification intimates long recovery from climate change

It may takes tens of thousands of years for oceans to recover from the acidity caused by increased levels of carbon dioxide

By David Biello
July 22, 2010 Single-cell life-forms thrive throughout the world’s oceans—and have for hundreds of millions of years. Tiny varieties known as calcareous nanoplankton build exuberant, microscopic shells—resembling wagon wheels, fishlike scales, even overlapping oval shields decorated with craggy explosions at their centers—known as “coccoliths”. The ability to form these shells rests on the amount of calcium carbonate (CaCO3) dissolved in the seawater—and that amount depends on the concentrations of atmospheric carbon dioxide (CO2). CO2 is the ubiquitous greenhouse gas emitted by human activity, particularly fossil-fuel and forest burning. As levels rise in the atmosphere (currently at 390 parts per million and counting), the ocean’s surface waters absorb more of the molecule. This water–CO2 mixture forms carbonic acid, which slightly lowers the ocean’s overall pH (the lower the pH, the more acidic). More acidic ocean water means less calcium carbonate—and less material for shell-building plants and animals of all sizes, including the nannoplankton that constitute the base of the food chain. Of course the present era is hardly the first time the planet has seen higher levels of CO2. In fact, roughly 121 million years ago—during an age known as the early Aptian—global CO2 levels were likely higher than 800 ppm (and possibly as high as 2,000 ppm) thanks to cataclysmic volcanic eruptions. Now new research published in Science July 23 shows how ancestors of today’s nannoplankton fared in those acidic oceans of long ago. It was a time of “severe global warming,” paleobiologist Elisabetta Erba of the University of Milan and her colleagues wrote, after studying the carbon isotopes embedded in deep seabed cores drilled in the Pacific Ocean and locations in the ancient Tethys Ocean, which existed during the Mesozoic era. The records reveal that acidification proved a big problem for nannoplankton. “During the Aptian episode, marine calcifiers experienced a major crisis due to increasing CO2-induced acidification,” Erba says. … That’s probably bad news for modern nannoplankton—and other shell-building microscopic life, such as foraminifera. Foraminifera responded similarly to the Aptian event, Erba says, although “data are still sparse.” Modern day experiments agree with the fossil record: High CO2 levels in lab tests prompt “selective coccolith malformation, dwarfism and decrease in calcification,” Erba notes, whereas these results have been conflicting at times, Bralower adds. …