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)