Changes in the Si / P weathering ratio and their effect on the selection of coccolithophores and diatoms

Abstract. The relative contribution of coccolithophores and diatoms to export production and burial influences the mechanisms of carbon (C) removal from Earth’s surface reservoirs. Whereas the supply of phosphorus (P) to the ocean basins controls export production over geological timescales, the availability of dissolved silica (Si) determines the relative contribution of diatoms to export production and increases the efficiency of the ocean’s biologically-driven C sequestration. Here we combine analyses of data from the sedimentary record and Earth system model simulations to investigate the coupling of P and Si weathering fluxes over the last 40 million years (My). Although both weathering fluxes increased on average through time, the relative increase of Si weathering over the last 20 My exceeded that of P. Our analysis suggests that the increase in P weathering under a regime of increasingly ocean turbulence allowed full utilization of increased Si supplies and links the ecological success of marine diatoms, and consequential decline of coccolithophores, to changes in the Si / P weathering ratio. These results allow us to envisage that ocean fertilization initiatives would be more effective if they were integrated within the framework of geo-engineering programs aimed at enhancing silicate weathering.