357
transfer of sulfur between the different reservoirs,
which is often associated with a change in the
oxidation state of the element. The main processes
to be considered in the sulfur cycle are the
riverine input of sulfate as a weathering product
of sulfur-bearing rocks, the precipitation of
evaporites from seawater and the biological
reduction of seawater sulfate and subsequent
formation of sedimentary pyrite (Fig. 10.10). The
associated fractionation mechanisms are therefore:
• isotope exchange reactions between sulfate
and sulfides
• kinetic isotope effects in the bacterial reduction of sulfate or in the oxidation of sulfides
Only a small fractionation is associated with
the precipitation of sulfates in seawater. Theoretical considerations as well as sulfur-isotope measurements in natural environments revealed an isotopic difference between seawater sulfate and
crystallized gypsum of 0 to +2.4‰ (Raab and
Spiro 1991). Therefore, within the generally observed variability in an evaporitic deposit, calcium
sulfate is assumed to record the isotopic composition of seawater sulfate at the time of deposition
(Claypool et al. 1980; Strauss 1997).
By far the largest fractionation is associated
with the bacterial sulfate reduction (see a recent
review by Canfield 2001). Due to the activity of
sulfate-reducing bacteria, such as Desulfovibrio
desulfuricans, organic matter is oxidized according to the following equation:
2 (CH 2 O) + SO 4
2- ⇒ H 2 S + 2 (HCO 3
- )
(10.23)
The resulting hydrogen sulfide reacts with
sedimentary iron, which is available in the reactive
non-silicate bond form (oxy-hydroxydes), and is
fixed as iron sulfide (e.g. pyrite). In general, a
substantial depletion of δ
34
S occurs due to
preferential utilization of the
32
S-isotope by the
sulfate reducing bacteria. Whereas the fractionation
Fig. 10.11 Variation of sulfate and sulfide concentration and δ 34 S values of pore water in core 2092 from the western
Baltic Sea (Hartmann and Nielsen 1969).
10.6
Geochemical Influences on 34 S/ 32 S Ratios
transfer of sulfur between the different reservoirs,
which is often associated with a change in the
oxidation state of the element. The main processes
to be considered in the sulfur cycle are the
riverine input of sulfate as a weathering product
of sulfur-bearing rocks, the precipitation of
evaporites from seawater and the biological
reduction of seawater sulfate and subsequent
formation of sedimentary pyrite (Fig. 10.10). The
associated fractionation mechanisms are therefore:
• isotope exchange reactions between sulfate
and sulfides
• kinetic isotope effects in the bacterial reduction of sulfate or in the oxidation of sulfides
Only a small fractionation is associated with
the precipitation of sulfates in seawater. Theoretical considerations as well as sulfur-isotope measurements in natural environments revealed an isotopic difference between seawater sulfate and
crystallized gypsum of 0 to +2.4‰ (Raab and
Spiro 1991). Therefore, within the generally observed variability in an evaporitic deposit, calcium
sulfate is assumed to record the isotopic composition of seawater sulfate at the time of deposition
(Claypool et al. 1980; Strauss 1997).
By far the largest fractionation is associated
with the bacterial sulfate reduction (see a recent
review by Canfield 2001). Due to the activity of
sulfate-reducing bacteria, such as Desulfovibrio
desulfuricans, organic matter is oxidized according to the following equation:
2 (CH 2 O) + SO 4
2- ⇒ H 2 S + 2 (HCO 3
- )
(10.23)
The resulting hydrogen sulfide reacts with
sedimentary iron, which is available in the reactive
non-silicate bond form (oxy-hydroxydes), and is
fixed as iron sulfide (e.g. pyrite). In general, a
substantial depletion of δ
34
S occurs due to
preferential utilization of the
32
S-isotope by the
sulfate reducing bacteria. Whereas the fractionation
Fig. 10.11 Variation of sulfate and sulfide concentration and δ 34 S values of pore water in core 2092 from the western
Baltic Sea (Hartmann and Nielsen 1969).
10.6
Geochemical Influences on 34 S/ 32 S Ratios