Monterey Bay Aquarium Research Institute
Celebrating our 20th Anniversary

Studying carbon dioxide in the deep sea

Celebrating 20 years

The burning of fossil fuels to create the energy our society depends on releases excess carbon dioxide (CO2) into the atmosphere where it, and other so-called "greenhouse gases," contribute to global warming. In hopes of isolating CO2 from the atmosphere, a blue-ribbon panel of the President's Council of Advisors on Science and Technology, in a 1996 report to President Bill Clinton, recommended the sequestration of CO2 in the deep ocean as a hydrate. The environmental impacts of sequestration were not thought through in the council's report, since the assumption was that the hydrate form would be stable and reside permanently as an inert solid.

But MBARI ocean chemist Peter Brewer and his colleagues at Stanford University realized early on that the hydrate would dissolve and form a region of CO2-enriched/low-pH water. Brewer and MBARI biologist Jim Barry and their team created and executed the first biological impact experiments in this field. The team tested theoretical predictions about the behavior of liquid CO2—the form that would be used for direct disposal—in the cold, extremely high-pressure environment of the deep sea.

If liquid carbon dioxide is released into seawater within a few hundred meters of the surface, it will float up toward the sea surface. In deeper water (several thousand meters), where temperatures are near freezing and pressures are very high, carbon dioxide reacts with water to form a solid ice-like compound called a clathrate hydrate. Based on laboratory experiments with CO2 hydrates, researchers imagined that liquid carbon dioxide released in the deep ocean would form a stable layer on the seafloor with a skin of solid hydrate as a boundary, like a pond covered by ice in winter.

Brewer and his colleagues took such experiments out of the lab and into the ocean. Using special instruments carried by the institute’s remotely operated vehicle (ROV) Ventana, the researchers generated CO2 hydrate from gas and liquid at depths ranging from 350 meters to 1000 meters in Monterey Bay. Experiments at these “shallow” depths showed that liquid CO2 would form a skin of hydrate and will rise toward the surface.

Liquid CO<sub>2</sub> overflows beaker
Liquid CO2 overflows beaker in experiment conducted at a depth of 3600 meters.

Brewer then took the experiments into the deeper ocean, using the deep-diving ROV Tiburon to study the behavior of liquid carbon dioxide at much greater depths. In these experiments, ROV Tiburon injected a few liters of liquid CO2 into a glass beaker at a depth of 3600 meters. Tiburon’s video camera relayed surprising information back to the researchers—the liquid CO2 reacted with the surrounding seawater and expanded, significantly increasing in volume during the first hour of the experiment.

As water molecules combined with CO2 molecules, gas hydrate formed and accumulated at the bottom of the beaker. The expanding volume of hydrate plus remaining liquid CO2 caused globules of liquid CO2 to spill over the top of the beaker, where they bounced to the seafloor and were easily carried away by the currents. According to Brewer, "Nothing like this was predicted."

The researchers also observed that the dissolution of carbon dioxide into seawater increased its density so much that CO2-rich seawater penetrated into the sediments to form dramatic hydrate “frost heaves” in the sediment surface.

Photograph of carbon dioxide pool and associated experimental gear at 3,600 meter depth.

Since those early experiments, Brewer and colleagues have continued to break new ground with novel experiments using MBARI’s ROVs to study greenhouse gases. The team has developed new tools such as an underwater Laser Raman Spectrometer to obtain in situ measurements of chemical properties in the ocean and collaborated with ecologists to study the effects of liquid CO2 on deep-sea organisms.

Led by Jim Barry, a team of MBARI biologists has been evaluating the response of typical deep-sea organisms to the acidification of seawater that is expected to occur near CO2 release sites.Results thus far indicate that many deep-sea animals exposed to even modest reductions in pH (~0.2 units) do not survive.  Survival was higher for smaller changes in ocean acidity.  Surprisingly, some fishes held within one meter of CO2 pools survived month-long exposure.

Brewer’s continuing efforts have also yielded the first direct measurements of the dissolution rate of liquid carbon dioxide on the deep seafloor. This information can be used to predict the lifetime of carbon dioxide released in the deep ocean. .

MBARI’s studies of CO2 sequestration are a shining example of the challenge David Packard set for MBARI—to develop new research tools, deploy them in the deep-water lab of Monterey Bay, and work on questions that were important to society.

MBARI contributors to carbon sequestration project: Jim Barry, Peter Brewer, Rachel Dunk, Jon Erickson, Gernot Friederich, Gene Massion, Noriko Nakayama, Lynn Orr, Ed Peltzer, Gregor Rehder, Mario Tamburri, Peter Walz

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