248
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
specific radioactivity also in this way. No wonder, therefore, that this method
often gives values about twice as high as the slurry method (Jorgensen 1978),
while the latter also somewhat exaggerates natural rates of sulfate reduction.
The users of the corer method often mention that sometimes the values thus
obtained are unrealistically high considering the available stock of labile
organic matter in the sediment (Thode-Andersen and Jorgensen 1989). Finally,
in the corer method, as has been used in practice, I do not like the apparatus
used for the extraction of sulfides from the sediments during their processing
after the incubation with the labeled sulfate (Jorgensen and Fenchel 1974;
Ivanov et al. 1976). The devices have too much empty space inside by the minor
contact of the driving gas flow with the sediment slurry. Taking the above
considerations into account, below is given a description of both methods
with possible corrections of the corer method which I consider necessary to
introduce.
5.6.2 Practical Implementation: Bottom Sediments
5.6.2.1 Preparation of the Working Solution of Labeled Sulfate
Batches of labeled Na- sulfate or H 2S04 with or without carriers must be purchased with the following scale of radioactivity in the batch: (1) for work in
freshwater with the core method 0.2mCi and with the slurry method 0.5mCi;
(2) for work in the sea with the core method 2 m Ci, and with the slurry method
20 m Ci. Correspondingly, they should be diluted to 20 ml for the core method
and to 50 ml for the slurry method, meaning that during the experiments with
the cores 40 J.lI of thus diluted working solution isotopic solution will be
injected via the holes, and with the slurry method 0.1-0.2ml will be injected
per sample. After the batch container is opened, I-2ml of the carrier Na2S04
solution is added into it, containing 2 mg/m Ci for freshwater and 5 mg/m Ci for
seawater. Then the mixture is extracted from the container and transferred
into distilled water or into the 4% NaCI solution for seawater. The volumes
for such a dilution have been given above. The working solution of labeled
sulfate thus prepared is filtered through the membrane filter, and distributed
by 0.5-ml portions into the I-ml glass ampules, which are sealed and sterilized
by boiling twice with a I-day interval for 15 min. In order to avoid quench corrections during routine work, the radioactivity of labeled sulfate in the
working solution should be measured under counting conditions which simulate those during the radioassay of sulfide extracted from experimental
samples (Rs cpmml-l). The contents of sulfate in the sediment cores (Ks) is
estimated in parallel core samples taken in the same place as the experimental ones. The pore water is squeezed out from the whole 10-cm core column
and the sulfates are determined in it using a standard gravimetric method such
as BaS04. When using the slurry method, the Ks values are obtained from the
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
specific radioactivity also in this way. No wonder, therefore, that this method
often gives values about twice as high as the slurry method (Jorgensen 1978),
while the latter also somewhat exaggerates natural rates of sulfate reduction.
The users of the corer method often mention that sometimes the values thus
obtained are unrealistically high considering the available stock of labile
organic matter in the sediment (Thode-Andersen and Jorgensen 1989). Finally,
in the corer method, as has been used in practice, I do not like the apparatus
used for the extraction of sulfides from the sediments during their processing
after the incubation with the labeled sulfate (Jorgensen and Fenchel 1974;
Ivanov et al. 1976). The devices have too much empty space inside by the minor
contact of the driving gas flow with the sediment slurry. Taking the above
considerations into account, below is given a description of both methods
with possible corrections of the corer method which I consider necessary to
introduce.
5.6.2 Practical Implementation: Bottom Sediments
5.6.2.1 Preparation of the Working Solution of Labeled Sulfate
Batches of labeled Na- sulfate or H 2S04 with or without carriers must be purchased with the following scale of radioactivity in the batch: (1) for work in
freshwater with the core method 0.2mCi and with the slurry method 0.5mCi;
(2) for work in the sea with the core method 2 m Ci, and with the slurry method
20 m Ci. Correspondingly, they should be diluted to 20 ml for the core method
and to 50 ml for the slurry method, meaning that during the experiments with
the cores 40 J.lI of thus diluted working solution isotopic solution will be
injected via the holes, and with the slurry method 0.1-0.2ml will be injected
per sample. After the batch container is opened, I-2ml of the carrier Na2S04
solution is added into it, containing 2 mg/m Ci for freshwater and 5 mg/m Ci for
seawater. Then the mixture is extracted from the container and transferred
into distilled water or into the 4% NaCI solution for seawater. The volumes
for such a dilution have been given above. The working solution of labeled
sulfate thus prepared is filtered through the membrane filter, and distributed
by 0.5-ml portions into the I-ml glass ampules, which are sealed and sterilized
by boiling twice with a I-day interval for 15 min. In order to avoid quench corrections during routine work, the radioactivity of labeled sulfate in the
working solution should be measured under counting conditions which simulate those during the radioassay of sulfide extracted from experimental
samples (Rs cpmml-l). The contents of sulfate in the sediment cores (Ks) is
estimated in parallel core samples taken in the same place as the experimental ones. The pore water is squeezed out from the whole 10-cm core column
and the sulfates are determined in it using a standard gravimetric method such
as BaS04. When using the slurry method, the Ks values are obtained from the