Once back on the ship, Harry Roberts excitedly describes the findings from deep submergence vehicle Alvin Dive 4181 to the rest of the science party. Click image for larger view and image credit.

The distinct interface between a very extensive mussel bed on the perimeter of a large mud flow and brine seep feature at Atwater Valley Site 340. Click image for larger view and image credit.

Mussel Mound

May 17, 2006

Harry Roberts
Louisiana State University

27°38.84 N
088°22.41 W

Video of the interface between a very extensive mussel bed on the perimeter of a large mud flow and brine seep feature. (Quicktime, 1.94 Mb.)

Today we visited Atwater Valley Site 340 (AT 340), water depth of about 2,170 m, where three previous dives have been made. AT 340 is a great site, with living tubeworms, mussels, and other benthic (bottom-dwelling) organisms like urchins and soft corals. The site was selected by studying geophysical maps of the bottom and also subsurface profiles that show the geology below the sea floor. The subject of this dive was an inviting target slightly to the northwest of the areas investigated earlier. This target had all the geological characteristics that suggested a productive site: highly reflective sea floor, indicating hard-bottom habitat, and a subsurface pathway to allow fluids and gases escape at the sea floor.

Our target was a mound that is about 10 m above the surrounding sea floor. We landed the deep submergence vehicle Alvin to the northeast of the target and immediately started running toward the mound. We soon started seeing evidence of hydrocarbon seepage, such as seep-related rocks, mussel shells, and small white bacterial mats. When we finally encountered the base of the mound it was truly an exciting experience! There were more tubeworm bushes and mussels than we had seen at any previous site during this cruise. The mound itself was made of mussels. Over the years, one layer of mussels after another had grown and died. The result was an entire mound of thousands of mussels cemented together. We named the site “Mussel Mound.”

When we reached the top of the mound, we found living mussel beds and tubeworms growing out of cracks in the rocks as well as in free-standing bushes. Mussel shells outnumbered the live mussels. The sheer number of the tubeworms was astonishing. Since the main objective of the dive plan was to collect mussels, we focused on mussel-pot collections and saved the tubeworms for later.

An expanse of what appears to be Beggiatoa bacterial mats. Beggiatoa uses hydrogen sulfide, which is commonly found at seeps, for energy. Click image for larger view and image credit.

During several dives, scientists have witnessed large aggregations of sea urchins plowing through the bottom sediments near seep sites. This image shows just a few of the hundreds of urchins seen during the May 17 DSV Alvin dive. Click image for larger view and image credit.

Despite a strong current running over the top of the mound our pilot, Mark Spear, maneuvered the Alvin into position. After gathering samples, we moved away from the mound and encountered an enormous mussel bed next to a large brine pool and mudflow. It was an awesome area! The brine, a supersaturated salt solution, probably originates from a salt mass that lies beneath the surface of the sea floor. We spent a few minutes reconnoitering, but because time was short and battery power was low, we had to leave the site quickly. Again, we stumbled upon another amazing area. Hundreds of urchins were “plowing” through the bottom like little trucks mining the bottom sediments. They left contorted trails that made the bottom look like the confluence of a bunch of meandering super-highways.

Unfortunately we couldn’t stay on the bottom any longer. We soon began our ascent. My excitement and desire to tell everyone else about our findings made it seem like a long trip back to the surface. Today’s dive site seems to be the most biologically productive site in the deep water areas of the Gulf of Mexico that we’ve visited so far. Our initial cruise plan called for us to move on, making our way westward across the Gulf and leaving this site behind, but today’s findings seem significant enough to amend the plan. The decision has been made to go back. On Friday, we’ll have one last dive at AT 340!

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