Mass wasting processes along the Owen Ridge (Northwest Indian Ocean)

International audienceThe Owen Ridge is a prominent relief that runs parallel to the coast of Oman in the NW Indian Ocean and is closely linked to the Owen Fracture Zone, an 800-km- long active fault system that accommodates today the Arabia-India strike-slip motion. Several types of mass failures mobilizing the pelagic cover have been mapped in details along the ridge using multibeam bathymetry and sediment echosounder. Here we present a synthetic map of the different types of mass wasting features observed along the ridge and we further establish a morphometric analysis of submarine landslides. The spatial variation of failure morphology is strongly related to the topography of the basement. The highest volumes of multi-events generated slides are mobilized along the southern portion of the ridge. There, the estimated volume of evacuated material during a slide is up to 45 km3. Combining these new observations with re-interpreted ODP seismic lines (Leg 117) documents sporadic mass wasting events through time along the southern segment of the ridge since its uplift in the Early Miocene, with a typical recurrence rate of the order of 105-106 years. Although seismicity may still be the final triggering process, mass wasting frequency is mainly controlled by the slow pelagic sedimentation rates and hence, time needed to build up the 40-80 m thick pelagic cover required to return to a mechanically unstable pelagic cover

Tectonics of the Dalrymple Trough and uplift of the Murray Ridge (NW Indian Ocean)

International audienceThe Dalrymple Trough is a 150-km-long, 30-km-wide basin located at the northern termination of the Owen Fracture Zone (OFZ), which is the present-day active India-Arabia plate boundary. The Dalrymple Trough is closely associated with the Murray Ridge, a complex of prominent bathymetric highs located on its eastern flank. Recent multibeam mapping of the connection between the Dalrymple Trough and the OFZ revealed a horsetail structure, which suggests a close relationship between geological histories of both structures. However, the 3-6 Ma age of initiation of the OFZ contrasts with the commonly accepted Early Miocene emplacement of the Dalrymple Trough. Recent seismic lines document a new tectonic history of the Dalrymple Trough, involving two major episodes of deformation along the India-Arabia plate boundary at ~ 8-10 Ma and ~ 1.9 ± 0.9 Ma. The 8-10 Ma episode is marked by a system of folds linked to the main uplift of the southern Murray Ridge and the first uplift of the northern Murray Ridge. This episode is related to a global plate reorganization event in the Late Miocene, well expressed by intraplate deformation in the Central Indian Ocean. The Dalrymple Trough opened at ~ 1.9 ± 0.9 Ma subsequently to the formation of a stepover at the India-Arabia plate boundary, coeval with the regional M-unconformity in the Oman abyssal plain, which marks a structural reorganization of the Makran accretionary wedge, and the last uplift of the northern Murray Ridge

Ocean productivity in the Gulf of Cadiz over the last 50 kyr

Reconstructions of ocean primary productivity (PP) help to explain past and present biogeochemical cycles and climate changes in the oceans. We document PP variations over the last 50 kyr in a currently oligotrophic subtropical region, the Gulf of Cadiz. Data combine refined results from previous investigations on dinocyst assemblages, alkenones, and stable isotopes ( 18O, 13C) in planktonic (Globigerina bulloides) and endobenthic (Uvigerina mediterranea) foraminifera from cores MD04-2805 CQ and MD99-2339, with new isotopic measurements on epibenthic (Cibicides pachyderma–Cibicidoides wuellerstorfi) foraminifera and dinocyst-based estimates of PP using the new n = 1,968 modern database. We constrain PP variations and export production by integrating qualitative information from bio-indicators with dinocyst-based quantitative reconstructions such as PP and seasonal sea-surface temperature and information about remineralization from the benthic 13C (difference between epi- and endo-benthic foraminiferal 13C signatures). This study also includes new information on alkenone-based SST and total organic carbon which provides insights into the relationship between past regional hydrological activity and PP regime change. We show that PP, carbon export, and remineralization were generally high in the NE subtropical Atlantic Ocean during the last glacial period and that the Last Glacial Maximum (LGM) had lower 13C than the Heinrich Stadials with sustained high PP, likely allowing enhanced carbon sequestration. We link these PP periods to the dynamics of upwelling, active almost year-round during stadials, but restricted to spring-summer during interstadials and LGM, like today. During interstadials, nutrient advection through freshwater inputs during autumn–winter needs also to be considered to fully understand PP regimes.info:eu-repo/semantics/publishedVersio

