93
gravity corer and with the piston corer display the
same stratigraphical depth zones of the sediment. It
is therefore assumed that the cores sampled with
the gravity corer are ‘somewhat compressed’ especially in the lower parts, whereas the piston corer
produces cores that are ‘somewhat extended’ in
length. How this comes into effect and what
conclusions are to be drawn with regard to the pore
water samples is yet completely uncertain. In the
examined pore water profiles of sediments extracted
with the gravity corer, there were no indications as
to any noticeable compression to date.
Both sampling tools can only be operated from
the decks of larger ships that are equipped with
steel ropes and winches designed for managing
weights of 10 - 15 tons. The author would personally always prefer using the gravity corer
because its handling is easier, safer and faster; and
because, especially in low latitudes, every minute
counts in which the core is unnecessarily exposed
to high temperatures on board the deck of the ship.
At least the upper 10 to 30 cm of the core
length obtained with either tool is usually
adulterated in that it is not appropriate for pore
water analysis. The multicorer, high-momentum
gravity corer, or at least the box corer should be
employed in a parallel procedure to ensure that
this layer will also be included as part of the
sample. It should not be overlooked that, especially in the deep sea, sampling with two different
tools ‘at the same site’ might imply a distance of
several 100 m on the ocean floor. From this
deviation considerable differences in pore water
composition, and in some of the biogeochemical
reactions close to sediment surface, are likely to
result. Hence the specification as to ‘same site’
must be acknowledged with caution.
After the usage of either tool - the gravity corer
and the piston corer - the sediment core obtained is
immediately dissected into pieces of 1 m in length
within the tube. Usually, the one meter long tubular
pieces, tightly sealed with caps, are stored at in
situ temperature prior to the subsequent further
processing which is to be carried out as quickly as
possible.
The Box-shaped Gravity Corer (Kastenlot)
In principle, the box-shaped gravity corer is not
dissimilar to the above mentioned gravity corer.
Here, a metal box with a core length of about 10 m
and a lateral dimension of about 0.1 m up to 0.3 m
is used instead of a steel tube with a plastic liner.
The core box is shut by valves upon being pulled
out of the sediment and is then hoisted upwards
through the water column. The advantage of
using the box-shaped gravity corer consists in
providing a large core diameter, less perturbations
due to the metal box’s thinner walls, and by removing one side of the box it allows the sediment
stratigraphy to be examined. The essential disadvantage in examining pore water samples consists
in the fact that the core is distinctly less accessible, and that processing of the core under an
inert atmosphere (glove box) is hardly feasible.
The Harpoon sampler
Sayles et al. (1976) have described a device that
allows collection of pore water samples under in
situ conditions. To achieve this, a tube is pressed
about 2 meters deep into the sediment, like a harpoon. Then the pore water is withdrawn from
various sections of the tube, through opened
valves, and concomitantly passed through filters.
The crucial step of separating sediment and pore
water thus happens under in situ conditions. The
obtained water samples are then ready to be analyzed ex situ. The concentrations which were measured by Sayles et al. (1976), however, do not
differ greatly from those which were obtained
‘from the same location’ by expressing pore water
under ex situ conditions.
Jahnke et al. (1982) used the same (or, at least a
quite similar) tool in 4450 m deep waters of the
equatorial Pacific, at approximately 0° to 10° N,
and approximately 140° W (MANOP-site). Here,
the distances between the locations in which the
harpoon sampler was used and in which, for
reasons of comparison, pore water was obtained
by compression after the sampling was carried out
with the box corer, amounted to 300 m to up to
3000 m. In measurements, which were corroborated several times, Jahnke et al. (1982) found similar
concentrations of nitrate, nitrite, silica, pH, and
manganese. Yet, the concentrations of ∑CO 2 , alkalinity and phosphate were distinctly higher and
displayed statistical significance.
