76 2 Isotope Fractionation Processes of Selected Elements
the fractionation factor. As shown in Fig. 2.21, H
2
S may become isotopically heav-
ier than the original sulfate when about 2/3 of the reservoir has been consumed. The
δ
34
S-curve for “total” sulfide asymptotically approaches the initial value of the orig-
inal sulfate. It should be noted, however, that apparent “closed-system” behavior of
covarying sulfate and sulfide δ
34
S-values might be also explained by “open-system”
differential diffusion of the different sulfur isotope species (J
´
ørgensen et al. 2004).
In recent years additional informations on sulfur isotope fractionation mecha-
nisms have been obtained from the analysis of the additional isotopes
33
S and
36
S
(Farquhar et al. 2003; Johnston et al. 2005; Ono et al. 2006, 2007). For long it
was thought δ
33
S and δ
36
S values carry no additional information, because sulfur
isotope fractionations follow strictly mass-dependent fractionation laws. By study-
ing all sulfur isotopes with very high precision these authors could demonstrate that
bacterial sulfate reduction follows a mass-dependent relationship that is slightly dif-
ferent from that expected by equilibrium fractionations. On plots Δ
33
Svsδ
34
Smix-
ing of two sulfur reservoirs is non-linear in these coordinates (Young et al. 2002).
As a result samples with the same δ
34
S-value can have different Δ
33
S and Δ
36
Sval-
ues. This opens the possibility to distinguish between different fractionation mech-
anisms and biosynthetic pathways (Ono et al. 2006, 2007). For instance, bacterial
sulfate reduction shows slightly different fractionation relationships compared to
sulfur disproportionation reactions (Johnston et al. 2005). Thus multiple sulfur iso-
tope analyses might have great potential in identifying the presence or absence of
specific metabolisms in modern environment or to have a fingerprint when a partic-
ular sulfur metabolism shows up in the geologic record.
Finally it should be mentioned that sulfate is labeled with two biogeochemical
isotope systems, sulfur and oxygen. Coupled isotope fractionations of both sulfur
and oxygen isotopes have been observed in experiments (Mizutani and Rafter 1973;
Fritz et al. 1989; B
¨
ottcher et al. 2001) and in naturally occurring sediments (Ku
et al. 1999; Aharon and Fu 2000; Wortmann et al. 2001). Brunner et al. (2005)
argued that characteristic δ
34
S-δ
18
O fractionation slopes do not exist, but depend
on cell-specific reduction rates and oxygen isotope exchange rates. Despite the ex-
tremely slow oxygen isotope exchange of sulfate with ambient water, δ
18
Oinsulfate
obviously depend on the δ
18
O of water via an exchange of sulfite with water.
2.9.2.2 Thermochemical Reduction of Sulfate
In contrast to bacterial reduction thermochemical sulfate reduction is an abiotic pro-
cess with sulfate being reduced to sulfide under the influence of heat rather than
bacteria (Trudinger et al. 1985; Krouse et al. 1988). The crucial question, which has
been the subject of a controversial debate, is whether thermochemical sulfate reduc-
tion can proceed at temperatures as low as about 100
◦
C, just above the limit of mi-
crobiological reduction (Trudinger et al. 1985). There is increasing evidence from
natural occurrences that the reduction of aqueous sulfates by organic compounds
can occur at temperatures as low as 100
◦
C, given enough time for the reduction to
proceed (Krouse et al. 1988; Machel et al. 1995). S isotope fractionations during