Abstract
Abstract. The deep outer margin of the Gulf of Lions and the adjacent basin, in the western Mediterranean Sea, are regularly impacted by open-ocean convection, a major hydrodynamic event responsible for the ventilation of the deep water in the western Mediterranean Basin. However, the impact of open-ocean convection on the flux and transport of particulate matter remains poorly understood. The variability of water mass properties (i.e., temperature and salinity), currents, and particle fluxes were monitored between September 2007 and April 2009 at five instrumented mooring lines deployed between 2050 and 2350-m depth in the deepest continental margin and adjacent basin. Four of the lines followed a NW–SE transect, while the fifth one was located on a sediment wave field to the west. The results of the main, central line SC2350 ("LION") located at 42°02.5′ N, 4°41′ E, at 2350-m depth, show that open-ocean convection reached mid-water depth (≈ 1000-m depth) during winter 2007–2008, and reached the seabed (≈ 2350-m depth) during winter 2008–2009. Horizontal currents were unusually strong with speeds up to 39 cm s−1 during winter 2008–2009. The measurements at all 5 different locations indicate that mid-depth and near-bottom currents and particle fluxes gave relatively consistent values of similar magnitude across the study area except during winter 2008–2009, when near-bottom fluxes abruptly increased by one to two orders of magnitude. Particulate organic carbon contents, which generally vary between 3 and 5%, were abnormally low (≤ 1%) during winter 2008–2009 and approached those observed in surface sediments (≈ 0.6%). Turbidity profiles made in the region demonstrated the existence of a bottom nepheloid layer, several hundred meters thick, and related to the resuspension of bottom sediments. These observations support the view that open-ocean deep convection events in the Gulf of Lions can cause significant remobilization of sediments in the deep outer margin and the basin, with a subsequent alteration of the seabed likely impacting the functioning of the deep-sea ecosystem.
Highlights
Albeit the deep-sea is the largest ecosystem on Earth, not mrouncmheinstaklncoownndiatiboonusTtchohonwetroiCltliinsrgyaftofheescctpeydhclbeinyrgceohfabnigoegseionchenevmi--ical compounds, the distribution of deep-sea habitats or the functioning of ecosystems
Nutrients in the surface layer, (ii) an increase of chl a concentration during the fall planktonic bloom owing to erosion of the seasonal thermocline and, (iii) a minimum chl a concentration and production during winter due to the intense vertical mixing, and (iv) a large increase in late winter/early spring associated with the large planktonic bloom that takes place when the restratification of the surface layer occurs and the surface layer has been enriched in nutrients from deeper waters
We consider the seasonal variability of currents and particle fluxes and interpret them in light of previous observations gathered in the northwestern Mediterranean Sea
Summary
Albeit the deep-sea is the largest ecosystem on Earth, not mrouncmheinstaklncoownndiatiboonusTtchohonwetroiCltliinsrgyaftofheescctpeydhclbeinyrgceohfabnigoegseionchenevmi--ical compounds, the distribution of deep-sea habitats or the functioning of ecosystems. The Mediterranean Sea constitutes a remarkable marine domain where exchanges with the Atlantic Ocean through the Strait of Gibraltar and deep-water formation drive thermohaline circulation in the different sub-basins and in the whole basin, in turn controlling the distribution of biogeochemical compounds and shaping the ecosystem (MERMEX Group, 2011). Studies conducted in the Mediterranean Sea have shown how significant the role of dense deep-water formation and associated winter vertical mixing could be. The occurrence of an efficient transfer of particles from the surface layer to depth linked to peak fluxes because of coastal and open-ocean dense water formation has been shown by several authors in the Gulf of Lions and in the nearby Ligurian Sea, both in the northwestern Mediterranean Sea, in winter 1999, 2005 and 2006 (Heussner et al, 2006; Sanchez-Vidal et al, 2009; Miquel et al, 2011). The main export of such large particles occurs in winter during episodes of enhanced vertical mixing (Durrieu de Madron et al, 1999; Stemmann et al, 2002)
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