Acidifying oceans

  • Core samples from the South Atlantic show the typical pale-colored sediment indicative of a healthy carbonate-rich seafloor, interrupted by a dark-colored band of clay, representing a blip in geologic time of 100,000 years, when the ocean was highly acidic, about 55 million years ago.

    Courtesy James Zachos, Uc-Berkeley
 

James Zachos fishes around his desk and pulls out a plastic bag filled with chunks of deep-sea sediments. The sediments, wrested from the South Atlantic in 2003, are 55.5 million years old and 'deep red in color because they are almost entirely clay. Missing is the abundance of shelly residue that gives abyssal sediments their typically pallid complexions. "This is what you end up with when the ocean is being acidified," the University of California, Santa Cruz, paleo-oceanographer says.

The acidification, he explains, was a byproduct of the ocean's stalwart performance as regulator of the planet's geochemistry. As carbon dioxide rises in the atmosphere, the ocean mops up much of the excess. Over the past two centuries, scientists estimate that its vast blue waters have absorbed something like 40 percent of the carbon dioxide we've thus far emitted. But this assist to the atmosphere comes at a price. In water, carbon dioxide turns into carbonic acid, the same weak corrosive found in soft drinks.

During the PETM, the ocean contained enough carbonic acid to make life difficult for many shell-building organisms. At times, shells may have dissolved as fast as marine organisms could construct them. Among those most affected, Zachos thinks, were benthic foraminifera or forams, bottom-dwelling organisms the size of sand grains. During this tumultuous time, more than 30 percent of benthic foram species are thought to have gone extinct.

Today, the ocean is already less alkaline than in pre-industrial times, by about 0.1 units of pH. Some 55 million years ago, the pH shift was more extreme, in the neighborhood of 0.4 units. But, says Zachos, we are now on track to surpass that shift by the start of the next century and to double it by the year 2300. If that happens, the pH of the global ocean, currently around 8.0, will ever more closely approach 7.0, the dividing line between alkalinity and acidity.

More than anything, it's the rate of change that has scientists worried.  It's one thing to add a big load of carbon dioxide to the ocean over a few millennia, quite another to shock the ocean by adding a similar amount in just a few centuries. "We do not know with certainty what the consequences will be," says Ken Caldeira, a climate expert with the Carnegie Institution Department of Global Ecology at Stanford University. "But we are now adding carbon so fast that, chances are, the disturbance to the ocean will be even more extreme." 

Eventually, of course, the ocean of 55.5 million years ago recovered. Rain falling on the land slowly weathered rock into acid-buffering compounds, which washed into rivers that emptied into the seas. The rebound is visually apparent, Zachos says, projecting a slide that shows the sedimentary sequence. And there it is, the color change -- from red to beige -- that marks the end of one disturbing chapter in our planet's history.

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