Regional controls on holocene sedminentation patterns along the Peru contonental margin; long term implications for El Niño-Southern Oscillation

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A marine heat wave drives massive losses from the world\u27s largest seagrass carbon stocks.

Seagrass ecosystems contain globally significant organic carbon (C) stocks. However, climate change and increasing frequency of extreme events threaten their preservation. Shark Bay, Western Australia, has the largest C stock reported for a seagrass ecosystem, containing up to 1.3% of the total C stored within the top metre of seagrass sediments worldwide. On the basis of field studies and satellite imagery, we estimate that 36% of Shark Bay’s seagrass meadows were damaged following a marine heatwave in 2010/2011. Assuming that 10 to 50% of the seagrass sediment C stock was exposed to oxic conditions after disturbance, between 2 and 9 Tg CO2 could have been released to the atmosphere during the following three years, increasing emissions from land-use change in Australia by 4–21% per annum. With heatwaves predicted to increase with further climate warming, conservation of seagrass ecosystems is essential to avoid adverse feedbacks on the climate system

Meso and small-scale variations of Pb-210 fluxes on the Northwestern Mediterranean continental margins

Pb-210 was analysed in samples collected from (16) sediment traps, deployed during I year on the Northwestern Mediterranean continental margins, within the framework of the (MTP 1)-EUROMARGE-NB programme. The traps were moored within (5) submarine canyons and their adjacent open slope, corresponding to contrasting conditions of particle inputs (fluxes and constituents). The major meso-scale observation (at the margin) is the variation in Pb-210 fluxes along the slope. increasing by a factor of 2-3 between the entrance and the exit of the slope, in relation to the general (water) circulation. At a smaller scale (canyon), the Pb-210 fluxes showed trends which were common to the various sites. i.e. a seaward decrease at 500m depth and an increase, with depth, in the canyons. All these features are related to mass flux variations, except for periods with huge mass fluxes, when Pb-210 fluxes reached a constant value. Pb-210 activities decreased with increasing mass flux; then did not show clear relationship with the concentration of the major constituents of the flux. Pb-210 fluxes, obtained from the traps were mainly in excess of the theoretical Pb-210 flux, available in the overlying water column. Since Pb-210 inventories, measured on the basis of the shelf and slope bottom sediments. were also in excess in relation to the available flux. The margin, as a whole, appears as a sink for Pb-210. This boundary-scavenging process appears to be controlled completely by the mass flux of particles. Differences were observed between Pb-210 fluxes in the near-bottom traps and in the underlying sediments; there can be linked to mass flux and/or morphobathymetry (the trap flux is higher than the sediment flux in the canyon, but lower on the open slope). Overall, the differences were not in excess of 50%, confirming good representation of data collected by the sediment traps. However, this finding Must be taken into account when comparing organic carbon or other constituent fluxes, between the traps and the sediment

Biotracers and geotracers of depositional events in NW Mediterranean margin over the past two centuries

