NW Eifuku: Submarine Volcanoes and Global CO₂ Increase

May 8, 2006

David Butterfield
Chemical Oceanographer
Joint Institute for the Study of the Atmosphere and Ocean Program, University of Washington
NOAA Vents Program, Pacific Marine Environmental Laboratory – Seattle, Washington

The CO₂ Champagne vents at NW Eifuku. (Quicktime, 1.3 Mb.)

Mussels and shrimp colonies at NW Eifuku. (Quicktime, 1.6 Mb.)

Scientists study the potential impact of increased CO₂ levels in the marine environment. (Quicktime, 2.3 Mb.)

NW Eifuku, a relatively small and deep volcano by the standards of the Mariana arc, packs a chemical impact way beyond its size. It takes us into a pressure/temperature space where carbon dioxide (CO₂) can exist as a liquid. CO₂ is that newsworthy, global-warming molecule that we breathe and pump into the atmosphere at a phenomenal rate by burning fossil fuels. CO₂ is a gas at room temperature and normal atmospheric pressure; and most people are familiar with dry ice, the solid form of carbon dioxide that quickly evaporates at room temperature. But, in order to observe the liquid form of carbon dioxide, you have to go to an environment of high pressure and cool temperature; in other words, the deep ocean environment.

Six gallons of CO₂ gas at sea level will compress to about 3 tablespoons of CO₂ liquid in the deep ocean. Liquid CO₂ remains intact in droplets and does not mix with water, but at temperatures below about 10°C (50°F), and elevated pressure, water and CO₂ form an ice-like hydrate. The physical/chemical properties of CO₂ have given some people the idea that the deep ocean can be used as a place to get rid of the excess atmospheric CO₂ from fossil fuel burning.

NW Eifuku is one of two places on earth where liquid CO₂ has been observed — the other is the Okinawa Trough. About 60 m below the summit of NW Eifuku (summit depth is 1,535 m, or more than 5,000 ft), is an area of continuous venting of grape-sized droplets of liquid CO₂ with a CO₂ -hydrate "skin." From the carbon (and helium) isotope signature of the droplets, we can tell that there is a large component (about 90%) of ancient marine carbonate in the gas. This carbonate had to be subducted, then incorporated into magma (fluid that forms igneous rock, when cooled) that feeds NW Eifuku volcano, then "degassed" and cooled to form the CO₂ -rich vents. The droplets come out of small cracks and crevices in the sea floor and rise up spinning and deformed through the water. Based on visual tracking of the droplets and properties measured in the water above the volcano, the droplets appear to dissolve over approximately 200 m of vertical rise. CO₂ is an acidic gas and lowers the pH of seawater when it dissolves, favoring calcium carbonate dissolution.

One of the scientific goals on this expedition is to study the effect of lowered pH from volcanic CO₂ emission on the abundant mussel communities that live on rocky ridges near the acid-producing vents. Click image for larger view and image credit.

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This is one of three sites where we sampled mussels and the surrounding water to understand the volcanic habitat. The shells of dead mussels dissolve very quickly, but the community maintains a high density. Squat lobster and shrimp are also abundant at NW Eifuku. Click image for larger view and image credit.

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