Research papers

The current filters are: Starting year = 2018, Ending year = 2019
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Ercilla G., Juana C., Periáñez R., Alonso B., Abril J.M., Estrada F., Casas D., Vázquez J.T., d’Acremont E., Gorini C., El Moumni B., Do Couto D., Valencia J. (2019)
Deep-Sea Research. Part I: Oceanographic Research Papers, 144, 1-16. DOI: 10.1016/j.dsr.2018.12.002. (BibTeX: ercilla.etal.2019a)
Abstract: See
This is an interdisciplinary study that combines morphoseismics, sedimentology and numerical modelling to elucidate at different scales of resolution the influence of alongslope processes on the turbidite systems (TSs) and canyons in the Alboran Sea (southwestern Mediterranean). Nine TSs are mapped in the Spanish margin (La Linea, Guadiaro, Baños, Torrenueva, Fuengirola, Salobreña, Sacratif, Calahonda and Almeria) and two in the Alboran Ridge (Piedra Escuela and Al-Borani). In the Moroccan margin, there are only two canyons (Ceuta and Nekor). Distinctive morphoseismic and sedimentological signatures from TSs and canyons have enabled three regional models of alongslope influence to be distinguished: a) Alongslope processes are dominant. This scenario characterizes the canyons of the Moroccan margin. The diagnostic signature is the lack of leveed channels and lobes at the Ceuta and Nekor Canyon mouths. b) Different degrees of interplay exist between alongslope and downslope processes. This scenario occurs in the TSs of the western Spanish margin. Here, the alongslope influence on TSs (La Linea, Guadiaro, Baños, Torrenueva and Fuengirola) is evidenced by the lack of overbank deposits in the La Linea and Guadiaro Canyons and an alongslope trend in the morpho-architecture of the channelized lobes and in the textural distribution of canyon/channel deposits (mass-flow deposits and turbidites). Both signatures indicate sandier TSs as well as Bouma turbidite sequences lacking the finest levels towards the Strait of Gibraltar. Local intercalations of contourites are also present in the Guadiaro lobe deposits. c) Downslope processes are dominant. This scenario characterizes the TSs of the eastern and central Spanish margin and Alboran Ridge. There, TSs seem to be controlled solely by the characteristics of the downslope gravity flows that transport sediment. The hydrodynamic and sediment dispersion models confirm that the main oceanographic factors governing the variable alongslope influence in TSs and canyons are the following: a vigorous WMDW flow along the Moroccan margin and the energetic Atlantic Jet, western Atlantic anticyclonic gyre and general acceleration of the Mediterranean waters towards the Strait of Gibraltar, along the western Spanish margin. This study demonstrates the pivotal role that alongslope processes can play in the onset and formation of TSs and canyons at continental margins.
Keywords: Turbidite system; Morphoseismic; Sediments; Alongslope processes; Downslope processes; Alboran Sea
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Ercilla G., Schwenk T., Bozzano G., Spiess V., Violante R., Estrada F., Ianniccheri F., Spoltore D.V., Alonso B. (2019)
Marine Geology, 417, 106028. DOI: 10.1016/j.margeo.2019.106028. (BibTeX: ercilla.etal.2019c)
Abstract: See
New insights into the Cenozoic depositional architecture based on the seismic stratigraphy of the northern Argentine continental slope, off Bahia Blanca, allow us to reconstruct its sedimentary evolution. Five major seismic boundaries, B1 to B5 (from oldest to youngest) represent the main discontinuities bounding six seismic units: I (Eocene), II (Oligocene to early Miocene), III (middle Miocene), IV (late Miocene), V (Pliocene), and VI (Quaternary). These units comprise deposits and features (mass-flow deposits, (hemi)pelagites and/or low-density turbidites, contourite drifts and moats, sediment waves, and canyon deposits), of which, through time, the contourites are ubiquitous. The depositional stratigraphic architecture reflects the expansion and relocation of the contourites, which are used to divide the sedimentation history into three major periods: Eocene to Early Miocene; Middle Miocene; and Late Miocene to Recent. Each period is interpreted as having occurred in response to palaeoceanographic changes in the Southern Component Deep Water (SCDW). These changes were regulated by palaeogeodynamic variations, regionally related to the Andean orogeny and remotely related to the opening stages of the Drake Passage and Central American Seaway, as well as the consequent increases in deep-water flux to the southern Atlantic. Additionally, we also consider their interplay with local and global sea-level changes. These three main periods reveal changes in the alongslope dynamics of the SCDW on the Argentine continental slope.
