Sediment Dynamics in the Gulf of Lions: the Impact of Extreme Events

ARTICLE IN PRESS

Continental Shelf Research 28 (2008) 1867–1876

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Continental Shelf Research

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Introduction Sediment dynamics in the Gulf of Lions: The impact of extreme events

1. Introduction many small rivers along their western coasts. The Po and Rhoˆne, whose watersheds originate from the Alps, show a prevailing Investigating the fate of riverborne or resuspended sediment seasonal variability with a spring increase in discharge due to that is transported across continental margins is a fundamental snowmelt. However, extreme flood events associated with intense task in developing our understanding of the factors impacting precipitation also occur with sub-decadal recurrence intervals. benthic habitats and ecosystems, the dispersal and sequestration The small rivers on the western coasts are largely event of chemical elements (e.g., carbon, contaminants), and, in the long dominated. Flood sediment is primarily deposited on prodeltas term, the construction of sedimentary strata and evolution of with some redistribution along the shelf under the effect of a continental margin morphology. general cyclonic drift of the shelf circulation. Canyons at the Sediment transport on continental margins depends on a wide southwest (SW) terminus of the shelves intercept the transport variety of processes, including surface waves, bottom-boundary- pathways and control the part of the sediment that is transferred layer currents, and fluid stratification; and parameters such as directly to the deep basin. particle input rates, seabed characteristics, accumulation rates, The Adriatic Sea and the Gulf of Lions also differ in important and slope stability. Large sediment-transporting events lead to the respects, particularly in their morphology. The northern Adriatic formation, burial, and preservation of event beds in sediment Sea is an elongated epicontinental sea extending southwards to deposits (Wiberg et al., 1996; Wheatcroft et al., 2007). about 700 km, whereas the Gulf of Lions has a crescent-shaped Past research has generally favored the study of individual shelf with maximum width of 70 km and a slope indented by processes and parameters through spatially and temporarily numerous submarine canyons. The SW end of the Gulf of Lions disjointed research efforts that did not lend themselves to a near the Cap de Creus Promontory is narrower than the SW end of broader understanding and quantification of their respective roles the northern Adriatic shelf near the Gargano Promontory, and or interconnections for sediment transport and strata formation. thus is able to more effectively block the southerly transport of A more holistic approach requires that the study of sediment water and sediment on that part of the shelf, favoring export transport and accumulation be linked with that of stratigraphy, towards the slope. thus providing a better understanding of the mechanisms A first joint North-American–European study was carried out responsible for stratigraphic architecture of continental margins, in the northern Adriatic from late 2000 to mid 2003 (see Milligan as well as a valuable database for groundtruthing the modeling of and Cattaneo, 2007, for details), with a particular focus on the sediment transport from source to sink. dispersal and deposition of sediment delivered to the basin during Studies linking short-term (i.e., hours–weeks) sediment trans- a 100-yr flood of the Po River in October 2000. This was followed port processes to the formation and preservation of event beds in by a second study centered in the Gulf of Lions, which took place shelf sediment deposits and stratigraphic geometry of continental from November 2003 to April 2005. margin sediment were central to the STRATAFORM project The main focus on the Gulf of Lions studies was to develop an conducted on US margins in the 1990s (Nittrouer and Kravitz, understanding of the sediment transport processes and pathways 1996; Nittrouer, 1999; Nittrouer et al., 2007). A European from river mouths to the continental shelf and ultimately to the counterpart of STRATAFORM, called EuroSTRATAFORM (European slope though submarine canyons that incise the margin. The tasks Margins Strata Formation) was subsequently developed to under- were aimed at (1) evaluating the temporal variability and stand sedimentary processes, deposits, and stratigraphy in areas composition of riverine sediment fluxes, (2) understanding the of the northeast Atlantic and Mediterranean systems representa- impact of short-term oceanic processes and their longer-term tive of the European continental margin (Syvitski et al., 2004; variability (i.e., seasonal, interannual, climatic), as well as the Weaver et al., 2006). The North-American contribution to this impact of human-induced activities on the sediment erosion, program was designed to focus on multidisciplinary studies of transport, and deposition, (3) documenting the formation, two Mediterranean margin systems: the northern Adriatic Sea and modification, and preservation of sediment strata, including event the Gulf of Lions. beds, as a product of processes acting with spatial and temporal Both the Adriatic Sea and the Gulf of Lions are temperate, heterogeneity, and (4) describing the stratigraphic signatures near deltaic margins exhibiting important similarities: small tidal the main sediment source (the Rhoˆne River) and at the main currents, variable wind-driven circulation, and dense-water outlet to the slope (SW end of the shelf). formation. They have a primary siliciclastic sediment input from This 2-yr study of the Gulf of Lions provided most of the a large fluvial dispersal system in the northern part of the basin ingredients for understanding and modeling the impact of very (the Po and Rhoˆne Rivers, respectively) and secondary inputs from energetic events on sediment dynamics. The 2003–2004 period

