Isotopic composition of disseminated sulfur in Precambrian sedimentary rocks

Isotopic analyses of trace sulfur in Precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that δ34S measurements of Archean sedimentary sulfur cluster mostly around mantle-like values and that a significant overall dispersion of 34 S 32 S ratios commenced at about 2.7 Ga ago. The outstanding feature of the Proterozoic record is the predominance of heavy δ34Ssulfide values, mostly in excess of 0%. CDM. Consideration of these data, in combination with the sparse δ534S record for Proterozoic sulfates, suggests that Δ34Ssulfide-sulfide may have been smaller than was the case during the Phanerozoic. A possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner. In the Archean, it was coupled to and controlled by the magmatic endogenic cycle. The Proterozoic, for the most part, represents a transitional stage into a Phanerozoic-like exogenic cycle, the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere. The indication that the mean combined δ34S of Proterozoic sulfate and sulfide may have significantly exceeded the Phanerozoic mean value of +2%. CDM is enigmatic and poses problems for balance considerations.