The emergence of oxygenic photosynthesis through the lens of carbonate diagenesis

地球历史上最重要的进化创新是含氧光合作用的出现。这种微生物代谢的进化最终推动了大气中氧气的增加，从根本上改变了所有的生物地球化学循环，并为随后的多细胞生命进化奠定了基础。然而，这种进化奇点的时间、原因和后果在很大程度上仍未得到解决。目前，关于含氧光合作用起源的理论存在分歧。虽然一些研究支持含氧光合作用的早期起源，大约比大气中氧气的增加早6亿年，但另一些研究认为，含氧光合作用在大氧化事件之前就出现了（大约23 - 34亿年前）。这场争论与解释数十亿年前太古宙岩石的地球化学数据的复杂性有着内在的联系。地球上氧气的历史部分是利用来自古代沉积物（如碳酸盐）的地球化学数据重建的。这些化学沉积物由阳离子（如Ca2+、Mg2+）和碳酸盐离子组成，其组成至少在最初反映了同时期海水的化学和同位素组成。然而，使用这种地球化学档案的主要限制之一是碳酸盐沉积物对成岩作用的敏感性，这一过程是未石化的沉积物转化为我们今天可以研究的岩石的过程。对于十亿年历史的碳酸盐岩来说，成岩蚀变是规律而非例外，广泛的再结晶改变了岩石记录中保存的地球化学信号。这些沉积后的过程是我们准确推断GOE前氧气水平的主要障碍。与试图避免成岩作用的常见尝试相反，本提案中概述的研究计划旨在通过绘制和更好地了解成岩过程，从古代碳酸盐沉积物中提取主要化学信息。通过钙同位素测量、同步x射线分析和数值成岩模型的结合，可以限制碳酸盐的成岩历史。当将这些技术与结构观测相结合时，就有可能“看穿”成岩作用，更准确地重建过去的海水化学。通过碳酸盐成岩作用重建古代环境的追求不仅与太古宙研究有关，而且与所有使用碳酸盐地球化学作为古代环境条件代用物的研究有关。因此，这一框架既具有高度原创性，又有可能在我们理解含氧光合作用的出现和整个地球历史上的生命进化史方面取得突破性进展。

The most important evolutionary innovation in Earth history was the emergence of oxygenic photosynthesis. The evolution of this microbial metabolism eventually drove the rise of oxygen in the atmosphere, fundamentally changing all biogeochemical cycles, and setting the stage for the subsequent evolution of multi-cellular life. However, the timing, causes, and consequences of this evolutionary singularity are largely unresolved. Currently, theories on the origin of oxygenic photosynthesis are divided. While some studies favor an early origin of oxygenic photosynthesis, ~600 million years prior to the rise of oxygen in the atmosphere, others have argued that oxygenic photosynthesis arose just prior to the Great Oxidation Event (~2.3-3.4 billion years ago). This controversy is intrinsically linked to the complications associated with interpreting geochemical data from billion-year-old Archean rocks. The history of oxygen on Earth has in part been reconstructed using geochemical data from ancient sediments, such carbonates. These chemical sediments are composed of cations (e.g., Ca2+, Mg2+) and carbonate ions whose composition, at least initially, reflects the chemical and isotopic composition of contemporaneous seawater. However, one of the main limitations in using this geochemical archive is the susceptibility of carbonate sediments to diagenesis, the process where unlithified sediments are transformed into the rocks we can study today. For billion-year-old carbonates diagenetic alteration is the rule rather than the exception and extensive recrystallization has modified the geochemical signals that are preserved in the rock record. These post-depositional processes are major obstacles for our ability to accurately infer oxygen levels before the GOE. In contrast to common attempts of trying to avoid diagenesis, the research program outlined in this proposal seeks to extract the primary chemical information from ancient carbonate sediments by mapping out and better understanding the diagenetic processes. By combining calcium isotope measurements, synchrotron X-ray analyses of Mn and Fe minerals, and numerical diagenetic models, it is possible to constrain the diagenetic history of carbonates. When paring these techniques with textural observations, it is possible to `see through' diagenesis and more accurately reconstruct past seawater chemistry. The pursuit of reconstructing ancient environments through the lens of carbonate diagenesis is not only relevant to Archean studies, but is pertinent to all studies using the geochemistry of carbonates as proxies for ancient environmental conditions. Consequently, this framework is both highly original and likely to lead to ground-breaking advances in our understanding of the emergence of oxygenic photosynthesis and the evolutionary history of life throughout Earth history.