ALBERT

Description: High-resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw〉6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW-verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along-strike structural variability as well as fault trend suggests that the HF is composed by three main sub-segments: North (N25), Central (N50) and South (N45). Swath-bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20º) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre-stack depth-migrated multichannel seismic reflection profiles across the HF and neighboring areas allowed us to constrain their seismo-stratigraphy, structural geometry, tectono-sedimentary evolution from Upper Jurassic to present-day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst-case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behavior and near-shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.

Description: IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor-sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved.

Description: Recently acquired high-resolution multichannel seismic profiles together with bathymetric and sub-bottom profiler data from the external part of the Gulf of Cadiz (Iberia-Africa plate boundary) reveal active deformation involving old (Mesozoic) oceanic lithosphere. This area is located 180 km offshore the SW Iberian Peninsula and embraces the prominent NE-SW trending Coral Patch Ridge, and part of the surrounding deep Horseshoe and Seine abyssal plains. E-W trending dextral strike-slip faults showing surface deformation of flower-like structures predominate in the Horseshoe Abyssal Plain, whereas NE-SW trending compressive structures prevail in the Coral Patch Ridge and Seine Hills. Although the Coral Patch Ridge region is characterized by subdued seismic activity, the area is not free from seismic hazard. Most of the newly mapped faults correspond to active blind thrusts and strike-slip faults that are able to generate large magnitude earthquakes (Mw 7.2-8.4). This may represent a significant earthquake and tsunami hazard that has been overlooked so far.

Description: The potential for a dynamical impact of Saharan mineral dust on the North Atlantic Ocean large-scale circulation is investigated. To this end, an ocean general circulation model forced by atmospheric fluxes is perturbed by an idealized, seasonally varying, net shortwave flux anomaly, as it results from remote sensing observations of aerosol optical thickness representing Saharan dust load in the atmosphere. The dust dynamical impact on the circulation is assessed through a comparison between perturbed and an unperturbed run. Results suggest that, following the dust-induced shortwave irradiance anomaly, a buoyancy anomaly is created in the Atlantic offshore the African coast, which over the course of the time propagates westward into the interior Atlantic while progressively subducting. Changes in the large-scale barotropic and overturning circulations are significant after 3 years, which coincides with the elapsed time required by the bulk of the buoyancy perturbation to reach the western boundary of the North Atlantic. Although small in amplitude, the changes in the meridional overturning are of the same order as interannual-to-decadal variability. Variations in the amplitude of the forcing lead to changes in the amplitude of the response, which is almost linear during the first 3 years. In addition, a fast, but dynamically insignificant, response can be observed to propagate poleward along the eastern boundary of the North Atlantic, which contributes to a nonlinear response in the subpolar region north of 40°N.

Description: We present a new classification of geological domains at the Africa-Eurasia plate boundary off SW Iberia, together with a regional geodynamic reconstruction spanning from the Mesozoic extension to the Neogene-to-present-day convergence. It is based on seismic velocity and density models along a new transect running from the Horseshoe to the Seine abyssal plains, which is combined with previously available geophysical models from the region. The basement velocity structure at the Seine Abyssal Plain indicates the presence of a highly heterogeneous, thin oceanic crust with local high-velocity anomalies possibly representing zones related to the presence of ultramafic rocks. The integration of this model with previous ones reveals the presence of three oceanic domains offshore SW Iberia: (1) the Seine Abyssal Plain domain, generated during the first stages of slow seafloor spreading in the NE Central Atlantic (Early Jurassic); (2) the Gulf of Cadiz domain, made of oceanic crust generated in the Alpine-Tethys spreading system between Iberia and Africa, which was coeval with the formation of the Seine Abyssal Plain domain and lasted up to the North Atlantic continental breakup (Late Jurassic); and (3) the Gorringe Bank domain, made of exhumed mantle rocks, which formed during the first stages of North Atlantic opening. Our models suggest that the Seine Abyssal Plain and Gulf of Cadiz domains are separated by the Lineament South strike-slip fault, whereas the Gulf of Cadiz and Gorringe Bank domains appear to be limited by a deep thrust fault located at the center of the Horseshoe Abyssal Plain.

Description: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18±2.59 µmol m−2 d−1) suggests that in May-July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O. This article is protected by copyright. All rights reserved.

