Northwestern Iberian Margin Research Articles

Modern conditions and recent environmental evolution of the industrialized inner Ría of Ferrol (Galicia, NW Spain)

Galician rías provide several ecosystem services of great ecological and economic significance in the northwestern Iberian margin, requiring a good environmental quality for sustainable harnessing. More paleoenvironmental reconstructions extending to their preindustrial state are needed to predict their evolution under natural and human-induced perturbations, such as ongoing anthropogenic global warming. This study's aim is twofold: first, characterize the current environmental conditions governing the inner Ría of Ferrol (Galicia, NW Spain) and address its recent natural and anthropogenic evolution. Therefore, a multiproxy approach (benthic foraminifera, grain size, pollen content, trace metals, Al, total organic and inorganic carbon contents, total nitrogen content, δ13C, magnetic susceptibility, 210Pb and 137Cs radiotracers, and microplastics) has been applied to intertidal surface and sediment core samples. The benthic foraminiferal results exhibit typical inner ría assemblages, primarily driven by natural ría-estuarine dynamics, with a higher dominance of brackish species (H. germanica, A. morphogroup tepida), fewer living individuals and marine allochthonous taxa toward the continental end-member. The foraminiferal standing crops and surface trace metal concentrations do not reflect strong anthropogenic impacts, although areas with elevated magnetic susceptibility have been detected, probably associated with nearby industrial activities. The recent sedimentary deposits reveal anthropogenic impacts at local and regional scales, with different environmental shifts. Local impacts were triggered by physical interventions in the inner ría, with the construction of the As Pías Bridge in 1968 and the great urban-industrial development of Ferrol city and surrounding localities experienced since the late 19th century. Land-use changes have driven regional scale changes, corresponding to reforestation plans with the introduction of anthropogenic plant species for industrial purposes initiated in the 1940s. The stratigraphic analysis of sediment cores has unveiled high levels of contaminants (Zn and microplastics) in the innermost sector as the primary environmental concerns. In this sediment-infilling and restricted area, potential dredging activities could release them leading to their possible bioavailability. Although past adverse environmental conditions cannot be discarded, sedimentary intervals with negligible presence of microfauna in the innermost area have mostly been attributed to taphonomic processes involving calcareous dissolution and disaggregation of agglutinated tests, likely caused by carbonate-undersaturated conditions and organic matter metabolization by microbial activity respectively.

Fixed-point time series, repeat survey and high-resolution modeling reveal event scale responses of the Northwestern Iberian upwelling

The hydrography and circulation of the Northwestern Iberian Margin (NWIM) was studied over an annual cycle from November 2008 to December 2009. The study was focused on the Cape Silleiro border region that separates the rugged northern coastline of the Rías Baixas embayments and the smoother southern coastline, a source of major fresh water outflows. Monthly across-shelf transects off Cape Silleiro monitored hydrography and currents, while an upward-looking ADCP registered currents in 72 m depth on the shelf over the period. To provide greater detail of the variability both spatial and temporal, during the annual cycle, a ROMS-AGRIF configuration of the region was built. Validation of the model against the observations produced robust results that reproduced many aspects of the observed variability. It was therefore possible to examine features of the NWIM, such as the seasonal cycle, wind-induced upwelling and downwelling and their associated horizontal circulations, as well as nearshore countercurrents related to freshwater plumes, in unprecedented detail. Five different scenarios were investigated in the study year through examination of characteristic physical processes: (1) late autumn upwelling, (2) high energy winter conditions, (3) spring upwelling-downwelling cycles, (4) summer upwelling, and (5) autumn transition to high energy conditions. The demonstration of the robust capacity of this model configuration, to reproduce in detail the phenomena observed in this region, suggests that the physical model could be reliably extended to other years in order to provide the basis for multidisciplinary studies in relation to biogeochemical cycles and harmful blooms. Other applications could include transport of sediments, larvae, microplastics and other contaminants as well as operational application for general ocean forecasting.