A detailed sedimentological and geochemical investigation was performed on slope sediments collected from the Gulf of Lions and Catalonian margins and from the northern Balearic margin. The study was based on cores, about 50 cm long. Various physiographic sites (canyons, open slopes) were selected according to the available sediment accumulation rates obtained with the Pb-210 geochronological method. At a short time scale, various parts of the slope were characterized by muddy hemipelagic sediment. Sedimentological data suggested that deposition was continuous. Total organic carbon and its hydrolysable fraction decreased downcore. Significant increase of C/N ratio downcore resulted from an early diagenetic process in response to the burial effect. However in other parts, the values of the C/N ratio were found to be irregular and maxima could frequently be related to both higher sand content and relict coastal remains derived from erosion and reworking of outcropping deposits of the last glacial maximum (lowstand system tract). Such tracts crop out over much of the shelf slope break seafloor. They provide various biological and lithological indicators: relatively well-preserved Miliolidea, coastal or lagoonal foraminifera (Ammonia beccarii, Elphidilan crispum), Characea oogons, oxidized beach-rock debris, gypsum, previously mature glauconite (with cracks) and coarse vegetal debris. Throughout the sections, the degree of diagenetic overprinting of the organic matter frequently showed a positive correlation with the abundance of some coastal indicators supporting the hypothesis of sedimentation induced predominantly by gravity These downslope accumulation may correspond to 10-40 % of the total deposit. The gravity induced transfers have been shown to be negligible in the sediment record of the northern Balearic margin. It is suggested that a reduced sedimentation rate (half that observed in the Gulf of Lions) would be expressed by a higher stability of the slope. The consequence of low sedimentation would be early diagenetic solution/reprecipitation of carbonate near the redox transition layer.Une étude sédimentologique et géochimique détaillée des dépôts de pente du golfe du Lion, de la Catalogne et du Nord des Baléares à partir de carottes courtes ne dépassant pas 50 cm a été effectuée. Les sites présentés (canyons, interfluves) ont été choisis en fonction de l’existence de taux de sédimentation mesurés à l’aide de la méthode au 210Pb. À cette échelle de temps réduite, plusieurs régions de la pente sont caractérisées par une accumulation de vases hémipélagiques. Les observations sédimentologiques suggèrent un processus continu de la sédimentation. Le carbone organique total ainsi que sa fraction hydrolysable tendent à diminuer du haut vers le bas de la carotte ; à l’inverse une nette augmentation du rapport C/N est observée. Cette augmentation traduit les effets d’une diagenèse précoce conséquente de l’enfouissement. Cependant, dans certains sites, le profil vertical du C/N est irrégulier et les maxima peuvent être souvent corrélés à la fois avec des teneurs en sable plus élevées et avec l’abondance de débris issus de l’érosion et du remaniement des dépôts littoraux du dernier bas niveau glaciaire. Ces derniers qui tendent à affleurer à proximité de la rupture de pente entre le plateau et la pente sont reconnus grâce à plusieurs marqueurs littoraux : Miliolidea assez bien préservés, foraminifères côtiers ou lagunaires (Ammonia beccarrii, Elphidium crispum), oogones de Characées, fragments oxydés de grès de plage, gypse, glauconie évoluée et débris grossiers de végétaux. La fréquente association de ces indicateurs côtiers avec le degré avancé de l’évolution diagénétique correspond à des épisodes où l’accumulation gravitaire joue un rôle important pouvant représenter de 10 à 40 % du sédiment total. Sur la marge septentrionale des Baléares, ces processus gravitaires paraissent avoir joué un rôle négligeable. Il est suggéré que la vitesse de sédimentation réduite correspond à une plus grande stabilité de la pente. En conséquence de cette lenteur de la sédimentation, les carbonates sont l’objet d’une diagenèse précoce avec des processus de dissolution et de reprécipitation à proximité de la limite d’oxydoréduction

Last millenia sedimentary record on a micro-tidal, low-accumulation prodelta (Têt NW Mediterranean)

SUBSURFStatistical sequential analysis was performed on a series of sediment cores collected from the Têt prodelta in the Gulf of Lions, northwestern Mediterranean Sea, between October 2003 and October 2004. Seabed changes during that period were correlated to hydrodynamic conditions (waves and currents) and river discharge. Low sediment supply prevents full preservation of new sediment strata on this low-accumulation prodelta located on a microtidal, storm-dominated inner shelf. Severe meteorological events caused a rapid succession of erosion and deposition phases. For example, the December 2003 flood and storm produced a flood layer deposit that persisted for 2 months with only slight transformations due to early diagenesis and/or bioturbation, until a new storm event eroded this layer. A typical sedimentary sequence was observed for the secular deposits composed of a 10-cm-thick sandy layer overlaying siltyclayey layers. These characteristic features were used to analyse the last millennia sedimentary record of the Têt prodelta. The low preservation of freshly deposited sediments and variable sedimentation rates during the last millennia period yield a sedimentary sequence formed by the outcropping of muddy prodeltaic units intersected by heterogeneous siltysandy units similar to those formed under present day hydrodynamic conditions. No flood layer was found related to catastrophic flooding of the last century in the sedimentary record. The Little Ice Age ( 15501850 AD) probably favoured the formation of a well-developed muddy prodelta in the mouth of the Têt River. Later on, the decrease of sediment supply by rivers due to climate change and/or human activities (damming, irrigation), and the increase of the number of high-energy storms reaching the coast, induced a coarsening of the top sediment layer on this prodelta. This modern change of the substrate is also observed in the composition of benthic biota found in the substrat

Microbial activity at the deep water sediment boundary layer in two highly productive systems in the Western Mediterranean: the Almeria-Oran front and the Malaga upwelling

