Motivated by a series of recent discoveries, DEVOCEAN will provide the first comprehensive evaluation of the emergence of diatoms and their impact on the global biogeochemical cycle of silica, carbon and other nutrients that regulate ocean productivity and ultimately climate. We propose that the proliferation of phytoplankton that occurred after the Permian-Triassic extinction, in particular the diatoms, fundamentally influenced oceanic environments through the enhancement of carbon export to depth as part of the biological pump. Although molecular clocks suggest that diatoms evolved over 200 Ma ago, this result has been largely ignored because of the lack of diatoms in the geologic fossil record with most studies therefore focused on diversification during the Cenozoic where abundant diatom fossils are found. Much of the older fossil evidence has likely been destroyed by dissolution during diagenesis, subducted or is concealed deep within the Earth under many layers of rock. DEVOCEAN will provide evidence on diatom evolution and speciation in the geological record by examining formations representing locations in which diatoms are likely to have accumulated in ocean sediments. We will generate robust estimates of the timing and magnitude of dissolved Si drawdown following the origin of diatoms using the isotopic silicon composition of fossil sponge spicules and radiolarians. The project will also provide fundamental new insights into the timing of dissolved Si drawdown and other key events, which reorganized the distribution of carbon and nutrients in seawater, changing energy flows and productivity in the biological communities of the ancient oceans.