 Leg 3 Logbook - Gas Hydrates
Day 4 — Oil and gas and mud, oh my!
August 5, 2009 As I write this, my jeans are dappled with splotches of congealing brown ooze, which smells somewhat like kerosene. We just finished processing the vibracores we collected during today’s dive. It was a messy business. Today’s dive site, Barkley Canyon, is famous among geochemists because it is one of the few well-known places where methane hydrates are exposed right at the seafloor. The hydrates are exposed along the upper edge of Barkley Submarine Canyon (offshore of Barkley Sound), in an area with several faults and underwater landslides. Canadian researchers know this spot as “The Fishboat Site,” because it was first discovered by some Canadian fishermen in about 2001. They were dragging their net along the seafloor, when, much to their surprise, it started to rise up to the surface on its own. When they hauled the net onto their boat, they found that it was filled with a strange, ice-like material that smelled like diesel. They emptied most of the nasty stuff back into the water, but took some photos and saved a small sample in a plastic bag, which they brought back to researchers at the University of Victoria. By the time the UVic researchers got the sample, the bag had blown up like a balloon and then popped, as the methane hydrates had turned back into methane gas. All that was left inside was an oily film. Fortunately one of the researchers had recently seen a TV special on methane hydrates, and realized this was what might have been in the bag. During subsequent ROV dives, they found several places where methane hydrates were exposed and petroleum byproducts were bubbling out of the seafloor. For obvious reasons, these discoveries attracted a lot of attention. 
This image from ROV Doc Ricketts shows the Cliffhanger site. In the foreground you can see three push cores, which the ROV’s manipulator arm is about to pull out of the mud. In the background, the gray box contains Laura Lapham’s osmosampler. To the right of the box, you can just see a bit of tan material, which is oil-soaked methane hydrate. Our first stop today was at one of the most famous places where hydrates are exposed—a small mound on the seafloor known as “Cliffhanger.” As the ROV approached the mound, we could see a small cave at the base of the mound. Just above the cave were several “windows” in the muddy seafloor through which we could see a yellowish waxy substance. This was the exposed methane hydrate. Pure methane hydrate is actually white, but the outcrops in this area contain so much oil that they are yellow or even brown in color. The first order of business was to install Laura Lapham’s osmosamplers. We ended up wedging the box containing five of these instruments on a small ledge at the mouth of the cave at Cliffhanger. For the next year these instruments will slowly suck water from the sediments into kilometer-long coils of thin copper tubing. When Laura retrieves the instruments next year, she will cut up the copper tubing into two-meter segments. Each segment will contain six days worth of collected “pore water” from a different depth in the sediment. By measuring the amount of methane in each segment, Laura will be able to tell how methane concentrations at this site change over time. Every time we disturbed the sediment at Cliffhanger, for example by pushing in a core tube, streams of yellowish or greenish bubbles poured out of the mud. These bubbles were the same color as the light-grade motor oil you would put in your car. Back in the lab, when we emptied the mud from our push cores, the oil smelled nasty and the hydrates popped and fizzed as if the mud had been mixed with soda water. 
Like the abandoned robots R2D2 and C3PO in the movie “Star Wars,” these temperature probes have sat on the seafloor for a year, waiting for someone to come pick them up and bring them back to the lab. We were happy to fulfill that task. In the background you can see some of the small white corals we observed in this area.
After collecting quite a few cores at Cliffhanger, we searched the area for seven temperature probes that had been left on the seafloor a year ago by Ross Chapman. This is how we often do oceanography—leave instruments down on the cold, muddy seafloor, then come back a year later and hope that they worked and recorded useful data. The ROV pilots found six of the seven probes, which was pretty amazing considering that the probes were only a few inches long, and we had to search an area the size of several football fields. We then cruised around looking for additional methane seeps. These were usually easy to spot because they looked like bulls-eyes on the seafloor. In the center of the bulls-eyes were white mats of bacteria that eat methane. Surrounding these were rings of special clams that have bacteria in their guts that feed on hydrogen sulfide, a chemical produced when methane reacts with chemicals in seawater. In addition to the bacterial mats and seep clams, we saw patches of deep-sea corals and lots of beautiful jellies. Among these were flocks of purplish blue lobate ctenophores, their bodies flapping slowly like gelatinous butterflies. We were also “pestered” throughout the dive by bright red-orange medusae (jellyfish), ranging from the size of pennies to the size of dinner plates. 
This ROV image shows a typical methane seep with white mats of bacteria ringed by seep clams. In this case, several sea stars and fish seem to have claimed the high ground at the top of the mound. Exactly how such mounds form is one of the main questions we are trying to answer on this cruise. We started our dive at around 6:00 this morning and finished at 6:30 this evening. Charlie Paull was hoping to find areas where the sediment would be thick enough to provide thick sediment cores. The layers of sediment in such cores could provide a historical context for the seeps we saw. Unfortunately, in most areas the sediment was very thin, with hard material less than a meter or so below the surface. Of course, this in itself may be a clue to the geology of the area and the formation of the seeps. However, at this point we don’t know exactly how to interpret this clue. 
This dimly lit room is where we spend up to 12 hours a day, watching several dozen computer and video monitors as ROV Doc Ricketts flies around the seafloor 800 meters below us. It can be exciting or tedious, depending on your perspective. Thus ends another day on board. Tonight we are sailing north again, this time heading for another famous methane site called the “Bullseye Vent.” We’ll be there before dawn, and dive first thing in the morning. Once again we’ll look for methane hydrate seeps and try to understand how they formed. This time, we’ll try not to get hydrates in our cores, because all that bubbling and fizzing just creates a mess (and messes up any layers that may exist in the sediment). However, getting muddy is part and parcel of doing marine geology, so there’s no point in washing my jeans tonight! 
What do you do when your sediment cores are fizzing and giving off an odor of diesel? That is the question we are pondering in this photograph. You can tell from the clean lab coats that we have just started extracting the sediment from push cores such as the one in the lower left. (Photo by Yirang Cho) —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.
|