1868 Introduction / Continental Shelf Research 28 (2008) 1867–1876 included a major flood of all rivers, a 50-yr recurrence interval Table 1 flood of the Rhoˆne and two major marine storms. The 2004–2005 Previous related published papers from the Gulf of Lions EuroSTRATAFORM period included a major dense-water formation and cascading program event. These events were perceived as rare or extreme events, as Author/year Title Journala they had a pluri-annual to pluri-decadal recurrence interval. A number of papers, mostly dealing with the 2003–2004 observa- Inputs and fate of riverine sediment tional study, have already been published in a special issue of Bourrin and Durrieu Contribution to the study of coastal rivers Vie et Milieu—Life de Madron and associated prodeltas to sediment and Environment Marine Geology (Weaver et al., 2006) or independently (Table 1). (2006) supply in Gulf of Lions (NW This volume focuses primarily on the 2004–2005 period, but also Mediterranean Sea) includes papers on the 2003–2004 period and contributions based Bourrin et al. (2007) Last millennia sedimentary record on a Marine Geology on studies of water and sediment dispersal in the Gulf of Lions micro-tidal, low-accumulation prodelta conducted outside of the EuroSTRATAFORM program. (Teˆt W Mediterranean) Sabatier et al. Sediment budget of the Rhoˆne delta Marine Geology (2006) shoreface since the middle of the 19th century Maillet et al. (2006) Morphological changes and sedimentary Marine Geology 2. The Gulf of Lions processes induced by the December 2003 flood event at the present mouth of the Grand Rhoˆne River (southern France) 2.1. General setting Miralles et al. Radionuclide deposition in the Rhoˆne Marine Geology (2006) River Prodelta (NW Mediterranean sea) in The Gulf of Lions has a wide crescent-shaped continental shelf response to the December 2003 extreme bordered by the Pre-Alps to the northeast and the Pyrenees to the flood Garcia-Garcia et al. Shallow gas off the Rhoˆne prodelta, Gulf of Marine Geology SW, with a maximum shelf width of about 70 km. Its continental (2006) Lions slope is deeply incised by an intricate network of submarine canyons (Fig. 1). Characteristics of suspended particles Curran et al. (2007) Settling velocity, effective density, and Continental Shelf The Holocene sedimentary cover, contemporary with the last mass composition of suspended sediment Research sea-level high stand, is present on the inner- and mid-shelf and is in a coastal bottom boundary layer, Gulf of formed by alternating silty/clayey and sandy deposits (Got and Lions, France Aloisi, 1990). It is thickest next to the Rhoˆne River mouth (50 m) Tesi et al. (2007) Source, transport and fate of terrestrial Marine Chemistry and gets thinner towards the west. It is less than 10 m thick off the organic carbon on the western Mediterranean Sea, Gulf of Lions, France Roussillon shelf and does not exceed 1 m off the Cape Creus, allowing Quaternary rocky formations to outcrop on the inner Impact of storms shelf. Modern sedimentary units broadly form stripes parallel to Palanques et al. Suspended sediment fluxes and transport Marine Geology (2006) processes in the Gulf of Lions submarine isobaths (Fig. 3). A mid-shelf mud belt is present between 40 and canyons. The role of storms and dense 90 m depth. It is connected to a coastal sandy belt by a transitional water cascading area with a large gradient in mud content at about 30 m depth, Guilleń et al. (2006) Sediment dynamics during ‘‘wet’’ and Marine Geology which corresponds to the average limit of the wave action on the ‘‘dry’’ storm events on the Teˆt inner shelf (SW Gulf of Lions) seafloor. In front of river mouths, submarine deltas with muddy silty deposits form at depths around 20–25 m. The deepest shelf Impact of dense shelf-water cascading facies, formed by a mixture of relict sands and fine modern Canals et al. (2006) Flushing submarine canyons Nature Ulses et al. Dense shelf water cascading in the Geophysical sediment, is found on the outer shelf and at the shelf edge, (2008a–c) northwestern Mediterranean during the Research Letters particularly in the vicinity of the Pyrenean canyons of Lacaze- cold winter 2005: quantification of the Duthiers and Cap de Creus (Monaco and Mear, 1984). export through the Gulf of Lion and the Sediment supplied by rivers in the Gulf of Lions comes from Catalan margin the Rhoˆne River, one of the largest rivers in the Mediterranean, Heussner et al. Spatial and temporal variability of Marine Geology (2006) downward particle fluxes on a continental and from numerous torrential rivers along the Languedoc– slope: lessons from an 8-yr experiment in Roussillon western coast. The Rhoˆne River supplies most (more the Gulf of Lions (NW Mediterranean) than 90% on average) of the fluvial water and suspended-sediment Gaudin et al. (2006) Massive sand beds attributed to Marine Geology discharge to the Gulf of Lions. Mean annual water discharge of the deposition by dense water cascades in the Boucart canyon head, Gulf of Lions Rhoˆne River over the last decades is about 1700 m3 sÀ1, and (northwestern Mediterranean Sea) sediment discharge ranges between 7 and 10 Â 106 tyrÀ1 according to different estimates (Sempe´re´ et al., 2000; Pont et al., 2002; Modeling studies of water and sediment dynamics and budgets Bourrin and Durrieu de Madron, 2006). River discharge, which is Ulses et al. Interactions between storms and dense Journal of (2008a–c) water cascading on shelf-slope exchange Geophysical fed by oceanic fronts, snowmelt, and marine storms, exhibits in the Gulf of Lion (NW Mediterranean) Research marked summer lows, spring and autumn peaks, and strong interannual variability. The largest floods, which are generally Stratigraphy Bassetti et al. Sand bodies at the shelf edge in the Gulf of Marine Geology associated with storm events, transport most of the sediment; for (2006) Lions (Western Mediterranean): deglacial example, 80% of sediment discharge for the Rhoˆne occurs when history and modern processes water discharge exceeds 3000 m3 sÀ1 (Pont et al., 2002) and 78% of Dennielou et al. Vertical grain-size variability within a Marine Geology the solid flux of the Teˆt River over the period 1978–1999 occurred (2006) turbidite levee: autocyclicity or allocyclicity? A case study from the Rhoˆne in only 50 days (Serrat et al., 2001). neofan, Gulf of Lions, western The EuroSTRATAFORM observation period was marked by an Mediterranean exceptional flood event in early December 2003, which affected Jouet et al. (2006) Shoreface migrations at the shelf edge and Marine Geology all of the rivers and delivered about 6 Â 106 t of sediment, as well sea level changes around the last glacial as smaller floods during winter and early spring 2004 (Fig. 2). maximum (Gulf of Lions, NW Mediterranean) Only two moderate floods of the Rhoˆne, both during the winter, occurred during the next year (Fig. 2). a Full citations are provided in the References. ARTICLE IN PRESS

