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10 December 2001 American Geophysical Union Fall Meeting
Seafloor laboratories yield new insights for SAN FRANCISCO—This week at the American Geophysical Union Fall Meeting, scientists from the Monterey Bay Aquarium Research Institute will present new results from recent field experiments aimed at understanding the dynamics and impact of carbon sequestration in the deep ocean. Their experiments—involving gas hydrates, larger volume carbon dioxide injections, and studies of the seafloor animal community—show the success of MBARI's efforts to push the capabilities of deep-sea science and technology, as envisioned by founder David Packard.
"MBARI has demonstrated a novel ability to set up complex laboratories on the seafloor with fluids, valves, pumps, cameras, and sensors, " said ocean chemist Peter Brewer, lead scientist of MBARI's carbon sequestration studies. "We can now conduct careful experiments with remarkable precision in remote parts of the ocean." Carbon dioxide and methane, another important greenhouse gas, form ice-like material called hydrate when chilled to a certain temperature and put under pressure. Gas hydrates occur naturally in locations in the deep sea and represent a large potential energy deposit. The material itself is difficult to study, however, as it decomposes to water and gas when samples are brought to the surface. In February this year, MBARI scientists and colleagues from the Lawrence Livermore National Laboratory and the U.S. Geological Survey completed a set of novel in situ experiments aimed at measuring the dissolution rates of carbon dioxide and methane hydrates in the deep sea where, although the pressure and temperature conditions are met, the water is chemically undersaturated. Their results are critical for physical models of deep-sea carbon sequestration strategies and for knowledge of the fate of methane hydrates exposed on the seafloor. In the experiment, the researchers used ROV Ventana to place a custom-made rack on the seafloor at 1,000 meters depth and deposit laboratory-produced samples of methane and carbon dioxide hydrate. The ROV's high-resolution video camera imaged the samples for about two hours, and a time-lapse video system provided an additional 20 hours of observations. The scientists calculated the dissolution rate of the material by measuring the change in diameter of the samples over time. The experiments showed that the carbon dioxide hydrate samples dissolved more quickly than the methane hydrate samples, with a rate of 13-17 mole CO 2/m2/h for the carbon dioxide hydrate, and 1.2-1.4 mole CH4/m2/h for the methane hydrate. By the second day of the experiment, only the methane hydrate remained. The researchers concluded that the fast dissolution rate of the carbon dioxide hydrate was related to its very high solubility in sea water—10.5 times that of methane.MBARI's groundbreaking in situ carbon sequestration experiments in 1998 were limited to small amounts (< 10 L ) of liquid carbon dioxide. This summer, Brewer's team deployed and used a new device that allowed them to inject 20-30 L of liquid carbon dioxide in a set of experiments at 3,600 meters depth. Using ROV Tiburon, the liquid CO2 samples were placed into special corrals on the seafloor designed for chemical observations and for studies of the biological impacts. Returning to the experiment site 24 hours later, the researchers were surprised to find that the liquid carbon dioxide in one corral had penetrated the sediment and formed into hydrate. The second corral, only 20 meters away and receiving a replicate treatment, had the predicted result that the CO 2 remained as liquid blobs in the corral. Even though the transitional stages were radically different, the material in both corrals dissolved at roughly the same rate based on time-lapse camera measurements of the experiment.Collaborating with Brewer and colleagues during the summer experiments, MBARI ecologist Jim Barry led efforts to evaluate the biological responses of representative deep-sea organisms to changes in seawater chemistry caused by carbon sequestration. They investigated survival rates and the physiological condition of a common sea urchin and sea cucumber at experimental and control sites. In the experimental corrals, they measured a lower abundance of worms and crustaceans in the sediments over a period of five weeks. Another result was that the lowered pH of the surrounding seawater caused decalcification of sea urchin spines. The group is now looking at the response by the sediment-dwelling meiofauna, microbial community, and mobile scavenger fishes. These studies are aimed at calculating realistic estimates of the impacts of carbon sequestration on deep-sea ecosystems. Related presentations: Geomicrobiology and Biogeochemistry of Gas Hydrate Systems I
Biogeophysics of Global Warming Mitigation
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