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 biogenic carbonates, allows the detailed assessment of secular variations within the global sulfur cycle and the interaction between the sulfur and carbon cycles. It is superior to the evaporite-based dataset because it enables sampling of the entire biostratigraphic column. Discrete biological and environmental signals can be deciphered from a somewhat "noisy" sulfur isotope record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around -51 parts per thousand which appears to be constant throughout the entire Phanerozoic. Observable large spreads of delta(34)S(sulfide) for any given sedimentary unit are caused by environmental parameters, such as type and availability of organic carbon or availability of sulfate. In particular, the growing importance of land plants and their impact on the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition. (C) 1999 Elsevier Science B.V. All rights reserved.