Previous studies have addressed the occurrence of Acid Rock Drainage (ARD) affecting La Silva stream due to the generation of large dumps of Middle Ordovician black shales during the construction of a highway close to El Bierzo (León, Spain). This ARD was characterized by sulphated acid waters with high concentration of heavy metals and anomalies in dissolved thorium (Th) and uranium (U). In the present study, we analyse in depth black shales and water, streambed sediments and precipitates of La Silva stream and its tributaries using different petrographic, mineralogical and geochemical approaches. Black shales, with average Th and U contents of 20 and 3 μg/g respectively contain disseminated detritic micro-grains of high weathering-resistant minerals, such as monazite and xenotime, that present smaller amounts of yttrium and rare earth elements (REY) and other elements as Ca, U, Th, Si and F. Results of the affected waters by ARD show an enrichment in dissolved Th, U and REY of several orders of magnitude with respect to natural waters. Sampled precipitates were mainly schwertmannite (Fe₈O₈(OH)_8-2x (SO₄)_xO₁₆•nH₂O) and goethite (α-Fe³⁺O(OH)) that showed an enrichment of Th (up to 798 μg/g) and REY, due to the presence of dissolved anionic species (e.g. [Formula: see text] , [Formula: see text] ) that enables their adsorption. Furthermore, these black shales show a clear enrichment in REE (Rare Earth Elements) with respect to NASC (North American Shales Composite) normalized REE patterns. Likewise, normalized REE patterns of stream waters and precipitates clearly show convex curvatures in middle-REE (MREE) with respect to light- and something less than heavy-REE, indicating the trend towards MREE enrichment. These findings are essential to evaluate the impact of ARD of Mid Ordovician shales in the surrounding environment, and to start considering these site as potential source of REE and critical raw materials, activating a Circular Economy.

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.

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.

Selective precipitation of schwertmannite was identified in the water column of a chemically stratified acidic lake (pH 2.0 2.5). A set of analytical techniques was used to identify accurately the mineral phase (X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy coupled with energy dispersive spectroscopy). The lake has two clearly differentiated layers: a thin superficial layer that extends to ∼2 m in depth with less dissolved solid content and, therefore, lower density, which contrasts with a thicker lower layer that extends to 34 m in depth. The upper layer is strongly influenced by climatic factors, triggering dilution (rainfall), concentration (dry season) and intense seasonal thermal fluctuations, while the lower layer is more compositionally and thermally stable.Schwertmannite precipitate was initially observed only in the upper layer, adhered to a plastic buoy and the anchor line. The latter was confirmed through a test involving a precipitation device that consisted of maintaining a series of plastic plates at different depths over a period of several months. Only the plates located in the upper layer became covered with precipitate, whereas the plates that were submerged in the lower layer remained clean. These observations clearly differed from the saturation indices of schwertmannite (SI_sch) calculated usingPHREEQCand using Bighamet al.(1996) or Kawano and Tomita (2001) solubility products. Schwertmannite would not precipitate in the lake with the former but would precipitate in both layers with the latter. It is the constant provided by Yuet al.(1999) that makes SI_schin both layers approach the observed behaviour: SI_sch>0 displays oversaturation in the upper layer while SI_sch<0 displays undersaturation in the lower layer. The value of the product of solubility that better adjusts to this situation is log K_sch= 9.5 10. Using the method by Yuet al.(1999) to establish the apparent solubility, a range of log K_schvalues was defined between 10.5 and 11.5 for the system studied.