MULTIPLE ISOTOPE SULFUR FRACTIONATIONS PRODUCED DURING SULFATE REDUCTION: CONTEXT FOR INTERPRETATION AND ASTROBIOLOGICAL IMPLICATIONS

We have analyzed d³³S and d³⁴S in sulfide produced by archaeaglobus fulgidus as the first step in a study of mass-dependent isotopic fractionation relationships generated by biological processes.  Recent research has highlighted the fact that anomalous (mass-independently fractionated) sulfur isotope compositions are observed in the Archean geological record.   Recently, interest in mass-dependent fractionation relationships generated by different processes has also been rekindled because it has been recognized that some signatures may be unique to biological processes and therefore may be a good criteria for identifying the onset of microbial sulfate reduction and for identifying the onset of other metabolisms that use sulfur.

Different mass-dependent fractionation processes occur because different fractionation processes obey different physical laws (e.g., velocity separation of isotopes are related to 1/2mv², isothermal hydrostatic separation of isotopes are related to e^-mgz/kT, and equilibrium exchange are related to e^-hu/kT, where u is the vibrational frequency and has a mass dependence of its own).  Discussions of these differences have been presented by Hulston and Thode (JGR 1965), Matsuhisa et al. (GCA 1978), Young et al. (GCA 2002) and Miller (GCA 2002).  The manifestation of these different mass-dependent fractionation relationships is implicit in the exponent of the definition of the quantity D³³S  = d³³S –1000*((1+ d³⁴S/1000)^0.5155-1) which is the mass-dependent fractionation relationship that is extracted from analyses of phanerozoic sulfide and sulfate minerals.  The mass-dependent fractionation relationship observed in our experiments of sulfate reduction by archaeaglobus fulgidus is D³³S  = d³³S –1000*((1+ d³⁴S/1000)^0.5117-1) and the different exponent is thought to derive from kinetic steps in the sulfate reduction process.  Evaluation of this exponent can be made in the context of all steps that fractionate sulfur isotopes and can be thought of as a vector sum in the coordinate systems d³⁴S vs d³³S or d³⁴S vs D³³S.  An understanding of the factors that determine the value of this exponent will be critical for devising strategies to identify early sulfur metabolisms and also has possible astrobiological implications.