The Control Van: Sights from the Seafloor

In the deep sea, boulders and stone outcroppings provide habitat for corals, sponges and other life. This image was taken July 26 at a depth of 2,295 meters. NOAA RESEARCH

Location: South of Kanaga and Adak Islands

Posted 08.01.04 at 1:42 pm

Guest journal, Pt. II, by Gary Greene,
Marine Geologist, Moss Landing Marine Laboratories
and the Monterey Bay Aquarium Research Institute

The excitement for me in this cruise lies in the control van. Here, the atmosphere of exploration and adventure is high. The biologists are keenly viewing the multitude of TV screens in the attempt to identify and collect that new species of coral. The geologists are observing the seafloor in an attempt to identify the types of rock that crop out on the seafloor and understand the dynamic seafloor processes that are active here. For me, I am looking for confirmation of the seafloor shape and sediment type that was interpreted from images made from echosoundings.

Using these acoustic images, maps have been constructed that show the seafloor conditions and various habitats that may be promising to corals and sponges. This is now the time to look at the sea bottom and see what types of seafloor conditions exist, to see if the interpretations were correct, and to see what biology exists and where, and if certain species have a preference for one type of geology or another.

Using the Remotely Operated Vehicle (ROV) Jason II from Woods Hole Oceanographic Institute, the capable Jason II crew drop the ROV and its companion Medea to the deep, dark cold seafloor. In this very hostile environment, Jason II is slowly directed to areas of interest using its sonar and the acoustic image maps. Because it is not possible to visually observe the entire seafloor offshore of the Aleutian Islands, we need to use sound to produce the maps that can image all of the seafloor. Thus, sound is the critical tool for mapping and understanding seafloor conditions, while Jason II is the tool used to groundtruth these acoustic observations—finally allowing us to see the seafloor for ourselves.

While in the control van, I examine the video coming from Jason II. Is the bottom covered with mud, and do burrows and tracks of organisms disturb this mud? If the mud is undisturbed, it tells me that not many organisms are living on or in the sediment. If the mud is disturbed and the floor has many mounds and holes, it tells me that seafloor life is active and that oxygen and food is plentiful.

Another thing I look for is the condition of the sediment. Is it rippled like a river or stream floor, which would indicate strong seafloor currents? Do boulders that sit on the seafloor have scour depressions around them, which also indicate strong currents?

Are there cliffs or scarps, which may form from submarine landslides or fault offsets? Are the walls of submarine canyons eroded with little attached organisms, which may indicate recent erosion from sediment-laden currents (called turbidity currents) that swiftly travel down slope. Are we looking at the deposits left by a glacier?

During the dive this day, we see many odd-shaped boulders scattered over the seafloor that provide habitat for corals, sponges and crinoids, living fossils that look like many-armed starfish without flesh. These boulders most likely were brought to these deep waters of the Aleutian Islandsä» flank by ancient glaciers. Icebergs broke off from the glaciers and drifted offshore, carrying their load of sediment, including the boulders. Once offshore, they melted, dropping the boulders to the seafloor. Still unburied today, these boulders provide small, but significant, hard topographic relief to an otherwise flat, muddy seafloor.

Other boulders may have been rafted offshore by kelp. The seaweed generally grows in the shallow waters around the Aleutian Islands by attaching to rocks and boulders (called holdfasts). Storms often rip up the kelp and carry it far offshore, where it rots and loses its hold on the boulders. The boulders then sink to the seafloor, where we find them as dropstones.

So, what did we learn this day? We learned that glaciers played a major role in the construction of significant, and perhaps critical, habitat along a gently sloping seafloor in deep waters offshore of the Aleutian Islands. On this otherwise homogeneous mud seafloor, the dropstones from the glaciers and kelp have provided a hard habitat upon which corals, sponges and crinoids can attach. These little oases of hard substrate also provide predators of the deep-sea areas in which to forage. In other words: Geology influences biology.