SIMULATION OF EARLY DIAGENETIC PROCESSES IN CONTINENTAL-SLOPE SEDIMENTS OFF SOUTHWEST AFRICA - THE COMPUTER-MODEL COTAM TESTED

Pore water of marine sediments recovered from two stations of the cont inental slope off southwest Africa were investigated. We present compu ter simulations of in situ and laboratory concentration profiles of ox ygen as well as laboratory concentration profiles of nitrate, calcium, pH and alkalinity. The simulations were carried out with help of a nu merical model (CoTAM) to describe the transport and the reaction of di ssolved species in sediments. CoTAM is based on an operator-splitting approach comprising the independent calculation of transport and chemi cal reaction. The consumption rates of oxygen and nitrate were determi ned by optimal fits to the measured pore water profiles of these speci es. It could be shown that measured concentrations of nitrate in pore water correspond to a decomposition of organic matter with C/N ratios between 3 and 3.7. However, artificially increased subsurface nitrate concentrations due to core recovery cannot be excluded, but our result s show much greater deviations from expected concentrations (assuming Redfield stoichiometry) than previously reported from comparative stud ies (i.e. [Martin and Sayles, 1996]). Oxygen consumption in situ was s hown to be distinctively lower than measured in multicorer cores after recovery. Simulations with varying denitrification rates indicate red uced diffusive nitrate release into the bottom water by up to 50% comp ared to shipboard results. Effects of nitrification and denitrificatio n on pore water pH, carbonate alkalinity and calcium concentrations we re simulated by recalculating concentrations of these species with reg ard to calcite equilibrium. For these calculations we used the standar d software PHREEQE as a subroutine of CoTAM. Calcium and carbonate alk alinity increase due to solid phase calcite dissolution. Delta pH was calculated to be an order of magnitude lower within the zone of oxygen depletion than indicated by shipboard results. This difference is mai nly related to non-equilibrium conditions during pH measurements. (C) 1997 Elsevier Science B.V.