Vanished evaporites and carbonate formation in the Neoarchaean Kogelbeen and Gamohaan formations of the Campbellrand Subgroup,South Africa |
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| Institution: | 1. Dipartimento Scienze della Terra, Università di Siena, I-53100 Siena, Italy;2. Department of Geology, University of Leicester, Leicester LE1 7RH, UK;3. Geological Survey of Norway, N-7491 Trondheim, Norway;1. Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada;2. Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6, Canada;3. Geological Survey of Canada, Natural Resources Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada;1. Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON, Canada P3E 1R9;2. Department of Geology, Museum Building, Trinity College Dublin, Dublin 2, Ireland;1. Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia;2. School of Earth Sciences, The University of Queensland, St. Lucia, QLD 4072, Australia;3. EarthByte Group, School of Geosciences, University of Sydney, NSW 2006, Australia;1. Geology Department, Mascara University, 29000, Algeria;2. Natural History Museum of Geneva, Geology and Paleontology Department, CP 6434, 1211 Geneva 6, Switzerland;1. Department of Geology, Rhodes University, Makhanda, South Africa;2. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;3. Centre for Microscopy, Characterisation & Analysis, University of Western Australia, Perth, Australia;4. DSI-NRF CIMERA, Department of Geology, University of Johannesburg, Aukland Park 2006, South Africa |
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| Abstract: | Field, petrographic and stable isotopic evidence indicate the former presence of widespread evaporites in the Neoarchaean Campbellrand Subgroup of South Africa. Calcitization of the vanished but once laterally-extensive evaporites was apparently driven by bacterial sulphate reduction of solid sulphate in association with organic diagenesis and pyrite precipitation within platform-wide microbialites and sapropels. This counters current interpretations that much of the calcite was precipitated directly on the seafloor or in primary voids in open marine conditions controlled by regional seawater chemistry. Rather, large-scale microbial mediation of ambient waters across a shallow to emergent platform raised carbonate alkalinity and removed kinetic inhibitors to carbonate formation.The low preservation potential of Precambrian solid sulphate is related in part to bacterial sulphate reduction within the microbially-dominated ecosystems of which cyanobacteria were a major component. Evidence for the former presence of solid sulphate in shallow Neoarchaean seas includes pseudomorphs after selenite, also recorded from the contemporaneous Carawine Dolomite of Australia, together with rock fabrics and textures typical of evaporite dissolution. Importantly, sulphur isotopes of pyrite samples from the Cambellrand carbonates show a wide range of values indicating biogenic fractionation of sulphate, a signature also seen in the Neoarchaean Belingwe Greenstone Belt of Zimbabwe, and the Mt McRae and Jeerinah shales of Western Australia.Mass microbial colonization across extensive Neoarchaean epeiric seas witnessed the microbiogeochemical transformation of the Earth’s hydrosphere, atmosphere and biosphere. The consequences for a reducing ocean would be the progressive oxidation of the major dissolved species in surface seawater, most notably of reduced sulphur and iron. Cyanobacterial photosynthetic oxidation of surface seawater drove formation of aqueous sulphate and permitted the precipitation of extensive evaporites in restricted basins, perhaps beginning the process of ridding the oceans of reduced sulphur. The first dramatic explosion of carbonate precipitation can be related to intense bacterial sulphate reduction in association with anoxic organic diagenesis and pyrite formation within the decaying interiors of microbialites and in sapropels. |
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