Paleoarchean sulfur cycle and biogeochemical surface conditions on the early Earth, Barberton, South Africa

This study presents the first multiple sulfur isotope dataset on sulfides from the ca. 3.5-3.2 Ga Onverwacht Group in the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. In situ delta S-34(CDT) and Delta S-33 values of pyrite (n = 568) are reported from a wide range of hydrothermal, volcanic and sedimentary environments and are used to explore Mid-Archean biogeochemical sulfur cycling. Samples are from fresh drill core collected by the Barberton Scientific Drilling Project that intercepted cherts, metabasalts and sheared ultramafics of the similar to 3.3-3.35 Ga Kromberg Formation; the sedimentary units of the similar to 3.432 Ga Noisy formation; and the unconformably underlying metabasaltic pillow lavas of the similar to 3.472 Ga Hooggenoeg Formation. Pyrite in quartz-carbonate-veins in the lower diamictite of the Noisy sequence records the largest range and most negative delta S-34(CDT) values so far reported from an Archean terrain (delta S-34(CDT) = -55.04 to +27.46%o). The Noisy sediments also contain detrital and diagenetic pyrites with a significant variation in Delta S-33 of between 0.62 to +1.4%0 and delta S-34(CDT) of between 7.00 and +12.6 parts per thousand, interpreted to reflect tectonic exposure of these early sediments to atmospheric - shallow marine conditions. Early marine pyrites from the Kromberg Cherts also display strong positive Delta S-33 values up to +2.50 parts per thousand with narrow range in delta S-34(CDT) values (-6.00 to +1.50 parts per thousand), whereas hydrothermal veins in the basal ultramafic shear zones preserve magmatic values ( 0 parts per thousand). This study reveals a potential proto-tectonic control on atmospheric, geodynamic and hydrothermal environments available for early sulfate reducing and/or methanogenic microbes in the Paleoarchean. No evidence for microbial sulfate reduction or disproportionation was identified in the Kromberg Cherts, despite previous morphological claims for microbial life. Highly variable and negative delta S-34(CDT) values were found in the Noisy turbidites and Hooggenoeg pillow lava breccia supporting the presence of microbial sulfate reduction in early tectono-sedimentary basins and in the Paleoarchean sub-seafloor, respectively. In light of current controversies surrounding sulfur isotope studies in similar-aged rocks of the Pilbara Craton (West Australia), we argue that microbial elemental sulfur disproportionation was not a preferred metabolic pathway on the Paleoarchean earth. (C) 2013 Elsevier B.V. All rights reserved.