Study of Sulfate Reduction
245
with A VS in experiments with labeled sulfate added to bottom sediments and
salt marsh soils (Ivanov et a1.1976; Howarth 1979). The appearance of the label
in pyrite was recorded in samples of salt marsh soil incubated with labeled
sulfate for only 10-20h. To measure the radioactivity acquired by pyrite during
35S-sulfate reduction experiments, after AVS is extracted from them, the sediment samples are treated with hot aqua regia tu digest the pyrite, which is an
extremely stable compound and does not react even with strong acids. For the
same purpose, its reduction with Cr 2 + to H 2S was also employed according to
Zhabina and Volkov (1978), a method also employed by Ivanov et a1. (1976),
Howarth and Merkel (1984), Thode-Andersen and Jorgensen (1989) and
Fossing and Jorgensen (1989). The radioactivity found in pyrite + SO during the
experimental estimation of the rate of bacterial sulfate reduction, the single
products of which in short-term experiments should be only H 2S and FeS, was
respected as an underestimations of real sulfate reduction and was added to
that of AVS during the calculations of the results of the experiments. The possible underestimation of the sulfate reduction rates calculated without taking
the share of pyrite + SO into account, in accordance with data of the abovementioned research, can range between 0 and 80%, but most often it is
between 5 and 15%.
A rather high variability of this share itself points to a possible artifactnal
character in the labeling of these compounds and especially of pyrite. The
extremely slow rate of its formation in bottom sediments and in wetland soils
is well known. Its purely diagenetic character is proven by the very fact of
stable coexistence of rather high concentrations of its persecutors FeS and SO
(Jorgensen 1978). The H 2S produced during sulfate reduction reacts with ions
of ferrous iron Fe 2 +, thus forming the amorphous FeS. This exists in sediments
mainly as the mineral mackinavite and dispersed hydrotroilite, FeSH20, the
latter providing the black color in the reduced sediments. Hydrogen sulfide
dissolved in water reacts with ferrous ions Fe 3 +, being oxidized to the element
sulfur So: HS- + 2Fe 3 + ~ SO + 2Fe 2 + + H+. This sulfur is accumulated in sediments and slowly reacts with hydrotroilite, forming pyrite: FeS + SO ~ FeS2'
The reaction is accelerated in an acid medium (King 1983). Authors discussing
the miracle of the appearance of the label from H 2S in pyrite after several
hours of incubation most often complain that this phenomenon is still not
explainable; however, it becomes explainable if it is treated as an artifact, as
usually happens with a normal miracle. The most probable explanation is that
both labeled pyrite and labeled element sulfur are formed during the procedure of H 2S distillation from the strongly acidified sediments but not only
during the incubation of the samples with 35S04-2. The latter was proven experimentally by King (1983); but perhaps the most evident proof of pyrite labeling as an artifact can be found in the paper by Holmer and Nielsen (1997).
These authors experimented with eelgrass muds using the technique of Fossing
and Jorgenssen (1989) based on chromium reduction of total stock of reduced
sulfur in the sediment sample; at present, this has become very popular. In
accordance with their results, during sulfate reduction even in short-term
245
with A VS in experiments with labeled sulfate added to bottom sediments and
salt marsh soils (Ivanov et a1.1976; Howarth 1979). The appearance of the label
in pyrite was recorded in samples of salt marsh soil incubated with labeled
sulfate for only 10-20h. To measure the radioactivity acquired by pyrite during
35S-sulfate reduction experiments, after AVS is extracted from them, the sediment samples are treated with hot aqua regia tu digest the pyrite, which is an
extremely stable compound and does not react even with strong acids. For the
same purpose, its reduction with Cr 2 + to H 2S was also employed according to
Zhabina and Volkov (1978), a method also employed by Ivanov et a1. (1976),
Howarth and Merkel (1984), Thode-Andersen and Jorgensen (1989) and
Fossing and Jorgensen (1989). The radioactivity found in pyrite + SO during the
experimental estimation of the rate of bacterial sulfate reduction, the single
products of which in short-term experiments should be only H 2S and FeS, was
respected as an underestimations of real sulfate reduction and was added to
that of AVS during the calculations of the results of the experiments. The possible underestimation of the sulfate reduction rates calculated without taking
the share of pyrite + SO into account, in accordance with data of the abovementioned research, can range between 0 and 80%, but most often it is
between 5 and 15%.
A rather high variability of this share itself points to a possible artifactnal
character in the labeling of these compounds and especially of pyrite. The
extremely slow rate of its formation in bottom sediments and in wetland soils
is well known. Its purely diagenetic character is proven by the very fact of
stable coexistence of rather high concentrations of its persecutors FeS and SO
(Jorgensen 1978). The H 2S produced during sulfate reduction reacts with ions
of ferrous iron Fe 2 +, thus forming the amorphous FeS. This exists in sediments
mainly as the mineral mackinavite and dispersed hydrotroilite, FeSH20, the
latter providing the black color in the reduced sediments. Hydrogen sulfide
dissolved in water reacts with ferrous ions Fe 3 +, being oxidized to the element
sulfur So: HS- + 2Fe 3 + ~ SO + 2Fe 2 + + H+. This sulfur is accumulated in sediments and slowly reacts with hydrotroilite, forming pyrite: FeS + SO ~ FeS2'
The reaction is accelerated in an acid medium (King 1983). Authors discussing
the miracle of the appearance of the label from H 2S in pyrite after several
hours of incubation most often complain that this phenomenon is still not
explainable; however, it becomes explainable if it is treated as an artifact, as
usually happens with a normal miracle. The most probable explanation is that
both labeled pyrite and labeled element sulfur are formed during the procedure of H 2S distillation from the strongly acidified sediments but not only
during the incubation of the samples with 35S04-2. The latter was proven experimentally by King (1983); but perhaps the most evident proof of pyrite labeling as an artifact can be found in the paper by Holmer and Nielsen (1997).
These authors experimented with eelgrass muds using the technique of Fossing
and Jorgenssen (1989) based on chromium reduction of total stock of reduced
sulfur in the sediment sample; at present, this has become very popular. In
accordance with their results, during sulfate reduction even in short-term