The isotopic fingerprint of sulfidic and ferruginous environments in the sedimentary record

今天，地球的大气层和海洋充满了氧气，这使得多细胞生命的存在。然而，情况并非总是如此，在地球历史的前半部分，大气层或海洋中几乎没有氧气。在这个时候，在首次发现大气和海洋中存在少量氧气的证据之后的20亿年里，地球上的生命完全是微生物，由细菌和古生菌组成。这种微生物生命在地质记录中留下了一些化学痕迹，尽管我们很少有可供探索的证据。了解这些化学变化的时间和节奏，从一个海洋中没有氧气的世界，通过一个海洋中有适量氧气的世界，到我们的海洋中有丰富氧气的世界，是了解生命如何在地球上进化的核心。追踪这些环境变化主要是通过对古代沉积岩的化学分析来完成的。其前提是，不同类型的矿物将沉积在具有不同化学类型的海洋中。例如，在一个没有氧气但有大量溶解铁的海洋中，某些矿物质是稳定的，而其他矿物质则不是，我们希望在这个时期的沉积岩中发现大量的这些矿物质。根据这些分析，该社区确定，在地球历史的前半部分，海洋主要富含铁，然后交替富含铁和硫化物，直到在过去10亿年的某个时候氧气变得丰富。这种方法的根本问题是，当沉积物沉积下来时，它们在变成岩石之前会发生许多化学变化。这些变化广义上称为成岩作用。在沉积物变成岩石之前，解决可能改变沉积物化学成分的成岩变化，对于理解我们的地质记录如何忠实地记录地球历史上环境条件的变化至关重要。本提案试图理解这一点。我们研究了东盎格鲁盐沼的现代沉积物，主要是富含铁。这些沉积物类似于可能沉积在富含铁的海洋中的沉积物，它们充满了高活性的铁矿物，这些铁矿物在氧气或硫化物的存在下通常不稳定。我们之前已经证明，这些富含铁的沉积物可以在环境中（一些沉积物有臭味，充满硫化物）和实验室中变成富含硫化物的沉积物。该建议旨在了解沉积条件从富铁到富硫化物的变化如何影响沉积物的矿物学，地球化学和同位素组成。为此，我们正在应用和开发一种新的工具，这是由我们的研究团队成员开创的。我们正在从这些现代沉积物中提取各种组分，包括来自野外的沉积物和我们在实验室中研究过的沉积物，或者在实验室中培养的沉积物，并分别分析它们，而不是像目前的方法那样对沉积物进行大量消化。我们假设，这些矿物馏分将更好地记录在沉积物埋藏过程中发生的变化，并作为沉积物从富铁富硫化物的演变，我们将找到这个过程的地球化学示踪剂。我们建议的最后一部分是采用这种新工具，并将其应用于以前被解释为具有富铁和富硫化物特征的非常古老的岩石。该项目将最终使我们能够了解我们的沉积记录的哪一部分记录了真实的环境条件，哪一部分是在沉积后修改期间获得的。

Today, Earth's atmosphere and oceans are filled with oxygen, which allows for the presence of multicellular life. However, this was not always the case, and for the first half of Earth's history there was almost no oxygen in either the atmosphere or the ocean. At this time, and for the two billion years after the first evidence for small amounts of oxygen in the atmosphere and oceans, life on the planet was solely microbial, made up of bacteria and archaea. This microbial life leaves some chemical traces of its existence in the geological record, although there is very little fossilised evidence for us to explore. Understanding the timing and pacing of these chemical changes, from a world with no oxygen in the oceans, through one with a moderate amount of oxygen in the oceans, to our world with abundant oxygen in the ocean, is central to understanding how life evolved on the planet. Tracing these environmental changes is done largely through the chemical analysis of ancient sedimentary rocks. The premise is that different types of minerals will be deposited in oceans that have different types of chemistry. For example, in an ocean with no oxygen but lots of dissolved iron, certain minerals would be stable and others not, and we would expect to find these minerals abundant in our sedimentary rocks from this time period. Based on these analyses, the community has determined that oceans were largely iron-rich for the first half of Earth history, and then alternately iron-rich and sulfide-rich until oxygen became abundant at some point in the last billion years. The fundamental problem with this approach is that when the sediments are laid down, they will change chemically in many ways before they are lithified into a rock. These changes are broadly termed 'diagenesis'. Resolving the diagenetic changes that may change the chemical composition of sediments before they become rocks is essential for understanding how faithfully our geological record may be recording changes in the environmental conditions over Earth history. This proposal seeks to understand this. We have studied modern sediments from East Anglian salt marshes, which are dominantly iron-rich. These sediments are analogues for sediment that may have been deposited in iron-rich oceans, they are full of highly reactive iron minerals that are often not stable in the presence of oxygen or sulfide. We have previously documented that these iron-rich sediment can become sulfide-rich sediments both in the environment (some of the sediment is smelly and full of sulfide) and in the laboratory. This proposal seeks to understand how this change in sedimentary conditions from iron-rich to sulfide-rich influences the mineralogical, geochemical, and isotopic composition of the sediment. To do this we are applying and developing a new tool, which was pioneered by members of our research team. We are extracting various fractions from these modern sediments, both sediment from the field and those we have worked with, or incubated in the laboratory, and analysing them separately, rather than doing a bulk digestion of the sediment all together, which is currently the approach. We hypothesise that these mineral fractions will better record the changes that occur during the burial of sediments and that as sediments evolve from iron-rich to sulfide-rich, we will find a geochemical tracer of this process. Our final part of the proposal is to take this new tool and apply it to very old rocks which have previously been interpreted to have both iron-rich and sulfide-rich characteristics. This project will finally allow us to understand what part of our sedimentary record documents real environmental conditions and which part has been acquired during post-depositional modification.