Trace metal enrichment through Setْbal-Lisbon Canyon and the adjacent continental shelf (Portuguese Margin): Links to sediment transport and accumulation

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Jesus, Carlos۱; de Stigter, Henko۲; Boer, Wim۲; Miranda, Paulo۱; Rocha, Fernando۱; Oliveira, Anabela۳
Holding Date 03 September 2008

Submarine canyons are active conduits for the transport of particulate matter, including natural and anthropogenic trace metals, from the coastal zone to the deep sea.
The main goal of the present study is to evaluate the quality of sediments in an important submarine canyon and surrounding continental shelf of the Portuguese margin, in order to understand the overall fate of anthropogenic contaminants. Lead, Zn, Cu, Cr and Ni concentrations were analysed in surface and down-core sediments and in sediment trap particulate material, as part of the project HERMES (EU 6th FP). The Setْbal-Lisbon canyon system is situated just offshore the most densely populated and most industrialized area of Portugal. The head of Lisbon Canyon is located close to the mouth of Tagus River, one of the largest rivers of Iberian Peninsula. The head of Setْbal Canyon is located close to the mouth of Sado River, a small river south of the Tagus.
The comparison between canyon and open slope surface sediments showed consistent higher contents of trace metals in the canyon sediments at comparable water depths, which is in accordance with canyons as active conduits for sediment transport as already stated above.
Comparing with down-core sediments dated with 150 years, Cr and Ni seem to have no enrichment in surface sediments while Pb, Zn and Cu do show although, with different patterns and degrees of contamination. The highest enrichment factors (EFs) of Pb and Zn are found in surface sediments from mud sedimentary deposits off the Tagus and Sado Rivers and in the upper canyon system (including all Lisbon Canyon). In fact, EF (Pb and Zn) in the upper Lisbon Canyon are similar to the ones found in the upper Foix Canyon, a submarine canyon that receives sediment inputs from a river system that drains industrialized areas of Barcelona (Palanques et al., 2007). In the offshore direction, dilution of sediments combined with an ineffective down-canyon sediment transport make the contamination levels of these metals very low. On the other hand, the EF of Cu increases with water depth along the canyon because Cu accumulation in phytoplankton seems to be the dominant process controlling its distribution pattern.
Throughout the canyon system, trace metals in particulate material from individual sediment trap deployments span relatively narrow ranges and are comparable to values observed in surface sediments at similar water depths. Hence, to a large extent, sediment trap data accurately reflect long-term depositional conditions.
Reference:
Palanques, A.; Masqué, P.; Puig, P.; Sلnchez-Cabeza, J.A. ; Frignani M. ; Alvisi, F. (2007). Anthropogenic trace metals in the sedimentary record of the Llobregat continental shelf and adjacent Foix submarine canyon (northwestern Mediterranean). Marine Geology, doi: 10.1016/j.margeo.2007.11.001.