September 7, 2013
After another weather delay, work resumed today in the Eel River Canyon area, about 30 kilometers offshore of Eureka, California. Within minutes of the remotely operated vehicle’s arrival on the seafloor at a depth of 1,800 meters, we saw solid methane hydrate poking out of the sediment and small plumes of bubbles rising from the seafloor—just what the team needed to continue studying the behavior and composition of greenhouse gases in the ocean. Today they wanted to test whether gases dissolved within deep-sea oil deposits dissipate before the oil makes it to the surface or whether some of these gases might make it to the surface and enter the atmosphere.
The ROV was outfitted with the laser Raman spectrometer to decipher which gases, if any, were present in the sample. The ROV pilot held a glass tube over the oil drops, which were fluorescent in the ROV lights, until the top couple of inches of the tube were filled. The spectrometer’s laser was trained on the sample, but the fluorescence of the oil itself interfered with the signal and it was not possible to get a reading as to whether there were dissolved gases in the sample.
It turned out that the best way to find out if there were gases dissolved in the oil was to bring the sample up from depth. Any gases dissolved in the oil at depth, where the water is cold and the pressure is great, generally separate out at shallower depths as the water warms and the pressure eases. With the ROV’s high-definition camera focused on the sample in the glass tube the entire way up, it was evident to the naked eye that a big layer of gas was forming just above the oil.
To get quantitative data, the laser Raman spectrometer was used on the sample at three different depths on the way to the surface. The chemistry team—Peter Brewer, Ed Peltzer, and Peter Walz—were able to see immediately in the resulting graph that a significant amount of methane was present, but also other gases including carbon dioxide, nitrogen, ethane, and propane. Their next step will be to fully process the data so they more carefully analyze what was present and in what quantities.
What the team learns from such studies can shed light on the impacts of both natural and accidental discharges of oil from the deep sea.
— Nancy Barr