Impacts of dynamically simulated biogenic particles and iron on the marine carbon cycle

We present a novel mechanistic representation of both organic and inorganic marine particles within the Bern3D Earth system model of intermediate complexity, which is based on the columnar particle flux model MSPACMAM. This new approach moves away from the assumption of globally and temporally invariant sinking profiles. Instead, the new scheme calculates sinking speeds and remineralisation and dissolution rates based on local temperature, density, and seawater chemistry. When combined with an improved representation of dynamic iron release and scavenging, this scheme introduces new dynamic feedbacks in the response of the biological pump to climate and circulation changes in the Bern3D model. Particle remineralisation and dissolution rates are now functions of temperature, oxygenation, saturation state, and sinking speed. The sinking rate is in turn modulated by changes in export production (amount and composition) as well as the viscosity of seawater. In addition to light and macronutrients, export production is affected by iron availability, which is depending on the rate of iron removal through scavenging and iron release from decaying particles and sediments, both processes that depend on organic particle fluxes and local oxygen concentrations. We demonstrate the non-linear interactions between these new dependencies in transient and steady-state simulations of various climatic boundary conditions. The newly introduced particle concentrations sensitive to changes in temperature and density result in shallower carbonate dissolution and deeper organic particle remineralisation in the Southern Ocean under full glacial conditions. In idealized scenarios of anthropogenic climate change, there is a smaller decline of export production but faster oxygen depletion than with the old static particle decay scheme. In addition, organic particle fluxes affect sedimentary iron release, which can lead to a positive feedback on export production if the released iron reaches the surface ocean.