Ocean Science Experiment Could Fight Acidification

May 8, 2008 -- As any good chemistry student knows, if something is too acidic, add a base. Could that be the solution to the problem of ocean acidification?

Danny Harvey of the University of Toronto wanted to check it out. In a new study, published in the Journal of Geophysical Research, he calculated the effect of dumping massive amounts -- 4.4 billion tons per year -- of powdered limestone, or calcium carbonate, into the ocean to counteract the acidification caused by increased atmospheric CO2 dissolving in the ocean.

"I've been concerned for a long time about how we might draw down the atmospheric CO2 once we've phased out fossil fuels," Harvey said.

"The natural recovery would be very slow," he added. "If you can find ways to enhance uptake of CO2, you could draw it down faster. But then there's the question, is there any way we could accelerate the natural recovery of the ocean's acidity?"

It turns out that adding limestone could do both, but -- perhaps surprisingly to the chemistry student -- the dominant effect of adding limestone, a base, would be to sequester carbon, not to change the pH.

Here's how it would work.

The upper layer of the ocean is supersaturated with carbonate ions (a necessary situation for corals, certain plankton and other tiny organisms to make their shells).

As CO2 from the atmosphere dissolves in the surface layer, the interaction of various types of carbonate molecules decreases the degree of supersaturation of carbonate ions (which threatens the health of these organisms), and increases acidity.

Since the surface layer is supersaturated, limestone powder can't dissolve until it sinks into deeper water, where higher pressures allow the water to hold greater amounts of carbonate ions while still remaining unsaturated. If that water returned to the surface through upwelling, it would be recharged with carbonate and able to take up more CO2.

The net effect is that more CO2 is pulled into the ocean -- about one molecule for every molecule of calcium carbonate added. But since more acidifying CO2 gets pulled in with this cycle, the acidity decreases only slightly as limestone is added.

Because this process relies on sinking, dissolving and upwelling, Harvey's calculations predict it would take decades for the limestone to fulfill its potential for absorbing CO2.

Still, Harvey calculates that adding 4.4 billion tons per year of limestone to regions of the ocean where strong upwelling occurs would eventually result in the uptake of about 0.3 billion tons of carbon per year.