MICROBES USE BOTH ELEMENTAL SULFUR AND SULFATE AT 3.3 GA IN THE BARBERTON GREENSTONE BELT, SOUTH AFRICA

Microbial sulfate reduction and elemental sulfur disproportionation are amongst the earliest traceable microbial metabolisms in the geological record, and both were recently suggested in the North Pole barite from the Pilbara Block, NW Australia [1-3]. Here we attempt to identify, using multiple S isotope data (³²S, ³³S, ³⁴S, determined by SIMS), traces of one or both of these metabolisms in unweathered drill core material from the Lower Mapepe formation (3.26 to 3.23 Ga), Barberton Greenstone Belt, South Africa. The core was drilled beneath the main workings in the Baryte Valley Syncline on farm Heemsteede 378 JU. Samples are taken from a 40 m section of barite-rich cherts and barite horizons, and an overlying 60 m of shales, cherts, carbonates and conglomerate. S isotopes were measured in bulk rocks and individual minerals including interstitial pyrite from massive barite beds, syn-sedimentary microcrystalline pyrite layers in chert and reworked barite sand deposits, and isolated euhedral pyrites in massive chert and barite rich units.

Microscopic pyrites show a moderate range in δ³⁴S from +5 to -12 ‰ with highly variable Δ³³S (-1 to +4 ‰). Individual petrographic associations of pyrite and variations between adjacent samples reveal discreet δ³⁴S vs. Δ³³S mixing lines between four isotopic reservoirs: (1) sulfide overlapping sulfate as measured for the barites by Bao et al [4]; (2) sulfide produced by abiotic or microbial sulfate reduction; (3) magmatic sulfide and (4) sulfide derived from elemental S of photolysis origin. The last of these four reservoirs changes abruptly in composition through the stratigraphy. Our results are consistent with input of sulfide from microbial S⁰ disproportionation and S⁰ reduction as well as the utilization of sulfate, although the microbial role in sulfate reduction remains difficult to resolve from abiotic processes.

[1] Phillippot et al. (2007) Science 317, 1534-1537. [2] Ueno et al. (2008) GCA 72, 5675-5691. [3] Shen et al. (2009) EPSL 279, 383-391. [4] Bao et al. (2007) GCA 71, 4868-4879.