EXPLOSEA (CTM2016-75947-R): Exploration of submarine hydrothermal vents, mineral deposits and geobio-systems associated.

Gas hydrate systems in the Scotia Sea, the deep-water oceanic gateway between Antarctica and South America, exist in a unique polar setting characterized by: (1) Very low bottom water temperatures (e.g., as low as −0.5 °C at water depths of 2000-4000 m); (2) strong bottom currents sourced both from the ice stream discharge in the Weddell Sea and from the Antarctic Circumpolar Current that, combined with high sediment supply, generates giant contourite drifts; (3) high lateral variability of the geothermal gradient due to near-surface magmatism and hydrothermal activity, which is associated with a system of oceanic ridges and relict subduction arcs within the Scotia plate. In this area, the base of the gas hydrate stability zone as inferred from a gas hydrate-related bottom simulating reflector adapt to this changing environment, producing a peculiar association with gas-related structures in the subsurface. Abstract 9

The aim of this work is to make a synthesis at regional scale focused on the geophysical characterization of submarine faults around the Iberian margin to identify active structures and analyze their development in the framework of the present plate organization. Most of these submarine faults show seabed morphological expressions mapped with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel seismic profiles. Present active tectonics, deformation, seismicity, and tsunami-affected coastal areas is mainly focused on south Iberia at the Eurasian and Nubia plate boundary. Submarine active faults in these areas are represented by long strike-slip fault systems and arcuate fold-thrust systems. Their development takes place in response to present NW-SE convergence between the Eurasian and Nubia plates. We propose a strain partitioning model of the plate boundary into simple and pure shear zones to explain the distribution and mechanisms of active submarine faults along the Gulf of Cádiz, Gibraltar Arc and Alborán Sea in response to the present-day shear stress orientation. Nevertheless, deformation is also focused in the NW Iberian margin. Thus, along the Galician and Portuguese margin, several submarine faults mapped as thrust fault systems with high-seismic activity along the Iberian ocean-continent transition reflect the re-activation of former structures. We suggest that submarine active faults in the NW and W Iberia are also the response to the eastwards transfer of short-offset transform faults of the Mid Atlantic Ridge into the oceanic Iberian along a weakness as the former plate boundary between the oceanic Iberia and Eurasia domains. The distribution and activity of submarine faults mapped in this work from geophysical and bathymetric data are in good agreement with geodetic data and focal mechanisms.

In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.

Hydrothermal iron (Fe)-rich sediments were recovered from the Tagoro underwater volcano (Central Atlantic) that was built during the 2011–2012 volcanic event. Cruises in 2012 and 2014 enabled the monitoring and sampling of the early-stage establishment of a hydrothermal system. Degassing vents produced acoustic flares imaged on echo-sounders in June 2012, four months after the eruption. In 2014 during a ROV dive was discovered and sampled a novel hydrothermal vent system formed by hornito-like structures and chimneys showing active CO2 degassing and anomalous temperatures at 120–89 m water depth, and along the SE flank at 215–185 m water depth associated with secondary cones. Iron- and silica-rich gelatinous deposits pooled over and between basanite in the hornitos, brecciated lavas, and lapilli. The low-temperature, shallow-water hydrothermal system was discovered by venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic/pHotic zone at 50–70 m water depths. The basanite is capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediments and contain micro-cracks and degasification vesicles filled by sulfides (mostly pyrite). Mineralogically, the Fe-oxyhydroxide sediments consist of proto-ferrihydrite and ferrihydrite with scarce pyrite at their base. The Fe-rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits, and the sediments show some dilution of the Fe oxyhydroxide by volcanic ash. The Fe-oxyhydroxide phase with a mean particle size of 3–4 nm, low average La/Fe ratios of the mineralized deposits from the various sampling sites, and the positive Eu anomalies indicate rapid deposition of the Fe-oxyhydroxide near the hydrothermal vents. Electron microprobe studies show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits within the mineralized samples, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16 s rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes. On a geological scale, these volcanic eruptions and low-temperature hydrothermal vents might contribute to increased dissolved metals in seawater, and generate considerable Fe-oxyhydroxide deposits as identified in older hot-spot seamounts.

