Observation of wavelength‐sensitive mass‐independent sulfur isotope effects during SO₂ photolysis: Implications for the early atmosphere

Abstract

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

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…2d, e, Ohmoto and Felder, 1987). One of these two grains has moderately negative  33 S value, which is consistent with S contribution from photochemically-produced sulphate aerosols (Farquhar et al, 2001). Many grains from the Ventersdorp Contact Reef have both δ 34 S and likely reflect the presence of a core of rounded pyrite within the cubic grains, a feature that is common for the Ventersdorp Contact Reef (England et al, 2002;Frimmel, 2005).…”

“…Shen et al, 2001;Grassineau et al, 2006). However, photochemical S fractionation may also produce a large range in  34 S values (Farquhar et al, 2001) that can be preserved/transferred to sedimentary pyrite; consequently,  34 S values of Archaean sedimentary sulphide and sulphate minerals alone cannot be used as a direct proxy for bacterial sulphate reduction and local high seawater sulphate content. Additional constraints for an igneous vs. sedimentary origin of sulphides as well as photochemical vs. bacterial S isotope fractionation can be derived from the presence of a NMDF signal, which is a characteristic feature of sedimentary rocks older than 2.4 Ga. By using such arguments one has to keep in mind that time intervals may have existed in the Archaean when the atmospheric oxygen levels exceeded a threshold concentration for preservation of NMDF signal at pO 2 > 10 -5 PAL (Ohmoto et al, 2006;Ono et al, 2006a) or the atmosphere had a different CO 2 /CH 4 ratio and organic haze thickness resulting in a different pattern of photochemical fractionation of S isotopes Domagal-Goldman et al, 2008).…”

“…Non-mass dependent fractionation (NMDF) of S isotopes is caused by SO 2 photolysis under ultraviolet radiation (Farquhar et al, 2001). Although thermochemical sulphate reduction in the presence of amino acids could generate NMDF of S isotopes (Watanabe et al, 2009), it has not yet been demonstrated that this process can account for the full range of NMDF observed in Archaean and early Palaeoproterozoic sediments nor that it is applicable to a diverse range of geological environments on the early Earth.…”

Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars

“…The most precise laboratory technique for measuring sulphur isotope ratios involves the conversion of all sulphurbearing phases to SF 6 , which removes the potential for isobaric interference during mass spectrometric analysis 33,34 . This study was performed with a sequential chemical extraction procedure on powdered whole rock that allowed the measurement of sulphur isotope ratios for different mineral phases.…”

Potential of ozone formation by the smog mechanism to shield the surface of the early Earth from UV radiation

“…Kasting and Catling (2003) provide a review on the rise of atmospheric O2. Sulphur isotope data (Farquhar et al, 2001;Pavlov and Kasting, 2002) and trace sulphate data (Hurtgen et al, 2002) suggests that O2 rose to at least 21 ppmv 2.3 billion years (Ga) ago, termed the "Great Oxidation Event" (GOE) and believed to be associated with cyanobacteria. The GOE may have been favoured by high methane levels because methane can diffuse into the upper atmosphere where it photolyses and subsequently loses some of its hydrogen, which is a strong sink for O2, to space (Kerr, 2005).…”