SULFUR ON THE EARLY EARTH

Sulfur is a widespread agent in surface geochemical systems and is abundant in most rock types; it is present in volcanic gases and marine waters, and has served a key role in geobiological processes since the origin of life. Like other low atomic number elements, sulfur isotope ratios in various compounds usually follow predictable mass-dependent fractionation laws and these isotope fractionations serve as powerful tracers for trace igneous, metamorphic, sedimentary, hydrothermal and biological processes. Mass-independent sulfur isotope fractionation is a short-wavelength photolytic effect that occurs in space, as well as in gas-phase reactions in atmospheres transparent to deep penetration by ultraviolet light. Hence, crucial aspects of the chemical evolution of the early atmosphere can be followed by S isotopes in Archean metasedimentary rocks. Because of its reactivity in a vast range of oxidation states, sulfur is an important source of metabolic energy to certain microbial communities considered analogous to what prevailed in the early biosphere. Hydrothermal areas which generate a rich brew of Me-S complexes are prime candidates for the ‘chemical factories' gave rise to the first life. Subsequent transformations of the sulfur cycle by different metabolic styles of organisms in response to global changes in surface redox can also be traced with multiple S isotopes in the rock record. Based on this new understanding, all Archean sulfur minerals previously documented for their ³⁴S/³²S compositions warrant a comprehensive re-examination of their ³²S, ³³S, ³⁴S (and ³⁶S), sulfur isotope systematics.