THE EVOLUTION OF THE MARINE P CYCLE

Phosphorous is typically considered to be the ultimate nutrient limiting primary productivity in the oceans. Given the importance of P in fueling marine biogeochemical cycles, there is an obvious impetus to reconstruct the P cycle through Earth’s history. However, tracking the P cycle through time hindered by a suite of challenges. Foremost, P has only one stable isotope, and so isotope geochemistry cannot be used to track mass fluxes similar to what is done with other key biogeochemical cycles (e.g., C, S, N). Inverting for P budgets, therefore, relies heavily on creative ways of measuring ancient seawater phosphate concentrations and estimating P burial fluxes. In principle, it is possible to estimate fluxes in the P cycle directly from observations of P abundance in sedimentary rocks through time. The chemical composition of shales have been long and often utilized to track element fluxes in the oceans. The main appeal of this archive is the abundance of trace and major element concentration data from shales deposited throughout Earth’s history. Further, with a large database the composition of shales in the geologic record is likely to provide a geochemical record of typical sedimentation on the continental margin. Therefore, I will draw from a new compilation of shale geochemical data—with information from over 5000 individual samples—with the goal of reconstructing P burial on continental margin and more broadly tracking the evolution of marine P cycle through time. I will suggest that the Neoproterozoic marks a significant change in the global P cycle. Specifically, the Ediacaran appears to record a shift to ‘Phanerozoic style’ P burial in continental margin sediments. This shift in the P cycle is linked change ecosystem structure, marine carbonate chemistry, and marine redox conditions.