73
3
Quantification of Early Diagenesis:
Dissolved Constituents in Pore Water and
Signals in the Solid Phase
HORST D. SCHULZ
A chapter on the pore water of sediments and the
processes of early diagenesis, reflected by the
concentration profiles therein, can certainly be
structured in different ways. One possibility is,
for example, to start with the sample-taking
strategies, followed by the analytical treatment of
the samples, then a model presentation to substantiate the processes, and finally a quantitative
evaluation of the measured profiles with regard to
material fluxes and reaction rates. The reader who
would prefer this sequence is recommended to
begin with the Sections 3.3, 3.4, 3.5 and to
consult the Sections 3.1 and 3.2 later. However, in
our opinion, marine geochemistry departed from
its initial development stage of sample collection
and anlysis some years ago. This book is
therefore written with a different structure from
earlier texts. The well understood theoretical
knowledge on concentration profiles has to be
introduced from the beginning. Only then do
problems arise which necessitate the investigation of particular substances in pore water and
the application of specific methods of sampling
and analysis.
Particularly, two fundamental approaches are
conceivable for investigating diagenesis processes, both having their advantages and disadvantages. The classical way leads to a sedimentological, mineralogical, and in the broadest sense
geochemical examination of the solid phase. The
advantage of this approach is that sample
withdrawal and analytical procedures are generally easier to carry out and produce more reliable
results. On the other hand, in most cases this
approach will not provide us with any information
on the timing at which specific diagenetical
alterations in the sediment took place. Nothing
will be learnt about reaction rates, reaction kinetics, and mostly we do not even know whether
the processes, interpreted on account of certain
measurements and/or observations, occur today
or whether they were terminated some time
before. Furthermore, the distinction between the
primary signals from the water column and the
results of diagenetic reactions, often remains
unclear.
A section dealing with the concentrations of
elements in the solid phase of the sediment
should not be missing despite the mentioned
disadvantages and problems that come along
with a geochemical description of a sediment/
pore water system. Section 3.7 therefore contains
a brief presentation of the various sampleprocessing methods and analyses, next to some
examples of measured element profiles. One of the
greatest problems encountered in interpreting element concentrations in the sediment’s solid
phase will always be one’s ability to distinguish
between a primary signal from the water column,
i.e. from the introduction into the sediment, and
the diagenetic alteration of the material.
If the processes of diagenesis and their timedependency are to be recorded directly, then a
second approach appears to be more reasonable;
one that includes the measurement of concentration profiles in pore water. Figure 3.1 shows
various concentration profiles of substances
dissolved in the pore water of sediments taken at
a depth of approximately 4000 m, off the estuary
of the River Congo. The differently shaped
curves shown here directly reflect the reactions
that are happening today. The investigation of
pore water thus constitutes the only way to
determine the reaction rates and the fluxes of
material which they produce. Yet, this procedure
is rather laborious and unfortunately contains
numerous opportunities for errors. Most errors
are caused as soon as the sediment sample is
taken, others are caused upon preparing the pore
water from the sediment, or in the course of
3
Quantification of Early Diagenesis:
Dissolved Constituents in Pore Water and
Signals in the Solid Phase
HORST D. SCHULZ
A chapter on the pore water of sediments and the
processes of early diagenesis, reflected by the
concentration profiles therein, can certainly be
structured in different ways. One possibility is,
for example, to start with the sample-taking
strategies, followed by the analytical treatment of
the samples, then a model presentation to substantiate the processes, and finally a quantitative
evaluation of the measured profiles with regard to
material fluxes and reaction rates. The reader who
would prefer this sequence is recommended to
begin with the Sections 3.3, 3.4, 3.5 and to
consult the Sections 3.1 and 3.2 later. However, in
our opinion, marine geochemistry departed from
its initial development stage of sample collection
and anlysis some years ago. This book is
therefore written with a different structure from
earlier texts. The well understood theoretical
knowledge on concentration profiles has to be
introduced from the beginning. Only then do
problems arise which necessitate the investigation of particular substances in pore water and
the application of specific methods of sampling
and analysis.
Particularly, two fundamental approaches are
conceivable for investigating diagenesis processes, both having their advantages and disadvantages. The classical way leads to a sedimentological, mineralogical, and in the broadest sense
geochemical examination of the solid phase. The
advantage of this approach is that sample
withdrawal and analytical procedures are generally easier to carry out and produce more reliable
results. On the other hand, in most cases this
approach will not provide us with any information
on the timing at which specific diagenetical
alterations in the sediment took place. Nothing
will be learnt about reaction rates, reaction kinetics, and mostly we do not even know whether
the processes, interpreted on account of certain
measurements and/or observations, occur today
or whether they were terminated some time
before. Furthermore, the distinction between the
primary signals from the water column and the
results of diagenetic reactions, often remains
unclear.
A section dealing with the concentrations of
elements in the solid phase of the sediment
should not be missing despite the mentioned
disadvantages and problems that come along
with a geochemical description of a sediment/
pore water system. Section 3.7 therefore contains
a brief presentation of the various sampleprocessing methods and analyses, next to some
examples of measured element profiles. One of the
greatest problems encountered in interpreting element concentrations in the sediment’s solid
phase will always be one’s ability to distinguish
between a primary signal from the water column,
i.e. from the introduction into the sediment, and
the diagenetic alteration of the material.
If the processes of diagenesis and their timedependency are to be recorded directly, then a
second approach appears to be more reasonable;
one that includes the measurement of concentration profiles in pore water. Figure 3.1 shows
various concentration profiles of substances
dissolved in the pore water of sediments taken at
a depth of approximately 4000 m, off the estuary
of the River Congo. The differently shaped
curves shown here directly reflect the reactions
that are happening today. The investigation of
pore water thus constitutes the only way to
determine the reaction rates and the fluxes of
material which they produce. Yet, this procedure
is rather laborious and unfortunately contains
numerous opportunities for errors. Most errors
are caused as soon as the sediment sample is
taken, others are caused upon preparing the pore
water from the sediment, or in the course of