Keywords: Seismic stratigraphy; Contourites; Cenozoic sedimentary history; Argentine continental margin; Ewing Terrace
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Evans R.L., Benoit M.H., Long M.D., Elsenbeck J., Ford H.A., Zhu J., Garcia X. (2019)
Earth and Planetary Science Letters, 519, 308-316. DOI: 10.1016/j.epsl.2019.04.046. (BibTeX: evans.etal.2019b)
Abstract: See
A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of which can facilitate melting. Instead, we suggest that small scale mantle convection related either to pre-existing lithospheric thickness variations, or to lithospheric loss through delamination, coupled with relative plate motion with respect to the underlying asthenosphere, can trigger small amounts of melting. This melt migrates upslope, along the base of the lithosphere, potentially thermally eroding the lithosphere resulting in further thinning
Keywords: Magnetotellurics; Seismic; Receiver function; Lithosphere
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Garcia X., Julià J., Nemocón A.M., Neukirch M. (2019)
Gondwana Research, 68, 174-184. DOI: 10.1016/j.gr.2018.11.013. (BibTeX: garcia.etal.2019b)
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The lithospheric architecture of the western Borborema Province and northern São Francisco craton of NE Brazil has been investigated through analysis of long-period magnetotelluric data acquired along a 700 km long survey, using 12 instruments. The survey samples several tectonic terrains in the Province and penetrates into the adjacent São Francisco craton after crossing the Araripe Basin, an aborted rift basin filled with Mesozoic sediments that peak at ∼1000 m above mean sea level. High conductivities are observed at shallow depths under the main Precambrian shear zones that pervade the Province – consistent with tectonic reactivation – and as a small patch embedded within the high resistivities that characterize the São Francisco craton. High conductivities (∼25 Ωm) are also observed below 120 km depth between the Patos and Pernambuco lineaments – right under the Araripe Basin – flanked by resistive (>120 Ωm) material immediately to the north and south. This deep, highly conductive body is found consistent with the presence of melt and aqueous fluids, and is interpreted as shallow asthenospheric mantle bounded by thicker lithosphere. We propose that extensional stresses in the Mesozoic stretched and thinned the lithosphere under the Araripe Basin, causing passive upwelling of asthenospheric material and lateral flow of the overlying lithosphere, and resulting in thickening of the lithosphere under the flanks and uplift of the Araripe Basin. We also hypothesize that thermal weakening of the lithospheric mantle – perhaps sustained by channeling of asthenospheric flows under the basin – would have caused regional stresses to concentrate in the brittle upper crust and contribute to basin inversion. We thus propose that a combination of localized horizontal stresses and vertical buoyancy from underlying asthenospheric material are ultimately responsible for the actual topography of the Araripe Basin.
Keywords: Araripe Basin; Borborema Province; Magnetotellurics; Conductivity; Lithosphere; Asthenosphere; Basin inversion; Partial melt
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Isla E., Gerdes D. (2019)
Progress in Oceanography, 178, 102180. DOI: 10.1016/j.pocean.2019.102180. (BibTeX: isla.gerdes.2019a)
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Global warming is heating the Antarctic circumpolar deep water (CDW), which comes into direct contact with the diverse and abundant macrobenthic communities thriving on the continental shelf of the Weddell Sea (WS). A set of 16 current meters deployed along more than 3000 km coastline revealed that tidal currents drive CDW intrusions onto the WS continental shelf and they can increase the temperature near the seabed by ~2.7 °C. The ongoing ocean warming trend may expose macrobenthic assemblages to ambient temperatures >2 °C by the end of the century with dramatic consequences for communities which have evolved during millions of years in near geophysical isolation under rather constant environmental conditions with temperatures <0 °C. These stenothermal communities have long generation times (therefore, reduced opportunity to mutate) and require hundreds of years for adaptation. Results from 135 benthic stations along the study area showed that macrobenthic communities in the southeastern section of the WS are the most vulnerable to the increase of temperature near the seabed given their high component of sessile organisms. Besides a dramatic marine biodiversity loss, the eventual demise of these communities, which provide habitat structure for a large number of species that can build up >87 g C m−2, will cause the liberation of thousands of tons of carbon to the environment. Macrobenthic communities colonizing the recently opened shelf in the Larsen A and B bays may not have the chance to reach the type of mature assemblage inhabiting the eastern WS shelf. The highest temperatures derived from CDW intrusions were recorded in the Filchner-Ronne region, suggesting that the consequences of the thermal impact could develop faster here than in the rest of the WS. Thus, these macrobenthic communities may show the effects of warming earlier than those thriving in other regions of the WS shelf. Global warming seriously threats the abundant and highly diverse macrobenthic communities of the Antarctic continental shelf
Keywords: Continental shelf; Antarctica; Global warming; Tides; Climate change; Benthic ecosystem; Weddell Sea
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Liu S., Van Rooij D., Vandorpe T., González-Pola C., Ercilla G., Hernández-Molina F.J. (2019)
Deep-Sea Research. Part I: Oceanographic Research Papers, 149, 103054. DOI: 10.1016/j.dsr.2019.05.014. (BibTeX: liu.etal.2019a)
Abstract: See
The 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 basins.
Keywords: Bottom currents; Contourites; Southern Bay of Biscay; Small basin
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Maldonado M., López-Acosta M., Sitjà C., , Galobart C., Ercilla G., Leynaert A. (2019)
Nature Geoscience, 12, 815-822. DOI: 10.1038/s41561-019-0430-7. (BibTeX: maldonado.etal.2019b)
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Silicon (Si) is a pivotal element in the biogeochemical and ecological functioning of the ocean. The marine Si cycle is thought to be in internal equilibrium, but the recent discovery of Si entries through groundwater and glacial melting have increased the known Si inputs relative to the outputs in the global oceans. Known outputs are due to the burying of diatom skeletons or their conversion into authigenic clay by reverse weathering. Here we show that non-phototrophic organisms, such as sponges and radiolarians, also facilitate significant Si burial through their siliceous skeletons. Microscopic examination and digestion of sediments revealed that most burial occurs through sponge skeletons, which, being unusually resistant to dissolution, had passed unnoticed in the biogeochemical inventories of sediments. The preservation of sponge spicules in sediments was 45.2 ± 27.4%, but only 6.8 ± 10.1% for radiolarian testa and 8% for diatom frustules. Sponges lead to a global burial flux of 1.71 ± 1.61 TmolSi yr−1 and only 0.09 ± 0.05 TmolSi yr−1 occurs through radiolarians. Collectively, these two non-phototrophically produced silicas increase the Si output of the ocean to 12.8 TmolSi yr−1, which accounts for a previously ignored sink that is necessary to adequately assess the global balance of the marine Si cycle.
