Description: Particles sinking out of the euphotic zone are important vehicles of carbon export from the surface ocean. Most of the particles produce heavier aggregates by coagulating with each other before they sink. We implemented an aggregation model into the biogeochemical model of Regional Oceanic Modelling System (ROMS) to simulate the distribution of particles in the water column and their downward transport in the Northwest African upwelling region. Accompanying settling chamber, sediment trap and particle camera measurements provide data for model validation. In situ aggregate settling velocities measured by the settling chamber were around 55 m d**-1. Aggregate sizes recorded by the particle camera hardly exceeded 1 mm. The model is based on a continuous size spectrum of aggregates, characterised by the prognostic aggregate mass and aggregate number concentration. Phytoplankton and detritus make up the aggregation pool, which has an averaged, prognostic and size dependent sinking. Model experiments were performed with dense and porous approximations of aggregates with varying maximum aggregate size and stickiness as well as with the inclusion of a disaggregation term. Similar surface productivity in all experiments has been generated in order to find the best combination of parameters that produce measured deep water fluxes. Although the experiments failed to represent surface particle number spectra, in the deep water some of them gave very similar slope and spectrum range as the particle camera observations. Particle fluxes at the mesotrophic sediment trap site off Cape Blanc (CB) have been successfully reproduced by the porous experiment with disaggregation term when particle remineralisation rate was 0.2 d**-1. The aggregation-disaggregation model improves the prediction capability of the original biogeochemical model significantly by giving much better estimates of fluxes for both upper and lower trap. The results also point to the need for more studies to enhance our knowledge on particle decay and its variation and to the role that stickiness play in the distribution of vertical fluxes.

Description: On a section between 72°S and 42°S and a transect between 60°E and 10°E through the Weddell Sea and the southernmost eastern Atlantic Ocean, the water column was sampled on 72 stations, and the stable carbon isotopic composition of total dissolved inorganic carbon (delta13C(SumCO2)) as well as the stable oxygen isotopic composition of seawater (delta18O) was determined. These data were compared with potential temperature, salinity, dissolved oxygen and phosphate data from the same stations. The observed delta13C(SumCO2)/PO4[3-] relationship in the deep Weddell Sea strongly differs from the global Redfield-driven deep water relationship. We attribute this to enhanced thermodynamic fractionation at sites of bottom water formation that decouples the nutrient signal from the delta13C(SumCO2) signal not only in surface and intermediate water masses but also in deep and bottom water. Different, water-mass specific thermodynamic imprints due to different modes of bottom water formation are assumed to cause the observed deviation from the global delta13C(SumCO2)/PO4[3-] relationship in the deep Weddell Sea. The influence of increased photosynthetic fractionation, i.e., a more negative than low-latitude isotopic organic carbon composition, is shown to be minor. As a result, Recent Weddell Sea deep and bottom water delta13C(SumCO2) is by 0.4-0.5 per mil higher than expected if solely biologic fractionation would occur. A discussion of simple hypotheses of Weddell Sea deep and bottom water formation during glacial times reveals that regardless of what scenario is considered, the thermodynamic imprint on Southern Ocean deep water would increase. This makes it difficult to explain low glacial delta13C values observed in benthic foraminifera from the subpolar Southern Ocean as being calcified in Antarctic source bottom water and thus is in support of hypotheses looking for additional sites of deep water formation.