Geological evolution from isotope proxy signals - sulfur

A currently emerging sulfur isotope record for Phanerozoic seawater, based on structurally substituted sulfate in stratigraphically well constrained b iogenic carbonates, allows the detailed assessment of secular variations wi thin the global sulfur cycle and the interaction between the sulfur and car bon cycles. It is superior to the evaporite-based dataset because it enable s sampling of the entire biostratigraphic column. Discrete biological and e nvironmental signals can be deciphered from a somewhat "noisy" sulfur isoto pe record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around -51 parts per thousand which appears to be constant t hroughout the entire Phanerozoic. Observable large spreads of delta(34)S(su lfide) for any given sedimentary unit are caused by environmental parameter s, such as type and availability of organic carbon or availability of sulfa te. In particular, the growing importance of land plants and their impact o n the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition. (C) 1999 Elsevier Science B.V. All rights reserved.