Abstract
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 210 Pb 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.
Highlights
Submarine canyons are important morphologic features cutting the seafloor of the Portuguese continental shelf and slope, and act as important conduits of natural and contaminated sediments from coastal areas to deeper parts of the ocean (de Stigter et al, 2007; Arzola et al, 2008; Lastras et al, 2009; Richter et al, 2009)
This paper focuses only on one site within the Cascais Canyon, which appears to be representative of the transport of contaminated sediments through the canyon
In the geomorphological context of the Cascais Canyon (Lastras et al, 2009), this multi-core was recovered at the thalweg of the main branch of the Cascais Canyon (Fig. 1), one of the shortest canyons on the western Portuguese Margin
Summary
Submarine canyons are important morphologic features cutting the seafloor of the Portuguese continental shelf and slope, and act as important conduits of natural and contaminated sediments from coastal areas to deeper parts of the ocean (de Stigter et al, 2007; Arzola et al, 2008; Lastras et al, 2009; Richter et al, 2009) Though they were more active during low sealevel stands, submarine canyons can maintain significant activity during the present state of high sea-level stands, for fine sediment transport (van Weering et al, 2002; Palanques et al, 2006), resulting in high rates of fine-grained sediment deposition. Part of these metals was exported from the estuary and deposited on the adjacent shelf and slope, in fine-grained deposits of the Tagus prodelta
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