Spatial and temporal variability of downward particle fluxes on a continental slope: Lessons from an 8-yr experiment in the Gulf of Lions (NW Mediterranean (original) (raw)
Related papers
Seasonality and composition of particulate fluxes during ECOMARGE—I, western Gulf of Lions
Continental Shelf Research, 1990
As part of the ECOMARGE program (ECOsyst~mes de MARGE continentale), sequential sediment traps were deployed on the continental margin of the Gulf of Lions, in the northwestern Mediterranean Sea. Two sites k)catcd in the southwestern part of this region were selected for the first phase of the experiment: inner shelf (bottom depth: 27 m) and continental slope (Lacaze-Duthiers Canyon, bottom depth: 650 m). The canyon was selected as representative of the continental slope, because canyons comprise more than 50% of the slope area. Total mass, organic matter, opal, carbonate and siliciclastic residue fluxes were measured biweekly at 50, 10tL 300 and 600 m in the upper part of the Lacaze-Duthiers Canyon (from July 1985 to April 19861 and for longer periods (2 weeks to 2 months) at 10 and 25 m on the shelf (from May 1985 to June 1986). Mass fluxes increased generally with depth, reaching values as high as 2(I,000 mg m -2 d-i in the 600 m slope (canyon) trap; this indicated lateral transport of biogenic and abiogenic particulate matter, from local (adjacent shelf and upper slope waters) and distant origin (Rh6ne River). Shelfslope particulate transfer attested by the paracontemporaneity of high flux events on the shelf and the slope and by the continuity of the shelf benthic nepheloid layer and intermediate nepheloid layers over the canyon; this rapid and took generally less than 16 days (trap sample resolution time). Particle fluxes and compositions defined a layered system, in which the surface layers < 100 m) were essentially characterized by biogenic material (organic matter and opal), and deep layers chiefly characterized by the collection of elastic material (carbonate and siliciclastic material). Significant temporal increases in total mass and constituent fluxes showed two frequencies: a low seasonal frequency, which was related to seasonal variations of the Liguro-Provenqal Current and to the winter increase of the Rh6ne River and other coastal river discharges; variations at higher frequencies, which were related to various impulse events such as summer internal waves, autumn and winter storms and spring nutrient enrichment. The role of the Lacaze-Duthiers Canyon in the particulate transfer across the continental margin is discussed within the particular context of this advective system. Although the importance of local, intra-canyon resuspension of bottom sediments could not be estimated, it seems that the high suspended particulate matter concentrations and particle fluxes observed are essentially related to the fact that this canyon acts as a natural trap which collects particles from the entire Gulf of Lions. Comparison of the flux necessary to sustain the unsupported 2")pb inventory in the canyon sediments, with the mean 2mpb flux measured by traps and of 2m Pb-determined mass accumulation rates with total mass fluxes indicate that the canyon partly acts as a modern sediment depocenter of the particles swept into it from the shelf and the slope; also partly as a modern conduit of particles to the deep basin. Several biological mechanisms arc probably responsible for the rapid settling of particles onto the slope (within the canyon); these may, thereby, reduce lateral transfer from the shelf to the open ocean and constitute a kind of "biological barrier".
2002
Long-term observations of monthly downward particle fluxes and hourly currents and temperature were initiated in 1993 in two canyons of the continental margin of the Gulf of Lion. The goals of this survey were to estimate its contribution to the CO 2 global budget and to understand the role of forcing factors in the control of present-day particle exchange across this margin. A previous statistical analysis of the long-term time series suggested that variability in the transfer of particulate matter to the deep ocean could be the result of the effect of the meandering of the Northern Current and by dense water formation in winter rather than variations in the sources of matter. Numerical simulations have been carried out to consider these hypotheses. A model is used to examine the impact of local atmospheric forcing (wind stress, heat fluxes, precipitation-evaporation budget) on the variability of the oceanic circulation and of mass fluxes within the canyons from December to April, for five consecutive years between 1996 and 2001. Results show an east-west gradient of mass export on the shelf and a positive correlation between anomalies of dense water formation rates and interannual variability of particle fluxes. However, in the eastern part of the Gulf, the simulated mass export from the shelf is not significant, even during a winter of strong convection, when the measured particle fluxes are at maxima. Moreover, although the model suggests that the dense water formation could be the major hydrodynamic forcing factor, this process is not sufficient to completely explain the space and time variations of observed particle fluxes, especially at depth.