Introduction / Continental Shelf Research 28 (2008) 1867–1876 1869

a Massif Central

Mistral Tramontane Pyrenees Alps

500 m

0 -500 m

-1000 m

-1500 m -2000 m Marin b 44° Massif central Vidourle Alps

Rho

43°30 Herault ne

Orb

Aude

800 600 43° 400 200 0 Agly -200 Tet -400 -600 Tech -800 -1000 42°30 -1200 Pyrenees -1400 Depth Elevation -1600 Cap -1800 Creus -2000 -2200 2°30’3° 3°30’4° 4°30’ 5° 5°30’ -2400 5 cm/s

Fig. 1. (a) Gulf of Lions bathymetry and topography with rivers (in blue), prismatic Holocene sedimentary unit on the shelf (green gradation), and modern ﬁne sediment depocenters off river mouths (brown areas; adapted from Got and Aloisi, 1990). Predominant winds are superimposed. (b) Map of mean annual near bottom current ﬁeld computed for 2004 (courtesy of C. Estournel).

The distribution of near-bottom suspended particulate matter matter concentrations are low, generally between 1 mg lÀ1 and over the shelf is characterized by the persistence throughout the a few mg lÀ1. Suspended matter is composed of fine particles year of a bottom nepheloid layer up to 15 m thick, whose (clays and fine silts), which tend to form aggregates up to maximum location varies with the position of the sources 1 mm in size (Curran et al., 2007; Ferre´ et al., 2008). The of suspended sediment: river mouths and coastal resuspension most common aggregate sizes found in the bottom nepheloid during wintertime, inner- and mid-shelf during summer (Durrieu layer are micro-flocs (around 50 mm) and macro-flocs (around de Madron and Panouse, 1996). Average suspended particulate 150 mm). ARTICLE IN PRESS

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Fig. 2. Time chart of forcing conditions, monitoring, and timeline of the field work in the Gulf of Lions. Forcing conditions include Rhoˆne River and other river freshwater discharge (peaks are indicative of floods); significant wave height off Sete (peaks are indicative of storminess), and near-bottom temperature in the Cap de Creus Canyon (lows are indicative of dense-water cascading) during the September 2003–May 2005 period. Arrows indicated period of dense shelf-water cascading.

An intermediate nepheloid layer is present at the shelf break in Circulation and wave conditions in the Gulf of Lions reflect the continuity with the bottom nepheloid layer on the shelf (Durrieu distinctive effects of different wind regimes. Predominantly de Madron et al., 1990); smaller intermediate layers develop northerly (N, Mistral) and northeasterly (NE, Tramontane) winds deeper in canyons (Durrieu de Madron, 1994). Durrieu de Madron generally induce distinctive and opposite circulation cells on the et al. (1990) and Lapouyade and Durrieu de Madron (2001) shelf, favoring intrusion of slope waters in the eastern and central showed that the intensity and size of turbidity structures on the parts, and export of shelf water at the southwestern end of the slope increase by about one order of magnitude between the Gulf (Estournel et al., 2003; Petrenko et al., 2005). Due to the eastern and southwestern limit of the Gulf. The seaward extension reduced fetch, N–NW winds generate small waves (significant of the nepheloid structures on the slope is limited by the general wave heighto2 m, peak periodo6 s) on the inner shelf. Con- along-slope circulation that sweeps away the suspended particles versely, episodic and short-lived E–SE (Marin) winds induce a rise exported from the shelf. Concentrations are larger in the canyon in sea level at the shore and an intense cyclonic circulation on the heads than on the adjacent open slopes, revealing their role as shelf (Ulses et al., 2008a). These winds are associated with a long natural traps and conveyors of particulate matter to the mid- and fetch and large swell (significant wave height up to 10 m, peak lower slope. period up to 12 s). River floods often occur in conjunction with ARTICLE IN PRESS