This work presents a new high-resolution multibeam bathymetric map of a segment of active deep sea-floor spreading in the Atlantic Ocean, the northern Mid-Atlantic Ridge (MAR) at 45–46º N. New high-resolution bathymetry data were acquired using an Atlas multibeam echosounder onboard the research vessel Sarmiento de Gamboa during the EXPLOSEA-2 survey in 2019. The final map of the MAR (50 m cell grid size) at the original scale of 1:200,000 shows a segment of 140 × 35 km of the MAR, at water depths from 715 to 3700 m. This new high-resolution bathymetric map allows to better defining the submarine morphology of the Moytirra hydrothermal active field, the only high-temperature field identified between the Azores Archipelago (Portugal) and Iceland. ROV submarine observations reaching the deepest part of the system for the first time show giant anhydrite-sulfide chimneys up to 20 m high, active strong black smokers and polymetallic massive sulfides.

This work presents the preliminary result of the multidisciplinary cruise EXPLOSEA2 surveying the northern Mid-Atlantic Ridge and Azores Archipelago from 46 • 30 N to 38 • 30 N aboard the R/V Sarmiento de Gamboa and ROV Luso over 54 days (June 11 to July 27, 2019). In this cruise report, we detail the geophysical, hydrographic, geological, oceanographic, ecological, and microbiological data acquired and a brief of main findings. The cruise addressed the exploration and comprehensive characterization of venting sites, including the water column, the sediments and rocks that host the hydrothermal activity, and the associated mineralizations, biology, and microbiology. Deep hydrothermal chimneys and massive sulfide deposits (up 3,000 m in depth) within the Moytirra hydrothermal active field were identified on slopes that had not been explored previously. Another striking finding made during the EXPLOSEA2 cruise was the field of carbonate chimneys named the "Magallanes-Elcano" field, a potentially relict ultramafic-hosted hydrothermal site sourced by abiotic methane. This field is related to a serpentinite and gabbro rock outcropping on a dome-shaped massif named the "Iberian Massif." An outstanding finding of the EXPLOSEA2 survey was the identification of the first garden of soft corals growing after active submarine eruptions were reported in the Azores Archipelago composed by a high density of soft corals the suborder Alcyoniina at the summit and flanks of a recent volcanic cone at 160 m water depth developed during the 1957-1958 eruption of Capelinhos. Several cold-water coral habitats formed by colonial scleractinians (e.g., Lophelia pertusa and Madrepora oculata), coral gardens composed of mixed assemblages of black corals (Leiopathes sp.), and octocorals and dense aggregations of the glass sponge Pheronema carpenteri that may be classified Frontiers in Marine Science | www.frontiersin.org 1 November 2020 | Volume 7 | Article 568035 Somoza et al. Mid-Atlantic Ridge and Azores Multidisciplinary Cruise as vulnerable marine ecosystems (VMEs) have been discovered during the EXPLOSEA2 cruise along the northern Mid-Atlantic Ridge. This work reveals the importance of multidisciplinary surveys to the knowledge of deep-sea environments.

Siboglinids were sampled from four mud volcanoes in the Gulf of Cádiz (El Cid MV, Bonjardim MV, Al Gacel MV, and Anastasya MV). These invertebrates are characteristic to cold seeps and are known to host chemosynthetic endosymbionts in a dedicated trophosome organ. However, little is known about their tube as a potential niche for other microorganisms. Analyses by scanning and transmission electron microscopy showed dense biofilms on the tube in Al Gacel MV and Anastasya MV specimens by prokaryotic cells. Methanotrophic bacteria were the most abundant forming these biofilms as further supported by 16S rRNA sequence analysis. Furthermore, elemental analyses with electron microscopy and energy-dispersive X-ray spectroscopy point to the mineralization and silicification of the tube, most likely induced by the microbial metabolisms. Bacterial and archaeal 16S rRNA sequence libraries revealed abundant microorganisms related to these siboglinid specimens and certain variations in microbial communities among samples. Thus, the tube remarkably increases the microbial biomass related to the worms and provides an additional microbial niche in deep-sea ecosystems.

Siboglinid worms were sampled from four mud volcanoes in the Gulf of Cádiz (El Cid MV, Bonjardim MV, Al Gacel MV and Anastasya MV). These invertebrates are characteristic to cold seeps and are known to host chemosynthetic endosymbionts in a dedicated trophosome organ. However, little is known about their tube as a potential niche for other chemosynthetic and non-chemosynthetic microorganisms. Analyses by scanning and transmission electron microscopy showed dense biofilms on the tube in Al Gacel MV and Anastasya MV specimens by prokaryotic cells. Methanotrophic bacteria were the most abundant forming these biofilms as further confirmed by 16S rRNA sequence analysis. Furthermore, elemental analyses with electron microscopy and EDX point to the progressive mineralization of the biofilm and the tube in absence of nutrients. Environmental bacterial and archaeal 16S rRNA sequence libraries revealed abundant microorganisms related to these siboglinid worms and variation in microbial communities among samples. We argue that these differences must be related to variance in seepage activity, as it is the main source of nutrients. Thus, the tube remarkably increases the microbial biomass related to the worms and needs to be incorporated as an important part of the worm’s microbiota. Furthermore, empty tubes may still influence the composition of the active microbial community at those sites.

