The chemical evolution of the Proterozoic biosphere

The chemical evolution of the Proterozoic biosphere is not a well comprehended topic. Major gaps exist in the redox record which documents the ocean’s transition from a dominantly ferruginous state throughout most of the Proterozoic, to widespread deep ocean oxygenation during the Late Neoproterozoic. Between ~1.85 – 0.54 Ga, significant developments in biological evolution also occurred with the emergence of macroscopic eukaryotes followed by the radiation of metazoans. A strong link between stably oxygenated marine environments and the radiation of increasingly complex organisms is observed across the literature. However, there is little understood concerning nutrient availability during the Proterozoic, another major governing factor in biological development. Phosphorus is considered the ultimate limiting nutrient for oxygen production over geological timescales through organic carbon burial, and is thus vital to consider in discussing the chemical evolution of the Proterozoic biosphere. Through the application of multiple palaeoredox proxies (iron speciation and trace metal analyses) alongside a newly adapted phosphorus speciation method, this research aims to add to the current record of Proterozoic redox to assess if the chemical evolution of the ocean had a potential effect on the biogeochemical phosphorus cycle, and thus the development of complex organisms. The sedimentary successions from the ~1.8 – 1.4 Ga Yanshan Basin in North China, and the 1.6 – 1.0 Ga Bashkir Meganticlinorium in Russia uncovered dynamically evolving water column chemistry. Their palaeoredox signals contrast to other basins from this time period and highlight the significant spatial and temporal heterogeneity that occurred across the Proterozoic ocean. Analysis of phosphorus-bound phases within marine sediments from the Late Palaeoproterozoic Yanshan Basin and the Ediacaran White Sea (Kel’tma and Kostovo cores), Avalon Peninsula, Cariboo Mountains and the Upper Ungoolya Group, Officer Basin confirmed that water column redox, and a basin’s depositional environment, were the dominant controls on the Proterozoic phosphorus cycle. This is the first geochemical record of phosphorus burial mechanisms in Proterozoic marine sediments as all other studies have been modelling based, or focused on BIFs and total phosphorus content. The research presented here provides a crucial insight into the feedbacks between ocean redox and nutrient availability during a dynamic period of Earth’s history.