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On 18 May 2010, we revisted ODP 889 to install an Imagenex multibeam sonar. With it, scientists hope to study plumes of methane gas that bubble from the seafloor in a portion of the Bullseye Vent called Bubbly Gulch. This methane percolates upward from gas pockets associated with gas hydrates in the seafloor. Although the gas hydrates are a frozen solid, there are fissures through which the methane escapes.

After escaping, the bubbles show up strongly in sonar readings. The bubbles are theorized to be enclosed in a thin skin of gas hydrates where methane contacts the deep cold waters here. By the time they rise to 900m and shallower depths, the plumes largely disappear as the methane re-dissolves into the seawater.

Bubble Hunting

Finding the bubble plumes was not as easy as we expected. They show up clearly in sonar - if within range. During our descent, we kept a close eye on the ROPOS sonar display, but no plumes appeared. After reaching the seafloor, we then flew to a previously identified location to look for bubbles ... but could not find any. We continued searching in other locations, looking for the telltale signs of a methane seep: calcium carbonate crusts in the vicinity, bacterial mats, and an absence of sea pens. But even after 2 hours of hunting, we found no active bubble zones. ROPOS then ascended some 60m off the seafloor, and prowled around looking for plumes in the sonar.

Eventually, we found several plumes, and slowly approached them using the sonar for guidance. At times, it was like chasing a mirage, as every time we approached, the plume signature receded in the sonar. Finally, though, ROPOS found itself in the midst of a stream of bubbles, which we tracked back to the seafloor. Methane was bubbling out prolifically from a nondescript patch of sediment - with not one of the telltale methane vent markers: no nearby calcium carbonate crusts, no bacterial mats, and a sea pen living in the midst of the bubble zone!

Although this bubble patch was beyond the reach of our 550m extension cable, we placed the multibeam sonar as close to it as possible, and connected it to the ODP 889 Instrument Platform after laying the cable with Mini-ROCLS. It's hoped that in this location, the sonar will be able to detect not only this plume, but also future Bubbly Gulch plumes emerging at different times and nearby locations.

Bubbly Gulch Encounters

In addition to methane gas seeps, we encountered a number of interesting things in Bubbly Gulch. Small cave-like structures were frequently spotted - we don't know if these are inactive vents or if benthic fauna create them. But, we do know they appear to be popular hangouts for small rockfish living there. Other benthic fauna we observed include sea pens, brittle stars, starfish and corals.

We also found a couple of interesting man-made objects. One was an exterior housing of a sonobuoy. The inert metal housing falls away from the rest of the sonobuoy on impact with the water and sinks to the bottom.

Kraki's Lair

We also happened upon an abandoned rice cooker or crock-pot and screwdriver upon which sat a large crab. ROPOS pilot Reuben Mills suggested we take a closer look, and carefully opened the lid. Inside, we discovered a mother octopus with her brood of eggs! Collaborating scientist Michael Riedel suggested adopting this creature as the Bubbly Gulch mascot. We're calling her "Kraki" for now, but let us know if you have another name to suggest.

Photos

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