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E–SE storms as the transport of humid marine air over coastal and Teˆt prodeltas induced by flood and storm events were relief induces abundant precipitation. monitored using swath bathymetry and repeated coring. Near- Storm winds from the E–SE mostly occur between late autumn bottom sediment transport and hydrodynamic conditions were and early spring. Major storms are rare; e.g., events with monitored on the Teˆt prodelta and in seven submarine canyon significant wave heights larger than 5 m represent about 0.2% of heads distributed along the slope to investigate the synchrony the record at the Sete wave gauge (see location on Fig. 1b) over the of forcing events across the shelf and the spatial variation of last decade (data from the CANDHIS national wave database). sediment fluxes at the shelf edge (Fig. 3a). Storm frequency shows a large interannual variability. The The second observational period began in late September 2004 autumn 2003 and winter 2004 period was characterized by and ended in October 2005 (Fig. 2). The seabed was sampled with extreme storminess with 11 moderate storms with significant coring to examine sediment accumulation in critical areas (Rhoˆne wave heights larger than 3 m and two major storms with prodelta, SW terminus of the shelf, and nearby canyons), significant wave heights larger than 5 m. Moreover, the first major to delineate variations in sedimentological character and to storm, in December 2003, was concomitant with major flooding of characterize key dynamical properties of the seabed (grain all rivers. Conversely autumn 2004 and winter 2005 experienced size, porosity, erodibility). Off-shelf and down-canyon sediment only one moderate storm (Fig. 2). transport processes were investigated in detail in the south- The cold and dry N–NW winds during autumn and winter western part of the margin with simultaneous tripod and provoke the progressive disappearance of summer stratification mooring deployments, spanning inner-shelf to mid-canyon depths and cooling of shelf water, which eventually leads to the (Fig. 3b). formation of homogeneous coastal water denser than the offshore Numerical studies were performed to develop a regional water (Dufaud-Julliand et al., 2004; Durrieu de Madron et al., coupled model, producing realistic and validated oceanic circula- 2005a, b; Ulses et al., 2008b). The cold, dense coastal water forms tion, together with sediment transport, erosion, and deposition. a bottom gravity plume that propagates across the shelf and, once They were used to understand the impact of extreme natural it reaches the shelf edge, cascades down the continental slope events (flood, storms, cascading) and human activities (trawling) following topographic depressions to its horizon of equilibrium acting with spatial and temporal heterogeneity on the formation density. E–SE storms and dense shelf-water transport can interact and modification of event beds. Estimates of cross-margin and enhance the off-shelf export (Palanques et al., 2006), but they sediment fluxes and budgets for different forcing conditions were require adverse conditions to develop. River floods associated made based on the model results. with large storms generally decrease the salinity of surface waters and thereby limit the density increase of coastal water. The strongest episodes of dense-water formation are observed during 3. Overview of the Gulf of Lions sediment-dynamics special issue very dry and windy winters, which are not favorable to E–SE storms. The research described in this volume examines sediment Whereas dense shelf-water cascading generally causes the dynamics in the Gulf of Lions from the source (riverine sediment formation of intermediate water over the slope, winters with very input) to the most significant sink (the southwestern submarine dense shelf-water formation contribute to the formation of deep canyons). The papers are arranged along this progression, water. Such episodes occur with a sub-decadal recurrence interval addressing (1) inputs and fate of riverine sediment; (2) character- (Be´thoux et al., 2002; Canals et al., 2006). The EuroSTRATAFORM istics of suspended particles; (3) sediment erodibility; (4) impact observations captured both conditions, with moderate dense- of storms; (5) impact of dense-water cascading; and (6) modeling water formation during winter 2004 and extreme dense-water studies of sediment dynamics and budgets. Previously published formation during winter 2005 (Fig. 2). The latter was character- EuroSTRATAFORM Gulf of Lions studies, similarly arranged by ized by a large number of days with strong northerly winds, which topic, are listed in Table 1. are associated with large heat losses and resulted in the largest cumulative heat loss from December 2004 to March 2005 of all 3.1. Inputs and fate of riverine sediment winters since 1948 according to Lo´ pez-Jurado et al. (2005). Average discharge for all rivers to the Gulf of Lions between The EuroSTRATAFORM observational period included signifi- December and March was about 1460 m3 sÀ1, which is lower than cant floods both on the Rhoˆne River, the largest source of the December–March mean discharge (2180 m3 sÀ1) and the sediment to the Gulf of Lions, and on smaller rivers that drain lowest value since 1993. All these factors made winter 2004–2005 the western coast, such as the Teˆt River. The fate of sediment an exceptional period for dense shelf-water formation over the discharged into the coastal zone during such floods is an northwestern Mediterranean (Canals et al., 2006; Font et al., important piece of the puzzle of strata formation in the Gulf of 2007). Lions. Bourrin et al. (2008a, b) describe the results of event-response data collection during a 5-yr return interval flood in the Teˆt River 2.2. Strategy for the observational and modeling efforts basin and adjacent inner shelf in the Gulf of Lions in April 2004, which they combined with concurrent mooring data from nearby Observations took place during two successive autumns and canyon heads (presented in Palanques et al., 2006). Sediment winters, the most energetic periods for sediment supply by rivers delivery to the coastal zone during this flood represented more and sediment transport by storms and dense-water flows. than half of the mean annual discharge of the Teˆt River to the Gulf The first period of observation began in October 2003 and of Lions. This flood transported a large amount of sand in ended in May 2004 (Fig. 2). It focused on the monitoring of suspension (25% of the total suspended load) and as bedload sediment discharge from the Rhoˆne River and the Teˆt River (8% of the total load). Sand introduced in the nearshore was (Fig. 3a). River plume and suspended-sediment distributions transported northwards during the peak storm and nourished a across the shelf were studied by direct observations (profiles of small delta. Fine sediment was separated from coarse sediment at temperature, salinity, and turbidity), and remote sensing was used the river mouth and was advected southwards and seawards by to determine the response of the plume to the hydrodynamic the counter-clockwise general circulation. The finest sediment conditions. Textural and bathymetric modifications of the Rhoˆne was transported in a hypopycnal plume towards the Cap de Creus ARTICLE IN PRESS

