Scientists study Deep-Sea Disposal of Fossil-Fuel CO2

Carbon dioxide injected deep in the ocean may form an ice-like sheet that would remain stable on the seabed, according to the results of experiments published in the May 7 issue of the journal Science.

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Since the oceans naturally absorb carbon dioxide, one possibility for disposing the human-generated excess is to directly put it into the deep ocean, bypassing the atmosphere. Ocean chemist Peter Brewer and his colleagues at the Monterey Bay Aquarium Research Institute (MBARI) and Stanford University designed and executed some of the first experiments to test theoretical predictions about the behaviour of liquid CO2, the form that would be used for direct disposal, in the cold, extremely high-pressure environment of the deep sea.

Under cool temperatures and high pressures, carbon dioxide and other greenhouse gases react with water to form a solid ice-like compound called clathrate hydrate. At shallow depths liquid carbon dioxide will rise to the surface. But based on laboratory experiments with CO2 hydrates, researchers imagined that liquid carbon dioxide put deep in the 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 on 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. At these shallow depths, liquid CO2 with a skin of hydrate is less dense than seawater and will rise toward the surface and back into the atmosphere.

Brewer’s most recent work, conducted last summer, concentrated on the behaviour of liquid carbon dioxide at much greater depths where it is denser than seawater. In these experiments, MBARI researchers used the institute’s ROV Tiburon to inject several litres 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 was highly reactive with the surrounding seawater, significantly increasing in volume within 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 carried easily away by the currents. “Nothing like this was predicted,” Brewer emphasized.

Brewer and colleagues will conduct subsequent experiments this summer to test new hypotheses generated from previous results. They also have begun collaborations with MBARI ecologists to study the possible effects of liquid CO2 on deep-sea organisms.

Ocean disposal for excess atmospheric carbon dioxide is gaining interest internationally with the recent adoption of the 1997 Kyoto Protocols to the United Nations Framework Convention on Climate Change. The enormous natural carbon buffering system of the ocean suggests that the deep ocean could absorb the amount of carbon that would cause a doubling in atmospheric concentrations and change its concentration by only two percent.

Brewer is optimistic that ocean disposal, while expensive, is a safe and viable option for deterring the harmful build-up of atmospheric carbon dioxide.

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