The last glacial maximum Balearic Abyssal Plain megabed revisited

Megabeds are thick sedimentary layers extending over thousands square kilometres in deep sea basins and are thought to result from large slope failures triggered by major external events. Such deposits have been found in at least three areas of the Mediterranean Sea. Although their discovery dates back to the early 1980s, many questions remain, concerning their initiation, source area, extent, and the nature of their emplacement. One of the largest previously documented megabeds was emplaced during the Last Glacial Maximum across the Balearic Abyssal Plain with a thickness of 8-10 m in water depths of up to 2800 m. New 3.5 kHz sub-bottom profiles and sediment cores provide greater constraint on the lateral variability of the megabed and allow to map it beyond previous estimates, with a revised areal extent up to 90,000-100,000 km2. Megabed terminations show gradual pinch-out to the West and an abrupt eastward termination against the Sardinia steep margin. The megabed presents both in seismic profiles and in sediment cores a tripartite subdivision likely corresponding to changes in flow regimes across the basin with a central area of sandy facies and erosional base oriented NNE-SSW allowing renewed discussions about sources and trigger of the megabed

Seafloor instabilities in the Gulf of Lions, Western Mediterranean

Le golfe du Lion (GoL) est une marge passive d'environ 200 km de long et 70 km de large qui s'étend d’ouest en est du sud de la côte française jusqu’à la plaine abyssale des Baléares (BAP) dans la Méditerranée occidentale. Le talus du golfe du Lion est découpé par quinze canyons sous-marins, le plus grand étant le canyon du Petit Rhône qui canalise les sédiments apportés par le fleuve Rhône à travers le delta du Rhône. Les déstabilisations des fonds marins dans le GoL se produisent à l’embouchure et le long des flancs des canyons, mais aussi dans les zones de pente d'interfluve et les levées des principales vallées sous-marines. Cette thèse de doctorat vise à comprendre les processus de mise en place des trois plus grands dépôts d’accumulation en masse du GoL : ceux du Rhône occidental, du Rhône oriental et de la mégaturbidite de la plaine abyssale des Baléares. J’utilise un nouvel ensemble multidisciplinaire de données comprenant des données géophysiques, sédimentologiques, géotechniques in-situ et de laboratoire. La zone d'étude est caractérisée par de multiples escarpements de glissements. La morphologie des glissements est contrôlée par la présence de plusieurs failles listriques enracinées dans les couches messiniennes. La très haute résolution des données géophysiques nouvellement acquises, entre autre avec des engins tractés près du fond, montre la structure interne des glissements avec un niveau de détail sans précédent. La datation au radiocarbone des glissements de terrain étudiés donne un âge d'environ 20 kyrs, ce qui suggère un rôle potentiel de l'abaissement du niveau marin pendant le dernier maximum glaciaire. Le principal facteur de préconditionnement qui affecte la stabilité des fonds marins dans le GoL est la présence de sédiments riches en argile le long des surfaces à la base des glissements, qui présentent un comportement de baisse de résistance à la déformation et agissent comme possibles plans de cisaillement. Le mécanisme exact de déclenchement responsable de la mise en place de ces grands glissements n'est pas encore clair, mais une combinaison de processus tels que l'augmentation locale et régionale de la pente due à l'halocinèse et la génération d'un excès de pression interstitielle due à l'accumulation rapide de sédiments affecte probablement la stabilité de la pente.The Gulf of Lions (GoL) is a passive margin of about 200 km length and 70 km width that extends in W-E direction from the Southern French coast to the Balearic abyssal plain (BAP) in the deep western Mediterranean. The slope of the GoL is dissected by fifteen submarine canyons, the largest one being the Petit Rhone Canyon that funnels sediment supplied by the Rhone River through the Rhone delta. Seafloor instability in the GoL occurs in canyon heads and flanks, but also in the interfluve slope areas and in the levees of the main submarine valleys. This PhD thesis aims at understanding the emplacement processes of the three largest mass transport deposits in the GoL: the Rhone Western and Eastern Mass Transport Deposits, and the BAP megaturbidite located in the abyssal plain. I present a newly available multidisciplinary dataset including geophysical, sedimentological, in-situ and laboratory geotechnical data.The study area is characterized by multiple headwall scarps. The overall morphology of the slope failures is controlled by the presence of multiple listric faults rooted in the Messinian strata. The higher resolution of newly available geophysical data, especially deep-towed seismic lines, shows the internal structure of the mass-wasting deposits in unprecedented detail. Radiocarbon dating on top of the studied mass transport deposits yields an age of about 20 kyrs, suggesting a potential role of sea-level lowering during the last glacial maximum. The main preconditioning factor that affects slope stability in the GoL is the presence of clayrich sediments within the basal surfaces of the failures that show strain-softening behaviour. The exact triggering mechanism responsible for the emplacement of these large mass transport deposits is not yet clear, but a combination of processes such as local and regional slope steepening due to halokinesis and excess pore pressure generation due to rapid sediment accumulation likely affect the slope stability

