Quadruple sulfur isotope analysis of ca. 3.5 Ga Dresser Formation: New evidence for microbial sulfate reduction in the early Archean

Multiple sulfur isotope system is a powerful new tracer for atmospheric, volcanic, and biological influences on sulfur cycles in the anoxic early Earth. Here, we report high-precision quadruple sulfur isotope analyses (S-32/S-33/S-34/S-36) of barite, pyrite in barite, and sulfides in related hydrothermal and igneous rocks Occurring in the ca. 3.5 Ga Dresser Formation, Western Australia. Our results indicate that observed isotopic variations are mainly controlled by mixing of mass-dependently (MD) and non-mass-dependently fractionated (non-MD) Sulfur reservoirs. Based on the quadruple Sulfur isotope systematics (delta S-34-Delta S-33-Delta S-36) for these minerals, four end-member Sulfur reservoirs have been recognized: (1) non-MD sulfate (delta S-34 = -5 +/- 2%; Delta S-33 = -3 +/- 1%); (2) MD sulfate (delta S-34 = +10 +/- 3%; (3) non-MD sulfur (delta S-34 > +6%; Delta S-33 > +4%); and (4) igneous MD sulfur (delta S-34 = Delta S-33 = 0%. The first and third components show it clear non-MD signatures, thus probably represent Sulfate and sulfur aerosol inputs. The MD sulfate component (2) is enriched in S-34(+10 +/- 3%) and may have originated from microbial and/ or abiotic disproportionation of volcanic S or SO2. Our results reconfirm that the Dresser barites contain small amounts of pyrite depleted in S-34 by 15-22% relative to the host barite. These barite-pyrite pairs exhibit a mass-dependent relationship of delta S-33/delta S-34 with slope less than 0.512, which is consistent with that expected for microbial Sulfate reduction and is significantly different from that of equilibrium fractionation (0.515). The barite-pyrite pairs also show up to 1% difference in Delta S-36 values and steep Delta S-36/Delta S-33 slopes, which deviate from the main Archean array (Delta S-36/Delta S-33 = -0.9) and are comparable to isotope effects exhibited by sulfate reducing microbes (Delta S-36/Delta S-33 = -5 to -11). These new lines of evidence support the existence of sulfate reducers at ca. 3.5 Ga, whereas microbial sulfur disproportionation may have been more limited than recently suggested. (C) 2008 Elsevier Ltd. All rights reserved.