"This is the first evidence of omega aragonite undersaturation in deep basin surface waters," Fiona McLaughlin of the Institute of Ocean Sciences, Canada, told environmentalresearchweb. "In a 2009 publication models predicted that the surface waters might be undersaturated in the Arctic within a decade. We're making those observations now, because the ice has melted so fast. Essentially the papers are almost being written at the same time."

Omega is a measure of the saturation state of calcium carbonate; values below one indicate that the waters will dissolve the mineral while values greater than one indicate favourable conditions for forming shells and skeletons.

Oceans around the globe are becoming more acidic as they absorb some of the carbon dioxide that man has emitted into the atmosphere. The Arctic Ocean is also experiencing an indirect effect. "The sea ice has melted and the meltwater is very low in alkalinity and dissolved inorganic carbon, two of the anions that contribute to pH and also to this omega aragonite," explained McLaughlin. In 2007 the extent of Arctic ice was the lowest on record.

Together with colleagues from the Institute of Ocean Sciences, the Japan Agency for Marine-Earth Science and Technology, and Tokyo University of Marine Science and Technology, McLaughlin has monitored conductivity, temperature and depth at four locations in the Canada Basin region of the Arctic Ocean from 2002 to 2008. In 2007, when the team realised that the omega ratio was near one, it decided to do aerial sampling the following year to discover the spatial extent of any undersaturation.

In 1997, the surface waters had an aragonite omega reading of 1.4, leading to "no problem at all to organisms." But in 2008 the level was 1 due to the influx of sea-ice meltwater. "So organisms that live in the upper part of the water column, such as larva of pterapods, are at risk," said McLaughlin.

That's not the only effect of the sea-ice melt. "Because of the retreat of the sea ice so far away from the edges of the shelf, now winds can be at work – much like they are on the west coast of Canada – to bring the water that's at 150 m and quite undersaturated up onto the shelf, and affect benthic organisms like clams and molluscs."

As a result two parts of the water column are affected. "We've got undersaturation in the surface in a large area of the Canada Basin, and then on the shelf we've got potential for upwelling of this undersaturated water," said McLaughlin.

This has implications for the food web. "Organisms that have calcium carbonate in their shells will be at risk, things like pterapods and forams," said McLaughlin. "Certainly laboratory studies exposing organisms to waters of different pH show that this puts them at risk. Now we have evidence that those conditions exist."

Because the Arctic food web is quite simple and short, it could be extremely vulnerable to such changes. But McLaughlin says that it takes time to see how populations are decreasing. "I think this has identified that we need to go out and make counts and do a time series so that we can see whether there are effects and what these organisms' tolerance is," she explained.

Future outlook

The researchers' latest data, which covers a larger area, indicates that the region of undersaturation is even bigger than shown in their paper.

"As there is still ice to melt in the Arctic, this region of undersaturation will continue to grow, I think," said McLaughlin. "It's hard to even say this, but once the permanent icepack melts in summer, that will stop the input of meltwater."

What's more, McLaughlin says that within a decade these low-aragonite surface waters will be leaving the Arctic and entering the North Atlantic. That means the phenomenon could affect a much larger area, providing an additional source of undersaturation on top of the ocean acidification that is already happening.
 
McLaughlin and colleagues have funding for another five years of surveys in the Canada Basin. "These time series are incredibly powerful in being able to identify change," she said. "We've been doing these measurements in the Arctic since the late 1980s. We're fortunate that the ice has retreated so much because it allows us to survey such a large area in 4–6 weeks; the downside is that the ice is disappearing and becoming thinner. It's an interesting time as a researcher but as a person living on the planet it's more worrisome. The planet is changing much more rapidly than anyone had thought."

The researchers reported their work in Science.