Lateral particle transport in nepheloid layers - a key factor for organic matter distribution and quality in the Benguela high-productivity area.

Abstract

This thesis aims to identify and estimate the significance of lateral transport of particulate organic matter in the deep water-column of the Benguela upwelling system (BUS) offshore southwestern Africa. Although the BUS is regarded as the most productive of the four major eastern boundary systems of the ocean, and therefore of high importance for the global marine carbon cycle, little information is available on processes and fluxes involved in particulate matter transport in this area. Above the continental slope, water layers strongly enriched in particle content are identified in intermediate water depths and in the lowermost water column. These layers are termed intermediate and bottom nepheloid layers (INLs and BNLs, respectively). In such layers, the major part of the advective particle transport is supposed to take place. A pronounced BNL covers the entire study area with maximum intensity above the outer shelf and at the shelf break. The detachment of this BNL at the shelf break feeds a major INL in 250 to 400 m water depth at 25.5Ã °S, positioned at the lower boundary of the oxygen minimum zone. Together, these strong subsurface nepheloid layers are indicators of intensive lateral particle transport from the outer shelf towards a depocenter of organic matter (OM) on the upper continental slope. The results on distribution, transport and quality of organic matter presented in this thesis confirm the major significance of lateral transport of pre-aged, but well-preserved organic particles in the highly-productive Benguela upwelling area. Many indications suggest that the results from this study are also applicable to other continental margin settings. For example, high 14C-ages of SPM and the deposition of OC-rich sediments with high hydrocarbon potential on the upper continental slope is also known from other high-productive areas of the modern ocean. Thus, the results may improve our understanding of the genesis of black shales and petroleum source rocks in the geological record, to which these depocenters may represent a modern analogue. Additionally, it is shown that advective processes have the potential to displace areas of enhanced OC burial effectively from maximum production cells along the coast towards the slope and possibly the deep-sea. These results limit the validity of often used, merely vertically oriented particle flux models. The findings from the BUS identify lateral transport as an important, yet hardly recognized, secondary mechanism, effectively transferring carbon from the atmosphere to long-term sequestration in the sediments. Therewith, the results contribute to the improvement of global models to predict the impact of environmental changes on the oceanic system.