 Leg 3 Logbook - Gas Hydrates
Day 8 — Sleuthing our way around
August 9, 2009 Latitude 48 degrees 42.50 minutes N Geology is like detective work. You make an initial guess about who dunnit, but the hard part is finding conclusive evidence. In many cases, like Sherlock Holmes, you have to collect many little bits of seemingly unimportant or unrelated information, and then use logic to put them together to build your case. This morning we tried to track down methane plumes from an area called the Spinnaker Vents, but were frustrated by conflicting information from our seafloor maps, the ROV’s compass, and our sonar display. During the afternoon we tried to hunt down a fault under the seafloor which we think may be geologically related to the Spinnaker. We found a few tantalizing clues and some circumstantial evidence, but no smoking gun (or in our case, no place where the fault was visible on the seafloor). The morning started pleasantly enough. I finally got up early enough to watch from the control room as the ROV descended through the water column. It was a fascinating way to wake up. A few dozen meters below the surface, the ROV passed through a swarm of diaphanous cross jellies. Farther down we passed long blue siphonophores, like gelatinous freight-trains, pulsing through the water. About 250 meters down, we descended through a layer of water filled with giant larvaceans, their meter-wide mucus nets like transparent balloons floating by. Somewhere around 500 meters below the surface, tiny, transparent taonid squids and long, thin, midnight-black fishes darted by. 
We explored many low mounds on the seafloor today. This one (about four meters across) had an impressive crater at its summit, which occasionally burped bubbles of methane.
After about 45 minutes of descent, the ROV reached the seafloor, about 1,300 meters below the surface, and we got down to business. Although previous researchers had observed methane plumes in this area, no one had actually found the vents that were emitting this methane. The detailed sonar data we collected last month showed a small group of irregular, overlapping pits in the seafloor in this area. We spent all morning exploring these pits, which turned out to be intermixed with small mounds and slabs of carbonate rock. We found the carbonate slabs first. They looked a lot like the eroded carbonate beach rock you see in tropical areas such as Hawaii or the Caribbean. Charlie believes that, like beach rock, these slabs formed beneath the sediment surface, and were then exposed when the surrounding sediment eroded away. However they were formed, these slabs of rock provided homes for lots of big red crabs and two-inch-long marine snails in the genus Neptunia. Many of the rock outcrops were covered with the six-inch-high egg towers of these snails, which looked like miniature castle turrets. 
Many of the carbonate rocks at Spinnaker Vent were inhabited by Neptunia snails, who found them convenient places to lay their six-inch-high towers of eggs. One snail is perched at the top of its tower, in the process of laying its eggs.
Elsewhere at the Spinnaker Vent we found what looked like small craters. Outside of each crater, the seafloor was covered with undisturbed mud, but inside the crater, we could see carbonate rocks exposed. Once again, it appeared that the carbonates had formed in a layer below the surface, but had later been exposed by erosion. These pits also provide homes for tubeworm colonies, methane-seep clams, and other critters. 
These vestimentiferan tubeworms often burrow into the seafloor in places where methane is seeping out of the sediment. In this case, they have grown among the carbonate rocks that may once have been buried, but are now exposed on the seafloor.
We also saw lots of mounds that reminded us of the ones we’d seen earlier near Bullseye Vent. They reminded me of prehistoric Celtic tumuli—small rounded hills a meter or two high and several meters across. Often their summits had cracked and broken open. Many were covered with tubeworms or methane-seep clams. At one site we saw bubbles coming out of the mound. First, we collected a push core and watched excitedly as methane hydrate formed inside the core tube. Next we decided to collect a vibracore. After yesterday’s exciting vibracore experience, everyone in the control room held their breath as we pulled out the core. But nary a bubble came out. One of Charlie’s goals for the dive was to map the positions, or at least understand the general orientation, of the mounds and pits at the Spinnaker Vents. We had a hard time figuring out exactly where we were most of the time, so we were a little frustrated, despite the interesting geological features we had found. 
After collecting sediment in one of our push cores, we found that methane hydrate had formed ice-like crystals at the top of the core.
Our frustration continued during the afternoon’s dive. We looked and looked for evidence of a fault that showed up in cross-sections of the seafloor created by our mapping AUV (amazingly, this AUV can simultaneously map both the surface of the seafloor and the layers of rock underneath). We spent several hours looking for signs of this fault on the seafloor, but found only circumstantial evidence—a few clusters of methane-seep clams and a few scattered tubeworm colonies. It’s possible that these features formed a line on the seafloor, we couldn’t tell for sure. So Charlie collected a number of vibracores in the hope that the subsurface sediments would provide additional evidence for the fault. As I finish writing this, it’s a gray and rainy night out here off the coast of Vancouver Island. We dodged squalls while processing vibracores on the afterdeck. Now it’s time for a hot drink by the fire. But I’ll settle for a full night’s sleep in a gently rocking bunk. 
This beautiful pink fish, about 40 centimeters long, came to watch as we collected samples at one carbonate outcrop.
—Kim Fulton-Bennett
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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.
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