104
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
For the analysis of ammonium, the sample carrier stream consists of 10 mM NaOH + 0.2 M Nacitrate. This converts ammonium into NH 3 -gas
which penetrates the PTFE membrane and travels
into the gas receiver stream, in this case consisting of 50 µm HCl. Here as well, an electric
conductivity cell is used for the measurements. To
provide a steady and impulse-free flow of the
solutions, a multichannel peristaltic pump is used.
The flow rates in both cases were set to approx.
1.4 ml min
-1
.
These very reliable and easy to establish analytical procedures for the determination of these
two important parameters of marine pore water are
of special interest, because they require only small
sample amounts and because a reliable analysis is
obtained over a broad range of concentrations.
Ion-selective Electrodes
Ion-selective electrodes were accepted in water
analysis only with great reluctance, despite of the
advantages that had been expected on their introduction about 20 years ago. The reasons for this
were manifold. The analytical procedure is inexpensive only at first sight. As has been confirmed
for some electrodes (especially when its handling
is not always appropriate), aging sets in soon
after initial use which becomes noticeable with
decreased sensitivity, low stability of measured
values, and prolonged adjustment times.
With regard to marine pore water, ion-selective
electrodes were successfully applied in the determination of fluoride (standard addition method) and
in the analysis of sulfide within a mixture of sediment
and pore water processed into SAOB-buffer (cf.
Sect. 3.3.2). Figure 3.1 shows a typical profile of
fluoride measured with an ion-selective electrode in
pore water. The total sulfide profile shown in Figure
3.14 was measured in the way described above, by
using an ion-selective electrode. The measured
values occasionally display considerable variations,
probably on account of the sediment’s decompression and thereafter they might be produced
upon withdrawing the sample from the core. They do
not necessarily come about during the course of the
analytical measurement.
Ion-Chromatography
The analysis of chloride and sulfate in marine pore
water samples after an approximately 20-fold dilution is a standard method for normal ion-chromatography (HPLC), so that no further discussion is
needed here. Considering the very dilute sample
solution and the low sample amount required in
ion-chromatography, the applied quantity of
pressed or centrifuged pore water is negligibly
small. However, the high background of chloride
and sulfate which is due to the salt content in sea
water prevents, with or without dilution, the
determination of all other anion species by ionchromatography. The sulfate profiles measured
with this method are shown in the Figures 3.1, 3.6,
and 3.14. Since the chloride profiles are mostly not
very interesting, because they reflect practically
no early diagenesis reactions at all, they can be
consulted for control and eventually for correction of the sulfate profiles, whenever analytical
errors have emerged in the course of their concomitant determination, e.g. due to faulty
dilutions, or mishaps occurring in the injection
valve of the machinery.
ICP-AES, AAS, ICP-MS
Generally the alkali metals (Li, Na, K, Rb, Cs), the
alkali earth metals (Mg, Ca, Sr, Ba) and other
metals (e.g. Fe, Mn, Si, Al, Cu, Zn, Cr, Co, Ni) are
determined in the acid-preserved pore water
samples. Analytical details depend on the special
conditions provided by the applied analytical
instruments ICP-AES (Inductively Coupled Plasma
Atomic Emission Spectrometer), ICP-MS
(Inductively Coupled Plasma Mass Spectrometer),
or AAS (Atomic Absorption Spectrometer) of the
respective laboratory. A discussion of details is
not appropriate in this chapter. In most cases
dilutions of 1:10 or 1:100 will be measured
depending on the salt content in marine environments, so that the required sample amounts are
rather low.
Gas-Chromatography
The quantification of methane with the aid of gas
chromatography (FID-detection) is an excellent
standard method, which therefore does not need
any further discussion. The main importance is
the immediate withdrawal of a sediment/pore water
sample as already mentioned in Section 3.3.2. This
sample is placed into a 50 ml-headspace vial with a
syringe in which the sample is combined with 20
ml of a prepared solution of 1.2 M NaCl + 0.3 M
HgCl 2 . After equilibrium is reached in the closed
bottle between the methane concentration in the
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
For the analysis of ammonium, the sample carrier stream consists of 10 mM NaOH + 0.2 M Nacitrate. This converts ammonium into NH 3 -gas
which penetrates the PTFE membrane and travels
into the gas receiver stream, in this case consisting of 50 µm HCl. Here as well, an electric
conductivity cell is used for the measurements. To
provide a steady and impulse-free flow of the
solutions, a multichannel peristaltic pump is used.
