A high-resolution time-series analyses of particle fluxes in the Northern Benguela coastal upwelling system: carbonate record of changes in biogenic production and particle transfer processes

Abstract

Fluxes of biogenic particles at the shelf edge off Walvis Bay, Namibia, are investigated from a time-series sediment trap in order to evaluate the response of biogenic production to rapid changes in the dynamics of the upwelling process, as well as the importance of particle transfer processes on the nature of sediments accumulating at a high rate on the Namibian margin. Total mass flux displays a smooth trend of lowered values from the start to the end of the experiment despite evidence, from the SST and wind records, of a variable pattern of upwelling dynamics. Contributions of the various biogenic components (carbonates, opal, organic matter) to the total mass flux were relatively constant throughout the period of trapping, suggesting that changes in upwelling dynamics have no obvious implications on the nature of biogenic particles sedimenting at the trap location. A detailed examination of the carbonate fraction indicates a partitioning of its various contributors (coccolithophores and planktonic foraminifera) in terms of sources and mechanisms of transfer to depth. From these data, it is suggested that the bulk of the biogenic particles sedimenting on the slope is resuspended material from the outer shelf, and conversely that direct input from surface waters does not contribute to a high extent to the particle flux at depth. A highly partitioned, three-dimensional flow field is proposed to explain the observed pattern of particle flux as well as the suggested transfer processes affecting the biogenic components.