Thousands of cold-water coral mounds along the Moroccan Atlantic continental margin : distribution and morphometry

Coral mounds formed by framework-forming cold-water corals pierce the seabed along most continental margins of the Atlantic Ocean and new sites are continuously being discovered. Here, we describe an extremely high accumulation of coral mounds at the NW Moroccan Atlantic margin between 35 degrees N and 35.5 degrees N. Within an area of only 1440 km(2), > 3400 mounds were found exposed at the seabed. The coral mounds are nowadays characterized by an almost complete lack of living cold-water corals. In addition, numerous buried mounds were identified in hydroacoustic sub-bottom profiles, and are estimated to be similar to 3.7 times more frequent than the exposed mounds. Consequently, a total of similar to 16,000 buried and exposed mounds is estimated for the entire study area. The exposed mounds are rather small with a mean height of 18 m and show a conspicuous arrangement in two slope-parallel belts that centre in water depths between 720 and 870 m and 890-980 m, respectively, putting them among the deepest mound occurrences discovered so far in the Atlantic. The mostly elongated mounds largely stretch downslope pointing to a significant influence of internal waves in the mound formation process. Moreover, based on their average dimensions, the entire coral mound volume can be estimated as 1.3 km(3), which means the mounds store a considerable amount of coral carbonate highlighting their potentially important role as regional carbonate factories. In combination with further occurrences of coral mounds along the Moroccan margin, both in the Mediterranean and in the Atlantic Ocean, these new findings underline Morocco's role as a hotspot for the occurrence of cold-water coral mounds

Morphological features and associated bottom-current dynamics in the Le Danois Bank region (southern Bay of Biscay, NE Atlantic):A model in a topographically constrained small basin

17 pages, 11 figures, 1 appendixThe present-day morphology of the Le Danois Bank region has been investigated based on bathymetric and high to ultra-high resolution seismic reflection data. The involved bottom-current processes are associated with the Eastern North Atlantic Central Water, the Atlantic Mediterranean Water and the Labrador Sea Water. Sediments originating from various canyon systems along the Cantabrian Margin and the Asturias continental shelf are transported by downslope and alongslope processes towards the Le Danois intraslope basin. The background flow velocities of bottom currents are all below the threshold (8–10 cm/s) of generating plastered and mounded geometries of contourite drifts. However, bottom currents are locally accelerated (up to 25 cm/s) due to the presence of the Le Danois Bank and the Vizco High, creating a furrow and three moats and generating six plastered drifts, three elongated mounded and separated drifts at different depth intervals. The extension and distribution of the drifts are controlled by slope morphology and/or bottom current velocities. Unlike contourite drifts along other continental slopes, a single contourite drift (the Gijón Drift) with a lateral variation in drift geometry and internal structure indicates the interaction of bottom currents with different flow dynamics. Additionally, scouring of active bottom currents and rapid sedimentation rate of contourite drifts may be at the origin of slope instability events. Besides contourite drifts, internal waves may have induced the formation of sediment waves. In the Le Danois intraslope basin, multiple sedimentary processes work together and shape the present-day seafloor. Bottom currents are focused due to deflection on complex topographical obstacles within a relatively small basin setting, and create a wide variety of sedimentary features, including contourite drifts. The resulting sedimentary features thus have more frequent lateral variations, a feature typical for topographically constrained small basinsThis study was carried out within the framework of a Chinese Scholarship Council “CSC Grant” (201506410062). The research was conducted in collaboration with “The Drifters Research Group” of the Royal Holloway University of London (UK) and it is related to the projects CTM 2012-39599-C03, CGL2016-80445-R, and CTM2016-75129-C3-1-R. [...] These research campaigns framed within the ECOMARG (REN2002-00916/MAR) and MARCONI (REN2001-1734 C03-01/M) projects. This study also builds upon achievements of project ESF Euromargins MOUNDFORCE, EC FP5 RTN EURODOM and EC FP6 HERMES (GOCE-CT-2005-511234-1)Peer Reviewe

