Deep CTD Cast, Part 3. What do we do with all this water? Ocean acidification in the Arctic

Date: August 25, 2010
Time: 1500 hours Pacific Daylight Time
Latitude: 81°49.16’N
Longitude: 135°20.50’W
Air temperature: 1.1°C (34°F)
Sea temperature: -1.7°C (29°F)
Wind speed and direction: 16 knots from the south-southwest
Ship’s speed over the ground: 6.3 knots
Water depth: 3,703 m

On August 21, we lowered a rosette of water-sampling bottles to 3,750-m water depth, nearly to the floor of Canada Basin in the Beaufort Sea. When we winched it back up, it contained 288 liters (about 75 gallons) of seawater, more than enough to keep the ocean acidification team aboard Healy busy for several hours.

USGS scientist Chris DuFore, University of South Florida (USF) research associate Xuewu (Sherwood) Liu, and USF graduate student Mark Patsavas are collecting and analyzing water samples throughout the cruise to study the carbon dioxide (CO2) system in the Arctic Ocean, with a particular emphasis on ocean acidification.

The concentration of CO2 in the atmosphere has been steadily increasing, primarily due to the combustion of fossil fuels. Simultaneously, the waters of the oceans are removing some of this excess CO2 through natural geochemical processes. As a result, the chemistry of the ocean is changing, causing a decrease in the pH of surface waters. This “ocean acidification” reduces the availability of carbonate ions (CO32-), which are important for building the shells and skeletons of calcifying marine organisms.

To study the CO2 system in the Arctic Ocean, Chris, Sherwood, and Mark are collecting and analyzing seawater throughout the mission, both while Healy is underway and while on station. Their aim is to document the carbonate chemistry of the Arctic waters and, among other things, to determine the waters’ saturation state with respect to calcium carbonate.

For underway measurements, they are using a specially designed instrument called a Multiparameter Inorganic Carbon Analyzer (MICA) developed at the University of South Florida, which continually measures partial pressure of CO2 (pCO2), pH, and total dissolved inorganic carbon in seawater. The MICA analyzes seawater from a flow-through system built into Healy that takes in seawater and circulates it through the lab before discharging it back into the ocean. Additional sensors built into Healy’s flow-through system measure temperature, conductivity, dissolved oxygen concentration, blue-green algae cell count, and chlorophyll concentration. Additionally, a LICOR gas analyzer, located at the bow of the ship, is collecting air samples to measure pCO2 in the atmosphere. This will allow researchers to compare CO2 levels in the water and the air to determine the storage capacity of the Arctic Ocean for atmospheric CO2.

Every 2 hours, the scientists collect discrete seawater samples from the flow-through system and use two bench-top spectrometers to measure pH and carbonate ion (CO32-) concentrations while onboard, using methods developed in the USF laboratory of Dr. Robert Byrne. Each of these measurements provides important information to characterize the entire seawater carbon dioxide system. Knowing the CO32- concentration of seawater provides immediate information relevant to the saturation state with respect to aragonite and calcite, the predominant minerals that compose the shells and skeletons of calcifying organisms.

Occasionally the ship stops to deploy the rosette, giving the geochemists the opportunity to collect seawater samples throughout the water column. Commonly it takes Chris, Sherwood, and Mark 3 to 4 hours to take subsamples from the Niskin bottles for a range of analyses. The subsampling went much faster on August 21, when they had the help of the two teachers aboard Healy, PolarTREC teacher Bill Schmoker and NOAA Teacher at Sea Caroline Singler. Here’s a photo account of the subsampling process: visit the site to see.

The data collected aboard Healy combined with the data from onshore analyses of samples shipped to St. Petersburg will provide comprehensive integrated information on the chemistry of Arctic waters. This study is the first of its kind to be conducted during the U.S.-Canada joint mapping missions.

Helen Gibbons, ECS Project. 25 August 2010. Article.

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