MESA Archives: Aleutian Islands Deep Water Corals Cruise, July 30, 2004
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Guest journal by Gary Greene Marine Geologist, Moss Landing Marine
Laboratories and the Monterey Bay Aquarium Research Institute
(Read how sound is used to explore the seafloor:
Part I and II)
Part I: Blind As A Bat:
Sounds of Science at Sea
Jason II electrical engineer Chris Taylor sits before a bank of video
panels and controls in the Deep Submergence control van. When Jason
II is diving, researchers work shifts to observe the video feed and
direct the technical team members who pilot the vehicle and control
its collecting arms. A single dive may last more than 24 hours. Photo by Sonya Senkowsky.
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.
An unidentified octopus sits between rocks, in front of bamboo coral.
The sighting was made at about 500 meters depth.
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 Islands' 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
Part I >>>