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Neukirch M., Rudolf D., Garcia X., Galiana S. (2019)
Geophysics, 84, 5, B299-V302. DOI: 10.1190/geo2018-0352.1. (BibTeX: neukirch.etal.2019b)
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The introduction of the Phase Tensor marked a breakthrough in the understanding and analysis of electric galvanic distortion effects. It has been used for (distortion free) dimensionality analysis, distortion analysis, mapping and subsurface model inversion. However, the Phase Tensor can only represent half of the information contained in a complete impedance data set. Nevertheless, to avoid uncertainty due to galvanic distortion effects, practitioners often choose to discard half of the measured data and concentrate interpretation efforts on the Phase Tensor part. This work assesses the information loss due to pure Phase Tensor interpretation of a complete impedance data set. To achieve this, a new MT impedance tensor decomposition into the known Phase Tensor and a newly defined Amplitude Tensor is motivated and established. Additionally, the existence and uniqueness of the Amplitude Tensor is proven. Synthetic data is used to illustrate the Amplitude Tensor information content compared to the Phase Tensor. While the Phase Tensor only describes the inductive effects within the subsurface, the Amplitude Tensor holds information about inductive and galvanic effects that can help to identify conductivity or thickness of (conductive) anomalies more accurately than the Phase Tensor. Furthermore, the Amplitude and Phase Tensors sense anomalies at different periods and thus the combination of both provides means to evaluate and differentiate anomaly top depths in the event of data unavailability at extended period ranges, e.g. due to severe noise
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O’Reilly B.M., Prada M., Lavoué F., Lebedev S. (2019)
Geophysical Journal International, 219, 2, 1421-1430. DOI: 10.1093/gji/ggz378. (BibTeX: oreilly.etal.2019a)
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Gravitational compaction of thick (2–10 km) sediment accumulations in sedimentary basins is controlled by the interplay of mechanical and chemical processes that operate over many orders of magnitude in spatial scale. The compaction of sediments into rock typically involves a density increase of ≈500 to 1000 kg m−3, occurring over a depth-scale of several kilometres. The volume decrease in the compacting sediments releases vast volumes of water, which plays an important part in the global hydrological cycle and also in tectonic and geochemical processes; including the formation of hydrocarbon and mineral deposits. This study utilizes recently developed tomographic seismic images from the Porcupine Basin, which lies in the deep-water North Atlantic Ocean. A generic method for predicting fluid pressure variations that are driven by gravitational compaction is developed over the scale of the entire sedimentary basin. The methodology is grounded upon both observational evidence and empirically based theories, relying on geophysical measurements and relationships between sediment porosities and densities. The method is based upon physical concepts that are widely used in the petroleum industry and applied extensively in models of overpressure development in sedimentary basins. Geological and geophysical data from exploration wells are used to test the predictions of the method at two locations within the basin and are found to be in good agreement with the theory.
Keywords: Permeability and porosity; Europe; Seismic tomography; Sedimentary basin processes
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Ortuño M., Corominas O., Villamor P., Zúñiga R.F., Lacan P., Aguirre-Díaz G., Perea H., Štěpančíková P., Ramírez-Herrera M.T. (2019)
Geomorphology, 326, 17-37. DOI: 10.1016/j.geomorph.2018.07.010. (BibTeX: ortuno.etal.2019)
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The Acambay Graben, within the central part of the Trans-Mexican Volcanic Belt, is one of the major sources of continental earthquakes in Mexico. To date, the activity and paleoseismological history of the axial faults of the graben are not well constrained. We provide morphological, structural and sedimentological evidence of the seismogenic nature of two of the axial structures, the Temascalcingo and the Tepuxtepec fault systems. Faults consist of multiple parallel scarps with en echelon and horse-splay patterns. Fault systems extend for 60 km and displace Quaternary to Upper Miocene volcanic edifices and volcano-sedimentary materials. Surface lengths of individual fault traces range between 3 and 25 km, and observed throws reach a minimum of 150–200 m. The long-term and short-term slip rate of the Temascalcingo fault system in the studied section presents similar values, ranging from 0.06 ± 0.02 (minimum long term) to 0.12 ± 0.02 mm y−1 (maximum value of average short-term). Only the long-term slip rate of the Tepuxtepec system could be constrained in 0.01–0.02 mm/y, being a minimum estimate. The Holocene fault rupture history at two sites provided evidence of six ruptures since 12,500–11,195 BCE, among which three ruptures should have occurred between 11,847 ± 652 BCE and 11,425 ± 465 BCE Variable single event displacements (SEDs, between 6 and 77) are interpreted as the result of fault interdependences and/or the interaction with the latest volcanic activity. Also, small displacements triggered by activity on other faults probably contributed to slip variability, i.e., faults display primary and secondary behavior
Keywords: Temascalcingo fault system; Tepuxtepec fault system; Paleoseismology; Fault complexity
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Ramón M., Simarro G., Galimany E., Lleonart J. (2019)
Regional Studies in Marine Science, 31, 100763. DOI: 10.1016/j.rsma.2019.100763. (BibTeX: ramon.etal.2019b)
