Briony,

I agree with Max that microbial sulfate reduction is most probably the 
only process you have to deal with. We observed a similar range of 
heavy delta-values for pyrite nodules from french black shales.

However, I don t think that different sulfate reduction rates should 
be responsable for the observed 34S enrichment. Probably, a 
separation in space of hydrogen sulfide formed under early and late(r) 
diagenetic conditions from the same aqueous solution may lead to the 
heavy pyrite nodules. The isotopically heavy barites formed in 
diagenetic "baryte fronts" are an example for solide sulfur-bearing 
phases formed from dissolved (isotopically heavy) residual sulfate. If
dissolved residual sulfate is reduced at a place separated in space 
from the early microbial sulfate reducing activities, this would 
lead to the observed results.

Michael




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Dr. Michael E. Boettcher
Institute of Chemistry and Biology of the Marine Environment (ICBM)
Carl von Ossietzky University
P.O. Box 2503
D-26111 Oldenburg
F.R. Germany
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