Sulfur isotopes mass independent

Baroni M, Thiemens MH, Delmas RJ, Savarino J (2007) Mass-independent sulfur isotopic composition in stratospheric volcanic eruptions. Science 315 84-87 Barth S (1993) Boron isotope variations in nature a synthesis. Geologische Rundschau 82 640-651... 

Farquhar J, Savarino J, Airieau S, Thiemens M (2001) Observation of wavelength sensitive mass independent sulfur isotope effects during SO2 photolysis Applications to the early atmosphere. J Geophys Res Planets 12 32829-32839... 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen - A novel isotope effect and its cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson T, Zipf EC, Erdman PW, van Egmond C (1995) Carbon dioxide and oxygen isotope anomalies in the mesophere and stratosphere. Science 270 969-972 Thode HG, Monster J (1964) The sulfur isotope abundances in evaporites and in ancient oceans. In Vinogradov AP (ed) Proc Geochem Conf Commemorating the Centenary of V I Vernadsku s Birth, vol 2, 630 p... 

Sulfur isotopes also show mass-independent effects that are probably produced by the same photochemical mechanism as oxygen effects in the Earth s upper atmosphere. Mass independent variations in sulfur from Martian meteorites have been interpreted to result from volcanic injections of SO2 and H2S into the Martian atmosphere followed by photolysis, which fractionates the sulfur isotopes. There is also evidence from ancient terrestrial sediments that the same photo lytic process was operating on sulfur in the Earth s atmosphere prior to 2.4 Ga, before oxygen began to accumulate in the atmosphere (see review by Thiemens, 2006). 

Sulfur isotope compositions in sulfonic acids show a mass-independent enrichment in 33S. Determining fractionations that are not controlled by mass is made possible because sulfur has so many stable isotopes. This feature has been attributed to ultraviolet irradiation of carbon disulfide in space, prior to its reaction to make sulfonic acid. 

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. 

Another possible source of chemical MIF is self-reaction of the SO radical, which either yields S and SO2 as disproportionation products [41], or the SO dimer as an association product [42]. Two of the four lowesf energy singlef sfafes of the SO dimer [43] have a reduced point group symmetry upon sulfur isotope substitution. If sulfur atom exchange is another possible outcome in the self reaction of SO, then a symmetry-derived mass-independent effect analogous to O3 formation may be possible. A similar conclusion may be drawn for the triplet states of the SO dimer. 

Pavlov, A. A., and Kasting, J. F. (2002). Mass-independent fractionation of sulfur isotopes in archean sediments Strong evidence for an anoxic archean atmosphere. Astrobiology 2, 27—41. 

There are several review papers on mass-independent chemical processes and their applications. Thiemens and Weston reviewed the progress in understanding the physical chemistry of gas-phase mass-independent processes and their observation on Earth and meteorites. Thiemens et al. (2001) reviewed the observations of mass-independent isotopic composition in various solid reservoirs of Earth and Mars, including both oxygen and sulfur isotopes. A more recent review (Thiemens, 2002) has summarized the theoretical and laboratory studies of the physical chemistry of mass-independent isotope effects and their observation on Earth and Mars, subsequent to the review of Thiemens et al. (2001). 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

In contrast, Ohmoto (1996, 1997) and Beukes et al. (2002) argue that the Great Oxidation Event interpretation does not take into account the reducing power of biological activity within Precambrian paleosols, and that O2 levels were close to present levels from 3,000 Ma to 1,800 Ma. An intermediate view of rising, but fluctuating atmospheric oxidation also is compatible with available paleosol data (Retallack, 2001a), and with limited evidence from mass-independent fractionation of sulfur isotopes (Farquhar et al., 2002). 

During the past few years the most exciting new development bearing on this question has been the discovery of the presence of mass-independent fractionation (MIF) of the sulfur isotopes in sulfides and sulfates older than ca. 2.47 Ga. Figure 11 summarizes the available data for the degree of MIF (A S) in sulfides and sulfates during the past 3.8 Ga. The presence of values of > 0.5%o in sulfides and sulfates older than ca. 2.47 Ga indicates that the O2 content of the atmosphere was <10 PAL prior to 2.47 Ga... 

Kasting (2001) argues in support of the view of Farquhar et al (2000) (but see also Ohmoto et al, 2001) that sulfur isotope fractionation changed around 2.3 Ga. This opinion is based on the claim, from comparison of sulfur isotopes, that so-called mass independent fractionation occurred as a result of gas-phase photochemical reactions, particularly photolysis of SO2. Such fractionation would be much more likely to occur in a I0W-O2 atmosphere in which sulfur was present in a variety of oxidation states. Thus, the claim that fractionation changed around 2.3 Ga ago can be seen as supporting the notion that there was a substantial rise in O2 around this time. This, however, raises the question if cyanobacterial oxygen production had been sufficient to create the mbisco fingerprint in carbonates as early as 2.7-3.0Ga ago, why did the rise of free O2 only occur 400-700 Myr later ... 

Other evidence for low Archean atmospheric oxygen concentrations come from studies of mass-independent sulfur isotope fractionation. Photochemical oxidation of volcanic sulfur species, in contrast with aqueous-phase oxidation and dissolution that characterizes the modem sulfur cycle, may have been the major source of sulfate to seawater in the Archean (Farquhar et al., 2002 Farquhar et al., 2000). Distinct shifts in and in sulfide and sulfate from... 

Thiemens, M., G. M. Michalski, A. Romero and J. R. McCabe Mass independent oxygen and sulfur isotopic compositions of environmental sulfate and nitrate. A new probe of atmospheric, hydrospheric and geological processes, Geophys. Res. Abstracts 5 (2003) no. 04359. 

An exciting new development in the field of stable isotope geochemistry has been the recent recognition of the importance of mass-independent fractionation in the sulfur isotope... 

Laboratory studies have shown that the most likely explanation for mass-independent sulfur isotope fractionations within the Earth system is through reactions which take place within the gas phase and thereby provide an important geochemical fingerprint of atmospheric processes (Farquhar et al., 2000, 2002). A particularly important reaction is the photochemical oxidation of sulfur in the atmosphere. Today this reaction is prevented by the presence of ozone and oxygen in the atmosphere which shield the lower atmosphere from the ultraviolet radiation required for this reaction. Experimental studies show that only tiny amounts of atmospheric oxygen are needed to prevent the photochemical oxidation of sulfur, indicating that photochemical oxidation can only take place in an atmosphere with very low levels of oxygen. 

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