1872 Introduction / Continental Shelf Research 28 (2008) 1867–1876

investigate this depositional evidence, Drexler and Nittrouer (2008) collected cores seawards in the Rhoˆne subaerial delta during cruises in October 2004 and April 2005. Laminated beds preserved in water depths o30 m within a crescent-shaped region 2.5 km from the river mouth and 7Be in the surface sediment of all but the most distal coree from the river mouth are evidence of episodic deposition of flood sediment. Flood deposits on the Rhoˆne subaqueous delta were identified using a dilution signature in 210Pb profiles (i.e., increased clay content, decreased 210Pb activity), with apparent accumulation rates in the range of 2.5–410 cm yrÀ1. In water depths greater than 30 m and further away from the river mouth, bioturbation erased these signatures except for those produced by the most extreme events.

3.2. Characteristics of suspended particles

Carbon and nitrogen stable isotope (d13C, d15N) concentrations and their ratio can be used to characterize the origin (biological, sedimentary) of particulate matter in the marine environment. Tesi et al. (2007) used known signatures for biological and sedimentological end members to determine the contribution and trace the fate of terrestrial organic carbon in the sediment of the Gulf of Lions shelf from the Rhoˆne prodelta to its SW outlet. Harmelin-Vivien et al. (2008) refined the estimate of the d13C and d15N concentrations for microphytoplankton, and demon- strate that values for surface water particulate organic matter (POM) cannot be used as a phytoplankton proxy in coastal areas subjected to high riverine inputs. Significantly different C and N isotopic signatures were estimated between surface water POM and microphytoplankton collected at the fluorescence maximum in May and November 2004. Surface water POM presented lower d13C and higher d15N values than those of microphytoplankton due to a higher content of continental and detrital material. While d13C and d15N signatures of phytoplankton did not vary with seasonal variations of environmental and biological parameters, d15N of surface water POM varied due to changing composition of the organic matter issued from the Rhoˆne River. The isotopic signature of marine phytoplankton was used afterwards by Sanchez-Vidal et al. (2008) to characterize the origin of the suspended particles collected by sediment traps in the SW canyons during an intense cascading event.

3.3. Sediment erodibility Fig. 3. Lithology of the superficial sediment for the Gulf of Lions (adapted from Monaco and Aloisi, 2000). (a) The upper map shows the experimental effort made Much of the sediment delivered to the coastal ocean by river in 2003–2004 over the whole of Gulf of Lions, comprising stations on the Rhoˆne and Teˆt Rivers, inner shelf, and upper slope. (b) The lower map zooms in on the flooding is stored, at least temporarily, in deltaic deposits offshore observational network set up in the southwestern part of the Gulf during the of river mouths or in mid-shelf mud belts like the one in the Gulf 2004–2005 observational period. of Lions (Fig. 3). Reworking of these deposits, as well as the general redistribution of sediment within the Gulf of Lions, depends on the frequency, magnitude, and duration of sedi- Canyon, where it exited the shelf within hours after the flood ment-transporting events on the shelf. Whether, and how much, peak. These results, which suggest that deposition on the Teˆt sediment is transported depends on the magnitude of wave- and prodelta is limited even during flood events, complement those of current-driven bottom shear stresses and the erodibility of seabed the sediment-dynamics study of Guilleń et al. (2006), which sediment. showed the reworking of the upper centimeters of prodeltaic Dufois et al. (2008) analyzed 1 yr (2001) of simulated current- sediment by waves and currents during major storms, and the and wave-generated bed shear stresses and compared their sedimentological analysis made by Bourrin et al. (2007), which spatial distribution to the grain-size distribution of seabed documented the absence of preservation of distinct flood layers sediment in the Gulf of Lions. They found that onshore winds over the last millennia in this low-accumulation prodelta. induce erosion and transport over most of the shelf whereas Whereas morphological changes and sediment deposits asso- potential erosion during offshore winds is limited to the coastal ciated with the December 2003 major flood were extensively zone. Wave-generated shear stresses are highest in the sandy studied at the Rhoˆne River mouth by Maillet et al. (2006) and inner shelf and decrease offshore. Current-generated shear Miralles et al. (2006), deposition of sediment discharged from the stresses can be high on the inner shelf during wind events, Rhoˆne River on the distal prodelta is less well documented. To are low on the muddy mid-shelf and increase again on the ARTICLE IN PRESS

