INVITED: BACTERIOGENIC SULFIDE FORMATION: STABLE ISOTOPE SYSTEMATICS OF IRON-SULFUR-ORGANIC CARBON IN THE ARCHEAN-PALEOPROTEROZOIC BLACK SHALES AND CRETACEOUS BLACK SHALES

The availability of reactive Fe, sulfate, and metabolizable organic matter (OM) are the dominant factors that control pyrite formation by sulfate-reducing bacteria (SRB). Variations in Fe-S-C_org contents and S isotope compositions of sediments and sedimentary rocks have been widely used to constrain the environmental conditions necessary for pyrite formation and the activity of SRB. The ability to also analyze Fe isotope compositions in conjunction with Fe-S-C_org systematics and S isotopes will provide additional constraints on the environmental factors leading to pyrite formation. Importantly, the Fe isotope composition of reactive Fe will be able to constrain the provenance of the Fe and if the Fe has been extensively redox-cycled [Yamaguchi et al. (2004), GCA 68S, 795]. We applied such Fe-S-C_org isotope systematics to Archean-Paleoproterozoic black shales from South Africa and Australia and Cretaceous (OAE2) black shales from Italy. In addition to our previous discovery of negative correlation between isotope compositions of Fe and S in 2.7 Ga deep-facies black shales (Yamaguchi et al., 2004), we report similar negative correlations occurring in black shales of 3.3 Ga Fig Tree Group, 2.9 Ga West Rand Group, 2.6 Ga Hamersley Group, and 2.2 Ga Pretoria Group. Exceptions include 2.7 Ga shallow-facies black shales and Cretaceous black shales, where week positive correlations were found. These negative and positive correlations are found to be independent of C_org contents and isotope compositions. These observations may be explained by a combination of limitation in sulfate and reactive Fe for bacteriogenic pyrite formation, which is likely to have occurred in the marine environments since at least 3.3 Ga before.