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Parastichopus regalis is an epibenthic holothurian common in the Mediterranean Sea and the NE Atlantic, which feeds on the upper layer of the sediment playing a significant role on soft-bottom dynamics. Whether or not P. regalis is able to select the sediment ingested by size is the question of this study. For this purpose, a comparison between grain size distributions of the seabed sediments and the digestive contents of sea cucumbers were carried out. We performed the comparisons among sediment distributions through the median diameter and the granulometric dispersion . The results showed that the size of the sediment within the holothurians was significantly smaller and more uniform than the ones in the seabed. Evidence showed that P. regalis select sediment by particle size during feeding, choosing the smaller particles. This finding reports novel information on the feeding behavior of this species, a fishery resource of local interest and importance in the Western Mediterranean region
Keywords: Holothurian; Feeding; Grain size selection; Mediterranean Sea
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Salvadó J., Grimalt J.O., López J.F., Palanques A., Canals M. (2019)
Science of The Total Environment, 647, 597-605. DOI: 10.1016/j.scitotenv.2018.07.458. (BibTeX: salvado.etal.2019b)
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The significance of the offshore vertical convection currents in the transport and sinking of water-soluble organic pollutants into marine deep basins has been evaluated. For this purpose, sediment cores were collected in the Gulf of Lion (GoL) at sites between 26 and 2330 m water depth. The top core layers were analyzed for aromatic and aliphatic hydrocarbons and organochlorine compounds. Organic compounds with logKAW (air water partition coefficient) between −2 and −4, e.g. lindane, PCB 28, PCB 52, phenanthrene, methylphenanthrenes, dimethylphenanthrenes, C14–C23n-alkanes, are found in higher concentrations or exhibit relative concentration increases in the sediments deposited in the continental rise as consequence of the open-sea convection processes associated with the formation of Western Mediterranean Deep Water (WMDW). In contrast, the organic pollutants with intermediate air-water distribution coefficients, logKAW between −2 and 0, and high octanol water distribution coefficients (logKow > 6), e.g. highly chlorinated PCBs, DDTs, DDEs, DDDs, C25–C35n-alkanes, and polycyclic aromatic hydrocarbons with molecular weight higher than 200, occur in association to sediment particles, which are mainly transported by the Northern current along the continental shelf forming the mud belt. The Rhône prodelta is therefore the area of the GoL showing the highest concentrations of this group of organic compounds, which are preferentially associated with water particles. Overall, the results show that vertical open-sea convection processes related with offshore formation of WMDW may have an important role in the transport and accumulation of water soluble pollutants to deep marine environments of the GoL (>2000 m water depth).
Keywords: Organochlorine compounds; Aliphatic hydrocarbons; Polycyclic aromatic hydrocarbons; Marine sediments; Western Mediterranean; Deep water formation
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Teixeira M., Terrinha P., Roque C., Rosa M., Ercilla G., Casas D. (2019)
Marine Geology, 410, 88-108. DOI: 10.1016/j.margeo.2018.12.011. (BibTeX: teixeira.etal.2019b)
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An integrated analysis of multibeam bathymetry and single- and multichannel seismic records were used to image the morpho-stratigraphy of the Alentejo Margin (Southwest Portuguese Continental Margin). The complex interaction of several alongslope and downslope processes in the area leads to the formation of various bottom current driven depositional and erosive features (moat) as well as gravity-driven features (gullies and landslides) in the Alentejo Margin. These processes feed each other contributing for the active sedimentary pattern on the area. Sines Contourite Drift (SCD) is a ~2311 km2 sedimentary feature with a perimeter of 303.9 km, 98 km length and 35 km width. Landslides occur either on steep and on gentle slopes between ~200 and 3200 mwd (meters water depth) in the study area, which has a total extent of ~85 km × 82 km. Scar concentration is higher in the middle- and lower-slope, whereas the thickest debris deposits are found in the slope basin area - Lebre Basin (LB). Translational landslides, with planar failure planes parallel to the slope surface are the main landslide typology verified in the study area. Several triggering and pre-conditioning factors, resulting from the interaction of alongslope and downslope processes, contribute for varied landslides scar concentration. Steep-slope and high sedimentation rates favour sediments under-consolidation, promoting excess pore water pressure and weak layers formation. In this paper, we show that the interaction of bottom currents with pre-existing tectonic structures promotes the complex interaction of both alongslope and downslope processes thus promoting an active and diversified geomorphological evolution and generalised slope instability.
Keywords: Sedimentary processes; Morphosedimentary features; Contourite drift; Bottom currents; Slope instability; Submarine landslides; Triggering and pre-conditioning factors
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Ugalde A., Gaite B., Ruiz M., Villaseñor A., Ranero C.R. (2019)
Seismological Research Letters, 90, 4, 1565-1576. DOI: 10.1785/0220180353. (BibTeX: ugalde.etal.2019a)
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In November 2014, a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (eastern Canary Islands). This region is characterized by low‐seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument‐seafloor system generated by bottom water currents in the area, was commonly detected on the ocean‐bottom seismometer (OBS) recordings. The marine network data also revealed dozens of nonseismic short‐duration signals per day that appear similar to other events on OBS recordings throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.
