359
Cariaco Basin. Sulfide isotopic compositions in
deep waters of the Black Sea were roughly constant near –41.5‰, while those of the Cariaco
Trench averaged –31‰. In contrast, in the uppermost 50 m of the Black Sea’s sulfidic waters (that
is in water depths between 100 and 150m) the
sulfide isotopic compositions change significantly
in a region of sulfide consumption, increasing up
to 5‰ vs. the deep-water background. These increases may be due to sulfide oxidation mediated
by MnO 2 or oxygen, but are not consistent with
sulfide oxidation by photosynthetic bacteria (Fry
et al. 1991). Growth experiments with sulfate-reducing bacteria suggested that part of the
increase in sulfide isotopic compositions could be
attributed to rapid rates of sulfate reduction in the
oxic/anoxic interface region.
10.6.2 δ
δ δ
δ δ 34 S in Marine Sediments
Evaporitic Sulfate
The general aspects of isotopic fractionation during the evaporation of sulfates have been addressed in the previous section. Due to a minor
fractionation during precipitation, marine evaporitic sulfates are assumed to reflect the original
seawater signature. However, exceptions might occur in sulfates precipitated during late stages of
evaporation. Sulfur isotope measurements from the
Permian Zechstein in Germany (e.g. Nielsen and
Ricke 1964) and experiments by Raab and Spiro
(1991) indicate that sulfate deposited within the
halite or even the potash-magnesia facies is depleted in 34 S in comparison to sulfate precipitated
within the calcium sulfate facies. Progressive crystallization of 34 S-enriched sulfate minerals results
in a residual brine depleted in 34 S, by possibly as
much as 4‰. Therefore, to obtain a true record of
seawater sulfate isotopic composition in the past
no late-stage sulfates should be included.
Sedimentary Pyrite
Modern sedimentary pyrites are generally depleted in 34 S relative to ocean water sulfate with
average δ 34 S-values around –20‰ and a range between +20‰ and –50‰. The majority of isotopic
compositions of pyrites in modern sediments indicate the sedimentary sulfides having formed
through bacterial sulfate reduction under open
system conditions with respect to sulfate availability. These measurements agree well with ex10.6
Geochemical Influences on 34 S/ 32 S Ratios
perimental data as discussed above. However,
higher values have been reported for pyrites
formed under anoxic bottom-water conditions like
in the Black Sea (Vinograd et al. 1962; Lyons and
Berner 1992). They are interpreted to result from
an open-system diagenesis combined with the
effect of differential diffusion (Jørgensen et al.
2004).
During Earth history, the isotope fractionation
between contemporaneous seawater sulfate and
sedimentary reduced sulfur compounds like pyrite
increases (Shen et al. 2001). With a few
exceptions, sulfur isotope fractionations seem to
have been negligible during the Archean but
increased during the Proteozoic, presumably as a
consequence of increased oceanic sulfate
concentrations (Habicht et al. 2002). In sediment
deposits younger than about 800 Myr, as in modern anoxic systems like the Black Sea, sulfur
fractionations often exceed what appears to be
the maximum reached during sulfate reduction in
surface sediments as well as in cultures of sulfate
reducers, as has been mentioned above. The isotope record for sedimentary sulfides of
Phanerozoic age shows a much higher variability
than observed for marine sulfates (Strauss 1997).
Beside a long-term trend with a Cambrium maximum of about +2 ‰, a Permian to Cretaceous minimum around –30 ‰ and an increase to about
–10 ‰ to –20 ‰ in the modern ocean sediments,
there is a considerable variation as high as 80 ‰
within a single sedimentary unit. The observed
range clearly reflects that in addition to the factors which control the sulfate isotopic composition in ancient seawater, changes in the geochemical conditions during sulfate reduction and
pyrite formation may affect the d
34
S of sulfides.
For example, changing bottom-water conditions
(oxic to anoxic and vice versa) during the time of
deposition may alter the conditions for pyrite formation. The amount of pyrite formed in sediments
is known to be limited by (1) the amount of
metabolizable organic matter (2) the amount of
sulfate and (3) the amount of reactive iron. Under
normal marine conditions, the amount of organic
matter limits the reduction process (Raiswell and
Berner 1986). This is documented in a positive
correlation of organic carbon and pyrite sulfur
through geologic history (Berner and Raiswell
1983). In anoxic environments, reactive iron appears to be the limiting factor for pyrite formation,
since the amount of H 2 S exceeds its removal by
the reaction with iron (Raiswell and Berner 1985).
