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

在我们的日常生活中，大气中氧气的存在是我们认为理所当然的。然而，含氧大气层的形成是一个复杂而漫长的过程。在2.4 Ga的大氧化事件（GOE）之后，氧气不得不一路奋斗到顶部，直到它成为地球大气层的组成元素之一。产氧底栖微生物生态系统中的黄铁矿氧化可能是造成这一延迟的一个可能原因，因为氧化会立即消耗氧气。南非巴伯顿绿岩带（BGB）穆迪群保存完好的沉积序列，不仅提供了3.2 Ga时产氧光合作用的独特证据，而且还记录了黄铁矿氧化的直接耗氧。穆迪带状铁地层被发现与微生物席和大量硅化石膏结核密切相关。我们认为，在底栖生物垫中产生的氧气氧化丰富的碎屑黄铁矿形成硫酸盐和铁氧化物。当硫酸盐被输送到桑迪洪泛平原时，氧化铁被冲刷到前三角洲和泻湖相，形成穆迪BIF。在这项建议中，这种机制的可行性将通过跨学科的方法进行测试，涉及地球化学，地质微生物学和应用岩石学。我们的目标是：（1）找到黄铁矿氧化的证据和产氧细菌的细菌遗骸。(2)测试产氧细菌如何应对太古代条件下黄铁矿氧化过程中产生的酸性条件。(3)测试变质作用期间在酸性条件下生长的细菌的分解潜力。(4)测试Moodies BIF的条带是否由生物垫中的pH变化产生。为此，我们打算进行特定的同位素和微量元素的签名分析，分析穆迪生物垫的微体化石的遗骸，进行生态生理实验与蓝藻在酸性，太古代条件下，并将这些细菌的P/T条件相关的BGB。我们期望我们的研究结果将有助于将GOE前氧化风化的证据与大气化学的历史联系起来，并支持在GOE之前的光合作用的合理古老性。

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.