Microbiological and biogeochemical studies were carried out in sediments and near bottom waters in the upwelling area off the Malaga coast and in the Almeria-Oran frontal zone. In these nutrient-rich conditions, metabolic activity is potentially limited by oxygen availability through the sediment depth. In the surficial sediments of the frontal zone, however, oxygen penetrated to a depth of 4 cm, allowing oxic mineralisation processes to occur throughout the layer. In the surficial sediments of the upwelling area, oxygen penetration was limited to the top 2.5 cm, leading to anoxic conditions. Glutamate respiration and global oxygen uptake rates were clearly higher than in the frontal zone. In the superficial sediments of the frontal zone, bacteria were less abundant and showed the lowest potential rate for mineralisation processes, but the highest rate for proteolysis. This discrepancy is probably due to differences in the quality of organic inputs into the two areas, with labile organic compounds reaching the sea bottom in the shallower upwelling zone. Such inputs enhance the mineralisation of low molecular weight monomers, whilst inhibiting the polymer hydrolysis processes. Conversely, in the deeper frontal zone, labile monomers become exhausted, decreasing the mineralisation rates. Concomitantly, bacteria have to develop ectoenzymatic activities in order to extract their carbon and energy from the available semi-labile polymers. Consequently, the theoretical relationship between the mineralisation and hydrolysis processes are tightly coupled in the upwelling area, and uncoupled in the frontal zone.La distribution de facteurs biogéochimiques et de divers processus microbiens a été étudiée dans les eaux proches du fond et les sédiments superficiels dans la zone de l'upwelling de Malaga et au niveau du front Almeria-Oran. Dans ces conditions relativement eutrophes, les activités métaboliques sont limitées par la disponibilité de l'oxygène moléculaire dans l'épaisseur sédimentaire. Toutefois, dans les sédiments de la zone frontale l'oxygène pénètre jusqu'à 4 cm de profondeur, permettant le développement de processus oxiques à travers cette couche. Par contre, dans la zone d'upwelling, où la vitesse potentielle de respiration du glutamate et la consommation globale d'oxygène sont nettement supérieures à celles de la zone frontale, la pénétration de l'oxygène est limitée à 2,5 cm, conduisant à l'anoxie à partir de ce niveau. Dans les sédiments superficiels de la zone frontale, les bactéries sont moins abondantes, et moins actives dans le processus de respiration du glutamate, que leurs homologues de la zone d'upwelling. Par contre, leur potentiel d'hydrolyse des polypeptides est plus développé. Cette divergence est probablement due à la différence de qualité des apports en matières organiques dans les deux zones. Par rapport à la zone frontale, dans la zone d'upwelling moins profonde les apports au domaine benthique incluent une fraction plus labile. Ces apports en matériel labile dopent les processus de minéralisation des monomères, alors que par un effet de retro-inhibition ils ralentissent l'hydrolyse des polymères. À l'opposé, dans la zone frontale plus profonde, le matériel atteignant le domaine benthique a été épuisé de sa fraction labile, ce qui provoque un ralentissement des vitesses de minéralisation des monomères. Simultanément, les bactéries doivent développer des activités ectoenzymatiques pour pouvoir extraire des polymères semi-réfractaires disponibles le carbone nécessaire à leur métabolisme. En conséquence, les relations entre les processus d'hydrolyse et de minéralisation sont couplés dans la zone d'upwelling et découplés dans la zone frontale

Searching for the Record of Historical Earthquakes, Floods and Anthropogenic Activities in the Var Sedimentary Ridge (NW Mediterranean),

International audienceSubmarine landslides on continental slopes are triggered by diverse mechanisms such as sea-level variations, climate-driven sediment supply fluctuations, slope steepening related to long term tectonics, earthquakes and human activities. In the present work, we try to discriminate the origin of major gravity events recorded in three sediment cores collected on the Var Sedimentary Ridge (NW Mediterranean) by means of X-ray imaging, grain-size distributions and 210Pb-based chronologies supported by 137Cs and AMS 14C dating. An alternation of hemipelagic mud and sandy turbidite layers is apparent in all cores. In two of them, the topmost turbidite can be correlated to the well-known 1979 Nice-Airport landslide. A sub-surficial sandy layer is identified in all three cores with an approximate age of 120 years, suggesting for the first time a relationship with the largest earthquake in the region (1887; Mw = 6.9). Below the depth of that major event, sand beds are tentatively related to older local earthquakes and centennial floods