Origin of coastal facies Banded Iron Formation, Barberton Greenstone Belt (3.2 Ga)

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The existence of oxygen in our atmosphere is something that we take for granted in everyday life. The onset of an oxygenated atmosphere however, was a complex and protracted process. Oxygen literally had to fight its way to the top until it became one of the integral elements of earth's atmosphere after the Great Oxidation Event (GOE) at 2.4 Ga. Pyrite oxidation in oxygenic benthic microbial ecosystems could be a possible reason for this delay since the oxygen would have been immediately consumed by the oxidation. The well-preserved sedimentary successions of the Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, may not only present a unique evidence for oxygenic photosynthesis at 3.2 Ga but also document the immediate oxygen consumption by oxidation of pyrite. Moodies Banded Iron Formations are found in close association with microbial mats and largely silicified gypsum concretions. We propose that oxygen generated in the benthic biomats oxidized abundant detrital pyrite to form sulphates and iron oxides. While the sulphates were transported to sandy floodplains, iron oxides were washed into the prodelta and lagoonal-facies to form Moodies BIF. In this proposal, the plausibility of such a mechanism will be tested by an interdisciplinary approach, involving geochemistry, geomicrobiology and applied petrology. Our goal is to: (1) find evidence for pyrite oxidation and bacterial remains of oxygen producing bacteria. (2) Test how oxygen producing bacteria respond to the acidic conditions created during pyrite oxidation under Archean conditions. (3) Test the fossilisation potential of bacteria grown under acidic conditions during metamorphism. (4) Test whether the banding of Moodies BIFs was created by pH changes in biomats. To this end we intent to perform analysis of specific isotope and trace element signatures, analyse Moodies biomats for remains of microfossils, carry out eco-physiological experiments with cyanobacteria under acidic, Archean conditions and subject these bacteria to P/T conditions relevant for the BGB. We expect that the results of our study will help to connect evidence for pre-GOE oxidative weathering with the history of atmospheric chemistry, and support the plausible antiquity of oxygenic photosynthesis well before the GOE.