Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p52b..04f&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P52B-04
Mathematics
Logic
0400 Biogeosciences, 1040 Isotopic Composition/Chemistry, 6215 Extraterrestrial Materials
Scientific paper
Accurate evaluation of sulfur isotope fractionations as tracers of biological activity requires knowledge of: (1) the isotopic composition of reservoirs of inorganic sulfur species (e.g., sulfide, sulfate, elemental sulfur); (2) competing mechanisms that may produce sulfur isotope variations of non-biological origin; and (3) any isotopic signatures that can be uniquely linked to specific biological processes. We show how consideration of these three characteristics in terms of multiple sulfur isotopes may validate claims that certain sulfur isotopic signatures are biological. For the most part, studies that have focused on the biological implications of variations in sulfur isotope compositions have only considered the quantity δ34S. We propose that it is critical to take into account δ33S variations as well. The consideration of multiple sulfur isotopes may help identify the sources of sulfur used by organisms because certain sulfur reservoirs possess anomalous and characteristic Δ33S compositions (Δ33S expresses the deviation from a mass fractionation array through δ33S = 0 and δ34S = 0 and is approximated by δ33Smeasured - 0.515 x δ34Smeasured). Measurement of δ33S adds an additional mass-balance constraint on an organism's metabolic reaction network and may identify a particular pathway for sulfur through this network. Most importantly, the observation of systematic Δ33S variations may produce definitive evidence for or against specific biological fractionation processes. The latter of these is possible because of small but measurable differences in mass dependent fractionation arrays that are associated with different fractionation processes. For example, it has been shown that the kinetic fractionation attending certain biological processes imparts Δ33S signatures that are distinct from those produced by abiotic processes. Although further experimental study will be required to determine whether similar Δ33S differences are characteristic of all biological fractionation processes, many biological processes have kinetic components and, accordingly, should produce a unique, case-specific fractionation of multiple sulfur isotopes.
Farquhar James
Johnston David T.
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