Cariaco Basin. Sulfide isotopic compositions in
deep waters of the Black Sea were roughly constant near –41.5‰, while those of the Cariaco
Trench averaged –31‰. In contrast, in the uppermost 50 m of the Black Sea’s sulfidic waters (that
is in water depths between 100 and 150m) the
sulfide isotopic compositions change significantly
in a region of sulfide consumption, increasing up
to 5‰ vs. the deep-water background. These increases may be due to sulfide oxidation mediated
by MnO 2 or oxygen, but are not consistent with
sulfide oxidation by photosynthetic bacteria (Fry
et al. 1991). Growth experiments with sulfate-reducing bacteria suggested that part of the
increase in sulfide isotopic compositions could be
attributed to rapid rates of sulfate reduction in the
oxic/anoxic interface region.
10.6.2 δ
δ δ
δ δ 34 S in Marine Sediments
Evaporitic Sulfate
The general aspects of isotopic fractionation during the evaporation of sulfates have been addressed in the previous section. Due to a minor
fractionation during precipitation, marine evaporitic sulfates are assumed to reflect the original
seawater signature. However, exceptions might occur in sulfates precipitated during late stages of
evaporation. Sulfur isotope measurements from the
Permian Zechstein in Germany (e.g. Nielsen and
Ricke 1964) and experiments by Raab and Spiro
(1991) indicate that sulfate deposited within the
halite or even the potash-magnesia facies is depleted in 34 S in comparison to sulfate precipitated
within the calcium sulfate facies. Progressive crystallization of 34 S-enriched sulfate minerals results
in a residual brine depleted in 34 S, by possibly as
much as 4‰. Therefore, to obtain a true record of
seawater sulfate isotopic composition in the past
no late-stage sulfates should be included.
Sedimentary Pyrite
Modern sedimentary pyrites are generally depleted in 34 S relative to ocean water sulfate with
average δ 34 S-values around –20‰ and a range between +20‰ and –50‰. The majority of isotopic
compositions of pyrites in modern sediments indicate the sedimentary sulfides having formed
through bacterial sulfate reduction under open
system conditions with respect to sulfate availability. These measurements agree well with ex10.6
Geochemical Influences on 34 S/ 32 S Ratios
perimental data as discussed above. However,
higher values have been reported for pyrites
formed under anoxic bottom-water conditions like
in the Black Sea (Vinograd et al. 1962; Lyons and
Berner 1992). They are interpreted to result from
an open-system diagenesis combined with the
effect of differential diffusion (Jørgensen et al.
2004).
During Earth history, the isotope fractionation
between contemporaneous seawater sulfate and
sedimentary reduced sulfur compounds like pyrite
increases (Shen et al. 2001). With a few
exceptions, sulfur isotope fractionations seem to
have been negligible during the Archean but
increased during the Proteozoic, presumably as a
consequence of increased oceanic sulfate
concentrations (Habicht et al. 2002). In sediment
deposits younger than about 800 Myr, as in modern anoxic systems like the Black Sea, sulfur
fractionations often exceed what appears to be
the maximum reached during sulfate reduction in
surface sediments as well as in cultures of sulfate
reducers, as has been mentioned above. The isotope record for sedimentary sulfides of
Phanerozoic age shows a much higher variability
than observed for marine sulfates (Strauss 1997).
Beside a long-term trend with a Cambrium maximum of about +2 ‰, a Permian to Cretaceous minimum around –30 ‰ and an increase to about
–10 ‰ to –20 ‰ in the modern ocean sediments,
there is a considerable variation as high as 80 ‰
within a single sedimentary unit. The observed
range clearly reflects that in addition to the factors which control the sulfate isotopic composition in ancient seawater, changes in the geochemical conditions during sulfate reduction and
pyrite formation may affect the d
34
S of sulfides.
For example, changing bottom-water conditions
(oxic to anoxic and vice versa) during the time of
deposition may alter the conditions for pyrite formation. The amount of pyrite formed in sediments
is known to be limited by (1) the amount of
metabolizable organic matter (2) the amount of
sulfate and (3) the amount of reactive iron. Under
normal marine conditions, the amount of organic
matter limits the reduction process (Raiswell and
Berner 1986). This is documented in a positive
correlation of organic carbon and pyrite sulfur
through geologic history (Berner and Raiswell
1983). In anoxic environments, reactive iron appears to be the limiting factor for pyrite formation,
since the amount of H 2 S exceeds its removal by
the reaction with iron (Raiswell and Berner 1985).