Four pure hydrogenetic, mixed hydrogenetic-diagenetic and hydrogenetic-hydrothermal Fe-Mn Crusts from the Canary Islands Seamount Province have been studied by Micro X-Ray Diffraction, Raman and Fourier-transform infrared spectroscopy together with high resolution Electron Probe Micro Analyzer and Laser Ablation Inductively Coupled Plasma Mass Spectrometry in order to find the correlation of mineralogy and geochemistry with the three genetic processes and their influence in the metal recovery rate using an hydrometallurgical method. The main mineralogy and geochemistry affect the contents of the different critical metals, diagenetic influenced crusts show high Ni and Cu (up to 6 and 2 wt. %, respectively) (and less Co and REY) enriched in very bright laminae. Hydrogenetic crusts on the contrary show High Co and REY (up to 1 and 0.5 wt. %) with also high contents of Ni, Mo and V (average 2500, 600 and 1300 μg/g). Finally, the hydrothermal microlayers from crust 107-11H show their enrichment in Fe (up to 50 wt. %) and depletion in almost all the critical elements. One hydrometallurgical method has been used in Canary Islands Seamount Province crusts in order to quantify the recovery rate of valuable elements in all the studied crusts except the 107-11H, whose hydrothermal critical metals’ poor lamina were too thin to separate from the whole crust. Digestion treatment with hydrochloric acid and ethanol show a high recovery rate for Mn (between 75% and 81%) with respect to Fe (49% to 58%). The total recovery rate on valuable elements (Co, Ni, Cu, V, Mo and rare earth elements plus yttrium (REY)) for the studied crusts range between 67 and 92% with the best results for Co, Ni and V (up to 80%). The genetic process and the associated mineralogy seem to influence the recovery rate. Mixed diagenetic/hydrogenetic crust show the lower recovery rate for Mn (75%) and Ni (52.5%) both enriched in diagenetic minerals (respectively up to 40 wt. % and up to 6 wt. %). On the other hand, the presence of high contents of undigested Fe minerals (i.e., Mn-feroxyhyte) in hydrogenetic crusts give back low recovery rate for Co (63%) and Mo (42%). Finally, REY as by-product elements, are enriched in the hydrometallurgical solution with a recovery rate of 70–90% for all the studied crusts.

Azooxanthellate cold-water corals (CWCs) have a global distribution and have commonly been found in areas of active fluid seepage. The relationship between the CWCs and these fluids, however, is not well understood. This study aims to unravel the relationship between CWC development and hydrocarbon-rich seepage in Pompeia Province (Gulf of Cádiz, Atlantic Ocean). This region is comprised of mud volcanoes (MVs), coral ridges and fields of coral mounds, which are all affected by the tectonically driven seepage of hydrocarbon-rich fluids. These types of seepage, for example, focused, scattered, diffused or eruptive, is tightly controlled by a complex system of faults and diapirs. Early diagenetic carbonates from the currently active Al Gacel MV exhibit δ13C signatures down to −28.77 ‰ Vienna Pee Dee Belemnite (VPDB), which indicate biologically derived methane as the main carbon source. The same samples contain 13C-depleted lipid biomarkers diagnostic for archaea such as crocetane (δ13C down to −101.2 ‰ VPDB) and pentamethylicosane (PMI) (δ13C down to −102.9 ‰ VPDB), which is evidence of microbially mediated anaerobic oxidation of methane (AOM). This is further supported by next generation DNA sequencing data, demonstrating the presence of AOM-related microorganisms (ANMEs, archaea, sulfate-reducing bacteria) in the carbonate. Embedded corals in some of the carbonates and CWC fragments exhibit less negative δ13C values (−8.08 ‰ to −1.39 ‰ VPDB), pointing against the use of methane as the carbon source. Likewise, the absence of DNA from methane- and sulfide-oxidizing microbes in sampled coral does not support the idea of these organisms having a chemosynthetic lifestyle. In light of these findings, it appears that the CWCs benefit rather indirectly from hydrocarbon-rich seepage by using methane-derived authigenic carbonates as a substratum for colonization. At the same time, chemosynthetic organisms at active sites prevent coral dissolution and necrosis by feeding on the seeping fluids (i.e., methane, sulfate, hydrogen sulfide), allowing cold-water corals to colonize carbonates currently affected by hydrocarbon-rich seepage.