Instabilités des fonds marins dans le Golfe du Lion, Méditerranée occidentale

The Gulf of Lions (GoL) is a passive margin of about 200 km length and 70 km width that extends in W-E direction from the Southern French coast to the Balearic abyssal plain (BAP) in the deep western Mediterranean. The slope of the GoL is dissected by fifteen submarine canyons, the largest one being the Petit Rhone Canyon that funnels sediment supplied by the Rhone River through the Rhone delta. Seafloor instability in the GoL occurs in canyon heads and flanks, but also in the interfluve slope areas and in the levees of the main submarine valleys. This PhD thesis aims at understanding the emplacement processes of the three largest mass transport deposits in the GoL: the Rhone Western and Eastern Mass Transport Deposits, and the BAP megaturbidite located in the abyssal plain. I present a newly available multidisciplinary dataset including geophysical, sedimentological, in-situ and laboratory geotechnical data.The study area is characterized by multiple headwall scarps. The overall morphology of the slope failures is controlled by the presence of multiple listric faults rooted in the Messinian strata. The higher resolution of newly available geophysical data, especially deep-towed seismic lines, shows the internal structure of the mass-wasting deposits in unprecedented detail. Radiocarbon dating on top of the studied mass transport deposits yields an age of about 20 kyrs, suggesting a potential role of sea-level lowering during the last glacial maximum. The main preconditioning factor that affects slope stability in the GoL is the presence of clayrich sediments within the basal surfaces of the failures that show strain-softening behaviour. The exact triggering mechanism responsible for the emplacement of these large mass transport deposits is not yet clear, but a combination of processes such as local and regional slope steepening due to halokinesis and excess pore pressure generation due to rapid sediment accumulation likely affect the slope stability.Le golfe du Lion (GoL) est une marge passive d'environ 200 km de long et 70 km de large qui s'étend d’ouest en est du sud de la côte française jusqu’à la plaine abyssale des Baléares (BAP) dans la Méditerranée occidentale. Le talus du golfe du Lion est découpé par quinze canyons sous-marins, le plus grand étant le canyon du Petit Rhône qui canalise les sédiments apportés par le fleuve Rhône à travers le delta du Rhône. Les déstabilisations des fonds marins dans le GoL se produisent à l’embouchure et le long des flancs des canyons, mais aussi dans les zones de pente d'interfluve et les levées des principales vallées sous-marines. Cette thèse de doctorat vise à comprendre les processus de mise en place des trois plus grands dépôts d’accumulation en masse du GoL : ceux du Rhône occidental, du Rhône oriental et de la mégaturbidite de la plaine abyssale des Baléares. J’utilise un nouvel ensemble multidisciplinaire de données comprenant des données géophysiques, sédimentologiques, géotechniques in-situ et de laboratoire. La zone d'étude est caractérisée par de multiples escarpements de glissements. La morphologie des glissements est contrôlée par la présence de plusieurs failles listriques enracinées dans les couches messiniennes. La très haute résolution des données géophysiques nouvellement acquises, entre autre avec des engins tractés près du fond, montre la structure interne des glissements avec un niveau de détail sans précédent. La datation au radiocarbone des glissements de terrain étudiés donne un âge d'environ 20 kyrs, ce qui suggère un rôle potentiel de l'abaissement du niveau marin pendant le dernier maximum glaciaire. Le principal facteur de préconditionnement qui affecte la stabilité des fonds marins dans le GoL est la présence de sédiments riches en argile le long des surfaces à la base des glissements, qui présentent un comportement de baisse de résistance à la déformation et agissent comme possibles plans de cisaillement. Le mécanisme exact de déclenchement responsable de la mise en place de ces grands glissements n'est pas encore clair, mais une combinaison de processus tels que l'augmentation locale et régionale de la pente due à l'halocinèse et la génération d'un excès de pression interstitielle due à l'accumulation rapide de sédiments affecte probablement la stabilité de la pente

Possible evidence for a relationship between gas hydrate dissociation and submarine slope failure in the Danube deep-sea fan, Black Sea

