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Expedition
to the Mariana forearc
Mar.
23 - May 4, 2003
Day
5, March 27
(click
on any image for the larger version)
| Day
5 - We arrived at Celestial Seamount at 0700. Celestial is a mud volcano
with several associated mounds to the west. We first went to one of
these mounds where the scientists wanted to get a mud/sediment sample
using a gravity
core. The coring process only took 2.5 hours--much faster than
the one we did at Deep Blue Seamount because we were in shallower
water. At Deep Blue we were working almost 4 miles deep, here the
core depth was 3419 meters (about 2 miles). The coring also went faster
because the winch operated at its optimal speed of 60 meters/minute.
The tension on the winch wire is measured and displayed throughout
the process. The tension increases as more and more wire is let out.
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| The
scientists have a fairly good idea if the core pipe penetrated the
sea floor by watching the tension display as the core pipe is lifted.
There is usually a greater tension as the pipe is pulled from the
mud. If the tension remains the same, it generally means the pipe
did not penetrate. Today the tension INCREASED as the winch started
to lift the pipe from the bottom. No one wanted to get too excited
but I could tell by the sparkle in their eyes that they expected mud.
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Corer
being recovered from the water. |
Plugged
up corer with end of core sticking out |
Mud
they got! Full grown adults with advanced science degrees dancing
with joy on the deck as the core pipe came onboard with mud protruding
from the end of the core pipe. Patty
Fryer, the Chief Scientist, was actually bouncing up and down
in her enthusiasm. I was pretty excited myself--my first look at real
mud from the famous mud volcanoes of the Mariana Forearc. |
| The
core pipe was 10-feet in length but the sample in the pipe was only
3 feet long. The sediment and mud got really hard by that depth and
the core pipe couldn’t go any deeper. It wasn’t rock but it was still
really hard. When the plastic liner in the core pipe is full of mud,
the plastic liner is cut lengthwise with a saw. Since there was just
a small section of mud in this one, the scientists brought the entire
plastic liner into the lab and knocked the sample out the end. What
a mess—mud and water everywhere but the basic core sample keeps its
shape. |
Chris
M assisting as Patty uses a metal knife to loosen the core |
|
The
chemists get to work with the mud first. They cut the core lengthwise
down the center and take mud/sediment samples every few inches.
The chemists want the pore water from the mud. They work quickly
because the water chemistry changes with the change in pressure
and temperature when the mud reaches the surface. The samples are
put into glass tubes and refrigerated to a temperature of 2 degrees
Celsius. The tubes are then put into the centrifuge for 5 minutes
at 13,000 RPM (revolutions per minute). This process separates the
mud from the water. The water is filtered and stored in small bottles.
The water is now stable and can be stored in the lab for analysis
at a later date.
The
geologists then observe the sample by eye and by microscope and
write up a preliminary description. The top 45 cm. of this core
is sediment that has settled on the seamount over a long period
of time. There is an ash layer in the sediment that is darker in
color. This ash probably settled after an eruption from of one of
the Mariana volcanoes. Below the sediment is the green serpentine
mud. The mud has rocks in it. Some of the rocks are peridotite,
the rock of the Earth’s mantle. The peridotite is what the scientists
call the protolith (before-rock). Peridotite becomes serpentine
when fluids rise through it and change the chemistry. Pieces of
peridotite come up with the mud. Also present in the green mud are
rocks that appear to have come from the subducted plate.
|
Aragonite
crystals. The smallest unit on the tape is 1 mm. |
At
the base of this core sample there are a lot of small, splinter-like
aragonite crystals. Aragonite is a form of calcium carbonate. The
crystals form when the sea water moving down into the sea floor by
diffusion meets the pore water moving up. The pore water comes from
the Pacific Plate as it is subducted and the increase in pressure
compresses and the plate and the water is squeezed out. When the two
different waters meet chemical reactions occur. In this case the calcium
carbonate precipitates as beautiful aragonite crystals. |
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Science
Summary - Day 5, March 27
Science
Objectives, Day 5:
The
fifth day of the cruise, March 27, after transiting north to Celestial
Seamount and collecting hydrosweep and EM300 bathymetry data over
gaps in previous surveys, we intend to core a small (3 km diameter,
300 m high) mound, hypothesized to be a serpentinite protrusion
a few km from the base of the main mud volcano, Celestial Seamount.
There are several mounds on the seafloor to the west of Celestial
Seamount and all are thought to be small protrusions of serpentinite
mud. The target for coring is a mound that shows slightly lower
backscatter on the side-scan imagery of the region. It is thought
that the lower backscatter is probably the result of the mound’s
having acquired a thin cover of sediment. It is suspected that the
mound is older than the higher-backscatter mound adjacent to it.
Both mounds are targets for coring, but the first one will be the
presumed older, sedimented mound so as both to verify the sediment
cover hypothesis and, assuming that hypothesis is correct, to provide
a more stable substrate for initial penetration of the core barrel.
Once the core is completed we will drop 3 transponders near the
summit of Celestial Seamount in preparation for a DSL120 survey
of the active conduit region at the summit of the mud volcano. Navigation
of the net and the beginning of the survey will take the remainder
of the day.
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