Modeling the Marine Silicon Cycle - Physics, Chemistry, and Biology

Abstract

A large part of the organic carbon transport into the deep ocean is carried out by diatoms. A unique diffusion-reaction model for theuptake of silicic acid by a single diatom cell is developed. For the first time, model results show concentration gradients of different silicic acid species, [H:sub:4:/sub:SiO:sub:4:/sub:] and [H:sub:3:/sub:SiO:sub:4:/sub::sup:-:/sup:], and of [H:img:=plus.gif:/img:] and [OH:sup:-:/sup:] in the diffusive boundary layer around the cell. It can be stated, that diffusion through the diffusive boundary layer is not a limiting factor for this diatom, even at low Si concentrations. To understand the role of silicic acid as a biomass limiting nutrient in the ocean a larger scale model for the Pacific is presented. The Silicon cycle in the equatorial Pacific has the potential to significantly enhance phytoplankton (i.e. diatom) growth during La Nina phases by H:sub:4:/sub:SiO:sub:4:/sub: transfer from the EUC into the ocean surface layer. This supports the theory of silica as a biomass limiting nutrient in this area. Despite some simplifications the physical and biological description of the 1988/1989 La Nina event was used as an analogue to get some understanding about the silicon sediment record inthe eastern equatorial Pacific at times of the last glacial maximum (LGM). A better approach to diatom productivity at the LGM is the use of silicon isotopes. For the first time, the overall distribution of silicon isotopes in the world ocean has been calculated by integration of a biogeochemical ocean circulation model, HAMOCC4. The model results can be used to predict opal silicon isotope compositions in the sediment. Due to the nature of the Pacific current system it might be valid to even apply a relationship between silicic acid concentrations in the surface and the silicon isotope signal in the sediment.