Mass-independent isotopic signatures for delta(33)S, delta(34)S, and delta(36)S from sulfide and sulfate in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago (Ma). Before 2450 Ma, the cycle was influenced by gas-phase atmospheric reactions. These atmospheric reactions also played a role in determining the oxidation state of sulfur, implying that atmospheric oxygen partial pressures were low and that the roles of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal. Atmospheric fractionation processes should be considered in the use of sulfur isotopes to study the onset and consequences of microbial fractionation processes in Earth's early history.

The present study arose out of an interest in resolving the anomalous sulfur isotopic compositions observed in the Archean sulfide and sulfates samples of Farquhar et al. [2000a, 2000c]. In these studies, mass-independent sulfur isotope compositions were observed in sulfide and sulfate minerals from SNC meteorites and in terrestrial samples older than 2450 Ma. The origin of these effects was hypothesized to be in the atmosphere; however, the reaction responsible for the effect was not identified. Experimental identification of the conditions and processes that generate these isotopic fractionation effects has the potential to be applied to planetary atmospheres and extended to past atmospheric evolution when combined with the rock record.Here we present the results from a series of photolysis experiments using sulfur dioxide at different UV spectral windows. We investigate the possibility of mass-independent isotopic fractionation effects associated with these photolysis windows and also investigate the possibility that such fractionation effects could be combined with the geological record and used as a geochemical sensor of changes in past atmospheric chemistry, composition, and transparency to UV radiation. We chose to concentrate on the photolysis of sulfur dioxide because its photochemistry involves electronically excited states, predissociation phenomena, and complex chemical reaction networks that involve not only SO2 but also SO, SO3, and S compounds [Okabe, 1978]. All of these 32829

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As recognized already by Charles Darwin, animals are geobiological agents. Darwin observed that worms aerate and mix soils on a massive scale, aiding in the decomposition of soil organic matter. A similar statement can be made about marine benthic animals. This mixing, also known as bioturbation, not only aides in the decomposition of sedimentary organic material, but as contended here, it has also significantly influenced the chemistry of seawater. In particular, it is proposed that sediment mixing by bioturbating organisms resulted in a severalfold increase in seawater sulfate concentration. For this reason, the evolution of bioturbation is linked to the significant deposition of sulfate evaporate minerals, which is largely a phenomena of the Phanerozoic, the last 542 million years and the time over which animals rose to prominence.Phanerozoic ͉ evaporite ͉ gypsum ͉ sulfate reduction W ith a current concentration of 28 mM, sulfate is the second most abundant anion in seawater. Sulfate enters the ocean mostly from river runoff, with minor contributions from volcanism (1). It leaves the ocean as either pyrite (FeS 2 ) buried in sediments, formed as a product of microbial sulfate reduction, or as sulfate minerals, mostly gypsum (CaSO 4 ⅐5H 2 O), in evaporite deposits (2). Gypsum precipitates before halite (NaCl) and becomes an important evaporite component when the ion product (IP CaSO4 ) between Ca 2ϩ and SO 4 2-in unevaporated seawater exceeds 23 mM 2 (3); presently IP CaSO4 is 280 mM 2. The concentration of Ca 2ϩ has varied between 10 and 40 mM over the last 550 million years (4), and if this range applies through Earth history, IP CaSO4 would exceed 23 mM 2 with relatively low SO 4 2-levels of 0.5 to 2 mM. If sulfate concentrations fall below this level or if IP CaSO4 becomes Ͻ23 mM 2 , the chances for gypsum supersaturation during evaporation of seawater is reduced. The total amount of gypsum deposition from any given parcel of seawater is also limited by sulfate concentration.Previous studies have documented little evidence for gypsum deposition before the Mesoproterozoic (1.6 to 1.0 billion years ago) suggesting reduced seawater sulfate concentrations before this time (5). This analysis is predicated on the assumption that observations of gypsum abundance faithfully represent the original magnitude of gypsum deposition. However, it is known that gypsum is easily dissolved during weathering (6), and alternative approaches have been developed to evaluate the history of gypsum deposition. One of these derives the history of gypsum deposition from the isotope record of sulfate and sulfide ( Fig. 1) (1), with the fraction of the total sulfur leaving the oceans as pyrite given by the equation: