A. Calafat

Composition and spatio-temporal variability of particle fluxes in the Western Alboran Gyre, Mediterranean Sea

Total mass and main constituents (carbonate, organic matter, biogenic opal and lithogenic fraction) flux series were obtained in the northern part of the Western Alboran Gyre during an annual cycle from July 1997 to May 1998, at 10 days sampling interval. Two mooring lines equipped with sediment trap–current meter pairs were deployed across the continental slope off Malaga. CTD, SeaWiFS and watershed fluvial discharge data sets were also obtained from several sources to establish their relations with particle flux data. Time-averaged fluxes for the whole period, including fluxes of organic carbon, increased slightly with depth in both locations as the result of the input of particulate matter by near-bottom nepheloid layers. Furthermore, time-averaged fluxes at mid waters were higher in the centre of the gyre than at its periphery, suggesting some kind of particle funnelling from the gyre periphery towards its centre. Temporal evolution of fluxes was highly variable throughout the year. All series showed an overall common evolution with a first peak at the beginning of summer 1997, a second broad composite peak during late autumn/winter and a third one in spring 1998. Composition of settling particles showed that summer and spring peaks were richer in biogenic constituents compared to the late autumn/winter peak. Particle fluxes to mid water depths in the northern part of the Alboran Sea seem to be mainly controlled by fluvial discharge and primary production. Fluvial discharge could be responsible for the higher lithogenic flux during autumn and winter, while high primary production could play a key role in generating biogenic particles during spring and summer. Regarding near-bottom fluxes, the temporal evolution was controlled both by the downward transfer of particulate matter from mid waters and the advective input of mostly lithogenic and carbonate matter. The increased advective input during maximum near-bottom fluxes is tentatively related to intense eddy-like activity recorded in the deep-water masses.

Accumulation rates of major constituents of hemipelagic sediments in the deep Alboran Sea: a centennial perspective of sedimentary dynamics

The accumulation rates of sediment and major constituents in three different geographic areas of the Alboran Sea have been assessed by means of 210Pb and 137Cs concentration profiles. Mixing is present in the top layer of the sedimentary column, with mixing coefficients estimated to range from 0.2 to 15 cm2 yr−1. Overall, apparent sedimentation rates for the last 100 years range from 0.014 to 0.182 g cm−2 yr−1 and show a tendency to decrease with distance to the coast and water column depth. However, specific characteristics of the sea floor for each physiographic environment govern the actual patterns of sediment accumulation. Particularly noticeable are the feeding role of submarine canyons, the trapping effect of slope terraces and the isolation caused by ridges. Sediment accumulation induced by turbiditic flows is also observed north of the Almeria–Oran area. Excess 210Pb inventories and surface concentrations reveal a net effect of sediment focussing and point to the role of bottom nepheloid layers in supplying a significant fraction of sediments to the deep areas. This is also evidenced by comparison of bottom sediment with the sediment trap data available from the Malaga area. We estimate that the particle advective input below 30 m above the bottom accounts for as much as 50–70% of the material that is ultimately deposited onto the sea floor. The greater abundance of lithogenic material in the Malaga zone (∼80%) reflects its larger input in the western Alboran Sea. Conversely, carbonate contents increase from less than 20% in this area to about 30% in the Almeria–Oran zone, reflecting the lesser importance of the dilution by lithogenic components. Biogenic silica was only detected in some surface samples, and no significant preservation was observed below the Eh boundary. Abundance and accumulation rates of organic matter are in accordance with the spatial patterns of primary production in the Alboran Sea: higher in the western part, due to the presence of the Western Alboran Gyre. However, near-bottom redistribution leads to the homogenisation of organic matter concentrations in bottom sediments of any given area. Finally, from comparisons with sediment trap data, the degradation of organic matter has been estimated to be approximately 30–40% of what reaches the bottom.

Enhanced carbon export to the abyssal depths driven by atmosphere dynamics

Long-term biogeochemical observations are critical to understand the natural ability of the oceans to fix CO2 into organic carbon and export it to the deep as sinking particles. Here we present results from a 3 year (2010–2013) sediment trap deployment that allowed detecting interannual variations of carbon fluxes beyond 4000 m depth in the Eastern Mediterranean Sea. Anomalous atmospheric conditions triggering strong heat losses in winter–spring 2012 resulted in convective mixing, nutrient uplifting, and a diatom-dominated bloom southeast of Crete. Phytoplankton growth, reinforced by the arrival of nutrients from airborne Etna volcano ash, was the highest in the last decade (satellite-derived Chl a concentrations up to 1.9 mg m−3). This situation caused carbon export to increase by 2 orders of magnitude (12.2 mg m−2 d−1) with respect to typical values, which demonstrates how pulses of sinking fresh phytodetritus linked to rare atmospheric processes can episodically impact one of the most oligotrophic environments in the world ocean.

