Leg 3 Logbook - Gas Hydrates
Day 10 — Pushing our luck at Bubbly Gulch
August 11, 2009
Latitude 48 degrees 40.26 minutes N
Longitude 128 degrees 50.20 minutes W
This morning we made our last ROV dive of the cruise. We knew we had to begin steaming south by 11:30 a.m., but Charlie wanted to fit in one more dive. His slogan for this dive was “Go for the gas.” We returned to the fault zone east of Bullseye Vent, which Charlie has nicknamed “Bubbly Gulch.” This is where we created the geyser of methane bubbles when we tried to take a push core (see the log entry for August 8 for details).
There is a saying about observations in science, “Once is interesting, twice is a coincidence, but three times is a trend.” Our goal for this dive was to create and observe at least two more gas eruptions. As before, we descended and looked around for gas plumes, which show up as ghostly blue trails on the ROV’s sonar screen. We saw some plumes on the way down, but lost them by the time we got to the seafloor.
After wending our way through the gardens of sea pens that cover the flat mud in this area, we found some familiar-looking cracks in the seafloor, marked by white mats of bacteria. We followed these cracks until we came to the area of low mounds we had explored previously. We searched for our first “victim.” Charlie decided to look for an area where the seafloor was not cracked, on the theory that cracks would let the gas escape, so there wouldn’t be much left below the surface. Eventually we found a slight bulge in the seafloor covered with dense bacterial mats and a few tubeworms. We inserted the core, pulled it out, and… no bubbles! (though we did get a nice long core). We were 0 for 1.
Next we tried a small mound with a nice bacterial mat on top, and lots of cracks. We landed right on top of the mound and, like a giant mosquito, inserted our core tube. At first the tube went in easily, then it stopped. We kept vibrating the core tube until it started moving again. At that point, bubbles began to pour out the sides of the core tube. Things seemed to be heating up, but when we pulled out the core tube, the bubbling essentially stopped. Perhaps some mud fell back into the core hole and blocked the flow of gas. But at least we felt like we were on the right track.
These two photos show the largest methane geyser we created today. Initially the gas pulsed out of the core hole, apparently under considerable pressure. After about an hour, however, the flow had subsided, and we could see individual bubbles rising into the water. These observations will help us estimate how much methane was stored just beneath the surface of this low mound.
For our next core, we landed directly over a crack in the seafloor, inserted the core, and hit pay dirt (or at least lots of gas). Bubbles swirled and pulsed up along the core tube (and probably ruined the core in the process). After we pulled the core out of the hole, we watched our geyser for perhaps 10 minutes, trying to figure out how much gas was coming out. It was a beautiful and captivating sight. Then, having won two out of three rounds (and our goal achieved) we moved on to our next victim.
The third core was from an area where lots of little streams of bubbles trickled out of a flat muddy plain. We figured we were sure to get gas here. But when we pulled out the core tube, we created just one more little stream of bubbles. We collected our last core on another low mound. This time, the core hit something hard less than a meter below the surface, and no gas came out. This last case notwithstanding, it appeared that substantial pockets of methane gas were most common where the seafloor had been pushed upward into mound-like structures.
Clouds of methane bubbles enveloped the vibracoring system on the ROV as we pushed into our third core of the day. As they rose, some of the bubbles combined with the surrounding seawater to form small chunks of white methane hydrate.
Our five vibracore tubes were all gone and our time was running out, so we went back to see how our little geyser-of-the day was faring. After about an hour, it had settled down to a just slow trickle of gas. We landed the ROV next to the core hole and watched for several minutes as bubbles popped out of the hole, one by one, and wiggled off up into the water column. Everyone in the control room was quiet, perhaps savoring the moment, thinking about all the amazing things we had seen in the last week, or just wondering what was for lunch. Breaking the silence, I asked “Well, Charlie, are you happy?” “I think at this point,” He replied, “I’m just tired.”
I don’t think anyone in the room would have disagreed with Charlie’s statement. But we still had a lot of work to do. We finished up the dive with a last transit across the cracked mounds and tumuli of “Bubble Gulch,” passed by our old friend, “Shannon” the whale carcass, and checked up one last time on Laura’s osmosampler (see Aug 7 log). Then, as the music swelled and the credits rolled, the pilots turned on the thrusters and the Doc Ricketts rose slowly back toward the surface.
Back on board, the real work began. All the samples from the trip had to be sorted, numbered, cataloged, and then stowed away safely for the trip back to MBARI. We had collected about two dozen rocks, four dozen push cores, and an equal number of vibracores. Most of the cores had been subsampled into hundreds of vials and bags.
Mary McGann sorts through vials of sediment samples that will be analyzed in laboratories back on shore. We spent most of this afternoon cataloging and packaging up hundreds of such samples.
Back on shore, these subsamples will be tested for Carbon 14, lead isotopes, and DDT, which will help us figure out how long ago the sediment was deposited. With the same goal in mind, Mary McGann will be picking through the sediment and counting species of microscopic marine animals called foraminifera. The cores will be subjected to x-rays, magnetic fields, and other high-tech analytical tools. Bill Ussler has already started extracting pore-water and methane gas from the sediment samples, creating yet more ampoules and vials for later study. The cruise may be almost over, but work on all the samples will continue for months, or even years.
I caught up with Charlie late this afternoon in the now-empty control room, as he worked over his notes and pondered the sonar maps of the areas we’d visited. I asked if the expedition had led to any great new insights into carbonate formation and methane gas deposits. He replied, “At this point, we have a lot of great observations, and a lot of ideas, but not a whole lot of data.”
Some of that data will trickle in as Charlie and the other researchers study the cores and the rock, water, and gas samples we collected. I have no doubt that a number of exciting and perhaps controversial scientific papers will come out of this cruise. But as always in science, by answering a few questions, we inevitably bring up half a dozen more. That’s what keeps it interesting.
The ROV pilots watch through their monitors in the control room as ROV Doc Ricketts is pulled up out of the water for the last time on this cruise. Although we scientists will go home in a few days, the ROV pilots and crew of the Western Flyer will head back out to sea for four more weeks of work in the Pacific Northwest.