Leg 3 Logbook - Gas Hydrates
Day 8 — Sleuthing our way around
August 9, 2009
Latitude 48 degrees 42.50 minutes N
Longitude 126 degrees 50.20 minutes W
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.