3.3.2
Pore Water Extraction from the
Sediment
In the following section, the separation of pore
water from the sediment will be described. The
necessary procedures will be described in the
sequence in which they are employed. This is to
3.3
Sampling of Pore Water for ex situ Measurements
gravity corer and with the piston corer display the
same stratigraphical depth zones of the sediment. It
is therefore assumed that the cores sampled with
the gravity corer are ‘somewhat compressed’ especially in the lower parts, whereas the piston corer
produces cores that are ‘somewhat extended’ in
length. How this comes into effect and what
conclusions are to be drawn with regard to the pore
water samples is yet completely uncertain. In the
examined pore water profiles of sediments extracted
with the gravity corer, there were no indications as
to any noticeable compression to date.
Both sampling tools can only be operated from
the decks of larger ships that are equipped with
steel ropes and winches designed for managing
weights of 10 - 15 tons. The author would personally always prefer using the gravity corer
because its handling is easier, safer and faster; and
because, especially in low latitudes, every minute
counts in which the core is unnecessarily exposed
to high temperatures on board the deck of the ship.
At least the upper 10 to 30 cm of the core
length obtained with either tool is usually
adulterated in that it is not appropriate for pore
water analysis. The multicorer, high-momentum
gravity corer, or at least the box corer should be
employed in a parallel procedure to ensure that
this layer will also be included as part of the
sample. It should not be overlooked that, especially in the deep sea, sampling with two different
tools ‘at the same site’ might imply a distance of
several 100 m on the ocean floor. From this
deviation considerable differences in pore water
composition, and in some of the biogeochemical
reactions close to sediment surface, are likely to
result. Hence the specification as to ‘same site’
must be acknowledged with caution.
After the usage of either tool - the gravity corer
and the piston corer - the sediment core obtained is
immediately dissected into pieces of 1 m in length
within the tube. Usually, the one meter long tubular
pieces, tightly sealed with caps, are stored at in
situ temperature prior to the subsequent further
processing which is to be carried out as quickly as
possible.
The Box-shaped Gravity Corer (Kastenlot)
In principle, the box-shaped gravity corer is not
dissimilar to the above mentioned gravity corer.
Here, a metal box with a core length of about 10 m
and a lateral dimension of about 0.1 m up to 0.3 m
is used instead of a steel tube with a plastic liner.
The core box is shut by valves upon being pulled
out of the sediment and is then hoisted upwards
through the water column. The advantage of
using the box-shaped gravity corer consists in
providing a large core diameter, less perturbations
due to the metal box’s thinner walls, and by removing one side of the box it allows the sediment
stratigraphy to be examined. The essential disadvantage in examining pore water samples consists
in the fact that the core is distinctly less accessible, and that processing of the core under an
inert atmosphere (glove box) is hardly feasible.
The Harpoon sampler
Sayles et al. (1976) have described a device that
allows collection of pore water samples under in
situ conditions. To achieve this, a tube is pressed
about 2 meters deep into the sediment, like a harpoon. Then the pore water is withdrawn from
various sections of the tube, through opened
valves, and concomitantly passed through filters.
The crucial step of separating sediment and pore
water thus happens under in situ conditions. The
obtained water samples are then ready to be analyzed ex situ. The concentrations which were measured by Sayles et al. (1976), however, do not
differ greatly from those which were obtained
‘from the same location’ by expressing pore water
under ex situ conditions.
Jahnke et al. (1982) used the same (or, at least a
quite similar) tool in 4450 m deep waters of the
equatorial Pacific, at approximately 0° to 10° N,
and approximately 140° W (MANOP-site). Here,
the distances between the locations in which the
harpoon sampler was used and in which, for
reasons of comparison, pore water was obtained
by compression after the sampling was carried out
with the box corer, amounted to 300 m to up to
3000 m. In measurements, which were corroborated several times, Jahnke et al. (1982) found similar
concentrations of nitrate, nitrite, silica, pH, and
manganese. Yet, the concentrations of ∑CO 2 , alkalinity and phosphate were distinctly higher and
displayed statistical significance.
3.3.2
Pore Water Extraction from the
Sediment
In the following section, the separation of pore
water from the sediment will be described. The
necessary procedures will be described in the
sequence in which they are employed. This is to
3.3
Sampling of Pore Water for ex situ Measurements