Planetary climate and surface chemistry are tightly coupled; atmospheric composition affects the transfer of solar and infrared radiation, and therefore climate, whereas the components of climate, including temperature and precipitation, strongly affect the chemical composition of the surface environment and the geochemical cycling of elements through it. An element of climatic, environmental and biological importance is sulfur. The modern biogeochemical sulfur cycle has been extensively studied and is relatively well understood. How the sulfur cycle may have differed early in the evolution of Earth and Mars, when the surface was anoxic and biological activity was geochemically unimportant, is less well understood and much more poorly constrained by data. In this PhD thesis we explore the early sulfur cycle using theory, experiments and models, emphasizing the differences from the modern and the implications for climate and the surface environment. On early Mars, we propose that the combination of a thick CO 2 atmosphere and vigorous volcanic outgassing, may have allowed SO 2 to accumulate to climatic importance. Developing and using a line-by-line radiative transfer model, we show that the radiative forcing supplied by a thick CO 2 greenhouse is uncertain but that part-per-million concentrations of SO 2 provide appreciable additional warming, perhaps enough to explain the early action of liquid water.

In addition, SO 2 may explain the observed Martian mineralogical record, winch contains abundant early sulfate, but no carbonate minerals. We show experimentally that sulfite minerals precipitate at the expense of carbonates at ratios of SO 2 to CO 2 as low as a few parts-per-billion, and that the sulfites transform to sulfates when exposed to oxidizing conditions. In contrast, rapid aqueous-phase destruction of SO 2 probably kept its concentration in Earth's early ocean much too low to matter climatically or mineralogically. Even so, with a simple model of the early Archean ocean-atmosphere we show that the record of mass-independent sulfur isotope fractionation can only be quantitatively interpreted by considering a full, anoxic sulfur cycle, and that such treatment provides explanations for salient features in this record. Finally, we present a coupled atmosphere-surface model, to be used for a detailed investigation of the sulfur cycle.