Trace metal enrichments in Portuguese submarine canyons and open slope: Anthropogenic impact and links to sedimentary dynamics

Abstract

In order to evaluate the role of submarine canyons as transport routes for the particulate matter, including anthropogenic trace metals, from coastal areas to the deep sea, we analysed contents of Ca, Al, Fe, Mn, Zn, Cr, Pb, Ni and Cu, organic carbon and calcium carbonate in surface sediments and sediment trap particulate material from canyon and open slope areas of the central Portuguese continental margin. The contribution of trace metals from anthropogenic sources was assessed by comparing present-day concentrations and average elemental concentrations for the industrial period (last ∼ 150 years) with pre-industrial baseline values. Baseline elemental concentrations are very similar for the Lisbon–Setubal and Nazare canyons, but varying degrees of anthropogenic trace metal enrichments are recorded in more recent sediments. The upper Lisbon and Cascais canyons appear the most affected by anthropogenic trace metal deposition; excess Pb, Zn and Cu amount ∼ 55%, ∼ 57% and ∼ 21%, respectively, of total inventories over the last 150 years. The upper Nazare canyon follows with 42%, 21% and 17%, respectively. Lower contributions of anthropogenic trace metals were found for the lower reaches of the canyons and adjacent open slopes, indicating that the upper-canyon areas act as trace metal depocentres, whereas dispersal to deeper canyon and slope areas is limited. The Tagus and to a lesser extent the Sado river appear the main sources of Pb, Zn and Cu to the canyons, but atmospheric dispersal of Pb seems also important, especially in the southern area. Anthropogenic enrichment of Pb and Zn in the Cascais and Lisbon canyons is comparable to values published for canyons in the NW Mediterranean and off Taiwan located in close proximity to industrialized areas. Different from Pb and Zn, present-day Cu enrichment appears to increase toward deeper water where pelagic sedimentation is predominant, suggesting that dispersal of this trace metal is at least partly linked with surface water plankton production.