A new temporal history of mass wasting processes for the west of the Canary volcanic province is presented. Its onset has been estimated in the middle–upper Miocene (∼13.5 ± 1.2 Ma), matching with a critical period of construction for this volcanic province. Seismic profiles show an emplacement longevity (from the Miocene to Quaternary) in multiple events, defined by stacked lobes of debrites, linked to the flank collapses and volcanic avalanches of the volcanic edifices (islands and seamounts). An evolution of pathways and source areas has been detected from east (Miocene) to west (Quaternary); as well as a migration of the activity to the northwest (west of the Canary Islands: e.g. El Hierro and La Palma). Six connected branches (I–VI), three of them described for the first time here, of Quaternary seismic units of mass transport deposits (MTDs) have been characterized. The Pleistocene makes up a huge buried MTDs system, until now unknown, pointing a new mass transport sedimentological scenario. Finally, the two southernmost branches (V–VI), up to now unknown, are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the Canary Island Seamount Province, apparently inactive from upper Cretaceous.

A new temporal history of mass wasting processes for the west of the Canary volcanic province is presented. Its onset has been estimated in the middle–upper Miocene (∼13.5 ± 1.2 Ma), matching with a critical period of construction for this volcanic province. Seismic profiles show an emplacement longevity (from the Miocene to Quaternary) in multiple events, defined by stacked lobes of debrites, linked to the flank collapses and volcanic avalanches of the volcanic edifices (islands and seamounts). An evolution of pathways and source areas has been detected from east (Miocene) to west (Quaternary); as well as a migration of the activity to the northwest (west of the Canary Islands: e.g. El Hierro and La Palma). Six connected branches (I–VI), three of them described for the first time here, of Quaternary seismic units of mass transport deposits (MTDs) have been characterized. The Pleistocene makes up a huge buried MTDs system, until now unknown, pointing a new mass transport sedimentological scenario. Finally, the two southernmost branches (V–VI), up to now unknown, are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the Canary Island Seamount Province, apparently inactive from upper Cretaceous.

Chemosynthetic-­-based ecosystems linked to magmatic/hydrothermal vents (lava volcanoes, hydrothermal vents, and sills-­-induced supercritical hydrothermal fluids) and biogenic/abiogenic methane seeps (mud volcanoes, pockmarks, brine pools, serpentinite diapirs) are living in large range of geological settings, from continental margins, oceanic crust basins, seamounts to mid oceanic ridges shaped by diverse oceanographic dynamics and geological processes. Chemosynthetic-­-based communities require distinct habitats but two main factors are common to maintain their populations: a) the release of deep seated fluids containing biogenic/abiogenic methane or other hydrocarbons, sulphur, iron, carbon dioxide; and b) hard substrate habitats as hard grounds (e.g. ferromanganese crusts), hydrothermal chimneys or methane derived-­-authigenic carbonates (MDACs) chimneys or pavements. Since 2000, extensive research on cold seeps in the southern NE Atlantic Ocean has resulted in the discovery of more than 60 mud volcanoes as well as of several mounds containing MDACs chimneys that are gathered in different fields. Chemosynthetic populations include sulphate reducing bacteria and methanotrophic archea (forming symbioses for AOM, anaerobic oxidation of methane), sulphur-­-oxidizing bacterial mats, tubeworms and extensive beds of clams and deep-­-sea mussels like Bathymodiolus sp. In addition, cold-­-water corals (CWC) living reefs, mounds and ridges, mainly composed of Lophelia pertusa, are extensively identified in association with cold seep build-­-ups as mud volcanoes. In areas of such extensive hydrocarbon seeps, massive authigenic carbonates are the dominant substrate for CWC colonisation and reef formation. Moreover, in the Macaronesia volcanic archipelagos (Canary-­-Azores-­-Madeira-­-Cape Verde), recent submarine eruptions as El Hierro volcano have allowed to identify the growth of new ecosystems associated with emissions of large quantities of iron and carbon dioxide from hydrothermal submarine vents. The seabed mapping of these deep-­-water habitats require a broad variety of geophysical system as multibeam echo-­-sounder (MBES) including bathymetry and backscatter data, parametric sub-­-bottom profilers, high-­-frequency echo-­-sounders and Remote Operated Vehicle (ROV). These techniques allowed us to map these seabed hotspot habitats hosting chemosynthetic communities and associated fauna.

