 Leg 3 Logbook - Gas Hydrates
Day 5 — A hole in the bottom of the sea
August 6, 2009 Latitude 48 degrees 40.02 minutes N We spent all day today exploring a hole in the bottom of the sea. It wasn’t a very deep hole—only about eight meters (25 feet) deeper than the surrounding seafloor. It wasn’t even a very large hole—about 250 meters long and 60 meters across. But on our high-resolution sonar images, it appears as a very distinctive and somewhat mysterious-looking feature. So of course Charlie wants to know how it formed. This hole in the sea is called “Bullseye Vent” because on earlier seafloor surveys, the site looked like a round, fuzzy spot. As for being a vent, one or two researchers have claimed to detect methane in the water above the site, so they claimed it was a vent, but one where methane only diffused out slowly, or perhaps was released only in short bursts. But no one has actually seen methane bubbling out of the seafloor. Charlie’s goal was to take a close look at the seafloor within Bullseye Vent, and to take a lot of sediment cores to try to understand what’s going on beneath the surface. To that end, we spent all morning moving slowly across the long axis of the site, from one end to the other. It was much less dramatic than it looked in the sonar image—more of a shallow trench than a deep hole. Even the steep-looking sides turned out to be just gentle, muddy slopes. By lunch time, we had used up all of our vibracore tubes, so we brought the ROV back to the surface. 
During our traverse of Bullseye Vent we saw a number of gaping holes such as this one. We believe they were made by human drilling rigs or cores, but they sure look weird. At the surface, the ROV pilots and scientists swarmed around the ROV like the pit crew at a stock-car race. The pilots checked the ROV’s vital systems and made minor changes where necessary. The researchers removed all the full core tubes and placed new ones on the vehicle. Within 20 minutes, the Doc Ricketts was back in the water and heading for the bottom. We spent the afternoon flying back down the length of the site, collecting more vibracores and measuring the amount of heat coming out of the seafloor sediments at different locations. The heat measurements will help researchers find out if fluids are rising through the sediments. Each vibracore and heat-probe measurement took us 10 to 15 minutes. Since we couldn’t move the ROV during these measurements, we spent a lot of time using the zoom lens on the ROV’s science camera to look at any animals on the nearby seafloor. 
A small (5-cm-across) brittle star climbs out of the smoking crater we made in the seafloor with one of our push cores. The “smoke” is fine mud carried out of the hole by the currents. Like the previous sites we’ve visited on this cruise, the Bullseye Vent has been studied by a lot of different researchers over the last few years. We saw all sorts of evidence of these visits, including small pits that looked like bomb craters. These were probably places where researchers have dropped large core tubes or inserted massive drill bits into the seafloor. We also saw parallel tracks that looked as if a pickup truck had driven across the muddy seafloor, but which were probably caused by some other research submarine. Overall, we traveled over a kilometer of seafloor and collected 10 vibracores and several dozen push cores. We found a few interesting rock formations, but no extensive clam fields or mats of bacteria, which are distinctive elements of just about every active methane vent we know. As usual, we had lots of muddy fun getting the cores out of the core tubes. A few of these cores even smelled like rotten eggs, but at least they weren’t fizzing and bubbling. 
A rattail fish swims past beautiful (at least to a geologist) layers of carbonates along the outer edge of Bullseye Vent. We collected samples of these rocks to help us figure out when and how this depression in the seafloor formed. Today was Patrick Mitts’s birthday, so he served a sumptuous steak and lobster dinner to celebrate. Unfortunately, many of us had to rush back to the ROV control room right after dinner, so we couldn’t stick around for the celebration. But we certainly enjoyed the food! After just one days' work, we still don’t know what’s going on at Bullseye Vent. Maybe the seafloor collapsed when methane hydrates turned into gas and destabilized the sediments. Or maybe, as Charlie has suggested, the seafloor cracked and methane hydrates within the seafloor floated upward, carrying bits of rock and sediment out of the pit (just as the hydrates lifted up the fishermen’s heavy nets at the Barkley Canyon site, as described in yesterday’s log). We’ll go back tomorrow and collect some more cores. Maybe data from these cores will help Charlie Paull figure out what geological processes created this particular hole in the bottom of the sea. 
Craig Joseph models the latest in geological field apparel on the deck of the Western Flyer. —Kim Fulton-Bennett
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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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