Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

BOGCM PISCES, model runs, sediment coupling (Community sediment model: sediment composition and benthic oxygen fluxes)

During OROIS, a sediment module was implemented to the standard PISCES model. The development of the sediment module closely followed Heinze et al. (1999). The description of POC remineralisation, limited to oxide degradation in Heinze et al. (1999), was extended to sub-oxide nitrate reduction. Off-line simulations were forced with bottom water conditions of dissolved Si, oxygen, phosphate, alkalinity, nitrate, and DIC, as well as yearly average fluxes of POC, clay, opal and carbonate provided by the PISCES standard version.

Stable distributions of dissolved and solid sedimentary tracers were obtained after 50kg of integration. The computed sediment composition was then used as initial conditions for the fully coupled simulations. Due to the long equilibration time of the coupled water-column/surface sediment system, the simulation is still running. Results of the coupled run are expected to be exploitable early in 2005.

In parallel and with the objective to validate the particle dynamics, we used yearly averaged fluxes computed by the PISCES standard version STD and the modified version STD2 to force the sediment module in an off-line mode. This approach takes advantage of the benthic compartment as the ultimate sediment trap to validate deep-water fluxes. Based solely on the bulk sediment composition, it is not possible to discriminate between PISCES STD and PISCES STD2.

The spatial pattern of SiO2 and CaCO3 distributions are reproduced by both versions. Computed values are however to low for areas rich in CaCO3 like for instance the mid-Atlantic ridge. The underestimation of maximum CaCO3 levels is stronger for PISCES STD2. On the contrary, this model version overestimates the POC concentration of surface sediments. Since results are represented on a weight percent basis, the increase in one component will result in a corresponding decrease of the other. The straightforward comparison between model output and observations of bulk sediment variables has thus a limited value.

We extended our evaluation of model output to the benthic oxygen fluxes. The latter are linked to the mineralization activity. They integrate POC supply, oxygenation of bottom waters and oxygen demand due to aerobic metabolism. Figure 4 compares estimates of benthic O2 fluxes by Jahnke (1996) to model output. Oxygen fluxes computed by the model forced with fluxes from PISCES STD are largely underestimated. The spatial pattern present in Jahnke's estimates is not reproduced. In comparison, the model version STD2 yields a satisfying spatial distribution of fluxes. They are however overestimated. These results are consistent with the conclusions drawn from the comparison of modelled and observed particle fluxes in the water column. They emphasize the correct representation of the fate of sinking fluxes in order to represent the coupled pelagic-benthic system.

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