Submarine geomorphology is the study of landforms and processes in the underwater domain. The ocean hosts a tremendous variety of forms that reflect the action of a range of tectonic, sedimentary, oceanographic, chemical and biological processes at multiple spatio-temporal scales. Submarine geomorphological mapping provides fundamental and effective means to characterize the seabed, an important interface between the geological substrate and the water column, where a range of biological, chemical and hydrodynamic phenomena are dependent on both the morphological and geological character of the seabed. In this way, a new extensive data set acquired of late-particularly for the Law of the Sea extended continental shelf (ECS) in the area of Galicia mapping purposes-provided the opportunity to analyse the 3D submarine geomorphology of the Galicia margin. We use artificial sun-illumination, shading and 3D rendering with digital bathymetric data (DTM's) to form natural looking and easily interpretable submarine landscapes of the main architectural elements that compose the submarine landscapes along the Galicia Margin. This seabed geomorphological mapping of the Galicia Margin allows us to establish the relationship between the processes (tectonic and sedimentary/oceanographic) and the resulting landforms. The submarine geomorphology of the Galicia Margin is inherited from former tectonic stages i.e. Variscan heritage as the Ibero-Armorican Arc, Mesozoic hyperextension of the Iberian-Newfoundland Atlantic conjugate margin, and Cenozoic convergence with subduction of the Bay of Biscay oceanic crust. Otherwise, the main oceanographic morphologies identified in the Galicia Margin related to sedimentary/oceanographic processes are: (i) System of gullies, canyons, channels and 83

A GIS catalogue of gas seeps around the Iberian Margin has been performed for the first time in the framework of the EMODNET-Geology-3 European project. Gas seep-related features include mud volcanoes and pockmarks but also other indicators of seabed fluid venting as the occurrence of methane-derived authigenic carbonates (MDACs) and chemosynthetic fauna hot-spots. The knowledge of these features and their distribution is relevant as they hold specific habitats, they are indicators of the presence of hydrocarbons at depth, they contain fluids rich in methane with a strong greenhouse effect, and they can induce changes in the physic-chemical properties of the sediments, causing instabilities. There are up to now 84 mud volcanoes in the southern Atlantic and Mediterranean margin, clustered into five main fields: 1) The Moroccan field (MF) offshore the Moroccan Atlantic margin; 2) the Guadalquivir Diapiric Ridge (GDR) field located on the southern Atlantic Iberian margin; 3) the TASYO field in the central GoC west off Gibraltar Strait; 4) the Deep Portuguese Margin field (DPMF) in the lower continental slope of the GoC: and (5) the West Alborán Basin (WAB) field (WAB) in the Mediterranean Sea. Gas hydrates have been collected in some mud volcanoes of GoC. In addition, extensive fields of MDAC chimneys and pavements have been identified as product of the anaerobic oxidation of methane (AOM) across the hydrocarbon seepages areas and chemosynthetic hot-spots as beds of Bathymodiulus Mauritanicus at the top of mud volcanoes. Finally, numerous pockmark fields have been detected on multibeam bathymetry and high-resolution seismic profiles across the Iberian continental margin. 198 pockmarks were identified in the Gulf of Cádiz (Atlantic Ocean) and 238 in the western Alborán Basin (Mediterranean Sea). Pockmarks, giant craters and oval depressions have also been identified in the northwestern Atlantic margin (Galicia and Cantabrian margin) and in the Murcia and Balearic Islands in the Western Mediterranean margin.

