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Expedition
to the Mariana forearc
Mar.
23 - May 4, 2003
Day
19, April 10th
(click
on any image for the larger version)
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Day
19 Chimneys
A
knock at my door at 0500, “Do you want to help with a piston core.”
Silly question, from a deep sleep to hardhat and life vest within
10 minutes and ready to go. My watch last night ended at midnight
when Medea lost a protective cover and Jason was brought to the
surface for repairs. We raided the refrigerator and then I slept
the early morning hours as we headed back to Conical Seamount.
A
spectacular rainbow arched over the core apparatus as we prepared
to deploy at Conical. There were a lot of comments about the placement
of the core, but skipping the pot-of-gold, we again went for the
blue mud. This core returned with 7.5 feet of brown sediments over
a section of dense, dark blue mud.
Jason
was back in the water at 1530. I figured he would be on location
and ready for the spectacular discoveries just in time from my watch
at 2000. At the start of the watch, the search was on for the core
holes left from the morning piston core. By 2200, we gave up on
finding the core holes and began a search for a chimney field that
had been seen in 1997.
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Jason's
collecting tray for bringing samples to the surface |
Chimneys
are formed when water seeping up to the sea floor, reacts with the
sea water and chemicals precipitate (solidify and fall out of solution).
Sometimes the precipitates form slowly and scatter with the currents.
Other times they precipitate quickly and build the formations we
call chimneys. The chimneys of the thermal vents found in marine
spreading centers grow to heights over 100 feet. These grow rapidly
as the hot water from the vents reacts with the cold sea water.
The predominant deposits of thermal vent chimneys are sulfur minerals.
The minerals forming these chimneys precipitate so rapidly and in
such large amounts that the chimneys often appear to be smoking.
They are called black smokers or white smokers depending on the
color of the minerals. These chimneys have been well studied.
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| Little
is know about the chimneys we’re looking for today. The chimneys of
the mud volcanoes grow more slowly as the reaction is not temperature
related and the flow is less rapid. The reacting waters are pore water
and sea water. The pore water is squeezed from the subducted plate.
As it rises it picks up chemicals from the rock it passes though.
Chemically, it is very different from sea water. When is comes into
contact with the sea water, minerals precipitate and form chimneys.
These would not be called smokers; maybe puffers or whiffers. Only
on a very few occasions has a puff, or whiff, of pore water been seen
coming from one of these. It can be seen because it is less dense
than the surrounding sea water and appears as small waves, somewhat
like the heat rising from a hot surface. |
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Chimneys
of the seamounts here are made of calcium carbonate or brucite.
The seamounts nearest the trench have chimneys composed of brucite
(magnesium hydroxide). The seamounts farther from the trench, such
as Conical, have chimneys made of calcium carbonate. The geochemists
think the composition varies due to the differences in the depth
at which the pore water is squeezed from the subducting plate--different
depths result in different chemistry.
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Jason
collecting a sample of the chimney |
Inactive,
fallen chimney at Conical seamount |
We
found two chimneys at Conical that were no longer active and had
fallen over. Each was about a meter in length. They look solid but
crumble easily from the pressure of Jason’s grip. Matt,
the pilot, is an expert at manipulating Jason’s hand and was able
to collect great samples.
We
were relieved by the next watch at midnight knowing the great discovery
was about to be made, but needing some rest we tumbled off to bed.
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Science
Summary - Day 19, April 10th
Science
Objectives, Day 19:
The
nineteenth day of the cruise, Apr. 10, we will deploy Jason2/Medea
on Conical Seamount to do a Jason lowering on the summit near the
site of fluid seeps and chimney structures observed in 1987 with
the Alvin submersible and the site of 1990 ODP drill holes. We will
observe the seeps and the boreholes and collect samples.
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