The computer model CoTAM (Hamer and Sieger, 1994) was developed to describe transport and chemical reaction of dissolved species in aquatic systems. Generally, it is possible to simulate one-dimensional advective, dispersive and diffusive transport processes coupled with a given set of redox reactions. The goal of this study was the application of CoTAM to early diagenetic mineralization processes in marine sediments. Measured pore water profiles of oxygen, nitrate, calcium, alkalinity, and pH were simulated in order to constrain the model.
A simulation with CoTAM is subdivided into discrete time and depth intervals. The numerical model comprises a separation of transport and reaction processes (Schulz and Reardon, 1983), which allows a step by step calculation of concentration changes due to pore water transport and consumption or production of any species. The results reported here are based on the assumption of predominantly diffusive transport of pore water. Transport characteristics of the sediment were described through diffusion coefficients, porosity and sedimentation rate. Decrease of oxygen and nitrate concentrations due to their use as terminal electron acceptors for organic matter decay was given through depth dependant consumption rates for both species. Formation of nitrate is coupled to the oxidation of organic nitrogen. Calcium, alkalininity and pH were not influenced by any consumption or production rates. Alkalinity and pH concentrations are calculated from initial concentrations and the amount of CO2 and H+ released through organic matter decay. Since higher CO2-concentrations and lower pH may cause dissolution of CaCO3 (if available) interactions within the carbon dioxide system have to be taken into account. To be able to evaluate these effects the computer software PHREEQE (Parkhurst et al., 1980) was linked to CoTAM. Therefore all concentrations calculated by CoTAM were recalculated in order to obtain thermodynamic equilibrium.
We present simulations from pore water measurements carried during METEOR cruise M20/2 along the continental slope off West Africa. In situ and laboratory data of oxygen were derived from J.K. Gundersen (pers. comm.) and Glud et al. (1994). Oxygen concentration profiles could be fitted exactly to the measured data.
Nitrification is coupled to oxygen consumption. Therefore, if other nitrogen sources like ammonium can be excluded, the amount of released nitrate is dependent on the composition of degraded organic matter. Our investigations from laboratory data revealed that nitrate production was in no case sufficient to explain measured data assuming Redfield-Ratio of degraded organic matter. Fitting of measured profiles could only be realized by decreasing the C/N-ratio (3-3.7) which significantly raised nitrate production. Such a preferential mineralization of nitrogen rich organic matter is already assumed in literature (Blackburn, 1980; Kristensen and Blackburn, 1987). Furthermore simulations for in situ measurements were carried out. Oxygen uptake rates in situ are lower than measured in laboratory (Glud et al., 1994). Because the oxygen uptake directly reflects the nitrification process, the differences in oxygen consumption should influence nitrate concentrations in pore water. Following our estimations in situ benthic nitrate fluxes are about 10 to 50% lower than calculated from laboratory data at the investigated stations.
Effects of nitrification and denitrification on pore water pH, alkalinity and calcium concentrations were also simulated. The results are increasing alkalinity and calcium concentrations due to CO2 release and calcite dissolution, as well as pH decrease due to H+ production. Simulation data are in agreement with measured concentrations of these species.
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Glud, R.N., Gundersen, J.K., Jørgensen, B.B., Revsbech, N.P. & Schulz, H.D., Deep Sea Res. 41, 1667-1788 (1994).
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