 Leg 3 Logbook - Gas Hydrates
Day 10 — Pushing our luck at Bubbly Gulch
August 11, 2009 Latitude 48 degrees 40.26 minutes N 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.
—Kim Fulton-Bennett
|
Track the ship
Equipment
 R/V Western FlyerThe R/V Western Flyer is a small water-plane area twin hull (SWATH) oceanographic research vessel measuring 35.6 meters long and 16.2 meters wide. It was designed and constructed for MBARI to serve as the support vessel for ROV operations. Her missions include the Monterey Bay as well as extended cruises to Hawaii, Gulf of California and the Pacific Northwest.
ROV Doc Ricketts is MBARI's next generation ROV. The system breaks new ground in providing an integrated unmanned submersible research platform, with many powerful features providing efficient, reliable and precise sampling and data collection in a wide range of missions.
R/V Zephyr is the primary support vessel for MBARI's autonomous underwater vehicle (AUV) program. This 26-meter vessel is also used to maintain environmental moorings, collect time-series data along the California Current, and support scuba divers as they study near-shore habitats.
The MBARI Mapping AUV is a torpedo-shaped vehicle equipped with four mapping sonars that operate simultaneously during a mission. The multibeam sonar produces high-resolution bathymetry (analogous to topography on land), the sidescan sonars produce imagery based on the intensity of the sound energy's reflections, and the subbottom profiler penetrates sediments on the seafloor, allowing the detection of layers within the sediments, faults, and depth to the basement rock.
A push core looks like a clear plastic tube with a rubber handle on one end. Just as its name implies, the push core is pushed down into loose sediment using ROV Tiburon's manipulator arm. As the sediment fills up the core, water exits out the top through one-way valves. When the core is pulled up again, these valves close, which (most of the time) keeps the sediment from sliding out of the core tube. When we bring these cores back to the surface, we typically look for living animals and organic material in the sediments.
The benthic elevator allows us to carry more than the ROV itself can carry. Loaded with sediment enrichers, it is deployed from the ship before the dive and free-falls to the bottom where the ROV pulls the equipment from the elevator for use. After the ROV is recovered, the elevator anchor's acoustic release is triggered from the ship, and the elevator freely ascends to the surface and is recovered.
Niskin bottles are used to collect water samples as well as the tiny bacteria and plankton in the water. The caps at both ends are open until the bottles are tripped, when the caps snap closed.
Held by the ROV's manipulator, the wire on the right is placed into the fluid emitted from a hydrothermal vent to record the temperature. Research Team
Charlie Paull has been a marine geologist and geochemical stratigrapher at MBARI since January 1999. The central theme of Charlie's work involves investigating the fluxes of fluids and gases through continental margins. Over the past decade his primary focus has been gas hydrate research on the Blake Ridge gas hydrate field on the continental rise off of southeastern North America. Assessing the global distribution of gas hydrate and interstitial gas is a continuing interest as well as the development of new techniques to detect the presence of gas hydrate in marine sediments. Charlie's other ongoing work is focused on the geology associated with seafloor seepage sites, including investigating the deposits associated with chemosynthetic communities, determining the processes that occur at the methane-sulfate boundary, and understanding the origin of pockmarks and other potential seafloor fluid venting sites.
During expeditions, Bill Ussler is primarily responsibility for the operation of the custom-built, portable chemistry lab van which contains a complete analytical laboratory for the analysis of the fluids and gases contained in marine sediments. Along with colleague Charlie Paull, Bill studies how methane (natural gas) forms and moves within seafloor sediments.
Michael Riedel was part of an international team of scientists supported by the Integrated Ocean Drilling Program (IODP) which completed a unique research expedition in 2005 aimed at recovering samples of gas hydrate, an ice-like substance hidden beneath the seafloor off Canada's western coast. As IODP Expedition 311's co-chief scientist, Michael explored his interest in gas hydrate; he believes such deposits have played an important role in ancient global climate change.
Ross's research interests are in seismo-acoustic propagation, with specific application to the study of marine gas hydrates, and development and application of acoustic inverse methods for estimation of geophysical properties of the ocean bottom and for source localization. (Note: At the last minute Ross was unable to participate in the cruise, although he did attend the initial science meeting before the ship left the dock.)
As a member of the USGS Coastal and Marine Geology Program, Mary McGann's professional interests include: foraminiferal and pollen biostratigraphy, paleoecology and biogeography; sedimentary paleoenvironment mapping, quaternary paleoclimatology; and AMS C-14 chronostratigraphy.
Laura's research is concentrated on studying methane cycling at cold seeps, biogeochemcial cycling of methane and sulfer in deep sea sediments, development of deep sea instrumentation to collect novel samples, stable isotope geochemistry, modeling of biogeochemical processes and temporal variability of dissolved methane concentrations. The focus of her research has been mainly on gas hydrate environments, but she is also interested in other systems that relate to the carbon cycle. Her research seeks to understand how methane is distributed between different pools, e.g. dissolved or hydrate phases, and also to understand how local biogeochemical processes affect this methane, mostly through anaerobic methane oxidation.
Kim helps people outside of the institute to understand MBARI's research and development efforts. He does this by writing news releases and articles about MBARI research, as well as by helping members of the press who want to write their own articles or create video stories about MBARI. His academic background is in marine geology, environmental planning, and science writing.
Yirang is an undergraduate at UC Davis in Environmental Studies and Ecological Engineering. She is an exchange student from Korea University who is interested in methane hydrates as an alternative energy source and in the ecological communities around methane seeps. She is very happy to have the opportunity to go on this cruise.
|
