A global oceanic sediment model for long‐term climate studies

A chemically reactive 10‐layer sediment module was coupled to a geochemical ocean general circulation model (the Hamburg Oceanic Carbon Cycle Model). The sediment model includes four solid sediment components (CaCO 3 , opal, organic carbon, and clay), and five pore water substances (dissolved inorganic carbon, total alkalinity, PO 4 3− , O 2 , Si(OH) 4 ) plus corresponding species containing 13 C and 14 C instead of 12 C. The processes, namely, particle deposition, pore water reactions, pore water diffusion and interaction with the open water column, vertical sediment advection, sediment accumulation, and bioturbation, are simulated through basic parametrizations. For the water column part the Si and C cycles are coupled by a formulation of the “rain ratio” Si:C(CaCO 3 ):C(POC), where POC is particulate organic carbon, in biogenic particle export production, with CaCO 3 frustrule production growing in parallel to a weakening of opal production during progressing deficiency of dissolved silicate in the surface layer. For two preindustrial velocity fields the model reproduces major features of observed water column and sediment tracer distributions parallel to a correct preindustrial CO 2 level close to 280 ppm. The model reacts sensitively to the formulation of the POC flux parametrization, the rain ratio, as well as the solubility of opal but is fairly insensitive to changes in the bioturbation rate as well as the amount of clay deposition. A simulation of the sediment distribution by use of a velocity field, which represents the ocean at conditions during the last glacial maximum, yields realistic glacial‐interglacial changes for the Atlantic Ocean, while discrepancies remain for the Indo‐Pacific region. A significant decrease of the atmospheric pCO 2 could be achieved through an additional change of water column inventories by a change in weathering input of Si and alkalinity.