by Lisa Robbins (USGS) and Jonathan Wynn (University of South Florida)

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Acidification of the Arctic Ocean is occurring faster than projected, according to new findings published in September 2013 in the journal PLOS ONE (“Baseline Monitoring of the Western Arctic Ocean Estimates 20% of Canadian Basin Surface Waters Are Undersaturated with Respect to Aragonite”). The increase in rate is being blamed on rapidly melting sea ice, a process that may have important consequences for health of the Arctic ecosystem.

Ocean acidification is the process by which pH levels of seawater decrease because of greater amounts of carbon dioxide (CO₂), an important greenhouse gas, being absorbed by the oceans from the atmosphere. Currently, oceans absorb about one-fourth of the CO₂ released into the atmosphere each year. Lower pH levels make water more acidic, and lab studies have shown that more acidic water decreases calcification rates in marine organisms that build shells or skeletons. These changes, in organisms ranging from corals to shrimp, have the potential to impact species throughout the food web.

A team of Federal and university researchers found that the decline of sea ice in the Arctic summer has important consequences for the surface layer of the Arctic Ocean. As sea-ice cover recedes to record lows, as it did late in the summer of 2012, the seawater beneath is exposed to atmospheric CO₂, which is the main driver of ocean acidification. In addition, the freshwater melted from sea ice dilutes the seawater, lowering pH levels still further and reducing the concentrations of calcium and carbonate ions, which are the constituents, or building blocks, of the mineral aragonite. All of these chemical changes lower the “saturation state” of seawater with respect to aragonite. This chemical index is defined in such a way that if the saturation state for a given mineral is greater than 1, the seawater is “supersaturated” and the mineral will not readily dissolve; but if the saturation state is less than 1, the seawater is “undersaturated” and the mineral will dissolve. Aragonite and other carbonate minerals make up the hard part of many marine microorganisms’ skeletons and shells. The lowering of saturation states for these minerals may impact the growth of such organisms and the many species that rely on them for food.

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Globally, Earth’s ocean surface is becoming acidified by the absorption of manmade CO₂. Ocean acidification models project that with increasing atmospheric CO₂, the Arctic Ocean will become undersaturated with respect to carbonate minerals in the next decade. In the recently published PLOS ONE paper, Robbins and the ocean acidification team members show that acidification in surface waters of the Arctic Ocean is happening faster than expected and is rapidly expanding into areas that were previously isolated from contact with the atmosphere by the former widespread ice cover. Already, approximately 20 percent of the Canada Basin, a deep basin in the Arctic Ocean, has become undersaturated with respect to aragonite in a time frame faster than models predicted. In fact, the new data indicate that undersaturation with respect to aragonite is occurring 30 times faster in the Arctic than in the Pacific Ocean. Nowhere else on Earth has such large-scale, rapid ocean acidification been documented.

The rapidity of ocean acidification in the Arctic is a result of climate change accelerating the melting of multiyear sea ice (ice that has survived more than one melting season) and the subsequent dilution of seawater. Not only is the ice cover removed, leaving the surface water exposed to manmade CO₂, the surface layer of frigid waters is now fresher, and this means that lower concentrations of calcium and carbonate ions are available for organisms. Researchers measured oxygen isotopes in the water to distinguish seawater, meltwater, and river runoff. This critical piece of information was used to document the extent of the meltwater: more than 370,000 square kilometers (140,000 square miles), an area the size of the State of Montana. The oxygen isotope data show that the freshwater from sea-ice melt is most closely linked to zones of carbonate mineral undersaturation.

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Compared with other oceans, the Arctic Ocean has been rather lightly sampled. The remoteness and extreme temperatures make it a challenging place to work and require instruments adapted to the harsh climate. The ocean acidification team was fortunate to work aboard the U.S. Coast Guard Cutter Healy, a polar icebreaker designed to support scientific research. The team investigated the chemistry of seawater continuously collected by the ship’s flow-through system, which takes in water from a port in the hull about 8 meters (26 feet) below the waterline. They were able to use new automated instruments that provided high spatial resolution by automatically analyzing seawater continuously every 2–7 minutes while the Healy was underway. The team also analyzed samples collected from the flow-through system at discrete locations. More than 34,000 water-chemistry measurements were made during 3 years of research cruises on the Healy in the Arctic Ocean.

“This unusually large data set, in combination with earlier studies, not only documents remarkable changes in Arctic seawater chemistry but also provides a much-needed baseline against which future measurements can be compared,” said Robert Byrne, a marine chemist at the University of South Florida (USF). He credits scientists and engineers at the USF College of Marine Science with developing much of the new underway technology, such as the Multiparameter Inorganic Carbon Analyzer (MICA). The MICA is capable of simultaneously analyzing three chemical parameters in seawater collected while a vessel is underway: pH, total dissolved inorganic carbon, and partial pressure of CO₂ (a measure of its content in the water, abbreviated pCO₂).

On the Healy, the ocean acidification researchers worked alongside other researchers involved in joint U.S.-Canada efforts to map the seafloor as part of the U.S. Extended Continental Shelf (ECS) Project. The ocean acidification research was funded by the USGS, the National Science Foundation, and the National Oceanic and Atmospheric Administration. 

Data sets for the 2010 and 2011 cruises are posted online.

The full citation for the recent report is:

Robbins L.L., Wynn, J.G., Lisle, J.T., Yates, K.K., Knorr, P.O., Byrne, R.H., Liu, X., Patsavas, M.C., Azetsu-Scott, K., and Takahashi, T., 2013, Baseline monitoring of the western Arctic Ocean estimates 20% of Canadian Basin surface waters are undersaturated with respect to aragonite: PLOS ONE, v. 8, no. 9, e73796, doi:10.1371/journal.pone.0073796.