Introduction / Continental Shelf Research 28 (2008) 1867–1876 1873 coarser-grained outer shelf owing to south-westward currents, heads. The high particle fluxes associated with the late autumn which can be strong during periods of onshore winds. storm were characterized by low concentrations of organic Law et al. (2008) used a Gust erosion chamber to examine size- carbon. Otherwise, organic matter collected during the fall–winter specific erodibility of sediment along a cross-shelf transect near period was relatively degraded and mostly of resuspended origin. the Teˆt River during three cruises in the winter/spring of Organic matter collected during the spring period was fresher, 2004–2005. In non-cohesive sandy silts, clay- and fine-silt-sized reflecting the combined effects of higher productivity and dense sediments were preferentially eroded at low shear stresses; shelf-water cascading, which rapidly advected newly formed progressively larger particles were eroded as shear stress organic matter to the canyon heads. increased. For finer cohesive beds with 47.5% clay, all grain sizes up to medium silts eroded at essentially the same rate regardless of shear stress (stresses up to 0.4 Pa). Law et al. (2008) suggest 3.5. Impact of dense shelf-water cascading that this difference in size sorting between non-cohesive and cohesive beds may be an important control on the sand–mud While storms are a ubiquitous driver of sediment transport on transition. continental shelves, the dense shelf-water cascading observed in the Gulf of Lions is an unusual but important mechanism affecting sediment dynamics on the shelf and canyons and the export of 3.4. Impact of storms suspended particulate matter from the shelf to the deep sea. As part of the 2004–2005 observational program, an array of tripods The transport of sediment from deltaic and other shallow- and moorings deployed on the inner- and outer shelf and in the water deposits to deeper shelf deposits, canyon heads, or the shelf canyons of the SW end of the Gulf documented the impact of a edge is generally accomplished by storms. One of the intriguing severe episode of dense shelf-water formation and cascading on questions in the Gulf of Lions, given the numerous canyons resuspension and transport. incising the shelf edge, is the importance of these canyons in Bourrin et al. (2008a, b) present observations of sediment sediment export from the shelf. dynamics on the Teˆt inner shelf during winter 2004–2005. Waves, As part of the EuroSTRATAFORM program in 2003–2004, near- current velocities, near-bottom water turbidity, bottom sediment bottom moorings were deployed on the Teˆt prodelta and in the grain-size and seafloor erosion/accretion data complemented the heads of the seven major submarine canyons along the Gulf 2003–2004 observations by Guilleń et al. (2006) and allowed of Lions margin. Palanques et al. (2006) previously showed the consideration of dense shelf-water formation on the shelf simultaneous response and the dramatic increase towards the separate from the influence of flood and/or storm events. These southwesternmost canyons of hydrographic, hydrodynamic, and observations provide evidence that currents caused by dense shelf suspended-sediment parameters due to storm and dense shelf- water are adequate to produce significant erosion and sediment water cascading events. Palanques et al. (2008) combined transport on the shelf. Hydrodynamic modeling showed that the observations collected in the Cap de Creus Canyon head and on core of the dense-water flow predominantly affects the inner the inner part of the adjacent continental shelf during the major shelf, but occasionally spreads over the mid-shelf mud belt and storm events in fall 2003 and winter 2004 to detail the shelf-slope the outer shelf. transport conditions. During the peak of the storms, high waves Ogston et al. (2008) compared measurements made with and currents resuspended sediment out to the canyon head. bottom-boundary-layer tripods in the heads of the southwestern Downwelling and the down-canyon transport of resuspended Lacaze-Duthiers and Cap de Creus Canyons to evaluate the sediment during the December storm ended suddenly due to processes involved in sediment delivery, resuspension, and restoration of shelf-water stratification; sediment resuspended on transport. They show that intense cold, dense-water flows carried the inner shelf did not reach the shelf edge. During the longer- most of the sediment down the canyons. Flows and suspended- lasting storm in February, dense shelf-water cascading continued sediment concentrations were weaker and more episodic in for several hours after the end of the storm, allowing sediment Lacaze-Duthiers Canyon than in Cap de Creus Canyon, emphasizing resuspended from the inner shelf to shelf to reach the canyon the preferential pathway of dense shelf-water flow towards the head and be exported down the canyon. southern end of the Gulf of Lions due to the important narrowing Bonnin et al. (2008) used near-bottom sediment-trap and of the shelf. current-meter data from these moorings to investigate variability Puig et al. (2008) deployed several instrumented moorings in in vertical particle fluxes in the canyon heads while Fabres et al. the upper portion of Cap de Creus Canyon during September (2008) used these data to examine the biogeochemistry of 2004–September 2005 to investigate the role of dense shelf-water particulate fluxes from the shelf to the slope. Bonnin et al. cascading in the transport of sediment through submarine (2008) found that vertical particle fluxes generally increased canyons. Cascading in this canyon began in late December and towards the southwestern end of the set of canyons, in keeping was limited to depthso200 m. Dense water reached 750 m by late with the general circulation on the shelf; the highest export fluxes January and cascading extended through the upper canyon from were associated with energetic storms and dense shelf-water late February to late March 2005 with steady currents 460 cm sÀ1. cascading. There were also seasonal differences in the nature of Increases in suspended-sediment concentration accompanied the fluxes. Vertical fluxes in the canyon heads were higher during cascading events, but peak concentration decreased through time. the late autumn storm than during the winter storm. The autumn Cumulative down-canyon transport was higher at 750 m than in storm was accompanied by high river discharges (Fig. 2) and the canyon head. Near-bottom sediment-trap data show that water-column stratification restricted downwelling flow to the these flows were able to transport significant quantities of sand in upper slope, allowing deposition of sediment in the head of the suspension. Puig et al. (2008) argue that sand transport by dense canyons. The winter storm took place while the water column was shelf-water cascades is actively eroding the canyon floor to form a well mixed and dense shelf-water formation was active, hence field of wide, flow-aligned sedimentary furrows. leading to stronger and deeper down-canyon transport and less To examine the role of SW canyons in the transport and storage deposition in the canyon heads. of sediment exported from the shelf, DeGeest et al. (2008) Fabres et al. (2008) also found seasonal and event-related collected cores within Cap de Creus Canyon and the adjacent variations in organic-matter export from the shelf through canyon shelf during five cruises from November 2003 to April 2005. These ARTICLE IN PRESS