LES SYSTEMES TURBIDITIQUES PROFONDS DE LA MARGE CELTIQUEARMORICAINE (GOLFE DE GASCOGNE) : PHYSIOGRAPHIE ET EVOLUTION AU COURS DES DERNIERS 30 000 ANS

Studies of acoustic data (3.5 kHz seismic and EM12 swath bathymetry data) and deepsea cores (Kullenberg) give new insight on the Celtic and Armorican margin sedimentation during the last 30 000 years BP. Our results show that the whole Celtic and Armorican turbidite systems corresponds to a mud/sand-rich multisource ramp. Indeed, the Celtic and Armorican deep-sea environments are fed by 8 main channels on the lower slope forming several small sized turbidite systems and contrast with the point-source submarine fan fed by a single feeder channel. Sedimentary facies and sequences analysis document the evolution of the sedimentation in relation to environmental conditions on the Celtic continental shelf during the Marine Isotopic Stage 1 and 2. During the last sea-level lowstand (Marine Isotopic Stage 2), overbank deposits give evidence of the occurrence of relatively low-density turbidity currents that would have originated at the front of the English Channel Delta on the Celtic continental shelf. During the last sea-level highstand (Holocene), despite the disappearance of rivers and deltaic edifices on the shelf, recent sandy layers indicate episodic high-density turbidity currents. These sandy holocene supplies seem derived from reworked outer shelf sand in relation to the high-energy (storms and spring tidal currents) conditions on the outer shelf and to the net sand transport from the near shore to the margin slope that prevail during high sealevel periods.Ce travail, basé sur l'analyse et l'interprétation de données acoustiques (sismique 3.5 kHz et multifaisceaux EM12) et de prélèvements (carottes Kullenberg), propose une reconstruction du fonctionnement sédimentaire récent (derniers 30 000 ans) du domaine profond de la Marge Celtique-Armoricaine. Les résultats obtenus ont permis d'interpréter l' ensemble des systèmes turbiditiques localisés au sein de la marge, comme un système multi-source de type "rampe". En effet, contrairement à la majorité des systèmes turbiditiques modernes, alimentés par une source ponctuelle, le domaine profond de la Marge Celtique-Armoricaine semble équitablement alimenté et abrite plusieurs systèmes turbiditiques de taille réduite, localisés au débouché de chaque canyon majeur. L' analyse détaillée des facièse t séquences sédimentaires a permis de mettre en évidence un fonctionnement sédimentaire lié aux conditions environnementales régnant au sein de la plateforme continentale Celtique. En bas niveau marin, la position du Delta de la Mer Celtique associé à de forts courants tidaux, semble représenter le facteur dominant contrôlant les apports sédimentaires vers le domaine profond. Cette configuration a entraîné, au cours du Stade isotopique 2, d' importants apports sédimentaires d' origine fluv-dioeltaïque qui se sont traduits par des séquences turbiditiques argilo-silteuses déposées sur les levées. En haut niveau marin, malgré la disparition de la totalité des réseaux hydrographiques, des apports sableux récents témoignent d' une alimentation épisodique du domaine profond. Ces apports, mis en évidence par des niveaux sableux centimétriques à métriques intercalés dans des argiles d' origine hémipélagique, témoignent d' écoulements turbiditiques de haute densité. Ce deuxième type de fonctionnement semble lié à la présence de corps sableux tidaux localisés sur la plateforme externe, ainsi qu' au transport sableux résiduel, orienté actuellement pour l' ensemble de la Mer Celtique vers le rebord de pente

Les systèmes turbiditiques profonds de la marge celtique-armoricaine (Golfe de Gascogne) : physiographie et évolution au cours des derniers 30 000 ans