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Brackenridge R.E., Stow D.A.V., Hernández‐Molina F.J., Jones C., Mena A., Alejo I., Ducassou E., Llave E., Ercilla G., Nombela M.A. (2018)
Sedimentology, 65, 7, 2223-2252. DOI: 10.1111/sed.12463. (BibTeX: brackenridge.etal.2018a)
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This work presents a detailed study of CONTOURIBER and Integrated Ocean Drilling Program 339 sediment data targeting sand‐rich contourites in the Eastern Gulf of Cadiz. All of the collected sediments are interpreted as contourites (deposited or reworked by bottom currents) on the basis of oceanographic setting, seismic and morphometric features, and facies characteristics. A variety of sandy and associated facies are found across the study area including: (i) bioturbated muddy contourites; (ii) mottled silty contourites; (iii) very fine mottled and fine‐grained bioturbated sandy contourites; (iv) massive and laminated sandy contourites; and (v) coarse sandy/gravel contourites. The thickest sands occur within contourite channels and there is a marked reduction in sand content laterally away from channels. Complementary to the facies descriptions, grain‐size analysis of 675 samples reveals distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (<20 μm) show normal grain‐size distributions, poor to very poor sorting, and zero or low skewness. These are deposited by settling from weak bottom currents with a fine suspension load. Muddy to fine sandy contourites (20 to 200 μm) trend towards better sorting and initially finer and then coarser skew. These are typical depositional trends for contourites. As current velocity and carrying capacity increase, more of the finest fraction remains in suspension and bedload transport becomes more important. Clean sandy contourites (>200 μm) are better sorted. They result from the action of dominant bedload transport and winnowing at high current speeds. The results highlight the importance of bottom current velocity, sediment supply and bioturbational mixing in controlling contourite facies. Despite growing interest in their hydrocarbon exploration potential, contourite sands have remained poorly understood. This research therefore has important implications for developing current understanding of these deposits and aiding the correct interpretation of deep marine sands and depositional processes.
Keywords: Contourites; Deep-water sands; Grain size; Gulf of Cadiz; Sediment facies model
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Casas D., Pimentel A., Pacheco J., Martorelli E., Sposato A., Ercilla G., Alonso B., Chiocci F. (2018)
Journal of Volcanology and Geothermal Research, 356, 127-140. DOI: 10.1016/j.jvolgeores.2018.02.017. (BibTeX: casas.etal.2018a)
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High-resolution bathymetric data and seafloor sampling were used to characterize the most recent volcanic eruption in the Azores region, the 1998–2001 Serreta submarine eruption. The vent of the eruption is proposed to be an asymmetric topographic high, composed of two coalescing volcanic cones, underlying the location where lava balloons had been observed at the sea surface during the eruption. The volcanic products related to the 1998–2001 eruption are constrained to an area of ~0.5 km2 around the proposed vent position. A submarine Strombolian-style eruption producing basaltic lava balloons, ash and coarse scoriaceous materials with limited lateral dispersion led to the buildup of the cones. The 1998–2001 Serreta eruption shares many similarities with other intermediate-depth lava balloon-forming eruptions (e.g., the 1891 eruption offshore Pantelleria and the 2011–2012 eruption south of El Hierro), revealing the particular conditions needed for the production of this unusual and scarcely documented volcanic product.
Keywords: Submarine volcanism; Lava balloons; Multibeam bathymetry; Terceira; Serreta Ridge; Cone formation
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Chen C., Watremez L., Prada M., A. Minshull T., A. Edwards R., M. O\'Reilly B., J. Reston T., Wagner G., Gaw V., Klaeschen D., M. Shannon P. (2018)
Journal of Geophysical Research: Solid Earth, 123, 10, 8312-8330. DOI: 0.1029/2018JB016375. (BibTeX: chen.etal.2018)
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The deep structure and sedimentary record of rift basins provide an important insight into understanding the geological processes involved in lithospheric extension. We investigate the crustal structure and large‐scale sedimentary architecture of the southern Porcupine Basin, offshore Ireland along three wide‐angle seismic profiles, supplemented by 13 selected seismic reflection profiles. The seismic velocity and crustal geometry models obtained by joint refraction and reflection traveltime inversion clearly image the deep structure of the basin. Our results suggest the presence of three distinct crustal domains along the rifting axis: (a) continental crust becoming progressively hyperextended from north to south through the basin, (b) a transitional zone of uncertain nature, and (c) a 7‐ to 8‐km‐thick zone of oceanic crust. The latter is overlain by an ~8‐km compacted Upper Paleozoic‐Mesozoic succession and ~ 2 km of Cenozoic strata. Due to the lack of clear magnetic anomalies and in the absence of well control, the precise age of interpreted oceanic crust is unknown. However, we can determine an age range of Late Jurassic to Late Cretaceous from the regional context. We propose a northward propagating rifting process in the Porcupine Basin, resulting in variations in strain along the rift axis.
