Sulfur isotopes mass dependent

Recent discoveries of oxygen and sulfur fractionations (e.g., Farquhar et al. 2000 Thiemens et al. 2001) that appear to have unusual mass dependence has renewed interest in variations in the mass dependence of different fractionation mechanisms (Gao and Marcus 2002 Young et al. 2002). Usually, mass-dependent fractionations scale in proportion to differences in isotopic mass ... 

In recent years additional informations on sulfur isotope fractionation mechanisms have been obtained from the analysis of the additional isotopes and (Farqnhar et al. 2003 Johnston et al. 2005 Ono et al. 2006, 2007). For long it was thonght and valnes carry no additional information, because sulfur isotope fractionations follow strictly mass-dependent fractionation laws. By studying all snlfnr isotopes with very high precision these authors could demonstrate that bacterial snlfate redaction follows a mass-dependent relationship that is slightly different from that expected by equilibrium fractionations. On plots vs mix-... 

The mass difference between the two isotopes of sulfur can alter the isotope ratio of a system during chemical and physical transformations (17-21) a process referred to as "fractionation." Quite simply, the zero point energy of a molecule, and hence its rotational and vibrational energy, is mass dependent (22). Therefore, molecules containing different isotopic masses will nave different reactivities and fractionation will occur during mass dependent transformations. 

The origin of mass-dependent fractionation (MDF) in isotope systems lies in the mass dependence of the molecular properties (e.g., zero-point energy) and physical processes (e.g., evaporation) affecting the compound. If a compound comprised of atoms with 3 or more stable isotopes, such as oxygen or sulfur, deviates from a mass-dependent relationship, the compound is said to exhibit mass-independent fractionation (MIF). MIF signatures are not affected by mass-dependent processes, and so are excellent tracers of the small number of mass-independent processes that exist in nature. 

Oxidation of sulfur-bearing gases in the atmosphere can lead to non-mass-dependent isotopic fractionation, manifested as excesses or deficiencies in Farquhar et al. (2000) found small deficits in a nakhlite, and proposed that this was an isotopic signature of martian atmospheric chemistry. This subtle non-mass-dependent signal was not seen in other martian samples, but it bolsters the idea that outgassed volatiles are fractionated in the atmosphere and then returned to the lithosphere. 

It is not surprising that the geochemical cycle of sulfur during the I0W-O2 Archean differed from that of the present day. As shown in Figure 5, the mass-dependent fractionation of the sulfur isotopes in sedimentary sulfides was smaller prior to 2.7 Ga than in more recent times. Several explanations have been advanced for this observation. The absence of microbial sulfate reduction is one. However, the presence of... 

In a similar way, mass-dependent sulfur isotopic fractionation has been used for tracking sulfur metabolism through time, with light sulfur,... 

There are four stable isotopes of sulfur 32S, 33S, 34S, and 3<5S, which have relative abundances of 95.1, 0.74,4.2, and 0.016%, respectively. The relative abundance of the various isotopes varies slighdy, depending on the source of the sulfur the ratio of 32S to 34S is 21.61—22.60. Three radioactive isotopes of masses 31,35, and 37 having half-lives of 2.6 s, 87 d, and 5 min, respectively, have been generated artificially. 

Sulfur is one of the few exceptions to the constancy of isotopic proportions, in that there is sufficient variation, dependent upon the source of the sulfur, to cause a variation in its atomic mass by approximately 0.01%. For normal stoichiometric calculations, however, this small variation is unimportant. 

The natural isotope distribution of sulfur S32 S34 is about 25 1. Therefore each peak in the mass spectrum of a sulfur compound which corresponds to a sulfur-containing fragment is accompanied by a second, shifted by 2 MU (mass units) to higher mass numbers. The intensity ratio of these two peaks depends upon the number of sulfur atoms present in the ion, if the contribution of other fragments to these peaks can be excluded it is 25 1 if only one sulfur is present, 25 2 for two, and so on (Beynon,13 p. 300). Therefore the presence of a sulfur-containing compound and the number of S atoms may be easily deduced from the ratio of the M+ and (M + 2)+ ions. 

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