Sensitivity of the marine biospheric Si cycle for biogeochemical parameter variations

A systematic quantitative assessment of the marine silicon cycle is presented, based on a prognostic coupled water column-sediment global biogeochemical ocean general circulation model (HAMOCC). The resulting tracer distributions are compared with a comprehensive marine Si database of measurements. The model parameters which govern the Si cycle within the model world are optimized through a linear response model. The functional relationships between the Si cycle parameters and the Si tracer distributions are derived from a series of sensitivity experiments addressing opal export production, particle flux through the water column, porewater chemistry, and external biogeochemical forcing. The most important parameters for a further quantitative improvement of the simulation are depth-dependent opal dissolution kinetics, a productivity-dependent opal settling velocity, a general change in maximum Si uptake velocity Vmax opal, and the clay as well as the Si input from continental weathering. The modeled Si budget shows a larger global export production, larger opal deposition rates onto the sediment surface and higher diffusive transports of porewater silicic acid into the open water column as estimated by Tre´guer et al. [1995]