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Druet M., Muñoz‐Martín A., Granja‐Bruña J.L., Carbó‐Gorosabel A., Acosta J., Llanes P., Ercilla G. (2018)
Tectonics, 37, 5, 1576-1604. DOI: 10.1029/2017TC004903. (BibTeX: druet.etal.2018c)
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The magma‐poor rifted continental margin of Galicia has an extremely complex structure. Its formation involved several rifting episodes that occurred ultimately during the early Cretaceous near a ridge triple junction, which produced a change in the orientation of the main structures in its transition to the north Iberia margin. In addition, there is a superimposed partial tectonic inversion along its northwest and northern border which developed from the Late Cretaceous to at least Oligocene times. The present study integrates a large volume of new geophysical information (mainly marine gravity data and 2D seismic reflection profiles) to provide insights on the formation of this rift system and on the development of its later inversion. The combined interpretation and modeling of this data enable the presentation of a new crustal and structural domains map for the whole Galicia margin. This includes the rift domains related to the extreme thinning of the crust and the lithospheric mantle (stretched, necking, and hyperextension and mantle exhumation (HME) domains), as well as a domain of intense compressional deformation. New constraints arise on the origin, the deep structure, and the characterization of the along‐ and across‐strike variation of the continent‐ocean transition of the margin, where a progressive change from hyperextension to partial inversion is observed. The development of both rifting and later partial tectonic inversion is influenced by the existence of former first‐order tectonic features. Most of the tectonic inversion is focused on the HME domain, which in some areas of the northwestern margin is completely overprinted by compressional deformation
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Estrada F., Galindo‐Zaldívar J., Vázquez J.T., Ercilla G., D\'Acremont E., Alonso B., Gorini C. (2018)
Terra nova, 30, 1, 24-33. DOI: 10.1111/ter.12304. (BibTeX: estrada.etal.2018a)
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The Alboran Sea constitutes a Neogene-Quaternary basin of the Betic-Rif Cordillera, which has been deformed since the Late Miocene during the collision between the Eurasian and African plates in the westernmost Mediterranean. NNE-SSW sinistral and WNW-ESE dextral conjugate fault sets forming a 75° angle surround a rigid basement spur of the African plate, and are the origin of most of the shallow seismicity of the central Alboran Sea. Northward, the faults decrease their transcurrent slip, becoming normal close to the tip point, while NNW-SSE normal and sparse ENE-WSW reverse to transcurrent faults are developed. The uplifting of the Alboran Ridge ENE-WSW antiform above a detachment level was favoured by the crustal layering. Despite the recent anticlockwise rotation of the Eurasian-African convergence trend in the westernmost Mediterranean, these recent deformations -consistent with indenter tectonics characterised by a N164°E trend of maximum compression- entail the highest seismic hazard of the Alboran Sea.
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Galindo‐Zaldivar J., Ercilla G., Estrada F., Catalán M., d\'Acremont E., Azzouz O., Casas D., Chourak M., Vazquez J.T., Chalouan A., Sanz de Galdeano C., Benmakhlouf M., Gorini C., Alonso B., Palomino D., Rengel J.A., Gil A.J. (2018)
Tectonics, 37, 8, 2513-2530. DOI: 10.1029/2017TC004941. (BibTeX: galindozaldivar.etal.2018c)
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The Eurasian‐African NW‐SE oblique plate convergence produces shortening and orthogonal extension in the Alboran Sea Basin (westernmost Mediterranean), located between the Betic and Rif Cordilleras. A NNE‐SSW broadband of deformation and seismicity affects the Alboran central part. After the 1993–1994 and 2004 seismic series, an earthquake sequence struck mainly its southern sector in 2016–2017 (main event Mw = 6.3, 25 January 2016). The near‐surface deformation is investigated using seismic profiles, multibeam bathymetry, gravity and seismicity data. Epicenters can be grouped into two main alignments. The northern WSW‐ENE alignment has reverse earthquake focal mechanisms, and in its epicentral region recent mass transport deposits occur. The southern alignment consists of a NNE‐SSW vertical sinistral deformation zone, with early epicenters of higher‐magnitude earthquakes located along a narrow band 5 to 10‐km offset westward of the Al Idrisi Fault. Here near‐surface deformation includes active NW‐SE vertical and normal faults, unmapped until now. Later, epicenters spread eastward, reaching the Al Idrisi Fault, characterized by discontinuous active NNE‐SSW vertical fractures. Seismicity and tectonic structures suggest a westward propagation of deformation and the growth at depth of incipient faults, comprising a NNE‐SSW sinistral fault zone in depth that is connected upward with NW‐SE vertical and normal faults. This recent fault zone is segmented and responsible for the seismicity in 1993–1994 in the coastal area, in 2004 onshore, and in 2016–2017 offshore. Insights for seismic hazard assessment point to the growth of recent faults that could produce potentially higher magnitude earthquakes than the already formed faults.
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Guerrero Q., Guillén J., Durán R., Urgeles R. (2018)
Marine Geology, 395, 219-233. DOI: 10.1016/j.margeo.2017.10.002. (BibTeX: guerrero.etal.2018a)
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A sand ridge field located over a retreating deltaic lobe in the Ebro Delta (NW Mediterranean) is characterized using three sets of co-located multibeam bathymetric data acquired in 2004, 2013 and 2015, measurements of near-bottom currents, high-resolution seismic profiles, and aerial photographs. The aim of this study is to illustrate the processes and timing involved in the initial stages of sand ridge development. The sand ridge field extends from the 5 to 15 m isobaths and the ridges have maximum heights and wavelengths of 2.5 and 400 m, respectively; they have straight crests that are arranged obliquely to the shoreline and are composed of fine sand. In general, the sand ridges are symmetric, although asymmetries with the lee side of the ridge facing to the SE and to the NW are also present. The genesis of the sand ridge field is closely related to the contemporary evolution of the Ebro River mouth. The change of the main river channel in the 1940s led to progressive abandonment of the former river mouth and to severe coastal retreatment (~ 37 m·yr− 1), making large amounts of sediment available for ridge formation. The persistent Mistral winds (NW) induce near-bottom currents flowing towards the SE, which are able to rework and transport sandy sediments. The sand ridges are currently active, with mean SE migration rates of ~ 10 m·yr− 1 most likely in pulses, when high-speed currents occur. Wave-storm events induce reverse flows (E-SE), which reshape the ridges to symmetric or opposite asymmetry geometries. The bedform morphologies, the oblique arrangement with respect to the shoreline, the angle between the strongest current and the crestlines and the sediment grain size match well with those of shoreface-connected (attached) sand ridges and, particularly, with the initial stages of sand ridge development on storm-dominated continental shelves. Sediment availability, shoreline retreat, relatively strong near-bottom currents induced by winds, seafloor irregularities and relative sea-level rise on the Ebro Delta combine to provide a suitable transgressive environment for sand ridge development. Time-scales related to sand ridges are usually of hundreds/thousands of years, but here it is demonstrated that the genesis of sand ridges can take place within a few decades. The studied bedforms are unlikely to persist in the absence of rapid sea-level rise, leading to sediment scarcity and wave and current reworking.