1874 Introduction / Continental Shelf Research 28 (2008) 1867–1876 were analyzed for grain size, accumulation rates, and sedimentary formation on the shelf due to increased water-column stratifica- structures to assess temporal and spatial patterns of sedimentation, with mixing consuming most of the dense water that is tion. The results indicate two depocenters on the shelf separated formed. Under the assumptions used by Herrmann et al. (2008), by a bypassing zone associated with the Cap Bear headland; their results indicate that dense shelf-water cascading to deep accumulation rates were 0–2.5 mmyrÀ1. Deposition of fine basin in the Gulf of Lions is virtually eliminated by 2100. sediment was found on the northern flank of the canyon, but not on the southern flank, which was characterized by coarse sediment and areas of apparent erosion. An ephemeral mud layer 4. Conclusions was present in the mid-canyon thalweg that appears to be periodically flushed down the canyon. DeGeest et al. (2008) With the occurrence of extreme floods, storms, and dense- estimate that 1% of the Rhone River’s sediment discharge into water formation, the EuroSTRATAFORM experiment provided a the Gulf of Lions is accumulating in the upper canyon on 100-yr unique opportunity to study the effects of some of the most time scales, but considerably more may be bypassing through the effective processes controlling sediment dynamics in the Gulf of upper canyon to the deeper parts of Cap de Creus Canyon. Lions’ continental shelf. These studies reveal a clear asymmetry in the inputs, deposition, or erosion areas and export of sediment. While major floods of the Rhoˆne River largely control the input of 3.6. Water and sediment dynamics and budgets investigated with sediment to the Gulf and produce identifiable deposits on its modeling proximal subaerial delta, evidences of flood deposits off small rivers like the Teˆt were ephemeral and barely preserved due to the Modeling provides an essential tool for enlarging the spatial recurrent reworking of the sediment by waves and currents. and temporal context of observational studies like those described Observations and modeling clearly emphasize the predominant above for the Gulf of Lions, as well as investigating effects of role of E–SE storms in wave-induced resuspension and along-shelf climate change and human disturbance. cyclonic transport of fine sediment on the inner shelf. However, Ulses et al. (2008c) combined observations with 3D modeling significant current-induced resuspension also occurs on the inner to resolve the spatial pattern and rates of sediment dynamics shelf during periods of dense-water formation driven by N–NW better during winter 2003–spring 2004, a period impacted by winds and on the outer shelf during E–SE storms. These two wind- floods and storms. They found that most of the particulate matter driven processes induce most of the export of suspended delivered by the Rhoˆne River was trapped on the prodelta. An sediment and associated elements from shelf, primarily at the estimated 48 Mt of shelf sediment was resuspended during the southwestern end of the Gulf. While hydrodynamic conditions large late autumn and late winter storms that year. Erosion was favor the formation of a mid-shelf mud deposit over most of the controlled by waves on the inner shelf and currents on the outer shelf south and west of the Rhoˆne River mouth, the narrowing of shelf, with deposition at depths between 50 and 100 m. Sediment the shelf and the proximity of canyon heads at the SW terminus export, which reached 9 Mt, took place mostly through the Cap de of the shelf funnel storm and dense-water currents and associated Creus Canyon. suspended sediment towards the canyons, preventing the deposi- The Gulf of Lions has one of the largest trawlable shelf areas in tion of new sedimentary strata on the SW shelf and eroding old the NW Mediterranean. Between 40 and 80 bottom trawlers are overconsolidated sediment outcrops. Some of the of sediment active all-year round in the Gulf and stir up almost the equivalent entering the canyons during these transport events forms of the area of the shelf (10,000 km2) annually. Durrieu de ephemeral deposits in the upper canyon that are periodically Madron et al. (2005) directly measured trawling-induced resus- flushed down the canyon, but much of it may pass through the pension of muddy sediment in the Gulf of Lions and found that upper canyon and be deposited deeper in the canyon system. bottom trawling altered the top few cm of the sediment, though Modeling suggests that during the most extreme events more only a small amount of that sediment is effectively resuspended. sediment leaves the shelf than the rivers supply. These events Ferre´ et al. (2008) used 3D model simulations to compare display a clear seasonality and large interannual variability, and trawling-induced sediment resuspension with naturally occurring are likely to be sensitive to climatic changes. resuspension due to waves and currents in the Gulf of Lions. Focussing on the period January 1998–April 1999, they found that whereas wave-current resuspension was episodic and concentrated on the inner shelf during fall and winter, resuspension from Acknowledgments trawling occurred more regularly throughout the year and was concentrated on the outer shelf. Trawling accounted for an order- The papers of this special issue are derived from programs of-magnitude less sediment export from the shelf than natural funded by the European Commission (Contract EUROSTRATA- processes during the study year, which included intense dense FORM number EVK-CT-2002-00079) and the US Office of Naval shelf-water cascading. However, the relative importance of Research EuroSTRATAFORM program. Additional support was sediment export due to trawling is estimated to be up to 20% of also provided by the Chantier Mediterraneé Nord-Occidentale total export during calm years. of the French Program National d’Environnement Coˆtier (PNEC), Herrmann et al. (2008) investigated the impact of atmospheric the Observatoire Re´gional Me´diterraneén de l’Environnement interannual variability and climate change on dense-water (ORME), and the HERMES program (Hotspot Ecosystems Research formation over the Gulf of Lions shelf, its export, and cascading on the Margins of European Seas, Contract number GOCE-CT- using a downscaling strategy, going from global climate models to 2005-511234-1). basin ocean circulation models, and finally to a high-resolution ocean model. For the ‘‘present period’’ (1961–1990), they found References that most of the dense water formed over the shelf sinks into the deep ocean by cascading during colder-than-average years, Bassetti, M.A., Jouet, G., Dufois, S., Berne´, S., Rabineau, M., Taviani, M., 2006. Sand whereas during warmer years, dense water is mainly consumed bodies at the shelf edge in the Gulf of Lions (Western Mediterranean): deglacial history and modern processes. Marine Geology 234, 93–109. by mixing on the shelf. For the ‘‘future period’’ (2071–2100), using Be´thoux, J.P., Durrieu de Madron, X., Nyffeler, F., Tailliez, D., 2002. Deep water in the IPCC A2 scenario, they found strong reductions in dense-water the western Mediterranean: peculiar 1999 and 2000 characteristics, shelf ARTICLE IN PRESS

