90
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
reactions in the oxic zone. Carbonate,
phosphate and nitrogen, as well as ammonia
are produced. The oxygen for the oxidation of
organic matter in this zone is derived from the
nitrate, produced in the oxic zone. In most
cases, this process oxidizes at least one order
of magnitude less organic matter than the
reactions in the oxic zone. For details of these
reactions, see Chapter 6.
• Below this zone, iron(III) oxides or iron(III)
hydroxides in the solid phase of the sediments act as electron acceptors. For details of
these reactions, see Chapter 7.
• Below this zone, dissolved sulfate serves as
electron acceptor for the oxidation of organic
matter, according to the ‘Froelich-model’.
Recent publications however showed that, in
most cases, not organic matter is oxidized in
this zone, but predominantly methane, which
diffuses up from the deeper parts of the sediment (Niewöhner et al. 1998). For details of
these reactions, see Chapter 8.
• The reaction with the lowest yield of standard
free energy is methane fermentation with the
products carbonate, methane, ammonia and
phosphate. For details of these reactions, see
Chapter 4.
3.3
Sampling of Pore Water
for Ex situ Measurements
Ideally, one would prefer to analyze pore water
exclusively under in situ conditions, as will be
explained in Section 3.5, for the parameters that
permit such procedure. The pressure change, and
frequently the change of temperature as well, are
usually coupled to ex situ measurement and exert
a number of influences of varying potential. However, the ex situ measurement will certainly remain
a necessity for quite a long time, with regard to
most of the substances dissolved in pore water
and especially for great depths below the sediment surface. This book is not the place to give a
general review on sediment sampling techniques.
Rather, the more common procedures for sediment
sampling will be introduced with an emphasis put
on pore water analysis. Then, the particularities,
the possible errors as well as problems arising in
the application of these sampling techniques, will
be discussed.
The following section is concerned with the
separation of the aquatic pore water phase from
the solid sediment phase. As with all particularly
problematic and error-inducing procedures, these
various techniques have, depending on the case
at hand, their specific advantages and disadvantages.
As a matter of course, one would want to analyze the obtained pore water as soon as it has
been separated from the sediment to quantify the
dissolved substances therein. In daily routine
proceedings, compromises must be made since
not all analyses can be carried out simultaneously,
and since each and every analytical instrument is
not present on board a ship. Thus, it will be
necessary to describe the state of knowledge
concerning pore water storage, transport and preservation.
3.3.1
Obtaining Samples of Sediment for
the Analysis of Pore Water
A number of different techniques are available to
withdraw samples from the marine sediment.
Depending on the scientific question under study,
a tool may be chosen that is either capable of
taking the sample without harming the sediment
surface or disturbing the supernatant bottom water (e.g. multicorer), or that punches out a large as
possible sample from the sediment surface area
(e.g. box corer), or one that yields cores from the
upper less solid meters of the sediment which are
as long as possible and most undisturbed (gravity
corer, piston corer, box corer).
The Box Corer
The generic term ‘box corer’ denotes a number of
tools of different size and design used in sampling marine sediments, mostly lowered from
ships by means of steel wire rope to the bottom.
All have a square or rectangular metal box in common which is pressed into the sediment by their
own weight, or perhaps by additionally mounted
weights. Upon lifting the tool from the sea-floor
the first pull on the steel ropes is used to close
the box by means of a shovel while it is still situated in the sediment. At the same time, an opening at the top is shut, more or less tightly, so
that the bottom water immediately above the
sediment is entrapped. The lateral dimensions of
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
reactions in the oxic zone. Carbonate,
phosphate and nitrogen, as well as ammonia
are produced. The oxygen for the oxidation of
organic matter in this zone is derived from the
nitrate, produced in the oxic zone. In most
cases, this process oxidizes at least one order
of magnitude less organic matter than the
reactions in the oxic zone. For details of these
reactions, see Chapter 6.
• Below this zone, iron(III) oxides or iron(III)
hydroxides in the solid phase of the sediments act as electron acceptors. For details of
these reactions, see Chapter 7.
• Below this zone, dissolved sulfate serves as
electron acceptor for the oxidation of organic
matter, according to the ‘Froelich-model’.
Recent publications however showed that, in
most cases, not organic matter is oxidized in
this zone, but predominantly methane, which
diffuses up from the deeper parts of the sediment (Niewöhner et al. 1998). For details of
these reactions, see Chapter 8.
• The reaction with the lowest yield of standard
free energy is methane fermentation with the
products carbonate, methane, ammonia and
phosphate. For details of these reactions, see
Chapter 4.
3.3
Sampling of Pore Water
for Ex situ Measurements
Ideally, one would prefer to analyze pore water
exclusively under in situ conditions, as will be
explained in Section 3.5, for the parameters that
permit such procedure. The pressure change, and
frequently the change of temperature as well, are
usually coupled to ex situ measurement and exert
a number of influences of varying potential. However, the ex situ measurement will certainly remain
a necessity for quite a long time, with regard to
most of the substances dissolved in pore water
and especially for great depths below the sediment surface. This book is not the place to give a
general review on sediment sampling techniques.
Rather, the more common procedures for sediment
sampling will be introduced with an emphasis put
on pore water analysis. Then, the particularities,
the possible errors as well as problems arising in
the application of these sampling techniques, will
be discussed.
The following section is concerned with the
separation of the aquatic pore water phase from
the solid sediment phase. As with all particularly
problematic and error-inducing procedures, these
various techniques have, depending on the case
at hand, their specific advantages and disadvantages.
As a matter of course, one would want to analyze the obtained pore water as soon as it has
been separated from the sediment to quantify the
dissolved substances therein. In daily routine
proceedings, compromises must be made since
not all analyses can be carried out simultaneously,
and since each and every analytical instrument is
not present on board a ship. Thus, it will be
necessary to describe the state of knowledge
concerning pore water storage, transport and preservation.
3.3.1
Obtaining Samples of Sediment for
the Analysis of Pore Water
A number of different techniques are available to
withdraw samples from the marine sediment.
Depending on the scientific question under study,
a tool may be chosen that is either capable of
taking the sample without harming the sediment
surface or disturbing the supernatant bottom water (e.g. multicorer), or that punches out a large as
possible sample from the sediment surface area
(e.g. box corer), or one that yields cores from the
upper less solid meters of the sediment which are
as long as possible and most undisturbed (gravity
corer, piston corer, box corer).
The Box Corer
The generic term ‘box corer’ denotes a number of
tools of different size and design used in sampling marine sediments, mostly lowered from
ships by means of steel wire rope to the bottom.
All have a square or rectangular metal box in common which is pressed into the sediment by their
own weight, or perhaps by additionally mounted
weights. Upon lifting the tool from the sea-floor
the first pull on the steel ropes is used to close
the box by means of a shovel while it is still situated in the sediment. At the same time, an opening at the top is shut, more or less tightly, so
that the bottom water immediately above the
sediment is entrapped. The lateral dimensions of