Quantitative characterisation of contourite deposits using medical CT

Five sediment cores, retrieved from four different depositional contouritic morphological settings (a sheeted drift, a confined mounded drift, a mounded elongated drift and a plastered drift) from the Northern Gulf of Cadiz and the Alboran Sea have been analysed using medical X-ray computed tomography (medical CT). A quantitative approach has been used, resulting in a workflow that delineates several radio-density ranges based on the Hounsfield Unit (HU) histogram of each core and tracks these ranges throughout the cores. In order to derive the geological significance, the radio-density ranges of all cores have been compared to non-destructive, continuous chemical and physical proxies as well as grain size measurements. The highest correlations occurred between high HU and proxies indicating elevated bottom currents, such as Zr/Al and sortable silt. Additionally, a continuous increase in average HU and inferred bottom current velocities, needed for the creation of the specific contourite setting, could be observed throughout the five cores. Despite imperfections and the requirement of additional research, promising results have been obtained which could improve the detection of diagnostic criteria for contourites. Moreover, the CT data can give more conclusive evidence on the nature of the (contourite) sedimentary sequence boundaries.Royal Holloway University of London | Ref. 1.5.247.13NMinisterio de Ciencia e Innovación | Ref. CTM 2012-39599-C03Ministerio de Economía | Ref. CGL2016-80445-RMinisterio de Economía | Ref. CTM2016-75129-C3-1-RMinisterio de Ciencia e Innovación | Ref. CTM2008-06399-CO4-02/MA

Quantitative characterisation of contourite deposits using medical CT

Five sediment cores, retrieved from four different depositional contouritic morphological settings (a sheeted drift, a confined mounded drift, a mounded elongated drift and a plastered drift) from the Northern Gulf of Cadiz and the Alboran Sea have been analysed using medical X-ray computed tomography (medical CT). A quantitative approach has been used, resulting in a workflow that delineates several radio-density ranges based on the Hounsfield Unit (HU) histogram of each core and tracks these ranges throughout the cores. In order to derive the geological significance, the radio-density ranges of all cores have been compared to non-destructive, continuous chemical and physical proxies as well as grain size measurements. The highest correlations occurred between high HU and proxies indicating elevated bottom currents, such as Zr/Al and sortable silt. Additionally, a continuous increase in average HU and inferred bottom current velocities, needed for the creation of the specific contourite setting, could be observed throughout the five cores. Despite imperfections and the requirement of additional research, promising results have been obtained which could improve the detection of diagnostic criteria for contourites. Moreover, the CT data can give more conclusive evidence on the nature of the (contourite) sedimentary sequence boundaries

Monitoring of a sediment plume produced by a deep-sea mining test in shallow water, Málaga Bight, Alboran Sea (southwestern Mediterranean Sea)

In this study different experimental designs for monitoring of sediment plumes produced by deep-sea mining are presented. Plumes of sediment stirred up from the seabed by mining machines are considered to represent a major environmental pressure which may extend far beyond the actual mining area. Two industry field tests with the scaled mining vehicle Apollo II of Royal IHC conducted in a relatively shallow setting offshore southern Spain provided valuable insights for anticipated monitoring of nodule mining activities in the deep Pacific. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude of around 5–10 cm s−1, and gently sloping seabed covered with fine muddy sediment provide a good analogue to operational conditions in the deep sea. The plume of suspended sediment mobilised by the mining vehicle was monitored with turbidity sensors deployed on a ship-operated CTD system and on a static array of moored sensors and monitored visually using a ship-operated ROV. It was found that the generated sediment plume extended no >2 m above the seabed close to the source (<100 m) but increased in height at greater distance. Furthermore, turbidity values decreased rapidly with increasing distance to the source. Even though plume monitoring suffered interference from bottom trawling activities in neighbouring areas, a distinct turbidity signal generated by the mining equipment could still be distinguished above background turbidity at 350 m away from the source. From the experience gained in shallow water, recommendations are made on how a combination of sensors operated from moving and moored platforms may be a suitable and successful strategy for monitoring man-made sediment plumes in the deep sea

