Organic sedimentation processes under two different upwelling systems over 23°C South America margins (Brazil & Chile).
Résumé
In South America, the Holocene is characterized by a high climate variability linked to the interconnexions between insolation, interannual Pacific and interdecadal Pacific and Atlantic variability. The phenomenon of waters upwelling is extremely important on climatic and ecological points of view. They may have a large retroactive influence on both the climatic environment developed close to the coasts, and the nutrient enrichment of the water column: a key factor that supports the regional fisheries activity. Upwelling zones, more frequent and intense on the western margins of continents, may also occur on some eastern margins. The Mejillones Bay (Chile, 23°S, 70.5°W) contains one of the more productive zones of the oceans due to the presence of a permanent upwelling system. Located on the western Pacific margin, this bay presents a specific geographical setting and a peculiar local current system, which induce an anoxic water-sediment interface. Therefore, laminated organic-rich sediments accumulate due to high productivity combined with a minimum disturbance and weak oxic degradation. The close continental arid climate drastically inhibits terrigenous inputs to the bay, so that the only sedimentary detrital fraction is related to wind-driven processes. Although less intense than other areas in the world, the Cabo Frio (Brazil, 23°S , 42°W) coastal upwelling cell is controlled by three main factors: 1) the topography and the orientation of the coastline, 2) the position of the Brazilian Stream axes and 3) the wind pattern. The upwelling events of this region have a strong seasonal signature, which is concentrated during austral spring and summer, mostly associated with NE-ENE winds. The topic of this project is to compare the sedimentary records of both permanent and seasonal resurgences submitted to different wind and climate systems in order to determine the large-scale processes that may affect the complex relationship between SACZ variability, cold fronts displacements and El Nino events. In this study, organic matter which is a good sedimentary marker for reconstructing upwelling variability will be determined through C, N and S contents, Rock-Eval parameters (TOC, HI), palynofacies composition, stable isotope analysis and alkenones. For this, a high-resolution sampling of the Holocene sediments was firstly realized on the continental shelf of Cabo Frio, collected at depths shallower than 150 m using Kullenberg piston cores. Additionnal coring in Mejillones bay will be performed during Spring 2004. First results show that conversely to Mejillones, the sediment is an homogeneous clay in which organic matter presents a variability in content and quality. The occurrence of humid climatic conditions on the continent drainage output not far from cape may be inducing an additional organic supply deriving from higher plants, which is superimposed to the marine organic matter. MHCG thanks the support of the Brazilian Education Ministry / CAPES Foundation (PhD grant), ISTO and IRD.