Sedimentation Processes on the Portuguese Margin: The Role of Continental Climate, Ocean Circulation, Sea Level and Neotectonics
Final Report Abstract
The SEDPORT project aimed to investigate the impact of strong climatic oscillations that have occurred in the North Atlantic on sedimentary processes and depositional features on the Portuguese shelf and upper slope. The influence of biological productivity, fluvial and submarine sediment transport mechanisms on the composition and properties of margin sediments should be determined, covering the last glacial-interglacial transition to unravel how these sedimentation processes may have changed under varying climate conditions that affected ocean circulation, sealevel, and continental weathering, vegetation and precipitation since the last Ice Age into the Late Holocene. The German part of the ESF EUROCORES Euromargins CRP SEDPORT attempted to establish paleoenvironmental reconstructions for changes in past temperatures, marine biological productivity and continental weathering, and to provide evidence for sediment transport mechanisms on Holocene fluvial sediment accumulation in the vicinity of the Tagus river, including the coastal zone, shelf and slope. Surface samples from four different regions along the western Portuguese shelf were analysed in terms of the provenance of their organic matter. In the North, off the Douro River mouth, river discharged, continental input is combined with marine produced material. Surface sediments inside the Tagus Estuary show a clear imprint of continental organic material. The decrease of continental organic contributions on the Tagus mud belt is visible by an increasingly marine source signature in the organic matter off the Tagus Estuary mouth. The southern sampling region off Cape Sines is characterised by exclusively marine input, due to the absence of major rivers. Camera and hydroacoustic investigation with a remotely operated vehicle (ROV) and with a parametric echosounder system along the coast revealed a highly variable sediment texture with sharp boundaries between different sediment textures. An evaluation of changing sediment properties during the past 3 000 years was obtained by a multi-proxy analysis of gravity cores from the Tagus Prodelta. The proxy data, mainly from core GeoB 8903, show no changes in the marine climate conditions nor in the quantity of organic matter contribution from land. In contrast, physical sediment properties such as magnetic susceptibility and grain-size indicate changes in sediment supply and transport mechanisms around 2 000 and 600 years BP. Grain-size analysis yielded three lithic modal size distributions (end-members), which are clearly related to organic and inorganic sediment properties. The magnetic susceptibility signal is carried by the finest end-member, organic carbon is related to an intermediate end-member and the calcium carbonate correlates with the coarsest sediment component. Variance in the different components shows a correlation with the North Atlantic Oscillation. A last Deglaciation to Holocene view on regional environmental changes is obtained by the analysis of a deep sea core (MD03-2698) in comparison to a piston core (D13882) from the Tagus mud belt. In the deep realm, changes in thermohaline circulation play a major role, particularly during the Deglaciation. Changes in sea level affect both settings by quantitatively changing the supply of terrigenous sediment to the deep sea and by changing the volume of sediment accommodation space on the shelf. Additionally, the postglacial sea level rise leads to a relocation of the upwelling cells from the shelf edge onto the shelf, which becomes evident by nutrient proxies. The effect of tsunamis on shelf sediments is spatially inhomogeneous, with stronger evidence for sediment reworking in sediments from the western Tagus mudbelt. The land-sea correlation of sediment properties and climate proxies records is still an open task and has to await the final data from-land drilled cores performed by the Dutch partner of SEDPORT.
Publications
- 2007. Provenance of organic matter and nutrient conditions on a river- and upwelling influenced shelf: A case study from the Portuguese Margin. Marine Geology, 243, 169-179
Alt-Epping, U., Mil-Homens, M., Hebbeln, D., Abrantes, F., & Schneider, R.R.
