By:
Chief Scientist, Bramley Murton.
The past couple of days we have been diving
with HyBIS over the northern side of the great rift that divides the Rio Grande
Rise in two. The Rio Grande Rise is a volcanic plateau formed 80 million years
ago when a hot spot in the mantle underlay the Mid-Atlantic Ridge. As the
African and South American tectonic plates separated, the hotspot caused an
excess of volcanism generating an island rather like Iceland. The trail of the
hotspot can be seen on each side of the Mid-Atlantic Ridge as the Walvis Ridge
in the east and the Rio Grande Rise in the west. With time, the island plateau
subsided beneath sea level to its present position we see today.
The
tectonic position of the Rio Grande Rise and its conjugate, the Walvis Ridge.
The
Rio Grande Rise, the Great Rift, and the location of our study area.
Prior to diving, we have to carefully plan
the dive so that we know exactly what we are aiming to see – and each dive aims
to test a hypothesis. In our case, we are looking to see if the cobalt rich
crusts vary in outcrop and thickness with increasing distance from the edge of
the Great Rift. The reasoning is simple: the Great Rift is over 1400m deep and
guides the direction of currents and tides within it. As these moving water
masses approach the walls of the rift, there is friction in the form of
turbulence and eddies that create a more energetic environment, suppressing
sediment accumulation and allowing the crusts to grow. This energetic
environment is likely to be different to the north and south of the rift, as a
result of geostrophic effects that cause the currents to turn. Further away
from the edge of the rifts, the energy is likely to be less, and we expect to
see an increase in the extent and thickness of the sediment cover. Hence our
dives are designed to survey and sample at different locations with increasing
distance from the rift walls.
Tim Le Bas, our survey and data manager,
plots the locations of the dives on our new bathymetry maps we have made with
our multibeam echo sounder system. We then transfer the maps to the HyBIS team.
After launching the HyBIS, we start-out across the seafloor making a detailed
video survey and sampling the rocks and biology.
Co-chief
scientists Paul Lusty (standing) advises Tim Le Bas where to place the tracks
for the next HyBIS dive mission.
Launching the HyBIS is a quick procedure. The
vehicle is prepared for the dive in a covered hanger inside the ship and
wheeled out under the gantry where it is lifted into the air and dropped into
the water. Floats are attached to the cable to keep it above the HyBIS when it
lands on the seafloor. The whole operation takes about ten minutes. Recovery is
the reverse, and is equally as fast.
The
deck crew launch our HyBIS robotic underwater vehicle over a calm sea from the
strarboard side of the side of the RRS Discovery as evening draws in.
The
science team watch as the HyBIS ‘flies’ across the seafloor.
Meanwhile, the AUV team have been
struggling to prepare the Autosub6000 robotic submarine for its first dive. One
of the risks with shipping delicate equipment across the world is that things
can be bumped in transit. We think this had happened to our internal navigation
module, and as a result the team have spend a lot of late nights integrating a
spare system in the vehicle. It is now ready for its first mission.
The
robotic submarine Autosub6000 being prepared for its first mission over the Rio
Grande Rise.
ENDs
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