Through the Looking-Glass, and What MEDUSA Found There

What lies beneath the seafloor can appear as unfamiliar as a world behind the looking glass. In our first exploration of the sedimentary “wonderland” underneath the seafloor in FESOM-REcoM with the sediment model MEDUSA, we began with a classical representation of early diagenesis, including oxic respiration and denitrification pathways for organic matter degradation, as used in many ocean biogeochemical models of comparable complexity. At first glance, the simulated sediments appeared reassuring: the inorganic components (calcite and opal) agreed reasonably well with observations. The organic carbon distribution was, however, less convincing. Despite reasonable global accumulation rates, there were pronounced biases in shallow regions and at high latitudes. To address these shortcomings, we explored several avenues. These included the processing of settling particulate flux in the water column prior to deposition, the addition of sulfate reduction as an additional degradation pathway, and an extension of the reactive sediment layer from 10 to 50 cm (increasing the vertical resolution from 21 to 71 grid points). We also revised the global sedimentation-rate distribution by introducing additional lithogenic input to improve bulk mass accumulation rates in shallow environments. Here we retrace the successive stages of this modelling effort and highlight lessons learned while venturing “through the looking-glass” of early diagenesis, including the importance of water-column processing, sediment redox pathways, and lithogenic inputs for representing sediment carbon dynamics in next-generation Earth system models.