Nearly 110 km of high-resolution multichannel seismic data from the Romanian Black Sea reveal slope failures and paleo canyons of the Danube deep-sea fan. The new data allow detailed investigation of a submarine landslide identified during a regional seismic and multibeam bathymetric survey. The landslide scar has a dimension of 3 x 2.5 km and covers an area of about 8 km². The volume of the mobilized material is estimated to approximately 0.36 km³. Based on average sedimentation rates in the Danube deep-sea fan, age of the landslide is estimated to be around 12 ka. The absence of deposits at the foot of the landslide and the vicinity of the paleo canyon suggest that the material removed during the slide event was drained through the paleo canyon. We observe an upward bending bottom simulating reflector (BSR) beneath and active gas expulsion from the seafloor encircling the landslide scar. The water depth of the slide area correlates with the upper limit of hydrate stability. Modeling the base of the gas hydrate stability zone (BGHSZ) for the paleo-water conditions during the last glacial maximum (LGM) reveals an upward bending trend. The modeled BGHSZ for LGM crops out in the vicinity of the headwall of the landslide whereas the present day BSR crops out at the foot of the failed area. Therefore, we propose that warming of the Black Sea might have caused dissociation of gas hydrates resulting in building increased sediment pore pressure, possibly triggering the landslide identified in our data

Late Eocene onset of the Proto-Antarctic Circumpolar Current

The formation of the Antarctic Circumpolar Current (ACC) is critical for the evolution of the global climate, but the timing of its onset is not well constrained. Here, we present new seismic evidence of widespread Late Eocene to Oligocene marine diagenetic chert in sedimentary drift deposits east of New Zealand indicating prolonged periods of blooms of siliceous microorganisms starting ~36 million years ago (Ma). These major blooms reflect the initiation of the arrival and upwelling of northern-sourced, nutrient-rich deep equatorial Pacific waters at the high latitudes of the South Pacific. We show that this change in circulation was linked to the initiation of a proto-ACC, which occurred ~6 Ma earlier than the currently estimated onset of the ACC at 30 Ma. We propose that the associated increased primary productivity and carbon burial facilitated atmospheric carbon dioxide reduction contributing to the expansion of Antarctic Ice Sheet at the Eocene-Oligocene Transition

Integrated geophysical, sedimentological and geotechnical investigation of submarine landslides in the Gulf of Lions (Western Mediterranean)

18 pages, 9 figures, 1 tableThe Gulf of Lions presents recurring mass-transport deposits (MTDs) within the Plio-Quaternary sediments, suggesting a long history of mass movements. The two large, surficial MTDs are located on the eastern and western levee of the Rhone canyon over an area exceeding 6000 km2 and volumes exceeding 100 km3. Both MTDs were emplaced 21 ka ago (peak of the Last Glacial Maximum), suggesting a common trigger. Here, we present a multidisciplinary high-resolution geophysical, sedimentological and in-situ geotechnical study of the source and deposit areas of both MTDs to characterize distinct expressions of sediment deformation as well as their spatial and chronological distributions. We show the internal structure of mass movements and resulting MTDs with unprecedented details that were previously represented in the conventional seismic data as transparent and chaotic facies. The combination of multidisciplinary approaches shows new insights into the nature of basal surfaces of the slope failures. In particular, we show that the basal surfaces of the failures consist of clay-rich material contrasting with the overlying turbiditic deposits, suggesting that a strong lithological heterogeneity exists within the strata. We suggest that this change in lithology between clay-rich sediments and turbiditic sequences most likely controls the localization of weak layers and landslide basal surfacesThis work and the PhD of Shray Badhani are funded by the European Union's Horizon 2020 research and innovation programme under the Marie-Skłodowska-Curie grant via project ITN SLATE (grant agreement no. 721403)Peer reviewe

Imaging seafloor instabilities using very high-resolution deep-towed multichannel seismic data in the Gulf of Lions (NW Mediterranean)