Continental Shelf Research, 1990
The first experiment of the ECOMARGE programme (ECOsyst6mes de MARGE continentale) was initiated in 1983-1984, in the Gulf of Lions (northwestern Mediterranean Sea). The objectives of the ECOMARGE-I experiment were: to quantify the transfer of particulate matter, in general, and of organic carbon, in particular, from its introduction to and formation in the waters of the continental shelf-to its consumption or sedimentation on the shelf or its transfer to the slope and deep sea; and to understand the processes involved in that transfer, consumption and sedimentation together with their variability in space and time. The results of that experiment, from 1983 to 1988, are presented in this Special Issue. The highlights of the results are summarised in this paper. These results indicate that, of the particles formed in the waters of the continental shelf and those introduced by rivers, some are deposited as sediments on the shelf. A portion is transported offshore, however, to the slope and deep sea. The Rh6ne River, in the northeastern part of the study area, is the major source of continental material; this is transported to sea in a benthic nepheloid layer and, mostly, alongshore to the southwest. Here, it largely leaves the shelf through the canyons, especially the Lacaze-Duthiers Canyon. In the offshore waters, particle concentrations and distributions show surficial, intermediate and benthic nepheloid layers. These turbid structures increase towards the southwest, corresponding to the seaward shift of the front between the coastal waters and the Liguro-Proven~al cyclonic gyre, a major forcing function in the Gulf of Lions. Considering the source and fate of particles (largely biogenic from the euphotic zone and abiogenic from deeper waters) a layered system is described, which is emphasized by the concentrations of natural and artificial elements and compounds. Of the flux of particles to the Lacaze-Duthiers Canyon, on a decadal scale, about 30% (as a minimum) is estimated to be stored as sediment; the remainder is transported down-canyon, towards the deep sea. The temporal variability of processes affecting this net seaward transport, of both biogenic and abiogenic material, is from hours, days to seasonal, and probably interannual, time scales. The response of the system to these variations is rapid, with pulses of increased discharge of particles from the adjacent shelf being detected in sediment traps in the Lacaze-Duthiers Canyon in less than 16 days (the temporal resolution of the traps). Based upon the study of tracers of particulate matter and environmental factors (i.e. river discharge and climatic conditions), it appears that the contribution from the Rh6ne River and its adjacent area is maximal during the winter; at this time, the flow of the Liguro-Proven~al Current also increases. In contrast, the maximum relative contribution of the adjacent southwesterly area to the flux in the Lacaze-Duthiers Canyon occurs in summer, during storm events.
Biogeosciences, 2010
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.
2003
Long-term observations of monthly downward particle fluxes and hourly currents and temperature were initiated in 1993 in two canyons of the continental margin of the Gulf of Lion. The goals of this survey were to estimate its contribution to the CO 2 global budget and to understand the role of forcing factors in the control of present-day particle exchange across this margin. A previous statistical analysis of the long-term time series suggested that variability in the transfer of particulate matter to the deep ocean could be the result of the effect of the meandering of the Northern Current and by dense water formation in winter rather than variations in the sources of matter. Numerical simulations have been carried out to consider these hypotheses. A model is used to examine the impact of local atmospheric forcing (wind stress, heat fluxes, precipitation-evaporation budget) on the variability of the oceanic circulation and of mass fluxes within the canyons from December to April, for five consecutive years between 1996 and 2001. Results show an east-west gradient of mass export on the shelf and a positive correlation between anomalies of dense water formation rates and interannual variability of particle fluxes. However, in the eastern part of the Gulf, the simulated mass export from the shelf is not significant, even during a winter of strong convection, when the measured particle fluxes are at maxima. Moreover, although the model suggests that the dense water formation could be the major hydrodynamic forcing factor, this process is not sufficient to completely explain the space and time variations of observed particle fluxes, especially at depth.
Biogeosciences Discussions, 2009
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.
