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Aspects of the early sulfur cycle and its effects on the climate and geochemistry of Earth and Mars

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Aspects of the early sulfur cycle and its effects on the climate and geochemistry of Earth and Mars
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  Aspects of the Early Sulfur Cycle and itsEffects on the Climate and Geochemistry of Earth and Mars A thesis presentedby Itay Halevy toThe Department of Earth and Planetary Sciencesin partial fulfillment of the requirementsfor the degree of  Doctor of Philosophy  in the subject of Earth and Planetary SciencesHarvard UniversityCambridge, MassachusettsApril, 2010  © 2010 by Itay HalevyAll rights reserved.  iiiDissertation Advisor: Professor Daniel P. Schrag Itay Halevy Aspects of the Early Sulfur Cycle and its Effects on theClimate and Geochemistry of Earth and Mars Abstract Planetary climate and surface chemistry are tightly coupled; atmospheric compositionaffects the transfer of solar and infrared radiation, and therefore climate, whereasthe components of climate, including temperature and precipitation, strongly affectthe chemical composition of the surface environment and the geochemical cycling of elements through it. An element of climatic, environmental and biological importanceis sulfur. The modern biogeochemical sulfur cycle has been extensively studied andis relatively well understood. How the sulfur cycle may have differed early in theevolution of Earth and Mars, when the surface was anoxic and biological activity wasgeochemically unimportant, is less well understood and much more poorly constrainedby data.In this PhD thesis we explore the early sulfur cycle using theory, experiments andmodels, emphasizing the differences from the modern and the implications for cli-mate and the surface environment. On early Mars, we propose that the combinationof 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 radiativetransfer model, we show that the radiative forcing supplied by a thick CO 2 greenhouseis uncertain, but that part-per-million concentrations of SO 2 provide appreciable ad-ditional warming, perhaps enough to explain the early action of liquid water. In  ivaddition, SO 2 may explain the observed Martian mineralogical record, which con-tains abundant early sulfate, but no carbonate minerals. We show experimentallythat 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 whenexposed to oxidizing conditions.In contrast, rapid aqueous-phase destruction of SO 2 probably kept its concentra-tion 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 thatthe record of mass-independent sulfur isotope fractionation can only be quantitativelyinterpreted by considering a full, anoxic sulfur cycle, and that such treatment pro-vides explanations for salient features in this record. Finally, we present a coupledatmosphere-surface model, to be used for a detailed investigation of the sulfur cycle.  Contents 1 Introduction 12 A Sulfur Dioxide Climate Feedback on Early Mars 73 Radiative Transfer in Carbon Dioxide-Rich Paleoatmospheres 304 Experimental Inhibition of Calcium Carbonate Precipitation by Sul-fur Dioxide 815 Explaining the Structure of the Archean Mass-Independent SulfurIsotope Record 1046 Sulfur Chemistry in Carbon Dioxide-Rich Paleoatmospheres 145A Longwave Absorption by High Abundances of Carbon Dioxide 168
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