Keywords: Ebro Delta, Coastal erosion, Shelf processes, Shoreface connected sand ridge, Morphodynamics, Bedform
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Isla E., Pérez-Albaladejo E., Porte C. (2018)
Scientific Reports, 8, 9154. DOI: 10.1038/s41598-018-27375-4. (BibTeX: isla.etal.2018a)
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Industrial activity generates harmful substances which can travel via aerial or water currents thousands of kilometers away from the place they were used impacting the local biota where they deposit. The presence of harmful anthropogenic substances in the Antarctic is particularly surprising and striking due to its remoteness and the apparent geophysical isolation developed with the flows of the Antarctic Circumpolar current and the ring of westerly winds surrounding the continent. However, long-range atmospheric transport (LRAT) of pollutants has been detected in the Antarctic since the 70’s along the Antarctic trophic food web from phytoplankton to birds. Still, no information exists on the presence of cytotoxic compounds in marine sediments neither at basin scales (thousands of kilometers) nor in water depths (hundreds of meters) beyond shallow coastal areas near research stations. Our results showed for the first time that there is cytotoxic activity in marine sediment extracts from water depths >1000 m and along thousands of kilometers of Antarctic continental shelf, in some cases comparable to that observed in Mediterranean areas. Ongoing anthropogenic pressure appears as a serious threat to the sessile benthic communities, which have evolved in near isolation for millions of years in these environments.
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Isla E., DeMaster D.J. (2018)
Geochimica et cosmochimica acta, 242, 34-50. DOI: 10.1016/j.gca.2018.08.011. (BibTeX: isla.demaster.2018a)
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Labile organic carbon (LOC) dynamics (i.e., of recently produced, planktonic material) and sediment dynamics were studied in the seabed using naturally occurring 14Corg and 210Pb measurements in the region where the Larsen Ice Shelves A and B were floating almost two decades ago. A non-steady-state diagenetic model was used to estimate sediment mixing coefficients as well as LOC fluxes to the seabed and LOC turnover times (i.e., mean residence times) in a suite of 14 sediment cores from the continental shelf, including a glacial trough. At four of the stations, cores were collected during 2007 and 2011 cruises, enabling a time-series approach for understanding the evolution of sedimentary processes and LOC dynamics in the deposits below a collapsed ice shelf. Sediment mixing coefficients, based on non-steady-state 210Pb profiles, varied between 0.01 cm2 y−1 and 1.6 cm2 y−1 in these post-ice shelf sediments. These values were similar to those found in polar deep-sea environments, where sedimentary conditions are less dynamic than in shallower provinces. LOC, whose abundance decreased uniformly with depth, was detected to depths ranging from 2 to 16 cm, with LOC seabed inventories varying from 1.5 to 22 mg LOC cm−2. Excess 210Pb and LOC fluxes were relatively uniform across the study area suggesting that similar particle fluxes have taken place within the Larsen system since the disintegration of the various ice shelves. The LOC mean residence time at the different stations varied from 3 y to >60 y. The 14Corg approach, calculating LOC content based on a two-end member model with planktonic 14Corg as the labile carbon end member, most closely correlated with the lipid content of the sediment, which has been considered one of the best descriptors of reactive organic matter readily available to benthic consumers. We suggest that the irregular combination of sea ice coverage, organic matter production and supply to the sea floor introduce scatter in the determination of sediment and LOC dynamics such that short-term temporal (<5 years) and spatial trends could not be readily resolved.
Keywords: Antarctica; Ice shelf collapse; Labile organic carbon; Biogeochemistry; Marine sediment; Larsen ice shelves; 14C; 210Pb
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Lafosse M., Gorini C., Le Roy P., Alonso B., d’Acremont E., Ercilla G., Rabineau M., Vázquez J.T., Rabaute A., Ammar A. (2018)
Marine and Petroleum Geology, 97, 370-389. DOI: 10.1016/j.marpetgeo.2018.07.022. (BibTeX: lafosse.etal.2018a)
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In active basins, tectonics can segment the continental shelf and control its stratigraphic architecture and physiography. Segmentation can explain the local evolution and morphology of the continental shelf because of sea-level variations, local tectonic segmentation and hydrodynamic processes. Here we investigate the tectonically active Morocco continental margin (southern Alboran Sea) using high-resolution seismic profiles and multibeam bathymetric data. The active faults bounding the transtensive Nekor basin triggered the segmentation of the shelf into three sectors showing different subsidence rates: a western sector corresponding to an extensive fault relay, a central sector corresponding to the subsiding Al-Hoceima Bay and an eastern sector corresponding to the footwall of the Trougout senestral normal fault. Results show that the staircase morphology of the shelf corresponds to successive submarine terraces at the shelf edge (ST1), mid-shelf (ST2) and inner-shelf (ST3) around −110 m,-80 m, and −40 to −20 m deep, respectively. The terraces correspond to the top of prograding wedges seaward and are erosive landward. They are correlated with stillstand from the Last Glacial Maximum to the Holocene highstand. Above the terraces, sub-aqueous dune fields are interpreted as degraded and deposited during the post-glacial transgression. In the central sector, typical delta front seafloor undulations on the shelf and crescent shaped bedforms at the head of marine incisions on the upper slope denote a fluvial influence during the Holocene. Seismic stratigraphy analysis revealed the preservation of six seismic units bounded by polygenic regional unconformities (S1, S2, S3, S4 and S5). Based on comparison with other Mediterranean margins, S1 to S5 are attributed to 4th order maximum regressive surface. We discuss the local preservation of the system tracts as a function of the vertical motion and the physiography of this tectonically active domain. This study provides useful clues for future local paleo-seismic analysis and to advance our understanding of sedimentary processes in active areas.
