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 delta-S-34 measurements of Archean sedimentary sulfur cluster mostly around mantle-like values and that a significant overall dispersion of S-34/S-32 ratios commenced at about 2.7 Ga ago. The outstanding feature of the Proterozoic record is the predominance of heavy delta-S-34(sulfide) values, mostly in excess of 0 parts per thousand CDM. Consideration of these data, in combination with the sparse delta-S-34 record for Proterozoic sulfates, suggests that DELTA-S-34sulfate-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 delta-S-34 of Proterozoic sulfate and sulfide may have significantly exceeded the Phanerozoic mean value of +2 parts per thousand CDM is enigmatic and poses problems for balance considerations.