Geochemistry of the 1.4 Ga Roper Group, Australia: An Ideal Window to the Redox Landscape of the Mid-Proterozoic Ocean

Understanding the nature of mid-Proterozoic oceanic redox chemistry is vital to unraveling the apparent biogeochemical stasis recorded during an estimated one-billion-year interval and its relationship to the early evolution of eukaryotic life. It is widely accepted that the chemistry of the early ocean and its life co-evolved. However, the true geochemical nature Earth's 'boring billion' remains poorly constrained. Surface waters in the mid-Proterozoic ocean were most likely oxygenated, while the deeper anoxic waters contained sulfidic zones at intermediate depths, with likely iron-dominated anoxic (ferruginous) conditions at greater depth. Our constraints on this structure remain regrettably limited in time and space, and many of the best data have come from the McArthur basin, Northern Territory, Australia. Specifically, black shales of the Roper Group in this region have emerged as one of our best windows to mid-Proterozoic ocean redox because they are well dated, minimally metamorphosed, and lie in the middle of the 'boring billion.' Most importantly, paleogeographic data suggest relatively stronger connection to the open ocean compared to the older Barney Creek Formation from the same region. Consequently, conditions in the Roper basin may reflect the redox state of the broader margin of the open ocean. Despite this potential, a high-resolution, fully integrated, large-scale geochemical study of the Roper has not been undertaken, and there are very few studies for the mid-Proterozoic in general. Of additional value, these data provide an essential backdrop for our ongoing organic geochemical study and specifically our search for the early record of eukaryotic organisms and their paleoenvironmental context. This project's primary aim is to generate a high-resolution geochemical history for samples from the Roper Group obtained by drill core to remove possible effects of weathering. The emphasis is on redox proxies, specifically TOC, pyrite-S isotopes, Fe speciation, and trace metal concentrations, all measured to provide a high-resolution perspective on the degree of oxygenation and its temporal variability, ultimately with an eye on coeval water-column redox structure through analysis of other cores. Also among the targets are degrees of basin restriction and, overall, the conditions for shallow and deep life. Uncertainties about global versus local redox conditions remain paramount and are often difficult to resolve. Nevertheless, preliminary data suggest that the Roper experienced at least episodic euxinia along the margin of an ocean that may have been dominated by dissolved Fe at greater depth.