April 27

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April 27th, 2003; Leg 5, Day #7

R/V Western Flyer transited overnight to a basin in the northwest of the Gulf.

The unique topography of the area helps create a warm, deep-water layer. Today ROV Tiburon�s CTD sensor measured temperatures of nearly 12 degrees Celsius at 800 meters depth. By this depth in Monterey Bay, the water is a chilly 4 degrees C.

These unique hydrographic properties provided Peter Brewer and Ed Peltzer a different temperature regime in which to replicate their liquid carbon dioxide dissolution experiments with Tiburon. The experiments use the high-resolution video capabilities of MBARI�s ROVs and a specially designed �bubble box� to image changes in gas samples as the ROV moves vertically in the water column.

Past experiments using this device on ROVs Ventana and Tiburon have yielded new insights into the dissolution and ascent rate of greenhouse gases and gas hydrates. These measurements are important for climate change models and for predictions of how release of gas material in the deep ocean�for example, by an underwater landslide that exposes a gas hydrate field�may impact the upper ocean and atmosphere. In Monterey Bay experiments, the cool water temperatures cause liquid carbon dioxide droplets to form a hydrate �skin� which slows down the dissolution rate. Brewer and Peltzer predicted that in warm water conditions, the liquid carbon dioxide droplets would dissolve and rise more quickly.

Today, local currents were too strong at the first dive site. The ship moved northward to a second site where Tiburon was put in the water again, diving down to 800 meters. Liquid carbon dioxide droplets were released into the bubble box at depth and were videotaped as the ROV ascended. The warm water and currents made the experiments more difficult to conduct than in Monterey Bay. The bubble box�s location on Tiburon�s frame also added extra turbulence which made it more challenging for the ROV pilots to track the droplets. By day�s end, however, Brewer and Peltzer were satisfied that the necessary data had been collected. Preliminary results support their prediction that dissolution and rise rates would be more rapid in a warm water environment.

Now the ship is heading south again to the Guaymas Basin, back to the transform fault area that we studied previously. The next two days will involve tests of the laser Raman spectrometer at a gas venting site. The ship transits have given us a chance to see new scenery, both day and night. Last night we saw an incredible display of bioluminescence in the water in front of the ship�s bow. Schools of fish of varying sizes could be discerned in vivid blue-green hue as they encountered the bioluminescent plankton in the surface waters.

� Debbie Meyer

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