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

大气中氧气的存在是我们日常生活中理所当然的事情。然而，含氧大气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，氧气不得不奋力往上爬，直到它成为地球大气中不可或缺的元素之一。富氧底栖微生物生态系统中的黄铁矿氧化可能是这种延迟的一个可能原因，因为氧气会立即被氧化消耗。南非巴伯顿绿岩带（BGB）中保存完好的Moodies组沉积序列，可能不仅为3.2 Ga时的含氧光合作用提供了独特的证据，而且还记录了黄铁矿氧化的直接耗氧。穆迪斯带状铁地层与微生物席和大部分硅化石膏结块密切相关。我们认为底栖生物体内产生的氧气氧化了丰富的碎屑黄铁矿，形成硫酸盐和氧化铁。当硫酸盐被输送到沙质洪泛平原时，氧化铁被冲进前三角洲和泻湖相，形成穆迪BIF。在本建议中，将采用跨学科的方法，包括地球化学、地球微生物学和应用岩石学，来检验这种机制的合理性。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残骸。(2)测试太古宙条件下，产氧细菌对黄铁矿氧化过程中产生的酸性条件的反应。(3)测试变质过程中酸性条件下生长的细菌的石化潜力。(4)检测modies BIFs的分带是否由生物体内pH值变化引起。为此，我们打算分析特定的同位素和微量元素特征，分析微化石遗骸的modies生物，在酸性和太古宙条件下对蓝藻进行生态生理实验，并将这些细菌用于与BGB相关的P/T条件。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之前的氧气光合作用的可信古老。

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.