Earthquake-Triggered Subaerial Landslides that Caused Large Scale Fjord Sediment Deformation: Combined Subaerial and Submarine Studies of the 2007 Aysén Fjord Event, Chile

On 21 April 2007 (Mw 6.2) an earthquake triggered more than 500 landslides near the epicenter along the Aysen fjord, Chile. One of the major failures occurred at the Punta Cola Valley involving a volume of 20.9 million cubic meters of rock. The main rockslide was followed by a rock/debris avalanche involving talus and glacio-fluvial deposits in the slope toe and valley floor that added a volume of 7.3 million cubic meters as entrained material. About half of the material involved in the rockslide-debris avalanche reached the shoreline and entered the fjord pushing deltaic deposits offshore while inducing a shoreline retreat of 100 m. The impact of the debris avalanche deformed an area of 7.6 km2 of the otherwise featureless and smooth sedimented fjord floor. The central part of the deformed area is currently deeper with respect to the undeformed floor, which suggests that between 1 and 10 m of sediment were eroded from an area of 1.85 km2 due to the direct impact of the avalanche. The combination of debris avalanche impact of this and other landslides, subaqueous failures and fjord floor deformation generated a series of displacement waves within the fjord with several meters to tens of meters high run-up along the shoreline.

Cold-Water coral distribution in an erosional environment: The strait of Gibraltar gateway

Publisher Summary The sill of the Strait of Gibraltar is the morphological, oceanographical, and ecological gateway between the Atlantic Ocean and the Mediterranean Sea for the post-Messinian crisis period. The seabed is composed of synorogenic Betic-Rif clayey flysch overlaid by Pliocene and/or Quaternary calcareous conglomerates and coral accumulations, as well as current transported sand and mud in the deepest parts. The Strait of Gibraltar is one of the busiest maritime zones in the world and is thus affected by invasive species, but the benthic community is poorly studied. Furthermore, the study zone is affected by benthic trawl fisheries on the shelf and near-shelf areas and by the laying of submarine cables. Overall, the naturalness of the study area is considered to be largely unmodified. Based on the Benthic Terrain model scheme, a morphological classification has been made of the bathymetric MBES data, indicating several morphological habitats in the area that can cause fragmentation in the main ecosystem. This classification is based on the rugosity, slope, curvature, depth, bathymetric positioning index (BPI) with annulus neighborhood of 1,125, 300, 120, and 60 m, and the range of standard deviation of the depth over a distance of 45 m based on a bathymetric grid of 15 m. The predicted textural distribution map is based on the rugosity, slope, and objectively classified morphological zones identified in combination with textural information of the sampled stations used in a maximum-likelihood statistic algorithm provided by ArcGIS. Coral distribution is based on grab samples and the layers of the derived morphological grids.

Cold-Water Coral Colonization of Alboran Sea Knolls, Western Mediterranean Sea

Publisher Summary The Alboran Sea is the westernmost basin of the Mediterranean Sea and represents a basin 350 km long and 150 km wide, with water depths between 0 and 2,000 m. The Alboran Sea is characterized by highly dynamic and variable water masses that make it one of the most productive areas in the oligotrophic Mediterranean Sea. Hydrodynamic features occur across all time and length scales, such as tidal motion, strong baroclinic jets, large-scale gyres, mesoscale eddies, upwelling regions, and frontal zones, all with important implications on the dynamics of plankton and benthic ecosystems [3]. The strong surface inflow of Atlantic water through the Strait of Gibraltar, known as the Atlantic Jet, maintains two semipermanent anticyclonic gyres consisting of a mixture of different proportions of Mediterranean and Atlantic waters that change in sympathy with tidal cycles. The knolls in the Alboran Sea are affected by benthic trawl fisheries, evidenced by trawl marks and lost fishing gear observed in the study area. Overall, the naturalness of the study area is considered to be modified. The topography of the Alboran Basin seafloor is characterized by pinnacles, knolls, banks, ridges, and troughs as a direct expression of the Pliocene-Quaternary compressive tectonic regime. Colonization in the past and present does not correspond to any particular side of the knoll flanks; hence, it appears there is no eznhanced source of nutrients nor any increased pressure related to sedimentation from any particular direction in the study area. Upscaling from detailed habitat maps of knolls to basin-wide predictive habitat maps reveals various potential CWC habitats that are not yet surveyed and might lead to new discoveries of relict and living CWC ecosystems in the basin.