The subaqueous landslide cycle in south-central Chilean lakes: the role of tephra, slope gradient and repeated seismic shaking

Subaqueous landslides are common features at active and passive ocean margins, in fjords and lakes. They can develop on very gentle slope gradients (<2) and the presence of sandy tephra layers seems to facilitate the development of translational failure. Despite numerous investigations, it remains elusive how different slope preconditioning factors act and interact over time and how different triggering mechanisms can lead to slope failure. In settings of low to moderate seismicity, stratigraphic sequences with sublacustrine mass-transport deposits (MTDs) have successfully been used for constructing prehistorical earthquake catalogs. In high seismicity areas, it is inferred that not all strong earthquakes succeed in triggering landslides on the investigated slope segments, and MTD records do not fully represent their complete recurrence pattern. Here, we present the spatio-temporal distribution of MTDs in two large glacigenic Chilean lakes (Villarrica and Calafquén) based on a detailed seismic-stratigraphic analysis and several radiocarbon-dated piston cores (up to 14m long). We find a strong influence of slope gradient on the occurrence and volume of landslide events; i.e. most (small) landslides take place on slopes of 5-20, whereas the few large (potentially tsunamigenic) landslides exclusively occur on slopes of <4. Liquefaction of sandy tephra layers facilitates the development of thin (<0.5m) in-situ deformations during earthquake shaking. When sandy tephra layers get progressively buried, liquefaction becomes unlikely, but repeated excess pore pressure transfer to overlying units facilitates the development of translational sliding. The occurrence of voluminous landslides seems to follow a “landslide cycle” which starts with the deposition of a tephra layer and the development of in-situ deformations directly on top. Once the slope sequence reaches a critical thickness, the end of the cycle is indicated by incipient scarp development, and subsequent major sliding event(s). The duration of the landslide cycle is defined by the rate of gradual sedimentation, but may be affected by sudden geological events (e.g., volcanic eruptions), expediting the end of the cycle. Despite the many methodological challenges inherent to the construction of a MTD stratigraphy, we propose that well-dated multiple MTD events can be used as positive evidence to strengthen and specify the regional paleoseismic record, concerning the largest events in a high-seismicity region. This method is most successful when targeting the base of relatively steep slopes (5-20) with frequent, minor landsliding, and complementing this with seismic-stratigraphic analysis of fluid-escape features and correlation with distal turbidite records.(VLID)3242017Submitted versio

The influence of biological maturity and competitive level on isometric force-time curve variables and vaulting performance in young female gymnasts

This cross-sectional study investigated isometric force-time curve variables and vaulting performance in young female gymnasts of varying maturity and competitive levels. One hundred twenty gymnasts aged 5–14 years were subdivided into maturity groupings and also according to their competitive level. Subjects performed isometric midthigh pulls (IMTP) before completing straight jump vaults that were recorded using two-dimensional video. All significance values were accepted at p < 0.05. Absolute peak force (PF abs) and force at various time epochs were significantly greater in more mature gymnasts, although no significant differences were observed in relative peak force (PF rel). When grouped by competitive level, elite gymnasts produced a significantly greater absolute rate of force development (RFD abs) at 0–150, 0–200, and 0–250 ms as well as relative RFD (RFD rel) at 0–200 and 0–250 ms than recreational gymnasts. Based upon regression analyses, force at 50 ms during the IMTP test explained 15% of vertical takeoff velocity during vaulting. Biological maturation seems to impact isometric force-time curve characteristics in young female gymnasts, and higher-level gymnasts produce greater RFD than those competing at a lower level. Vaulting vertical takeoff velocity seems to be largely independent of isometric force-time characteristics with only a small amount of variance explained by force at 50 ms