A finite-element coupled sea ice--ocean model (FESOM) has been developed at the Alfred Wegener Institute for Polar and Marine Research.The sea-ice component is a dynamic-thermodynamic model with an elastic-viscous-plastic rheology.The ocean component is the hydrostatic, primitive-equation Finite Element Ocean Model (FEOM).An eight-compartment model of the marine ecosystem, featuring nitrate and silicate cycles and considering possible iron limitation, has been implemented.The coupled model has been configured in a circumpolar domain covering the Southern Ocean between the coast of Antarctica and 48S, and on a global grid with $1.5^\circ$ mean resolution.Multi-decadal simulations have been performed in both configurations with a surface forcing derived from atmospheric reanalysis datasets.The model features a realistic representation of sea-ice coverage and large-scale ocean circulation.Results from a wide range of sensitivity studies confirm the crucial importance of a carefully chosen adaptive mixing scheme to parameterize vertical and horizontal mixing.Artificial passive tracers are used to identify water mass formation pathways.With the right choice of parameters and parameterizations, the model is able to reproduce observed water mass properties and formation processes realistically.