II ()
5 Spallal and Temporal DislrIhullOn Patterns
Following the experiences of the previous year, starch (tlour) was added to the
sediment in different quantities (0.45 kg C/m2 up to 2.7 kg C/m2) in four experimental treatments in 1995 (Langner 1997, Oelschlager unpub!. data). Mesocosm
studies, with different organic substrates had shown that starch caused the best
agreement with a natural load e.g. by macroalgae. In these field experiments, the
temporal development of total sulphur, dissolved sulphate and sulphide, iron (soluble and total), DOC, water-content, sediment density and POC were determined.
Directly after the experiment started, increased sulphide concentrations were
detected at the three sites with the highest enrichment. After four weeks increased
sulphide values were recorded at all experimental sites. Two months after the onset
of the experiment, sulphide values decreased in the treatments with the lowest
amounts of POC (S 1, 0.45 kg C m'\ whereas unchanged concentrations of sulphide were measured in the treatments S2 and S3 (medium load). At treatment S4
with the highest organic load (2.7 kg C m· 2 ) the rise of the sulphide concentration
continued. After a decrease during the winter months one year after the beginning
of the experiment the sulphide concentrations in treatment S4 increased again, to
the level of the previous year. In treatment S3, the sulphide concentrations were
slightly lower. while the values of the treatments S I and S2 corresponded to those
of the unpolluted reference sites. The results indicated that the regeneration times
depended on the magnitude of the organic load. The released DOC quantities were
also coupled to the amount of the load. The sulphate concentrations were reversely
correlated to the sulphide-values - with increasing sulphide concentrations, the
sulphate-values decreased. During the summer months, a complete exhaustion of
the sulphate-pool was frequently observed at several sediment depths. Without an
organic load, neither sulphide nor methane were emitted. Six weeks after the onset
of the experiment a linear relation was found between the released CO 2 -amounts
and the degree of the load, while the emissions of sulphide and methane showed no
linear correlation with the quantity of starch brought into the sediment. The production of sulphide was probably limited by the availability of sulphate. For methane, a clear difference was determined between day and night emissions, which did
not occur for sulphide and CO,.
In the context of the loading experiments effects on the isotope-geochemistry of
solid and dissolved phases were examined as well (Bottcher unpub!. data). In contrast to the reference site, the DOC and IHS-concentrations in the sediments enriched with organic matter were increased (Fig. 5.2.11). Probably because sufficiently large quantities of reactive iron complexes were missing in the sandy
sediments and/or the reaction rate of the iron-containing solid phases was too slow.
the formed IHS could not be precipitated as iron-sulphide. In contrast to the open
ocean, the dissolved residual sulphate was enriched with '4 S , as during the process
of sulphate-reduction the light S-isotope changes preferentially into the reduced
phase. Fractionation factors were between -5 to -25 %0. The decrease of the fractionation factor was the result of an increase in the absolute rate of sulphatereduction. The rate of sulphate-reduction is mainly a function of the availability of
organic substances, pore water sulphate and the ambient temperature. Indeed, with
increasing ambient temperature the isotope fractionation factor in pore water sulphate decreased.
5 Spallal and Temporal DislrIhullOn Patterns
Following the experiences of the previous year, starch (tlour) was added to the
sediment in different quantities (0.45 kg C/m2 up to 2.7 kg C/m2) in four experimental treatments in 1995 (Langner 1997, Oelschlager unpub!. data). Mesocosm
studies, with different organic substrates had shown that starch caused the best
agreement with a natural load e.g. by macroalgae. In these field experiments, the
temporal development of total sulphur, dissolved sulphate and sulphide, iron (soluble and total), DOC, water-content, sediment density and POC were determined.
Directly after the experiment started, increased sulphide concentrations were
detected at the three sites with the highest enrichment. After four weeks increased
sulphide values were recorded at all experimental sites. Two months after the onset
of the experiment, sulphide values decreased in the treatments with the lowest
amounts of POC (S 1, 0.45 kg C m'\ whereas unchanged concentrations of sulphide were measured in the treatments S2 and S3 (medium load). At treatment S4
with the highest organic load (2.7 kg C m· 2 ) the rise of the sulphide concentration
continued. After a decrease during the winter months one year after the beginning
of the experiment the sulphide concentrations in treatment S4 increased again, to
the level of the previous year. In treatment S3, the sulphide concentrations were
slightly lower. while the values of the treatments S I and S2 corresponded to those
of the unpolluted reference sites. The results indicated that the regeneration times
depended on the magnitude of the organic load. The released DOC quantities were
also coupled to the amount of the load. The sulphate concentrations were reversely
correlated to the sulphide-values - with increasing sulphide concentrations, the
sulphate-values decreased. During the summer months, a complete exhaustion of
the sulphate-pool was frequently observed at several sediment depths. Without an
organic load, neither sulphide nor methane were emitted. Six weeks after the onset
of the experiment a linear relation was found between the released CO 2 -amounts
and the degree of the load, while the emissions of sulphide and methane showed no
linear correlation with the quantity of starch brought into the sediment. The production of sulphide was probably limited by the availability of sulphate. For methane, a clear difference was determined between day and night emissions, which did
not occur for sulphide and CO,.
In the context of the loading experiments effects on the isotope-geochemistry of
solid and dissolved phases were examined as well (Bottcher unpub!. data). In contrast to the reference site, the DOC and IHS-concentrations in the sediments enriched with organic matter were increased (Fig. 5.2.11). Probably because sufficiently large quantities of reactive iron complexes were missing in the sandy
sediments and/or the reaction rate of the iron-containing solid phases was too slow.
the formed IHS could not be precipitated as iron-sulphide. In contrast to the open
ocean, the dissolved residual sulphate was enriched with '4 S , as during the process
of sulphate-reduction the light S-isotope changes preferentially into the reduced
phase. Fractionation factors were between -5 to -25 %0. The decrease of the fractionation factor was the result of an increase in the absolute rate of sulphatereduction. The rate of sulphate-reduction is mainly a function of the availability of
organic substances, pore water sulphate and the ambient temperature. Indeed, with
increasing ambient temperature the isotope fractionation factor in pore water sulphate decreased.