99
It should be noted that the choice of the filter
used in the extraction procedure will determine
what will be evaluated as dissolved constituent
and what will be evaluated as particle or solid
sedimentary phase. The entire field of colloids
which had not yet been investigated thoroughly
for marine sediments will therefore be either evaluated as part of the solid phase, or as dissolved
constituents, depending on the analytical method
employed, for instance, whenever this fraction is
eliminated by the addition of hydrochloric acid.
With respect to the available and customary
methods, a lot of work still remains to be done on
this subject.
Centrifugal Extraction of Pore water
Centrifugal extraction of pore water from sediments is a procedure not very frequently performed although it appears rather easy at first examination. Anoxic sediments must be loaded into
tightly sealed centrifugation tubes, to be filled
and capped under the inert atmosphere conditions
of a glove box. But as these plastic tubes are often
not sufficiently air-tight, the invasion of ambient
air might affect the sample. Moreover, a cooling
centrifuge must be regularly employed, since heating the sediment above the in situ temperature is
by no means desirable. According to the opinion
of most workers, the operation of a centrifuge on
board a ship is rather troublesome and, most
frequently, leads in various ways to the reception
of high repair bills.
In the course of centrifugation, the sediment
will be compacted in such a manner that a
supernatant of pore water can be decanted.
Generally, less pore water is gained as compared
to pressure exertion. At any rate, the water must
be passed through a filter possessing a pore-size
of 0.1 to 0.2 µm. As for anoxic sediments, all these
steps must be carried out inside the glove box and
under an inert atmosphere. The measurements
comparing centrifugal extraction and pressing
techniques, which have been done so far, indicate
that both methods yield quite identical values (see
Fig. 3.17).
Whole Core Squeezing Method
A simple method of pore water preparation from a
sediment core that is still kept in a plastic liner has
been described by Jahnke (1988). In this method,
which is also suitable for employing on anoxic
sediments, holes are drilled into the tubes used in
the multicorer, high-momentum gravity corer, or
for a partial sample derived from the box corer.
These holes are sealed prior to core sampling with
plastic bolts and O-rings. As soon as the tube is
filled with the sediment core, the device is
mounted solidly permitting a piston from above
and below to hold the core into position. Then,
the prepared drainage outlets are opened and
sample ports mounted to them, through which the
pore water is drained off and passed through a
filter. If the core is compressed either by the
mechanical force of the pistons, or by infusion of
gas, the filtered pore water runs off through the
prefixed apertures and can be collected without
coming into contact with the atmosphere, provided that such precautions were taken. Jahnke
(1988) compares concentration profile examples of
pore water obtained by the application of this
method with pore water extracted centrifugally.
Within the margins of attainable precision, these
profiles must be conceived as truly ‘identical’.
Pore-Water Extraction with Rhizons
Rhizons were originally developed to gain pore
water samples which were then used to analyze
the moisture of water in unsaturated soils (Meijboom and van Nordwijk 1992). A rhizon is supposed to function like an artificial root of a plant,
hence its name is derived from ‘rhiza’, the Greek
word for root. The device consists of thin tubes
of various lengths (several cm) measuring
approx. 2.5 mm in diameter. They are closed on
one end and possess a connector on the other,
over which a negative pressure is built up inside
the tube. The tube material consists of a
hydrophilic porous polymer, with a pore diameter
of about 0.1µm. The actually quite instable tube
is reinforced by an inserted piece of wire made of
inert metal or durable plastic, allowing it to easily
penetrate the soil or a sediment from the outside.
In case of hard soils or sediments, a hole must
first be punctured with a similarly shaped piece
of metal or durable plastic.