European Geosciences Union (EGU) General Assembly 2020, 4-8 May 2020The Gulf of Lions (GoL) is a passive margin of about 200 km long and 70 km wide with main sediment supply from the Rhone River supplying Alpine sediments to the Rhone delta. Submarine landslides in the GoL are widespread from the upper slope to the deep basin, within the canyon flanks and in the interfluves of major canyons. The two main submarine landslides present in the GoL are the Eastern Rhône Interfluve Slide (ERIS) and an unnamed slide complex on the western side of the Petit Rhone Canyon. Their resulting mass transport deposits (MTDs), the Rhone Eastern MTD (REMTD) and the Rhone Western MTD (RWMTD) have previously been described in detail in several studies. However, due to the lack of high-resolution multidisciplinary datasets, such as high-resolution seismic, sediment cores, and in-situ geotechnical measurements, a detailed analysis of weak layers and preconditioning factors was never performed. Here, we present a suite of a multidisciplinary dataset; particularly very high-resolution deep-towed multichannel seismic data acquired using Ifremer’s in-house acquisition system SYSIF (SYstème SIsmique de Fond) to assess seafloor instabilities in the GoL. The objectives of this study are twofold and aimed at 1) using deep-towed multichannel seismic data to capture the internal structure of the mass-wasting products previously imaged as seismically transparent or chaotic intervals in conventional seismic data; 2) using multidisciplinary dataset to analyse the basal surfaces of slope failures in the GoL. For the first time, the newly-acquired SYSIF data show in unprecedented detail the internal structure of mass-transport deposit along with small-scale slope failures. We present here an example of a failure that consists of slide blocks, folded and faulted strata with remnant stratigraphy previously associated with a transparent or chaotic facies in the conventional reflection seismic data. The combination of deep-towed seismic and sedimentological data, as well as in-situ measurements allowed us to analyse and characterize the nature of the basal surface of the slope failures in greater detail. We show that the basal surfaces of the recurring slope failures mainly consist of fine-grained clay-rich sediments as compared to turbiditic sequences typical of Rhone turbiditic system. Such observations suggest that greater degree of lithological heterogeneity in sedimentary strata promotes slope failure in the GoL, most likely related to higher liquefaction potential of coarser-grained material, excess pore pressure and possibly resulting variation in sediment strengthPeer reviewe

Morphology of retrogressive failures in the Eastern Rhone Interfluve during the Last Glacial Maximum (Gulf of Lions, Western Mediterranean)

The Gulf of Lions (NW Mediterranean Sea) is a SW-NE oriented passive continental margin formed since the Oligocene. It presents small to large scale mass movement features suggesting a long history of seafloor instability. Of particular interest are the two surficial large mass-transport deposits along the Rhone turbiditic levee, known as the Rhone Eastern and Western Mass-Transport Deposits (REMTD and RWMTD). With the help of the recently acquired multi-beam bathymetric, sub-bottom profiler, high-resolution seismic and sedimentological data, we investigate the morphology, timing, kinematics, and possible triggering mechanisms of the source area of the REMTD, which we refer to as the Eastern Rhone Interfluve Slide (ERIS). ERIS has an estimated run-out distance of approximately 200 km. It covers an area of about 700 km2 and the volume of the mobilized material is approximately 110 km3. Our data reveal four individual glide planes within the ERIS complex which were most likely generated by retrogressive failures. The basal surfaces of the ERIS coincide with high-amplitude seismic reflectors similar to those previously interpreted as the expression of condensed sections on the upper slope. The turbiditic sequences sandwiched between the condensed sections likely control the localisation of potential weak layers favouring the failures. AMS radiocarbon dating yields an age of approximately 21 ka cal BP for the failures, which falls within the peak of the Last Glacial Maximum. The toe area of the ERIS is incised by several active listric faults rooted in the Messinian strata, which control the location of the slide scarps. The combination of several factors such as slope steepening, halokinesis, and excess pore pressure generation due to rapid turbiditic sedimentation during the Last Glacial Maximum are considered as the possible candidates for the triggering of the failures in the investigated slope

The role of condensed sections on seafloor instability: an example of the Gulf of Lions, Western Mediterranean

American Geophysical Union (AGU) Fall Meeting, 9-13 December 2019, San FranciscoThe Gulf of Lions (GoL) is a 200 km long, 70 km wide passive margin where alpine sediments delivered by the Rhone River are deposited on the Rhone delta and deep-sea fan. Seismic data displays multiple sedimentary sequences bounded by high-amplitude reflectors in the upper slope interfluve areas and turbiditic levees downslope. These sedimentary sequences are attributed, in the upper slope, to 100 kyr-glacio-eustatic cycles that initiated during the Mid-Pleistocene Transition, marking the onset of the Milankovitch cyclicity in the Western Mediterranean. The high-amplitude reflectors are interpreted as condensed sections. The GoL margin presents recurring mass transport deposits throughout the Plio-Quaternary succession. Previous studies highlighted a strong stratigraphic control on the landslide basal surfaces, where the basal surfaces of the landslides coincide with the high-amplitude reflectors. In this study, we show a detailed sedimentological and geotechnical analysis of a sediment core that penetrated one of these high-amplitude reflectors. The combined analysis of sediment cores and in situ penetrometer data (CPTu) shows elevated values of porosity, consistency limits, clay content and undrained shear strength compatible with hemipelagic sediments that deposited during sea-level highstands. The sediments corresponding to these high-amplitude reflectors are stiff and less sensitive than those deposited rapidly during the glacial and deglacial periods. Our results suggest that due to higher stiffness and shear strength, these layers do not essentially fail and rather become a basal surface over which sliding occur