Influence of sedimentary environments on sulfur isotope ratios in clastic rocks: a review

Strata-bound sulfide deposits associated with clastic, marine sedimentary rocks, and not associated with volcanic rocks, display distributions of δS34 values gradational between two extreme types: 1. a flat distribution ranging from δS34 of seawater sulfate to values about 25‰ lower; and 2. a narrow distribution around value δS34 (sulfide)=δS34 (seawater sulfate) −50‰, and skewed to heavier values. δS34 (seawater sulfate) is estimated from contemporaneous evaporites. There is a systematic relation between the type of δS34 distribution and the type of depositional environment. Type 1 occurs in shallow marine or brackish-water environments; type 2 occurs characteristically in deep, euxinic basins. These distributions can be accounted for by a model involving bacterial reduction of seawater sulfate in systems which range from fully-closed “batches” of sulfate (type 1) to fully open systems in which fresh sulfate is introduced as reduction proceeds (type 2). The difference in the characteristic distributions requires that the magnitude of the sulfate-sulfide kinetic isotope effect on reduction be different in the two cases. This difference has already been suggested by the conflict between δS34 data for modern marine sediments and laboratory experiments. The difference in isotope effects can be accounted for by Rees' (1973) model of steady-state sulfate reduction: low nutrient supply and undisturbed, stationary bacterial populations in the open system settings tend to generate larger fractionations.