94
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
say, that we will begin with the filled tubes of the
multicorer, the high-momentum gravity corer, or
the meter-sized pieces of the gravity corer or piston corer. The entire processing steps described
in the following for the core material should be
performed at temperatures which should be kept
as close as possible to the temperatures prevailing
under in situ conditions.
In this context, the problem needs to be dealt
with as to how long a tightly sealed core, which is
exposed to the in situ temperature all the while,
can be left to itself before it is processed without
the occurrence of any essential perturbation in the
pore water fraction. In order to extract pore water
from a sediment core, such as is shown in Figure
3.1, and subsequently analyze and preserve it,
even a practiced team would require several days.
In most cases, a compromise needs to be found
therefore, between the highest number of samples
and most rapid processing.
A special situation prevailed in the case of the
core shown in Figure 3.1, as the ship was cruising
for a relatively long time after the core had been
taken. Then the experiment was conducted that
led to the results shown in Figure 3.1. The core
was analyzed with an almost unusual high
sampling density. As these procedures afforded
plenty of time, and since it was unsure whether
the pore water of the sediment core, which was
stored at in situ temperature in the meantime,
would change within the prospective processing
time of ten days, the single pieces measuring one
meter were intentionally not analyzed in a depth
sequence, but in a randomized sequence. Thereby, the variations in the processing time had to
become reflected as discontinuities in the corresponding data at the end of each meter interval.
This had not been the case at either interval, from
which it follows that a processing time of 10 days
was obviously quite innocuous to the quality of
the samples.
Analysis of Dissolved Gases
For some substances dissolved in pore water
everything will be too late for a reliable analysis
as soon as the sediment core lies freshly, but in a
decompressed state, on the deck of the ship. This
holds true predominantly for the dissolved gases.
In this respect, dissolved oxygen is relatively
easy to manage, a circumstance which becomes
evident upon comparing the in situ oxygen profile
with the ex situ profile, both measured at the same
sites (shown in Fig. 3.12). The reason for this
similarity is that the relatively low concentrations,
that are often below the saturation level, do not
significantly assume a condition of oversaturation
even after decompression. Notwithstanding, the
results shown in Figure 3.12 reflect the situation
too favourably, because differences of up to a
factor of 2 were also observed between in situ and
ex situ conditions with regard to the penetration
depth of oxygen, and thus to the corresponding
reaction rates as well. As to what extent these
differences between in situ and ex situ conditions
really exist, or whether such differences result
from measurements carried out at not exactly
identical sites, has not yet been sufficiently investigated.
The case is obviously similar for dissolved
carbon dioxide whose concentration is essentially
determined by the equilibrium of the aquatic
carbonate phases. As for the alkalinity, no remarkable variations were found in measured values,
even at high concentrations, when the measurements were performed successively on adjacent
parts of the same sediment core (cf. alkalinity
profile shown in Fig. 3.1).
The measurement of sulfide is much more
troublesome, especially at high concentrations (It
should be noted that H 2 S is strongly toxic and
that one can become quickly accustomed to its
smell after prolonged presence in the laboratory.
When working with sulfide-containing core material the laboratory should be ventilated thoroughly at all times!). As a general rule, everything already perceived by its smell is already lost
to analysis. Since sulfide is readily analyzed by
various methods in aqueous solution, most errors
arise from decompression of the core and the
subsequent separation of pore water from the
sediment. As soon as decompression begins, a
great quantity starts to degas, initially forming a
finely distributed effervescence. In this condition,
measurements can mostly still be carried out,
however with less satisfactory results, provided
that a specific volume of water-containing degassed sediment is punched out with a syringe and
immediately brought into an alkaline environment
(SAOB = Sulfur Anti-Oxidizing Buffer with pH >
13, after Cornwell and Morse 1987).
