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

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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.