Global trends in nutrient dynamics during the Ediacaran / Cambiran period as revealed in nitrogen isotope signatures

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The Ediacaran and the Cambrian periods record remarkably important environmental changes on Earth which are believed to be relevant for the most prominent bioradiation in Earth's history. The transition to a fully oxygenated surface environment occurred during the late Neoproterozoic and is believed to have influenced the nitrogen cycle in terms of the variable availability of nitrogen in reduced (NH4) or oxidised (NO3) forms for primary productivity. Coincident with these variations was the widespread deposition of giant phosphorite- and organic-rich sediments (black shales), notably in China. The previous project has focused to create detailed 15N records of the ~635 to ~500 Ma interval. It documented a strong negative nitrogen isotopic excursion in most of the studied sections from South China during the earliest Cambrian. This excursion is probably linked to the switch between the older small shelly fossil assemblages and the later "typical" fossil association including trilobites, molluscs, echinoderms, sponges and other "modern" type fossils.Based on these results, the proposed project aims to study in detail the nitrogen isotope record of shales and other sedimentary rocks from three well-exposed and well-documented Ediacaran to Early Cambrian sections in South China, closely cooperating with the other geochemical subprojects in a multi-proxy approach so that they can be used as key sections for the reconstruction of biogeochemical and paleoenvlronmental changes during the Precambrian-Cambrian transition. Additionally, samples using the same proxies from the Tarim block of NW China, from Kazakhstan, Spain and Namibia will be integrated in this study for global biogeochemical correlation, albeit at a lower resolution. First results from the Spanish sections indicate the existence of a negative nitrogen isotope excursion during the early Cambrian comparable to that documented from the South China sections. We will also analyze, in cooperation with our Chinese partners, organic and carbonate carbon isotope composition where carbonates are present.The new data, in combination with the existing records, will be integrated with results from paleobiology and stratigraphy to assess their possible role in "feeding" the global radiation event at the PCB.