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exchange in the Gulf of Lion (NW Mediterranean). Journal of Geophysical Xavier Durrieu de Madron Research 113, C02010. CEFREM, CNRS—University of Perpignan, 66860 Perpignan, France Ulses, C., Estournel, C., Puig, P., Durrieu de Madron, X., Marsaleix, P., 2008b. Dense shelf water cascading in the northwestern Mediterranean during the cold E-mail address: [email protected] winter 2005. Quantiﬁcation of the export through the Gulf of Lion and the Catalan margin. Geophysical Research Letters 35, L07610. Ulses, C., Estournel, C., Durrieu de Madron, X., Palanques, A., 2008c. Suspended Patricia L. Wiberg sediment transport in the Gulf of Lions (NW Mediterranean): impact of Department of Environmental Sciences, University of Virginia, extreme storms and ﬂoods. Continental Shelf Research. Weaver, P.P.E., Canals, M., Trincardi, F., 2006. Editorial. EUROSTRATAFORM special Charlottesville, VA, USA issue of Marine geology. Marine Geology 234, 1–2. Wheatcroft, R.A., Wiberg, P.L., Alexander, C.R., Bentley, S.J., Drake, D.E., Harris, C.K., Ogston, A.S., 2007. Post-depositional alteration of strata. In: Nittrouer, C., et al. Pere Puig (Eds.), Continental Margin Sedimentation: Transport to Sequence. Blackwell Institut de Cie`nces del Mar, CSIC, Barcelona, Spain Publication, Oxford, pp. 101–155. Wiberg, P.L., Cacchione, D.A., Sternberg, R.W., Wright, L.D., 1996. Linking sediment transport and stratigraphy on the continental shelf. Oceanography 9, 153–157.