The special property of the hydrophilic porous
polymer is that, after short soaking in water, it
becomes permeable to water - but not to air - once
a vacuum is applied inside. Wherever the rhizon
and the pore water come into contact, the negative pressure will draw the water into the interior,
without allowing air to enter or destroy the vacuum. The water thus drawn inside will be simul3.3
Sampling of Pore Water for ex situ Measurements
It should be noted that the choice of the filter
used in the extraction procedure will determine
what will be evaluated as dissolved constituent
and what will be evaluated as particle or solid
sedimentary phase. The entire field of colloids
which had not yet been investigated thoroughly
for marine sediments will therefore be either evaluated as part of the solid phase, or as dissolved
constituents, depending on the analytical method
employed, for instance, whenever this fraction is
eliminated by the addition of hydrochloric acid.
With respect to the available and customary
methods, a lot of work still remains to be done on
this subject.
Centrifugal Extraction of Pore water
Centrifugal extraction of pore water from sediments is a procedure not very frequently performed although it appears rather easy at first examination. Anoxic sediments must be loaded into
tightly sealed centrifugation tubes, to be filled
and capped under the inert atmosphere conditions
of a glove box. But as these plastic tubes are often
not sufficiently air-tight, the invasion of ambient
air might affect the sample. Moreover, a cooling
centrifuge must be regularly employed, since heating the sediment above the in situ temperature is
by no means desirable. According to the opinion
of most workers, the operation of a centrifuge on
board a ship is rather troublesome and, most
frequently, leads in various ways to the reception
of high repair bills.
In the course of centrifugation, the sediment
will be compacted in such a manner that a
supernatant of pore water can be decanted.
Generally, less pore water is gained as compared
to pressure exertion. At any rate, the water must
be passed through a filter possessing a pore-size
of 0.1 to 0.2 µm. As for anoxic sediments, all these
steps must be carried out inside the glove box and
under an inert atmosphere. The measurements
comparing centrifugal extraction and pressing
techniques, which have been done so far, indicate
that both methods yield quite identical values (see
Fig. 3.17).
Whole Core Squeezing Method
A simple method of pore water preparation from a
sediment core that is still kept in a plastic liner has
been described by Jahnke (1988). In this method,
which is also suitable for employing on anoxic
sediments, holes are drilled into the tubes used in
the multicorer, high-momentum gravity corer, or
for a partial sample derived from the box corer.
These holes are sealed prior to core sampling with
plastic bolts and O-rings. As soon as the tube is
filled with the sediment core, the device is
mounted solidly permitting a piston from above
and below to hold the core into position. Then,
the prepared drainage outlets are opened and
sample ports mounted to them, through which the
pore water is drained off and passed through a
filter. If the core is compressed either by the
mechanical force of the pistons, or by infusion of
gas, the filtered pore water runs off through the
prefixed apertures and can be collected without
coming into contact with the atmosphere, provided that such precautions were taken. Jahnke
(1988) compares concentration profile examples of
pore water obtained by the application of this
method with pore water extracted centrifugally.
Within the margins of attainable precision, these
profiles must be conceived as truly ‘identical’.
Pore-Water Extraction with Rhizons
Rhizons were originally developed to gain pore
water samples which were then used to analyze
the moisture of water in unsaturated soils (Meijboom and van Nordwijk 1992). A rhizon is supposed to function like an artificial root of a plant,
hence its name is derived from ‘rhiza’, the Greek
word for root. The device consists of thin tubes
of various lengths (several cm) measuring
approx. 2.5 mm in diameter. They are closed on
one end and possess a connector on the other,
over which a negative pressure is built up inside
the tube. The tube material consists of a
hydrophilic porous polymer, with a pore diameter
of about 0.1µm. The actually quite instable tube
is reinforced by an inserted piece of wire made of
inert metal or durable plastic, allowing it to easily
penetrate the soil or a sediment from the outside.
In case of hard soils or sediments, a hole must
first be punctured with a similarly shaped piece
of metal or durable plastic.
The special property of the hydrophilic porous
polymer is that, after short soaking in water, it
becomes permeable to water - but not to air - once
a vacuum is applied inside. Wherever the rhizon
and the pore water come into contact, the negative pressure will draw the water into the interior,
without allowing air to enter or destroy the vacuum. The water thus drawn inside will be simul3.3
Sampling of Pore Water for ex situ Measurements