Study of Sulfate Reduction
247
sediment resettles rather rapidly onto the bottom, thus restoring the ambiental situation of silt covered with a clear water layer. At the end of incubation,
the sample is fixed the ZnClz. Then it can be processed with only one operation of the extraction of labeled sulfide. This method needs neither measurement of radioactivity of sulfate in the sample (Rs) nor its absolute content (Ks)
to estimate its inverse specific radiuacLiviLy (Sr) in the;: e;:xperimental sample;:.
The first value is predicted by the radioactivity of sulfate in the volume of
working solution of 35SOl-. The second (Ks) is equal to the sulfate content in
the sample of natural water used in the experiments. For seawater its values
can be found in the oceanographic tables.
The core method deals with intact sediment cores taken with the aid of
plastic tubes of 2.5-3 cm in diameter. It is used in two versions: in one version,
the radioisotopic solution is injected into the core with the aid of syringe with
a long needle from its upper open end (Ivanov 1968). The needle is gradually
withdrawn during the injection, thus distributing the label along the core in its
center, assuming that with time it will be diffused horizontally in the sediment.
According to the other version (Jorgensen 1977), the sample is taken with the
aid of a corer tube having holes filled with silicone rubber on its sdie. The holes
are situated at 1-2-cm intervals and serve for horizontal injection of the label.
The latter is injected in micro quantities of 2-4 III with a micro syringe. For injection in this case a carrier-free solution of sodium sulfate or sulfuric acid is used.
The label is laid horizontally while the syringe is withdrawn. After incubation,
during which the label is presumed to have gradually diffused into the sediment column, the core is cut into 2-cm sectors, each of which is processed for
all three parameters: R;, R., and Ks. This means that with this method, for each
sample 15-20 individual analyses should be made instead of 1 or 2 with the
slurry method.
The main disadvantage of this latter method is its leng-thiness. If there
were any hope of obtaining adequate data on the rate of sulfate reduction
even this lengthy procedure would be acceptable; but it seems that also with
this method this is actually impossible. It can give no more than an approximation with a tendency to overestimation because of the extremely indefinite
character of specific activity of labeled sulfate in this case. In fact, we are
dealing here with specific radioactivity which is continuously changing during
exposure in space and in time. During the process of diffusion of injected
microvolumes of label solution from the place of injection, where initially it is
extremely high, it gradually decreases in the sediment column. Thus, the pool
of sulfates is labeled differently in different parts of the core at any given incubation time. The unknown character of the specific radioactivity of this pool
also results from the existence of isolated micropools inside the structures of
the undisturbed sediment. What is especially wrong with this method, from my
viewpoint, is the use of carrier-free solutions of labeled sulfate for the injection. Having an extremely high specific radioactivity, the nanoquantities of
strongly labeled sulfate may be instantly absorbed by minerals and colloids
and may enter the exchange reactions with sulfate minerals, decreasing its
247
sediment resettles rather rapidly onto the bottom, thus restoring the ambiental situation of silt covered with a clear water layer. At the end of incubation,
the sample is fixed the ZnClz. Then it can be processed with only one operation of the extraction of labeled sulfide. This method needs neither measurement of radioactivity of sulfate in the sample (Rs) nor its absolute content (Ks)
to estimate its inverse specific radiuacLiviLy (Sr) in the;: e;:xperimental sample;:.
The first value is predicted by the radioactivity of sulfate in the volume of
working solution of 35SOl-. The second (Ks) is equal to the sulfate content in
the sample of natural water used in the experiments. For seawater its values
can be found in the oceanographic tables.
The core method deals with intact sediment cores taken with the aid of
plastic tubes of 2.5-3 cm in diameter. It is used in two versions: in one version,
the radioisotopic solution is injected into the core with the aid of syringe with
a long needle from its upper open end (Ivanov 1968). The needle is gradually
withdrawn during the injection, thus distributing the label along the core in its
center, assuming that with time it will be diffused horizontally in the sediment.
According to the other version (Jorgensen 1977), the sample is taken with the
aid of a corer tube having holes filled with silicone rubber on its sdie. The holes
are situated at 1-2-cm intervals and serve for horizontal injection of the label.
The latter is injected in micro quantities of 2-4 III with a micro syringe. For injection in this case a carrier-free solution of sodium sulfate or sulfuric acid is used.
The label is laid horizontally while the syringe is withdrawn. After incubation,
during which the label is presumed to have gradually diffused into the sediment column, the core is cut into 2-cm sectors, each of which is processed for
all three parameters: R;, R., and Ks. This means that with this method, for each
sample 15-20 individual analyses should be made instead of 1 or 2 with the
slurry method.
The main disadvantage of this latter method is its leng-thiness. If there
were any hope of obtaining adequate data on the rate of sulfate reduction
even this lengthy procedure would be acceptable; but it seems that also with
this method this is actually impossible. It can give no more than an approximation with a tendency to overestimation because of the extremely indefinite
character of specific activity of labeled sulfate in this case. In fact, we are
dealing here with specific radioactivity which is continuously changing during
exposure in space and in time. During the process of diffusion of injected
microvolumes of label solution from the place of injection, where initially it is
extremely high, it gradually decreases in the sediment column. Thus, the pool
of sulfates is labeled differently in different parts of the core at any given incubation time. The unknown character of the specific radioactivity of this pool
also results from the existence of isolated micropools inside the structures of
the undisturbed sediment. What is especially wrong with this method, from my
viewpoint, is the use of carrier-free solutions of labeled sulfate for the injection. Having an extremely high specific radioactivity, the nanoquantities of
strongly labeled sulfate may be instantly absorbed by minerals and colloids
and may enter the exchange reactions with sulfate minerals, decreasing its