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Mil-Homens M.,National Laboratory of Energy and Geology | Costa A.M.,National Laboratory of Energy and Geology | Lebreiro S.M.,Geological Survey of Spain | Canario J.,Instituto National Of Recursos Biologicos | And 9 more authors.
Scientia Marina

The Cascais Canyon delivers contaminated sediments from the shelf to the deep marine environment. Multi-core PE252-32, located at 2100 m water depth in the canyon, records the latest 300 years of sedimentation. It was dated by 210Pb and analyzed texturally and geochemically for major elements and selected trace metals (Cu, Cr, Hg, Li, Ni, Pb and Zn). Cluster analysis performed on the down-core geochemical data identified four groups of variables related by grain-size, geochemical source and composition. Mercury, Pb and Zn were grouped in a cluster representing the anthropogenic component. Cluster analysis was applied again particularly to the latter cluster relatively to depth, in order to constrain the onset and temporal evolution of anthropogenic contamination. A second clustering, made on the basis of Hg, Pb and Zn, grouped samples by age and degree of anthropogenic contamination. One cluster contained relatively uncontaminated samples older than 1900 AD, and another cluster samples younger than 1900 AD with distinct metal enrichment. Maximum enrichments occurred during the early 1980s, followed by a slight recovery from the mid-1980s to the present. Mercury was the element with the highest enrichment factor (EFHg=5). Despite relatively low accumulation rates at this core location, our results show the importance of the Cascais Canyon as a transport route for contaminated sediments from the Tagus prodelta into the deep regions of the Portuguese Margin. Source

Costa A.M.,National Laboratory of Energy and Geology | Mil-Homens M.,National Laboratory of Energy and Geology | Lebreiro S.M.,National Laboratory of Energy and Geology | Richter T.O.,Netherlands Institute for Sea Research | And 9 more authors.
Marine Geology

Submarine canyons play an important role in the transfer of contaminated sediments from shelf areas to the deeper ocean. To evaluate the importance of submarine canyons adjacent to the Tagus and Sado estuaries (Portuguese Margin) as sediment pathway major and trace elements, 210Pb radionuclides, organic carbon, calcium carbonate and grain size were analyzed in fifteen short sediment cores from the Cascais and Lisboa-Setúbal submarine canyons and neighbor slopes at the Estremadura Spur and off Sines. Mass accumulation rates (MAR) vary between 0.02 and 0.40g cm-2 y-1 with the highest values in the upper Cascais and Lisboa canyons. Mercury and Pb enrichment factors exceed 2 and show a continuously increasing trend since 1950AD in the upper sections of the Cascais Canyon and in the Lisboa-Setúbal Canyon system, implying an anthropogenic source. Mercury and Pb anthropogenic inventories also present highest values in the canyon heads, which is consistent with its proximity to the Tagus and Sado river mouths. To a lesser extent, the Estremadura and Sines slopes and the deeper sections of the canyons also have slightly increasing Hg and Pb enrichment factor trends towards the present-day. For these areas, with a major influence of pelagic sedimentation, we infer atmospheric transport as a complementary source of contaminants. Therefore, our data suggest that although canyons are preferential conduits to transport sediments to the deep sea when compared to the slopes, Hg and Pb derived from human activities have reached both domains with different intensities down to a depth of 2000mwd. © 2011 Elsevier B.V. Source

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