Topics: Finding Telltale Hydrothermal Plumes
With MAPRs (Miniature Autonomous Plume Recorders)
land, its usually easy to spot volcanic activity. Erupting
lava is the most obvious sign, but volcanism also produces large
changes in the gases and liquids that reside in the Earth. Hot
springs and geysers in volcanic areas, such as at Yellowstone National
Park or on the islands of Hawaii and Iceland, also provide impressive
evidence of volcanic activity.
But ocean scientists cannot see into the deep ocean and it is very difficult
to dive into it. So they need to be clever about how they detect submarine volcanic
eruptions. We are using an ingenious new way to monitor volcanic activity on
the global mid-ocean ridge: the Autonomous Hydrophone Array (AHA). Then, to home
in on possible eruption sites along the East Pacific Rise, we will use five Miniature
Autonomous Plume Recorders (MAPRs), which were loaned to us by Dr. Ed Baker of
the NOAA Pacific Marine Environment Laboratory (PMEL) in Seattle, Washington,
one of our Shorebased Collaborators. Dr. Baker and his colleagues at PMEL are
the developers of the MAPR sensors.
Palomares, Scripps technician, prepares a Miniature Autonomous Plume
Recorder (MAPR) for deployment. The larger titanium cylinder he is holding
is a pressure housing that contains electronics and batteries. The smaller
metal cylinder near the bottom is the light sensor. The two narrow tips sticking
out of the pressure housing are the temperature and pressure sensors. The white
semi-circles indicate where the MAPR clamps on to the fiber-optic cable.
are small instruments that measure three things: First, they measure
ocean pressure (which goes up the further you go down) and thus tells
us how far the MARRs are below the ocean surface. They also measure
how warm and how clear the ocean waters are. Both of these measurements
can provide telltale signs of hydrothermal vents-which become very
active immediately after submarine volcanic eruptions.
Fluids flowing out of seafloor at hydrothermal vents on the mid-ocean ridge are
hotter and more buoyant than seawater, so they continue to rise through the ocean.
But as the hot hydrothermal fluids mix with the abundant surrounding seawater,
they begin to cool down. The fluids continue to mix with the ocean water until
they are diluted enough to have the same density as the surrounding ocean water.
At this so-called level of neutral buoyancy, the mixture of hydrothermal fluid
and ocean water stops going up and down and begins to spread out horizontally.
So, a single black smoker chimney venting hydrothermal fluids, or even a small
group of chimneys that might occupy an area no larger than a classroom, can produce
a hydrothermal plume that can extend across many kilometers of
the ocean near the seafloor. You can see a similar phenomenon on land. Smokestacks
of factories or power plants also produce plumes of smoke that rise to a certain
height and then spread out for miles and miles, sometimes moving downwind.
The hydrothermal plumes offer a great way to find black smokers. If you detect
a hydrothermal plume, you know that high-temperature hydrothermal activity is
going on nearby. By homing into the place where the plume is most concentrated,
you can usually home into the place where black smoker chimneys will be found.
Rob Palomares checks out one of the MAPRs. Data from the MAPRs is downloaded into the computer in the background, which also programs the MAPRs for their subsequent deployments.
A MAPR is hung on the fiber-optic cable 200 meters above the DSL-120 sonar
and lowered to the seafloor for the first time on the cruise.
Heres where the MAPRs come in. They can detect the warmer water temperatures
of a hydrothermal plume. Hydrothermal fluids are not only hot, they also contain
lots of dissolved particles, which makes the fluids look like black smoke spewing
from a chimney.
MAPRs also have a device to measure how light is scattered through
seawater. If the water contains hydrothermal particles, then light
sent out from a sensor on the MAPR will bounce strongly off the particles
and back to the sensor. That is a signal that the sensor has come
upon a plume. If the water is crystal clear, as is typical for most
deep ocean water, then very little light will scatter back. A similar
effect also occurs in air on land. When you drive though a foggy
area at night, a lot of light from your headlights will bounce back
from water droplets that comprise the fog. When the air is clear,
the light doesnt bounce back at all.
Hydrothermal plumes in the area we will be surveying typically rise between 100
and 300 meters above the seafloor. We will attach five MAPRs onto the fiber-optic
tow cable for the DSL-120 sonar and Argo II vehicles at heights that should pass
through any hydrothermal plumes that might be present. The light-scattering data
and the temperature data should give us a very good idea of where any hydrothermal
activity is occurring on the seafloor. Strong hydrothermal activity may mean
that there has been a recent eruption -- which is what we are looking for! We will
use the MAPR information to plan our Argo II surveys and rock sampling. If we
encounter very strong hydrothermal activity and actually see vents in the Argo
cameras, we will try to collect samples of the water using a CTD system (which
measures the waters Conductivity, Temperature and Depth). We will also
use a clamshell grab to collect samples of hydrothermal animals
living in the vent communities.
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