The flow rates in both cases were set to approx.
1.4 ml min
-1
.
These very reliable and easy to establish analytical procedures for the determination of these
two important parameters of marine pore water are
of special interest, because they require only small
sample amounts and because a reliable analysis is
obtained over a broad range of concentrations.
Ion-selective Electrodes
Ion-selective electrodes were accepted in water
analysis only with great reluctance, despite of the
advantages that had been expected on their introduction about 20 years ago. The reasons for this
were manifold. The analytical procedure is inexpensive only at first sight. As has been confirmed
for some electrodes (especially when its handling
is not always appropriate), aging sets in soon
after initial use which becomes noticeable with
decreased sensitivity, low stability of measured
values, and prolonged adjustment times.
With regard to marine pore water, ion-selective
electrodes were successfully applied in the determination of fluoride (standard addition method) and
in the analysis of sulfide within a mixture of sediment
and pore water processed into SAOB-buffer (cf.
Sect. 3.3.2). Figure 3.1 shows a typical profile of
fluoride measured with an ion-selective electrode in
pore water. The total sulfide profile shown in Figure
3.14 was measured in the way described above, by
using an ion-selective electrode. The measured
values occasionally display considerable variations,
probably on account of the sediment’s decompression and thereafter they might be produced
upon withdrawing the sample from the core. They do
not necessarily come about during the course of the
analytical measurement.
Ion-Chromatography
The analysis of chloride and sulfate in marine pore
water samples after an approximately 20-fold dilution is a standard method for normal ion-chromatography (HPLC), so that no further discussion is
needed here. Considering the very dilute sample
solution and the low sample amount required in
ion-chromatography, the applied quantity of
pressed or centrifuged pore water is negligibly
small. However, the high background of chloride
and sulfate which is due to the salt content in sea
water prevents, with or without dilution, the
determination of all other anion species by ionchromatography. The sulfate profiles measured
with this method are shown in the Figures 3.1, 3.6,
and 3.14. Since the chloride profiles are mostly not
very interesting, because they reflect practically
no early diagenesis reactions at all, they can be
consulted for control and eventually for correction of the sulfate profiles, whenever analytical
errors have emerged in the course of their concomitant determination, e.g. due to faulty
dilutions, or mishaps occurring in the injection
valve of the machinery.
ICP-AES, AAS, ICP-MS
Generally the alkali metals (Li, Na, K, Rb, Cs), the
alkali earth metals (Mg, Ca, Sr, Ba) and other
metals (e.g. Fe, Mn, Si, Al, Cu, Zn, Cr, Co, Ni) are
determined in the acid-preserved pore water
samples. Analytical details depend on the special
conditions provided by the applied analytical
instruments ICP-AES (Inductively Coupled Plasma
Atomic Emission Spectrometer), ICP-MS
(Inductively Coupled Plasma Mass Spectrometer),
or AAS (Atomic Absorption Spectrometer) of the
respective laboratory. A discussion of details is
not appropriate in this chapter. In most cases
dilutions of 1:10 or 1:100 will be measured
depending on the salt content in marine environments, so that the required sample amounts are
rather low.
Gas-Chromatography
The quantification of methane with the aid of gas
chromatography (FID-detection) is an excellent
standard method, which therefore does not need
any further discussion. The main importance is
the immediate withdrawal of a sediment/pore water
sample as already mentioned in Section 3.3.2. This
sample is placed into a 50 ml-headspace vial with a
syringe in which the sample is combined with 20
ml of a prepared solution of 1.2 M NaCl + 0.3 M
HgCl 2 . After equilibrium is reached in the closed
bottle between the methane concentration in the