Even more difficult is the sampling and the
analysis of sediments with a marked content of
methane gas. In this particular case, a considerable amount of degassing of the sample occurs
immediately upon decompression, thus not
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
say, that we will begin with the filled tubes of the
multicorer, the high-momentum gravity corer, or
the meter-sized pieces of the gravity corer or piston corer. The entire processing steps described
in the following for the core material should be
performed at temperatures which should be kept
as close as possible to the temperatures prevailing
under in situ conditions.
In this context, the problem needs to be dealt
with as to how long a tightly sealed core, which is
exposed to the in situ temperature all the while,
can be left to itself before it is processed without
the occurrence of any essential perturbation in the
pore water fraction. In order to extract pore water
from a sediment core, such as is shown in Figure
3.1, and subsequently analyze and preserve it,
even a practiced team would require several days.
In most cases, a compromise needs to be found
therefore, between the highest number of samples
and most rapid processing.
A special situation prevailed in the case of the
core shown in Figure 3.1, as the ship was cruising
for a relatively long time after the core had been
taken. Then the experiment was conducted that
led to the results shown in Figure 3.1. The core
was analyzed with an almost unusual high
sampling density. As these procedures afforded
plenty of time, and since it was unsure whether
the pore water of the sediment core, which was
stored at in situ temperature in the meantime,
would change within the prospective processing
time of ten days, the single pieces measuring one
meter were intentionally not analyzed in a depth
sequence, but in a randomized sequence. Thereby, the variations in the processing time had to
become reflected as discontinuities in the corresponding data at the end of each meter interval.
This had not been the case at either interval, from
which it follows that a processing time of 10 days
was obviously quite innocuous to the quality of
the samples.
Analysis of Dissolved Gases
For some substances dissolved in pore water
everything will be too late for a reliable analysis
as soon as the sediment core lies freshly, but in a
decompressed state, on the deck of the ship. This
holds true predominantly for the dissolved gases.
In this respect, dissolved oxygen is relatively
easy to manage, a circumstance which becomes
evident upon comparing the in situ oxygen profile
with the ex situ profile, both measured at the same
sites (shown in Fig. 3.12). The reason for this
similarity is that the relatively low concentrations,
that are often below the saturation level, do not
significantly assume a condition of oversaturation
even after decompression. Notwithstanding, the
results shown in Figure 3.12 reflect the situation
too favourably, because differences of up to a
factor of 2 were also observed between in situ and
ex situ conditions with regard to the penetration
depth of oxygen, and thus to the corresponding
reaction rates as well. As to what extent these
differences between in situ and ex situ conditions
really exist, or whether such differences result
from measurements carried out at not exactly
identical sites, has not yet been sufficiently investigated.
The case is obviously similar for dissolved
carbon dioxide whose concentration is essentially
determined by the equilibrium of the aquatic
carbonate phases. As for the alkalinity, no remarkable variations were found in measured values,
even at high concentrations, when the measurements were performed successively on adjacent
parts of the same sediment core (cf. alkalinity
profile shown in Fig. 3.1).
The measurement of sulfide is much more
troublesome, especially at high concentrations (It
should be noted that H 2 S is strongly toxic and
that one can become quickly accustomed to its
smell after prolonged presence in the laboratory.
When working with sulfide-containing core material the laboratory should be ventilated thoroughly at all times!). As a general rule, everything already perceived by its smell is already lost
to analysis. Since sulfide is readily analyzed by
various methods in aqueous solution, most errors
arise from decompression of the core and the
subsequent separation of pore water from the
sediment. As soon as decompression begins, a
great quantity starts to degas, initially forming a
finely distributed effervescence. In this condition,
measurements can mostly still be carried out,
however with less satisfactory results, provided
that a specific volume of water-containing degassed sediment is punched out with a syringe and
immediately brought into an alkaline environment
(SAOB = Sulfur Anti-Oxidizing Buffer with pH >
13, after Cornwell and Morse 1987).
Even more difficult is the sampling and the
analysis of sediments with a marked content of
methane gas. In this particular case, a considerable amount of degassing of the sample occurs
immediately upon decompression, thus not