34S
values vary within a range from -26.2
to 42.7; this variation of 70
of amplitude is registered between two samples separated in depth by only 3 m (Table
3).The four pyrite
concretions selected for sulfur isotopic analysis are presently located well
below the active zone of sulfate reduction. Their 34S
values vary within a range from -26.2
to 42.7; this variation of 70
of amplitude is registered between two samples separated in depth by only 3 m (Table
3).
Sedimentary sulfides are
generally formed during the bacterial reduction of sulfate (reactions 1 and 2),
when metal ions are available in the solutions to precipitate the sulfide
minerals. During this reaction, bacteria preferentially reduce the lighter
sulfate ions, producing sulfide that is depleted in 34S relative to
the original sulfate source. In recent marine sediments, the 34S
values of solid and gaseous sulfides range from -40
to 20, depending both on sulfate
diffusion and burial rates (Pierre, 1989).
The large range of 34S
values of the Blake Ridge pyrites argue for non-steady-state conditions where
sulfate was reduced in a rather closed system. This is typical of interstitial
environments when sulfate diffusion rates are much slower than burial rates. In
such systems, sulfide may become highly enriched in 34S if it is
formed from the 34S-rich sulfate remaining after a precursor stage of
sulfate reduction. These types of complex multistep sulfate reduction reactions
are thought to have occurred in the Blake Ridge sediments to produce pyrite with
exceptionally high 34S
values.
An alternative explanation
could be that the sulfur of pyrite derived from the bacterial decomposition of
hydrocarbons that are impregnating the sediments. Although the 34S
values of the hydrocarbons contained in the sediments from the Blake Ridge are
unknown, maximum values of about 30
and minimal values of -10 are
reported in the literature (Thode, 1970). These values might explain in part the
measured 34S
values of the Blake Ridge pyrites, if additional fractionations are assumed to
occur during the hydrocarbons decomposition.