Keywords: Seismic stratigraphy; Active tectonic; Pleistocene; Continental shelf; Swath bathymetry; Geomorphology; Western Mediterranean
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Palanques A., Puig P. (2018)
Marine Geology, 406, 119-131. DOI: 10.1016/j.margeo.2018.09.010. (BibTeX: palanques.puig.2018a)
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The effects of deep dense shelf water cascading and open sea convection on the sediment dynamics of the northwestern Mediterranean basin were studied by near-bottom moored instruments recording trapped particle fluxes, suspended particle fluxes, water properties and hydrodynamics from November 2011 to July 2012. During this period, near-bottom currents induced by winter dense water formation generated benthic storms that caused resuspension at 2450 m water depth, increasing by more than one order of magnitude the ambient suspended sediment concentrations, the trapped particle fluxes and the suspended sediment fluxes. During the preconditioning phase of the open sea convection, from December 2011 to mid-February 2012, currents (1–10 cm s−1), suspended sediment concentrations (<0.1 mg l−1), Chl-a fluorescence values (<0.063 μg l−1), trapped total mass fluxes (10–50 mg m−2 d−1) and trapped organic carbon fluxes (1–4 mg m−2 d−1) were low, and organic matter was mainly undegraded and of marine origin. Open sea convection was observed at the study site in mid-February, at the beginning of the violent mixing phase, increasing current velocities up to 26 cm s−1 and Chl-a fluorescence values up to 0.074 μg l−1, supplying particles with fresh marine organic matter content. During the last fortnight of February, two major dense shelf water cascading pulses generated Chl-a fluorescence increases (up to 0.116 μg l−1) and large suspended sediment concentration peaks (up to19 mg l−1), suspended sediment fluxes (up to 6500 mg m−2 d−1) and trapped total mass flux increases (up to 22,900 mg m−2 d−1), which were associated with benthic storms resuspension. During this phase, trapped organic carbon flux increased almost two orders of magnitude (up to 260 mg m−2 d−1), with pulses of both marine and terrestrial organic matter. The sinking and spreading phase occurred from early March to mid-June. The signal of deep dense shelf water cascading lasted past early April, and the spreading of the newly formed dense water maintained maximum currents of up to 25 cm s−1 and trapped particle fluxes of up to 2000 mg m−2 d−1 until mid-June. At the beginning of this phase, organic matter was terrestrial and several turbidity peaks occurred during current speed increases generated by benthic storms. At the end of this phase, the organic matter became less terrestrial, trapped organic carbon fluxes decreased from about 190 to 10 mg m−2 d−1 and turbidity peaks occurred with low current velocities indicating the arrival of storm tails at the mooring site. The large particle fluxes of fresh or relatively undegraded organic carbon induced by deep dense water formation during winter 2012, contributed to the “fertilization” of the northwestern Mediterranean deep benthic ecosystems.
Keywords: Benthic storms; Open sea convection; Dense shelf water cascading; Deep sea; Sediment transport; Seafloor resuspension
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Purroy A., Najdek M., Isla E., Župan I., Thébault J., Peharda M. (2018)
Marine Environmental Research, 142, 234-249. DOI: 10.1016/j.marenvres.2018.10.011. (BibTeX: purroy.etal.2018a)
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The trophic ecology of two bivalves, the clam Callista chione and the cockle Glycymeris bimaculata was studied using environmental and biochemical variables of the suspended particulate matter and the sediment. Samples were collected from two shallow sites, Pag and Cetina, in the coastal oligotrophic Mediterranean Sea, during a 17 month period. The temporal variation of the particulate matter reflected a mixture between marine and terrestrial sources throughout the year, with a clear marine influence during summer and fall, and predominance of terrestrial inputs during spring and winter. The digestive gland was a useful rapid turnover tissue, where the carbon isotope signal was species-specific and the nitrogen isotope one was site-specific. FA markers in the digestive gland revealed a mixed diet where Callista chione fed more upon fresh material than G. bimaculata which relied largely on bacteria-derived detritus. Overall, little feeding niche overlap was observed between the two species during the year, indicating resource partitioning, expected for a food-limited system. The present trophic ecology study in co-occurring species allowed identifying species-specific feeding adaptations to environmental variability.
Keywords: Feeding ecology; Bivalves; Suspension feeder; Biochemical variables; Stable isotopes; Fatty acids; Adriatic sea; Niche partitioning; Coastal ecosystems