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

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Sediment Dynamics in the Gulf of Lions: the Impact of Extreme Events ARTICLE IN PRESS Continental Shelf Research 28 (2008) 1867–1876 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/locate/csr 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 0278-4343/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2008.08.001 ARTICLE IN PRESS 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, Guille´n et al. (2006) Sediment dynamics during ‘‘wet’’ and Marine
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