[BCG09-P15] Unraveling the mystery of MIF-S in Archean sedimentary rocks
Keywords:Mass independent fractionation of S isotopes, hydrothermal synthesis of organic matter, thermochemical sulfate reduction, bacterial sulfate reduction, Fe-oxidizing bacteria
Based on theoretical and experimental investigations of multiple S isotope fractionations involving surface reactions, we have proposed that the MIF-S in sedimentary rocks was created by thermochemical sulfate reduction involving organic matter (OM) (e.g., Watanabe et al., 2009). However, questions have remained as to why MIF-S is rare in younger rocks and why some OM produce MIF-S, but others do not. We have realized that the generation of reactive OM is the key to these questions. At the JpGU 2017 meeting, we reported the discovery of two types of OM in the 3.45 Ga-old Marble Bar Chert from Western Australia. One is biogenic OM, the remnant of aerobic chemolithotrophic benthic Fe-oxidizing bacteria; and the other is abiogenic OM, produced by hydrothermal-seawater reactions. Both types of OM-syntheses were carried out through the reaction CO2 + 6Fe2+ + 0.5O2 = CH2O + 3Fe2O3 + 12H+ (R3), where Fe2+ was supplied by hydrothermal fluids, and CO2 and O2 by the local deep ocean water. The biogenic OM is large (~1-10 µm in size), protected by cell walls, and has maintained its bacterial morphology. In contrast, the abiogenic OM occurs typically as a ~10-20 nm-sized cluster of angstrom-sized OM particles. Such OM, compared to the biogenic OM, would be much more reactive and would therefore be preferentially utilized in thermochemical sulfate reduction to generate MIF-S. Before ~2.4 Ga, abiogenic OM production would have been much more important than today, because: (i). The Earth’s interior was hotter than today, and igneous and hydrothermal activities were more extensive in the oceans, which provided larger amounts of Fe2+ into the oceans; and (ii). The atmosphere were richer in CO2 (pCO2 >100 PAL) and already O2-rich (pO2 >~1 PAL).
Compared to the biogenic OM, abiogenic OM would have decomposed to generate the substrates (e.g., H2, lactic acids) for heterotrophs (e.g., sulfate-reducing bacteria) at much faster rates. The kinetic S isotopic fractionation during bacterial sulfate reduction (34DS = d34SSO4 - d34SH2S) decreases with increased rates of reduction. Consequently, the smaller 34DS values for pyrites in Archean sedimentary rocks, compared to those in the younger rocks, can also be explained by the higher abundance of abiogenic OM in the Archean oceans. Therefore, the MIF-S (33DS) and 34DS records of sedimentary rocks indicate the thermal- and tectonic histories of the Earth, rather than the atmospheric O2 history.