246
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
experiments practically only pyrite was formed, as evidenced by the share the
chromium reducible fraction in total radioactivity of reduced sulfur of over
98%, while the share of AVS was less than 5%. According to their data, the
stock of AVS in the sediments at a core depth of Scm was less than 10mgS 2 -
dm- 3 of mud, which is also clearly nonsense, because this stock in the seagrass
muds simply could not be less than 100mgdm--', and usually, it is two to three
times more. The cause of this and of AVS underestimation is the use of overstrong (12N) aggressive hydrochloric acid, which results in the instant formation of SO from hydrotrolite. Then the following treatment with chromium
reduces it again of H 2 S and is erroneously considered as pyrite.
The practical conclusions which must be drawn, are: (1) the procedure of
H Z 35 S distillation should be softened: the acidification should proceed gradually and the basic stock of H 2S should be first extracted by COz, which is
formed in any case in the reaction vessel from carbonates. If the sediments do
not contain enough carbonates, these should be added to the sample before
the H 2S-distillation procedure, (2) instead of HCl, a less aggressive 10% H 2S04
should be used for acidifying the samples, (3) the sulfate reduction rates measured by both the slurry and the core techniques (see below) are at best only
an approximation of the real level. In this case, approximately 5-10% of probable correction for the radioactivity lost with pyrite and SO is inexplicable,
moreover: (a) the procedure of their estimation is very complex and timeconsuming, (b) the heating of the sample, which is an element of this procedure, inevitably results in errors when working with easily oxidizable compounds like SO or HzS, and (c) the SO and FeSz may be formed also from sulfate
pools other than H 2S and thus their specific activities are actually unknown
(Howarth 1979).
The radioisotopic method for measuring the in situ rate of sulfate reduction in sediments is used in its two main modifications - the slurry modification
(Sorokin 1962) and the core modification (Ivanov 1965;Jorgensen and Fenchel
1974; Jorgensen 1977). To choose between them, it is necessary to know the
advantages and drawbacks combined with their practical use. First of all, these
methods are actually not alternative to each other but, rather, are complementary, because they can be applied for solving different problems. The slurry
method, being much simpler and standard, serves well to measure sulfate
reduction rates in the upper 0-5 cm sediment layers, which is important for
purposes of monitoring the ecological state of large areas in water basins. The
core method is much more complex and time-consuming; it needs greater
expertise and time, and serves mostly to solve the principal problems of the
sulfate reduction process in sediments, such as its vertical distribution or H 2S
production per 1 m- z or bottom area.
According to the slurry method, the sample of sediment is placed into a
40-60-ml test tube, which is completely filled with natural water with a certain
amount of sulfide (to keep the Eh low), with the radioisotopic working solution of labeled sulfate previously added to it. The content of the test tube is
then mixed, making a slurry. During the following period of incubation, the
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
experiments practically only pyrite was formed, as evidenced by the share the
chromium reducible fraction in total radioactivity of reduced sulfur of over
98%, while the share of AVS was less than 5%. According to their data, the
stock of AVS in the sediments at a core depth of Scm was less than 10mgS 2 -
dm- 3 of mud, which is also clearly nonsense, because this stock in the seagrass
muds simply could not be less than 100mgdm--', and usually, it is two to three
times more. The cause of this and of AVS underestimation is the use of overstrong (12N) aggressive hydrochloric acid, which results in the instant formation of SO from hydrotrolite. Then the following treatment with chromium
reduces it again of H 2 S and is erroneously considered as pyrite.
The practical conclusions which must be drawn, are: (1) the procedure of
H Z 35 S distillation should be softened: the acidification should proceed gradually and the basic stock of H 2S should be first extracted by COz, which is
formed in any case in the reaction vessel from carbonates. If the sediments do
not contain enough carbonates, these should be added to the sample before
the H 2S-distillation procedure, (2) instead of HCl, a less aggressive 10% H 2S04
should be used for acidifying the samples, (3) the sulfate reduction rates measured by both the slurry and the core techniques (see below) are at best only
an approximation of the real level. In this case, approximately 5-10% of probable correction for the radioactivity lost with pyrite and SO is inexplicable,
moreover: (a) the procedure of their estimation is very complex and timeconsuming, (b) the heating of the sample, which is an element of this procedure, inevitably results in errors when working with easily oxidizable compounds like SO or HzS, and (c) the SO and FeSz may be formed also from sulfate
pools other than H 2S and thus their specific activities are actually unknown
(Howarth 1979).
The radioisotopic method for measuring the in situ rate of sulfate reduction in sediments is used in its two main modifications - the slurry modification
(Sorokin 1962) and the core modification (Ivanov 1965;Jorgensen and Fenchel
1974; Jorgensen 1977). To choose between them, it is necessary to know the
advantages and drawbacks combined with their practical use. First of all, these
methods are actually not alternative to each other but, rather, are complementary, because they can be applied for solving different problems. The slurry
method, being much simpler and standard, serves well to measure sulfate
reduction rates in the upper 0-5 cm sediment layers, which is important for
purposes of monitoring the ecological state of large areas in water basins. The
core method is much more complex and time-consuming; it needs greater
expertise and time, and serves mostly to solve the principal problems of the
sulfate reduction process in sediments, such as its vertical distribution or H 2S
production per 1 m- z or bottom area.
According to the slurry method, the sample of sediment is placed into a
40-60-ml test tube, which is completely filled with natural water with a certain
amount of sulfide (to keep the Eh low), with the radioisotopic working solution of labeled sulfate previously added to it. The content of the test tube is
then mixed, making a slurry. During the following period of incubation, the