BioEd Online

Human-generated carbon dioxide has sunk down to a great depth in the North Atlantic Ocean, a new study has shown. The work, published in Proceedings of the National Academy of Sciences¹, suggests that the oceans store CO₂ for longer than expected — good news for reducing the risk of climate change, but bad news for marine life in the deep sea.

About half of the atmospheric CO₂ produced by human activities since the beginning of the industrial revolution has ended up in the ocean². The gas, along with oxygen and other compounds in the air, dissolves into the surface waters and is mixed around by the currents. Because the ocean is so huge, it has an enormous capacity to suck up gas.

Scientists have long known that CO₂ would eventually be transported to the deep sea. But previous studies have been unable to spot man-made carbon dioxide at depths of greater than 4,000 metres. How much was down deep was a great unknown.

Now Douglas Wallace, at the Leibniz Institute of Marine Sciences at the University of Kiel, Germany, and colleagues say that the concentration of man-made CO₂ in the western basin of the North Atlantic Ocean is at least 10% of that at the ocean surface. Wallace says that if the same holds true around the globe, then that would indicate a very large reservoir of man-made carbon in the ocean.

This study really emphasizes the "extraordinary invasion of what is now some 500 billion tons of fossil CO₂ into the ocean", says Peter Brewer, an ocean chemist at Monterey Bay Research Aquarium Institute in Moss Landing, California.

Tracking carbon

Until now, scientists have grappled with detecting exactly where in the ocean human-generated carbon dioxide is stored. Most studies have used tracers of elements such as chlorofluorocarbons (CFCs) or ¹⁴C from nuclear bombs. But while anthropogenic carbon has been entering the atmosphere since the late 1800s, many of these tracers have only been around since the end of World War II.

Wallace and colleagues set themselves the challenge of developing a different way to look at older anthropogenic carbon in the oceans. They measured dissolved inorganic carbon and watched how its concentration changed over time in relation to other factors, including temperature, pH, dissolved nutrients and oxygen. Several decades worth of data let them work out the relationship between these factors and anthropogenic CO₂ for different depths, down to 4000 metres.

Once CO₂ is carried downwards, the surface waters become free to take up and store more CO₂. And the deeper it goes, the longer it is expected to stay down. Although this buffers against global warming, it makes the deep sea more acidic — corroding and dissolving the armour and skeletons of deep-water corals and other marine creatures. "This is a sign that we are radically changing the pH of the deep ocean," says Wallace.

Tough times

The wildlife at risk includes organisms that construct hard parts from one of two forms of calcium carbonate: calcite or aragonite, both of which become soluble at a certain pH, temperature and depth. Deep-sea corals, for example, build their skeletons from calcium carbonate.

Wallace and colleagues show that aragonite is already unstable in the depths of the western basin of the North Atlantic Ocean. But in the eastern basin, the introduction of carbon is shifiting the horizon at which ocean life begins to dissolve. According to their calculations, this has shifted upwards some 400 metres since before industrial times, and is projected to rise up a total of 700 metres by 2050.

It is unclear what, exactly, this will do to the corals. Scott Doney, Senior Scientist at the Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution in Massachusetts, says that the rising of this horizon "will put in danger the deep-sea corals that are living at these depths". Brewer, on the other hand, says that "Pacific deep-sea corals appear to cope quite well. There are many examples from Pacific sea mounts where such corals are already well below the aragonite saturation horizon and thrive."

Wallace says that the potential effects of such "large chemical changes" on corals needs to be further looked into.