Ce travail, basé sur l'analyse et l'interprétation de données acoustiques (sismique 3.5 kHz et multifaisceaux EM12) et de prélèvements (carottes Kullenberg), propose une reconstruction du fonctionnement sédimentaire récent (derniers 30 000 ans) du domaine profond de la Marge Celtique-Armoricaine. Les résultats obtenus ont permis d'interpréter l'ensemble des systèmes turbiditiques localisés au sein de la marge, comme un système multi-source de type "rampe". En effet, contrairement à la majorité des systèmes turbiditiques modernes, alimentés par une source ponctuelle, le domaine profond de la Marge Celtique-Armoricaine semble équitablement alimenté et abrite plusieurs systèmes turbiditiques de taille réduite, localisés au débouché de chaque canyon majeur. L'analyse détaillée des faciès et séquences sédimentaires a permis de mettre en évidence un fonctionnement sédimentaire lié aux conditions environnementales régnant au sein de la plateforme continentale Celtique. En bas niveau marin, la position du Delta de la MerCeltique associé à de forts courants tidaux, semble représenter le facteur dominant contrôlant les apports sédimentaires vers le domaine profond. Cette configuration a entraîné, au cours du Stade isotopique 2, d'importants apports sédimentaires d'origine fluvio-deltaïque qui se sont traduits par des séquences turbiditiques argilo-silteuses déposées sur les levées. En haut niveau marin, malgré la disparition de la totalité des réseaux hydrographiques, des apports sableux récents témoignent d'une alimentation épisodique du domaine profond. Ces apports, mis en évidence par des niveaux sableux centimétriques à métriques intercalés dans des argiles d'origine hémipélagique, témoignent d'écoulements turbiditiques de haute densité. Ce deuxième type de fonctionnement semble lié à la présence de corps sableux tidaux localisés sur la plateforme externe, ainsi qu'au transport sableux résiduel, orienté actuellement pour l'ensemble de la Mer Celtique vers le rebord de pente.Based on interpretation of acoustic data (3.5 kHz seismic and EM12 swath bathymetry data) and bottom sampling (Kullenberg cores), this memoir proposes a review of Celtic and Armorican margin deep sea sedimentation during the last 30 000 years BP. Our results show that the whole Celtic and Armorican turbidite systems corresponds to a mud/sand-rich multisource ramp. Indeed unlike the point-source submarine fan fed by a single feeder channel, the Celtic and Armorican deep-sea environments are fed by 8 mains channels on the lower slope forming several small sized turbidite systems. Sedimentary facies and sequences analysis document the evolution of sedimentation linked to the environmental conditions on the Celtic continental shelf during the Marine Isotopic Stage 1 and 2. During the last lowstand of sea-level, the overbank deposits give evidence of the occurrence of relatively low-density turbidity currents that would have originated at the front of a deltaic environment on the Celtic continental shelf. During the high sea-level conditions, despite the disappearance of rivers and deltaic edifices on the shelf, recent sandy layers indicate episodic high-density turbidity currents. These sandy Holocene supplies seems derived from reworked outer shelf sand due to the high-energy conditions on the shelf and to the net sand transport from the near shore to the margin slope during high sea-level periods

The "Fleuve Manche" : the sub-marine sedimentary features from the outer shelf to the deep-sea fans

International audienceAbstract Bathymetric, seismic and coring surveys have been carried out since the beginning of the 1990s on the outer shelf, the slope and the continental rise of the Western Approaches margin. Most of the sedimentary features studied belong to a sedimentary system built up since the Oligocene and fed by a palaeoriver, the ‘Fleuve Manche’ that flowed in the English Channel during eustatic lowstands. A geomorphological map details the spatial distributions of the palaeovalleys on the outer shelf, the drainage‐basin‐like organised canyons on the slope and the channel–levee and lobe complexes on the rise. The obvious channel–canyon connections and the assessed palaeovalley–canyon links demonstrate that the ‘Fleuve Manche’ was the single source for terrigeneous fluxes of the Armorican Deep Sea Fan when it was one of two sources of the Celtic Deep Sea Fan. Amongst parameters controlling the sedimentation, two factors play a major role. (i) Subsidence controlled the possibly Messinian riverine incision, the turbidite activities during lowstands, as well as marine infilling of the seawards part of the ‘Fleuve Manche’. (ii) Quaternary sedimentary supplies were influenced by the melting ice‐sheets and the connections with the onshore catchment basins. Copyright © 2003 John Wiley & Sons, Ltd

