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

Abstract

The role of internal tides in driving the sedimentation and re-suspension of biogenic POM, has been investigated, and the investigation provided the basis to formulate the hypothesis that the benthic boundary layer turbulence at the shelf break zones of the Namibian shelf and probably most of the Benguela is dominated by both tidal bores and high frequency soliton activity. At the continental shelf of Namibia, sediments with high concentrations of POM are confined to three well-defined long shore belts of 500-800 km in length. Temperature time series data from the benthic boundary layer at three sites was used to assume that the dominant forcing mechanisms are internal tides and their interaction with the shelf break zones. The investigation further showed that the modulation of the amplitude is closely linked to the lunar and semi-diurnal barotropic tides but the frequency of variability of the baroclinic component also includes a supra-tidal frequency of less than an hour. To conclude observations have been made that Namibian upwelling systems, ecologically significant hypoxia events are seasonally ubiquitous in the inner shelf and often linked to, inter-annually more variable, extreme sulphide rich methane eruptions. Historical work in the Benguela system suggested that, below the gravity wave base, the POM distribution in the mud belts of the Namibian shelf sediments is linked to long period physical mechanisms. These include both intensification of Ekman driven currents and inertial flows near the bottom at the shelf break.