MIF-S RECORD IN SEDIMENTARY ROCKS: AN INDICATOR OF ATMOSPHERIC OR BIOLOGICAL EVOLUTION?

Many geoscientists consider the record of mass independent fractionation of sulfur isotopes (MIF-S: Δ³³S & ^33-34Θ values are outside the ranges of 0±0.2‰ & 0.0515±.005, respectively) in sedimentary rocks as the best indicator for atmospheric O₂ evolution, and that the dramatic change from an anoxic (pO₂<10^-5 PAL) to oxic (pO₂>10^-5 PAL) atmosphere occurred ~2.4 Ga ago. This is based on the acceptance of two dogmas: (I) atmospheric photochemical reactions are the only mechanisms to create MIF-S, and (II) only rocks >~2.4 Ga in age bear MIF-S signatures. However, there are many >~2.4 Ga rocks with no MIF-S and some <1.7 Ga rocks with MIF-S. The >2.4 Ga sedimentary rocks with large MIF-S studied here have unique geological and geochemical characteristics, including quite notable systematic relationships between MIF-S values and organic C content, its maturation, and hydrothermal proxies (e.g., Zn content & fluid inclusion). Based on these observations, we hypothesize that MIF-signatures were created by reactions among organic-rich sediments, S-bearing solid compounds, and S-bearing hydrothermal fluids at T = 100-200°C during the early diagenetic stage of sediments. To test this hypothesis and to understand the origins of MIF-S in sedimentary rocks, we conducted reduction experiments of S-bearing species (e.g., S⁰ & sulfate) by a variety of amino acids at T = 150-200°C. The reduction rate and S-isotope fractionation factor varies largely depending on temperature, difference in valence states between a S-source and products (e.g., H₂S), and type of amino acid used. The Δ³³S value increases with decreasing rate (maximum Δ³³S = 0.45‰), and the ^33-34Θ values between H₂S and SO₄^2- ranged from ~+0.33 to ~+1.14; these values clearly indicate MIF-S. By incorporating these values in Rayleigh fractionation and recycling models, we can explain the entire range of Δ³³S values (-2 to +7‰) observed in geologic samples. In addition, reactive amino acids (e.g., glycine & alanine) were probably more abundant in organic-rich sediments of Archean age, compared to younger sediments, because of the probable change in relative abundances of simple/complex amino acids through geologic time (e.g., Jordan et al., 2005). Therefore, the MIF-S record can be a good indicator for biological evolution rather than atmospheric O₂ evolution.