Biogeosciences, 2013
Particle fluxes (including major components and grain size), and oceanographic parameters (near-bottom water temperature, current speed and suspended sediment concentration) were measured along the Cap de Creus submarine canyon in the Gulf of Lions (GoL; NW Mediterranean Sea) during two consecutive winter-spring periods (2009-2010 and 2010-2011). The comparison of data obtained with the measurements of meteorological and hydrological parameters (wind speed, turbulent heat flux, river discharge) have shown the important role of atmospheric forcings in transporting particulate matter through the submarine canyon and towards the deep sea. Indeed, atmospheric forcing during 2009-2010 and 2010-2011 winter months showed differences in both intensity and persistence that led to distinct oceanographic responses. Persistent dry northern winds caused strong heat losses (14.2 × 10 3 W m −2) in winter 2009-2010 that triggered a pronounced sea surface cooling compared to winter 2010-2011 (1.6 × 10 3 W m −2 lower). As a consequence, a large volume of dense shelf water formed in winter 2009-2010, which cascaded at high speed (up to ∼ 1 m s −1) down Cap de Creus Canyon as measured by a currentmeter in the head of the canyon. The lower heat losses recorded in winter 2010-2011, together with an increased river discharge, resulted in lowered density waters over the shelf, thus preventing the formation and downslope transport of dense shelf water. High total mass fluxes (up to 84.9 g m −2 d −1) recorded in winter-spring 2009-2010 indicate that dense shelf water cascading resuspended and transported sediments at least down to the middle canyon. Sediment fluxes were lower (28.9 g m −2 d −1) under the quieter conditions of winter 2010-2011. The dominance of the lithogenic fraction in mass fluxes during the two winter-spring periods points to a resuspension origin for most of the particles transported down canyon. The variability in organic matter and opal contents relates to seasonally controlled inputs associated with the plankton spring bloom during March and April of both years.
Continental Shelf Research, 1990
An integrated study of particle transport, deposition and storage was carried out over the continental margin of the Gulf of Lions (northwestern Mediterranean Sea). The dispersal system of fine-grained sediment is discussed and a sedimentary budget of sediment transfer and accumulation is established. Sediment cores were collected from the continental shelf and slope, along the ECOMARGE-I transect. Accumulation rates were measured using radiochemical techniques. Sediment traps were deployed, from 1985 to 1986, at two sites: on the continental shelf (27 m) and in the Lacaze-Duthiers Canyon (645 m). This study revealed that the most important sedimentary processes on this margin are high seasonal variability of lithogenic particle fluxes and lateral advection. The major fluctuations are related to storm events, to an increase in rivcr discharge and the intensity of the regional current. Flux gradients increase exponentially toward the bottom, in shallow and deep water. However, in the deeper traps at the two sites annual lithogenic fluxes decrease seaward, from 25 to 3.5 g m-2 day i. Across this margin, sediment accumulation rates ranged from 2.9 g m-2 day-j , on the middle of the prodeltaic area, to 0.16 g m-2 day-I at 645 m in the canyon. Smectite was found to be a natural tracer for characterizing an annual cycle of sediment input, deposition, resuspension and shelf-slope exchange processes. At the shelf site, two intra°annual cycles of sedimentation and erosion of the surface sediments were recognized. Comparison between trap deposition rates and bottom sediment accumulation rates implies that the prodelta area acts as a zone of storage (11%) for the silty material and as a temporary sink for fine-grained sediment, that is eroded from the area in winter. In the canyon, variations in deposition rates, smectite content, and grain size are related to morphological features and the various modes of transport that control sedimentation. The discontinuity in the flux gradient between surface and deep layers, and the discrepancy between long-term accumulation rates based on 14C dating and trap deposition rates suggest that fluxes in the deeper layer in the canyon are in fact essentially controlled by settling from the intermediate nepheloid layers detached from the shelf break and by along-bottom transport; also that resuspension has a longterm impact on the entire canyon maintenance. On the other hand, comparing near-bottom trap 2J"pb fluxes to the fluxes required to support the excess 21°pb inventories in the sediments leads to the conclusion that much (70%) of the particulate flux passes through the canyon to the dcep sea on this short time scale (< I00 years).
Deep Sea Research Part I: Oceanographic Research Papers, 1999
Climatological, current and particulate #ux data were gathered in the Grand-Rho(ne canyon on the Gulf of Lions continental margin for one year (Jan. 1988}Jan 1989). Time series were analyzed to determine the in#uence of physical exchange processes on particulate matter at the shelf-edge, with a special emphasis on the Northern Current variability. The synoptic variability of the Northern Current was linked to meanders of 2}5 day period. Its meso-scale activity presented a seasonal signal with maximum values in early spring. Peaks of particulate #uxes in the upper traps were little a!ected by large river and atmospheric inputs, but rather by enhanced shelf-slope exchanges at the shelf edge due to intense cross-slope #uctuations of the Northern Current. These #uctuations caused crossisobath #ows near the bottom, which appeared to be a potential mechanism in transporting particles o! the shelf. At 900 m depth, high-#ux events measured by sediment traps were primarily linked to periods of higher cross-slope current oscillations. This correlation suggests that vertical motions caused by these oscillations contribute to the suspended particulate matter transport through the process of bringing higher suspended material concentrations from above to greater depths. Vertical velocity estimates were derived through temperature #uctuations combined with vertical temperature gradient and from the kinematic boundary condition. A simple di!usion model indicates that the vertical turbulent