The rise and fall of Archean atmospheric oxygen - Did temporary carbon burial as Fe(ox)-DOM complexes play a modulating role?

获得分子氧仍然被认为是地球上日益复杂的生命进化的主要驱动力。在这方面，产氧光合作用的出现和随后的大氧化事件(GOE)代表了地球历史上的两个关键事件。然而，关于大气中氧气的长期上升的细节仍然很少。例如，我们知道，在GOE期间，大气O2最初的大幅上升紧随其后的是新的下降。直到很久以后，在新元古代，大气中的O2才再次上升--这一次达到了稳定的接近现代的水平。是什么导致了最初的超调和重新缩水？在充氧光合作用过程中释放的氧化能力与二氧化碳固定到生物质中的碳量在化学计量上相对应。生物质的再矿化导致氧气再次下降，因此，如果没有碳埋藏和地壳碳库的初步建立，地球早期大气的氧化是不可能的。一旦生物质到达沉积物，就会逐渐转化为稳定的干酪根，通常在时间尺度上锁定在碳循环的地质部分&10^6年。但对于更不稳定的埋藏碳形式来说，这可能是非常不同的。我们知道太古宙海相溶解有机质(DOM)的规模一定比现在大得多，而且在太古宙期间沉积了大量的条带状铁建造(BIF)。BIF前驱体矿物亚铁水合物容易吸附大量的DOM，但由于稳定性较低，在变形的早期阶段它就倾向于重结晶。在这个项目中，我们将检验这样一种假设，即DOM-亚铁水合物可能暂时掩埋了足够的有机物，从而导致大气中氧气的大幅上升。与沉积干酪根不同，人们对DOM的地质命运几乎一无所知。存在这样一种可能性，即在最早的变质作用中，当铁水合物重结晶时，它可能已经从未成熟的BIF沉积物中消失了。对释放的DOM进行异养改造，随后会下调先前释放的氧气。我们将创建高铁水合物和纯蓝藻DOM的复合体，并在高压釜中进行模拟区域变质，以揭示DOM在后生和早期变质地质条件下的稳定性和行为。最终，我们将评估BIF中临时碳储存的可能性，作为太古代大气氧气水平的调节器--超调和重新下降--从而基本上评估构造控制氧气长期上升和有机体复杂性演化的可能性。

Access to molecular oxygen is still considered the primary driver for the evolution of increasingly complex life on Earth. In this respect the emergence of oxygenic photosynthesis, and the subsequent Great Oxidation Event (GOE) represent two pivotal incidents in Earth history. Details of the protracted rise of atmospheric oxygen are however still sparse. For example, we know that the initial, substantial rise of atmospheric O2 during the GOE was immediately followed by a renewed drawdown. Only much later, during the Neoproterozoic, did a renewed rise of atmospheric O2 occur - this time to stable near-modern levels. What caused the initial overshoot and renewed drawdown? The oxidizing power released during oxygenic photosynthesis corresponds stoichiometrically to the amount of carbon that is fixed into biomass from CO2. Remineralization of the biomass causes renewed O2 drawdown and hence the oxidation of Earth’s early atmosphere would not have been possible without carbon burial and the initial establishment of a crustal carbon reservoir. Once biomass reaches the sediment it is gradually converted to stable kerogen, and typically locked up in the geological portion of the carbon cycle on time scales >10^6 years. But this could have been very different for more labile forms of buried carbon. We know that the Archean marine pool of dissolved organic matter (DOM) must have been magnitudes larger than at present, and that enormous volumes of banded iron formations (BIF) were deposited during the Archean. The BIF-precursor mineral ferrihydrite is prone to adsorbing large amounts of DOM, but due to a low stability it tends to recrystallize already during earliest stages of metamorphosis. In this project we will test the hypothesis that DOM-ferrihydrite complexes could have temporarily buried sufficient organic matter to cause a substantial rise in atmospheric oxygen. In contrast to sedimentary kerogen, virtually nothing is known about the geological fate of DOM. The possibility exists that it would have been lost from immature BIF sediments upon recrystallization of the ferrihydrite during earliest metamorphism. Heterotrophic reworking of the released DOM would subsequently down-regulate the previously released oxygen. We will create complexes of ferrihydrite and pure cyanobacterial DOM, which will be subjected to simulated regional metamorphism in an autoclave to reveal the stability and behavior of DOM under geological conditions of catagenesis and early metamorphism. Eventually we will evaluate the possibility of temporary carbon storage in BIFs as a modulator for Archean atmospheric oxygen levels - both the overshoot and the renewed drawdown - thereby essentially evaluating the likelihood of tectonic control on the protracted rise of oxygen and the evolution of organismic complexity.