Impact of freshwater release in the Mediterranean Sea on the North Atlantic climate

International audienceSediment cores from the Mediterranean Sea have evidenced several periods of Sapropel deposition, which can be explained by events of anoxic bottom conditions. An explanation for such events calls for a very stratified sea, possibly related with freshwater input through increased precipitations and runoff discharges. It has been suggested that such a stratified Mediterranean Sea may in turn weaken the Atlantic meridional overturning circulation (AMOC) through changes in the Mediterranean overflow water (MOW). Nevertheless, models used to establish this result were relatively simple and the mechanisms leading to such an impact remained elusive. To improve on those previous studies, we analyse the impact of different freshwater releases with rates of 0.2, 0.1, 0.05 and 0.02 Sv (1 Sv=10 6 m 3 /s) in the Mediterranean Sea using the IPSL-CM5A-LR model in a few multi-centennial simulations. We focus the analysis on the impact of a decrease in the Mediterranean overflow water (MOW) on the large-scale Atlantic circulation. We find a consistent change in horizontal currents in the upper Atlantic Ocean in all simulations in the first century, with a large enhancement of the northward current west of Rockall in the northeast Atlantic. Concerning the AMOC response, we identify three different processes that impact its fate. The first is related to changes in geostrophic currents at depth induced by the disappearance of the MOW, which tends to weaken the AMOC. On the contrary, the second enhances the AMOC and is associated with the increase in northward currents in the horizontal upper circulation. The last process is due to the spread of surface freshwater anomalies out of the Mediterranean Sea that freshens the North Atlantic convection sites and weakens the AMOC. Depending on the rate of the freshwater release, the strength and balance of these three processes are different. For rates larger than 0.05 Sv, we observe a strong reduction of the AMOC, while for lower rates, we notice an enhancement in the upper cell. The climatic response follows that of the upper AMOC with a warming of the North Atlantic for rates lower than 0.05 Sv and a cooling for higher rates. Given that past estimates of freshwater release in the Mediterranean Sea indicate rates lower than 0.05 Sv, we argue that Sapropel events may have enhanced of the upper AMOC and warmed of the North Atlantic.

Are Past Sea-Ice Reconstructions Based on Planktonic Foraminifera Realistic? Study of the Last 50 ka as a Test to Validate Reconstructed Paleohydrography Derived from Transfer Functions Applied to Their Fossil Assemblages

Since its existence, paleoceanography has relied on fossilized populations of planktonic foraminifera. Except for some extreme environments, this calcareous protist group composes most of the silty-to-sandy fraction of the marine sediments, i.e., the foraminiferal oozes, and its extraction is probably the simplest among the currently existing set of marine fossil proxies. This tool has provided significant insights in the building of knowledge on past climates based on marine archives, especially with the quantification of past hydrographical variables, which have been a turning point for major comprehensive studies and a step towards the essential junction of modelling and paleodata . In this article, using the modern analog technique and a database compiling modern analogs (n = 1007), we test the reliability of this proxy in reconstructing paleohydrographical data other than the classical sea-surface temperatures, taking advantage of an update regarding a set of extractions from the World Ocean Atlas for transfer functions. Our study focuses on the last glacial period and its high climatic variability, using a set of cores distributed along the European margin, from temperate to subpolar sites. We discuss the significance of the reconstructed parameters regarding abrupt and extreme climate events, such as the well-known Heinrich events. We tested the robustness of the newly obtained paleodata by comparing them with older published reconstructions, especially those based on the complementary dinoflagellate cyst proxy. This study shows that the potential of planktonic foraminifera permits going further in reconstructions, with a good degree of confidence; however, this implies considering ecological forcings in a more holistic perspective, with the corollary to integrate the message of this fossil protist group, i.e., the obtained parameters, in light of a cohort of other data. This article constitutes a first step in this direction