The geology, geochemistry and biology of submarine canyons west of Portugal: Introductory remarks

Masson, Douglas G. and Tyler, Paul A. (2011) The geology, geochemistry and biology of submarine canyons west of Portugal: Introductory remarks. Deep Sea Research Part II: Topical Studies in Oceanography, 58 (23-24), 2317-2320. (doi:10.1016/j.dsr2.2011.08.008).

Abstract

Large submarine canyons are prominent features of the continental margin west of Portugal (Fig. 1). The largest, including Nazaré and Setúbal Canyons, begin almost at the coast and cut across the entire continental shelf and slope before merging with the deep ocean basin floor at depths >4500 m; in places they are incised >2000 m below the depth of the adjacent shelf and slope (Lastras et al., 2009). Canyon location is at least in part controlled by the geological structure of the margin, with ENE trending faults of Variscan age playing a prominent role ( [Vanney and Mougenot, 1990] , [Pinheiro et al., 1996] and [Lastras et al., 2009] ). Some Portuguese margin canyons, such as Lisbon and Setúbal Canyons, are situated immediately offshore from river mouths, suggesting a genetic relationship (Fig. 1). Others however, including Nazaré Canyon, have no present day river connection.
The Portuguese margin canyons have been a focal point for European canyon research for the past 15 years. The major advantage of this area for canyon research is that each canyon is a discrete feature, with strong contrasts in geological and sedimentological environments between canyons. This has allowed testing of canyon paradigms that include:

• Canyons are fast-track pathways for sediment transport from the continental shelf to the deep ocean.
• The activity of canyons is related to sea level, and many present day canyons are largely inactive.
• Canyons funnel nutrients down slope, thus creating biological ‘hotspots’.
• Canyons act as sinks for organic carbon from terrigenous and continental shelf sources.

Co-ordinated European research on the Portuguese Canyons began with the OMEX II programme (1997–2000), with the emphasis on ocean margin fluxes and the export of carbon from the shelf to the continental slope. Nazaré Canyon was the focus of work in this programme (van Weering and McCave, 2002). The role of canyons as conduits for sediment transport to the deep ocean was investigated by the EUROSTRATFORM programme (2002–2005), which targeted the contrasting sedimentary systems that were believed to characterise Nazaré and Lisbon/Setúbal Canyons (Weaver, 2006). HERMES (2005–2009) and HERMIONE (2009–2012) were broad multidisciplinary projects, spanning geology, biology and oceanography, with the overarching objective of increasing understanding of marine ecosystems on the European continental slope, including man’s influence on these ecosystems ( [Weaver et al., 2009] and [Weaver and Gunn, 2009] ). These also concentrated on Nazaré and Lisbon/Setúbal Canyons, with some additional work on Cascais Canyon. This volume mainly presents results from HERMES and HERMIONE, although these more recent findings clearly benefit from the long standing integrated research programme that has been directed at the Portuguese margin.

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