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Ocean reflux

Corrosive seawater burps up temporarily

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2:05pm, May 22, 2008
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Seawater with the potentially shell-disrupting chemistry predicted for the open ocean after 2050 has already surfaced along North America’s West Coast, scientists report.

In spring 2007, the corrosive, deep water rose temporarily to the Pacific surface some 40 kilometers roughly west of the California-Oregon border, says Richard Feely of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle. Elsewhere on the continental shelf, corrosive water rolled up but didn’t surge all the way to the surface, Feely and his colleagues report in an upcoming Science.

Deeper water normally swells upward at this time of year. But so much carbon dioxide — from natural and human-related processes — had dissolved in the water that the upwelling had a pH around 7.7. Surface water in the region typically has a pH of between 8.0 and 8.3 (a pH below 7 is acidic).

Gloomy estimates hadn’t predicted such a pH drop at the ocean surface until the second half of the century, Feely says. “This means that ocean acidification may be seriously impacting marine life on our continental shelf right now,” he says.

Feely blames human releases of greenhouse gas for creating the conditions that led to the upwelling. Deeper water naturally dips closer to acidity and carbonate scarcity, and human additions of carbon dioxide have expanded this zone upward.

“I was expecting that upwelling systems would be the first place where corrosive waters would reach the surface, but I hadn't really thought we were already there,” says Corinne Le Quéré of the University of East Anglia, in Norwich, England, after hearing about the work.

"What this study will really do is to point at where the biologists should come and study the impact of ocean acidification," says Le Quéré.

Just what the slosh of unusually low pH meant to the sea creatures isn’t clear, says Victoria Fabry of CaliforniaStateUniversity, San Marcos. Lab tests, including her work on free-swimming pteropod snails, suggest that many species in shallower waters fail to make proper calcium carbonate shells and skeletons in lowered pH waters, where carbonate is scarce. Mussels, oysters and other commercial species could be at risk. So might fish, such as juvenile salmon that fatten up on pteropods and other calcifying nuggets.

Feely and his colleagues discovered the extent of the upwelling during a research cruise last May and June. The research team sampled water along a series of paths sticking out from the shore like teeth on a somewhat splayed comb. The cruise data can’t tell them how the water moved along the coast in later days or months, but Feely says he’s seen signs that it rolled down toward San FranciscoBay.

Some colder water normally rises toward the sea surface along the coast at this time of year, says coauthor Christopher Sabine, also of NOAA’s Seattle lab. Spring and summer wind patterns nudge surface waters westward, drawing up water from 150 to 200 meters below the surface.

With the industrial age, extra carbon dioxide wafts into the atmosphere and gets picked up by the sea. The oceans now take up 30 million metric tons of carbon dioxide a day, Sabine says. The extra dose makes the pH decline in the water column more dramatic so lower pH water is closer to the surface, where the upwelling originates.

“The water it’s grabbing is now corrosive, and it wasn’t before,” he says.

The California report looks like only the second report of low-carbonate, or undersaturated, water on any sea surface, says Toby Tyrrell of the National Oceanography Centre at the University of Southampton in England. He and his colleagues reported the first, in the Baltic Sea, but he notes that the chemistry there is different because the brackish sea ranks between freshwater and open-ocean in salinity.

Other regions with water dynamics similar to those off the California coast might have their own corrosive upwellings, Fabry says. Waters along the eastern edges of oceans, such as those off South America or Africa, now need surveying.

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