Methane, CO2 and other greenhouses gases associated with sill intrussions in sedimentary basins may have driven catastrophic climate change in the geological past. Modeling of hydrothermal venting based on fossil volcanic sedimentary basins show that overpressure generated by gas release during kerogen breakdown in the sill thermal aureoloe may cause fracture formation. Fluid focusing and overpressure migration above the sill result in vent formation after only few decades. The amount of methane and other gases generated depends on TOC content, the kerogen and the age of the sedimentary sequence. Here we describe the morphological expressions such as mounds, craters and domes related to subsurface sills discovered on the seafloor in deepwater oceanic basins 3,000-5,000 m water depths (e.g. Canary Basin-NE Atlantic and Scan Basin-Scotia Sea, Antarctica). We identify several types of surface manifestations related to evolution of sill intrusions in sedimentary oceanic basins. Therefore, numerous sea-floor mounds discovered on the North Atlantic and Antarctica are are linked to sills intruded in the oceanic sedimentary sequence. In first instance gases (including also mantle-derived volcanic gases) moves upwards through the tips of the sills. Overpressure at its tip form hydro-fractures that can be reach the seafloor forming peripheral vents of mud flows and gases with hydrothermal mineralization. Secondly, methane generated give rise to an overpressure reservoir above the sills deforming progressively the overlying sediments and forming convex reflections and large scale seabed domes. Progressive uplift may elevated mounds in the surface throughout reverse thrusting faults formed on the former hydrofractures at the tip of the sills. Craters identified in the center of some mounds could be response to violent degassing in single events. The occurrence of diagenetic or gas hydrates front under the subseabed, especially in the Antarctica,may complicate the type of surface manifestations of fluid flow forming giant craters or collapses. These expressions of fluid flow show clearly differences with those from derived gas seeps in continental margins such as mud volcanoes or pockmarks. The type of these surface manifestations will be very common as the research activity increase in these huge unexplored oceanic deepwater areas between continental margins and mid-ocean ridges. In addition, sill intrusions increase the possibilities for exploration and discovery of important hydrocarbon reservoirs in oceanic deep-sea basins, and their importance as reservoirs. of methane or other greenhouse gases This research is funded by EXPLO-SEA Project CTM2016-75947-R.

Thick ferromanganese (Fe-Mn) crusts from four Cretaceous seamounts (The Paps, Tropic, Echo and Drago) at the southern Canary Island Seamount Province (CISP) in the northeastern tropical Atlantic were recovered along the flanks and summits from 1700 to 3000 m water depths. CISP is composed of > 100 seamounts and submarine hills, is likely the oldest hotspot track in the Atlantic Ocean, and is the most long-lived of known hotspots globally. The Fe-Mn crusts grow on basalt-sedimentary rock substrates below the northeastern tropical Atlantic core of the oxygen minimum zone (OMZ) with a maximum thickness of 250 mm at a water depth of 2400 m. The mineralogical and chemical composition of these Fe-Mn crusts indicate a hydrogenetic origin. The main Mn minerals are vernadite with minor interlayered todorokite and asbolane-buserite. Fe oxides are essentially ferroxyhyte and goethite. The Fe-Mn crusts show high average contents in Fe (23.5 wt%), Mn (16.1 wt%), and trace elements like Co (4700 μg/g), Ni (2800 μg/g), V (2400 μg/g) and Pb (1600 μg/g). Rare earth elements plus yttrium (REY) averages 2800 μg/g with high proportions of Ce (1600 μg/g). Total platinum group elements (PGEs) average 230 ng/g, with average Pt of 182 ng/g. Two main types of growth layers form the crusts: 1) a dense laminae of oxides with high contents in Mn, Co and Ni associated with vernadite and Cu, Ni, and Zn associated with todorokite; 2) botryoidal layers with high contents in Fe, Ti, V and REY associated with goethite. The Fe-Mn crusts from the CISP region show higher contents in Fe, V, Pb and REY but lower Mn, Co, Ni and PGEs contents than Pacific or Indian ocean seamount crusts. The oldest maximum age of initiation of crust growth was at 76 Ma (Campanian, Late Cretaceous). Using a combination of high resolution Co-chronometer and geochemical data along an Electron Probe Micro Analysis (EPMA) transect, four stages in morphology, chemical contents and growth rates can be differentiated in the the Cenozoic crusts since 28 Ma, which we interpret as due to changes in the ventilation of the North Atlantic OMZ and to the increase of Saharian dust inputs. An earliest growth period, characterized by similar contents of Fe and Mn in the interval 27.8–24.45 Ma (late Oligocene-early Miocene) reflects slow precipitation related to a thick OMZ. An intermediate laminated zone with higher contents of Fe, Si and P, high growth rates reaching 4.5 mm/Ma, and precipitation of Fe-Mn oxides during the interval 24.5–16 Ma is related to periods of ventilation of the OMZ by intrusion of deep upwelling currents. Significant increase in Fe contents at ca. 16 Ma correlates with the onset of incursions of Northern Component Waters into the North Atlantic. Finally, since 12 Ma, the very low growth rates (< 0.5 mm/Ma) of the crust are related to a thick North Atlantic OMZ, an increase in Sahara dust input and a stable thermohaline circulation.