Internal tide-shelf topography interactions as a forcing factor governing the large-scale distribution and burial fluxes of particulate organic matter (POM) in the Benguela upwelling system

This study investigates the role of internal tides in driving the sedimentation and re-suspension of biogenic POM on the Namibian shelf and give rise to stable 500-800 km long shore alternating bands of high and low POM concentrations. Temperature time series data (September 2000-March 2001) from the benthic boundary layer at three sites are used to hypothesise that the dominant forcing mechanism are internal tides and their interaction with the shelf break zones. Vertical displacements of the temperature structure by 100-150 m at the outer shelf break (depth 450 m) are shown to occur through out the 6-month time series. In contrast at non-shelf break sites the vertical displacements of temperature are negligible. The shear-stresses predicted landward of the shelf break zone from 100-150 m vertical displacements of the temperature structure are significantly higher (>0.1 Pa) than the critical shear-stresses which govern the re-suspension of biogenic particles and fine sediments (0.05-0.1 Pa). Short-term ADCP data was used to show that critical shear-stress distribution at the different POM areas is consistent with the predicted net accumulation and net erosional zones of POM across the Namibian shelf. This study hypothesises that the barotropic-baroclinic tidal coupling governs vertical particle flux dynamics whereas Ekman and inertial flows are thought to govern the horizontal advection scales that result in the observed POM distribution. The importance of this improved dynamical understanding has implications for both carbon burial efficiency as well as the variability in the suitability of benthic fisheries habitats.