Description: High‐throughput platforms for microalgae screening are not yet commercially available. In this study, the feasibility of 96‐well microplates was analyzed for microalgae research. Equivalence among wells, as culture microreactors, was investigated in controlled high CO2 conditions. Specific growth rates of two microalgae species, Scenedesmus sp. UTEX1589 and an environmental isolate, were significantly higher in border wells than in internal positions. Furthermore, growth rate gradients analyzed as contours throughout the platform were observed for Scenedesmus sp. However, the output variable exhibited high precision associated with a low coefficient of variation (CV), between 6.8 and 7.8%. In a demonstrative experiment to determine the effect of culture media dilution on six microalgae species, treatments were randomized in the central subset of a microplate. Results were consistent and statistically sound (CV 9.4–12.9%), and showed that microalgae species could grow with no detrimental effect in 50% (v/v) dilution of the culture medium. Provided border wells exclusion and a randomized design, 96‐well microplates are a practical and statistical robust platform for microalgae research.

Description: The Benguela Upwelling system (BUS) is the most productive of all eastern boundary upwelling ecosystems and it hosts a well‐developed oxygen minimum zone. As such, the BUS is a potential hotspot for production of N2O, a potent greenhouse gas derived from microbially‐driven decay of sinking organic matter. Yet, the extent at which near‐surface waters emit N2O to the atmosphere in the BUS is highly uncertain. Here we present the first high‐resolution surface measurements of N2O across the northern part of the BUS (nBUS). We found strong gradients with a three‐fold increase in N2O concentrations near the coast as compared with open ocean waters. Our observations show enhanced sea‐to‐air fluxes of N2O (up to 1.67 nmol m−2 s−1) in association with local upwelling cells. Based on our data we suggest that the nBUS can account for 13% of the total coastal upwelling source of N2O to the atmosphere.

Description: Hydroxylamine (NH 2 OH), a short-lived intermediate in the nitrogen cycle, is a potential precursor of nitrous oxide (N 2 O) in the ocean. However, measurements of NH 2 OH in the ocean are sparse. Here we present a data set of depth profiles of NH 2 OH from the equatorial Atlantic Ocean and the eastern tropical South Pacific and compare it to N 2 O, nitrate, and nitrite profiles under varying oxygen conditions. The presence of NH 2 OH in surface waters points toward surface nitrification in the upper 100 m. Overall, we found a ratio of 1:3 between NH 2 OH and N 2 O in open ocean areas when oxygen concentrations were 〉50 μmol/L. In the equatorial Atlantic Ocean and the open ocean eastern tropical South Pacific, where nitrification is the dominant N 2 O production pathway, stepwise multiple regressions demonstrated that N 2 O, NH 2 OH, and nitrate concentrations were highly correlated, suggesting that NH 2 OH is a potential indicator for nitrification.

Description: Accurate and reliable biodiversity estimates of marine zooplankton are a prerequisite to understand how changes in diversity can affect whole ecosystems. Species identification in the deep sea is significantly impeded by high numbers of new species and decreasing numbers of taxonomic experts, hampering any assessment of biodiversity. We used in parallel morphological, genetic, and proteomic characteristics of specimens of calanoid copepods from the abyssal South Atlantic to test if proteomic fingerprinting can accelerate estimating biodiversity. We cross-validated the respective molecular discrimination methods with morphological identifications to establish COI and proteomic reference libraries, as they are a pre-requisite to assign taxonomic information to the identified molecular species clusters. Due to the high number of new species only 37% of the individuals could be assigned to species or genus level morphologically. COI sequencing was successful for 70% of the specimens analysed, while proteomic fingerprinting was successful for all specimens examined. Predicted species richness based on morphological and molecular methods was 42 morphospecies, 56 molecular operational taxonomic units (MOTUs) and 79 proteomic operational taxonomic units (POTUs), respectively. Species diversity was predicted based on proteomic profiles using hierarchical cluster analysis followed by application of the variance ratio criterion for identification of species clusters. It was comparable to species diversity calculated based on COI sequence distances. Less than 7% of specimens were misidentified by proteomic profiles when compared with COI derived MOTUs, indicating that unsupervised machine learning using solely proteomic data could be used for quickly assessing species diversity.