Flux and composition of settling particles across the continental margin of the Gulf of Lion: the role of dense shelf water cascading

C Pasqual,M Canals,A Palanques,P Puig,X Durrieu De Madron,N Delsaut,A Sanchez-Vidal,A Calafat,D Zúñiga,S Heussner

doi:10.5194/bg-7-217-2010

Abstract

Abstract. Settling particles were collected using sediment traps deployed along three transects in the Lacaze-Duthiers and Cap de Creus canyons and the adjacent southern open slope from October 2005 to October 2006. The settling material was analyzed to obtain total mass fluxes and main constituent contents (organic matter, opal, calcium carbonate, and siliciclastics). Cascades of dense shelf water from the continental shelf edge to the lower continental slope occurred from January to March 2006. They were traced through strong negative near-bottom temperature anomalies and increased current speeds, and generated two intense pulses of mass fluxes in January and March 2006. This oceanographic phenomenon appeared as the major physical forcing of settling particles at almost all stations, and caused both high seasonal variability in mass fluxes and important qualitative changes in settling material. Fluxes during the dense shelf water cascading (DSWC) event ranged from 90.1 g m−2 d−1 at the middle Cap de Creus canyon (1000 m) to 3.2 g m−2 d−1 at the canyon mouth (1900 m). Fractions of organic matter, opal and calcium carbonate components increased seaward, thus diminishing the siliciclastic fraction. Temporal variability of the major components was larger in the canyon mouth and open slope sites, due to the mixed impact of dense shelf water cascading processes and the pelagic biological production. Results indicate that the cascading event remobilized and homogenized large amounts of material down canyon and southwardly along the continental slope contributing to a better understanding of the off-shelf particle transport and the internal dynamics of DSWC events.

Highlights

Continental shelves are bypass zones of lithogenic and biogenic material from their source in the continent to their sink in the deep sea floor
Results indicate that the cascading event remobilized and homogenized large amounts of material down canyon and southwardly along the continental slope contributing to a better understanding of the off-shelf particle transport and the internal dynamics of dense shelf water cascading (DSWC) events
In this paper we evaluate the behavior and characteristics of settling particulate matter collected during the HERMES (Hotspot Ecosystems Research on the Margins of European Seas) project along the Lacaze-Duthiers and Cap de Creus canyons and the adjacent southern open slope from October 2005 to October 2006

Summary

Introduction

Continental shelves are bypass zones of lithogenic and biogenic material from their source in the continent to their sink in the deep sea floor. High biological production takes place in shelf regions due to nutrients inputs from land and coastal upwelling. In those environments, settling material is subjected to biological, physical and chemical processes among which the remobilization basinwards supposes the principal way of shelf erosion. Experiments carried out in the Western Mediterranean Sea show differences of three orders of magnitude when comparing settling particles in the slope with the open basin, underlining the importance of continental shelves as a source of material for the adjoining slope environment (Heussner et al, 2006; Martın et al, 2006; Zuniga et al, 2009). Various studies carried out in the GoL have documented the rapid export of sediment from the shelf and upper slope to deeper environments by intense

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Discussion

Conclusion