Glider Observations of the Northwestern Iberian Margin During an Exceptional Summer Upwelling Season

AbstractGlider observations from the Northwestern Iberian Margin during the exceptionally strong 2010 summer upwelling season resolved the evolution of physical and biogeochemical variables during two upwelling events. Upwelling brought low‐oxygen Eastern North Atlantic Central Water from 190 m depth onto the shelf up to a depth of 50 m. During the two observed periods of upwelling, a poleward jet developed over the shelf break. The persistent upwelling favorable winds maintained equatorward flow on the outer shelf for 2 months with no reversals during relaxation periods, a phenomenon not previously observed. During upwelling, near‐surface chlorophyll a concentration increased by more than 6 mg m−3. Oxygen supersaturation in the near surface increased by more than 20%, 6 days after the chlorophyll a maximum.

Mapping of currents off the northwestern Iberian coast with the Regional Ocean Modelling System

ABSTRACT The structure and the temporal variability of modelled velocity fields at different depths are studied for the years 2002–2004 in the Northwestern Iberian Margin. The currents are simulated with the Regional Ocean Modeling System in a high resolution that is compatible with the input data from the HYbrid Coordinate Ocean Model reanalysis and the atmospheric forcing from the Climate Forecast System Reanalysis, as well as with the tidal forcing from the TPXO Global Tide Solution. After an initial validation of the numerical model results, the velocity field is mapped to identify the region's main flow characteristics and its seasonal and interannual variability, in order to find areas that have strong currents, which could be of interest for exploring tidal stream energy. There are three main regions within the numerical domain, at different levels, where velocity values reach higher values than their surroundings: (i) located at the surface flowing south around 9.5°W, related to the summer Upwelling Jet; (ii) located further offshore near the Galician Bank, flowing north at deeper water levels which is visible during most part of the year and; (iii) within Ría de Arousa, also visible all year round.

Geomorphological and sedimentary processes of the glacially influenced northwestern Iberian continental margin and abyssal plains

The offshore region of northwestern Iberia offers an opportunity to study the impacts of along-slope processes on the morphology of a glacially influenced continental margin, which has traditionally been conceptually characterised by predominant down-slope sedimentary processes. High-resolution multibeam bathymetry, acoustic backscatter and ultrahigh-resolution seismic reflection profile data are integrated and analysed to describe the present-day and recent geomorphological features and to interpret their associated sedimentary processes. Seventeen large-scale seafloor morphologies and sixteen individual echo types, interpreted as structural features (escarpments, marginal platforms and related fluid escape structures) and depositional and erosional bedforms developed either by the influence of bottom currents (moats, abraded surfaces, sediment waves, contourite drifts and ridges) or by gravitational features (gullies, canyons, slides, channel-levee complexes and submarine fans), are identified for the first time in the study area (spanning ~90,000 km2 and water depths of 300 m to 5 km). Different types of slope failures and turbidity currents are mainly observed on the upper and lower slopes and along submarine canyons and deep-sea channels. The middle slope morphologies are mostly determined by the actions of bottom currents (North Atlantic Central Water, Mediterranean Outflow Water, Labrador Sea Water and North Atlantic Deep Water), which thereby define the margin morphologies and favour the reworking and deposition of sediments. The abyssal plains (Biscay and Iberian) are characterised by pelagic deposits and channel-lobe systems (the Cantabrian and Charcot), although several contourite features are also observed at the foot of the slope due to the influence of the deepest water masses (i.e., the North Atlantic Deep Water and Lower Deep Water). This work shows that the study area is the result of Mesozoic to present-day tectonics (e.g. the marginal platforms and structural highs). Therefore, tectonism constitutes a long-term controlling factor, whereas the climate, sediment supply and bottom currents play key roles in the recent short-term architecture and dynamics. Moreover, the recent predominant along-slope sedimentary processes observed in the studied northwestern Iberian Margin represent snapshots of the progressive stages and mixed deep-water system developments of the marginal platforms on passive margins and may provide information for a predictive model of the evolution of other similar margins.

Circulation on the Northwestern Iberian Margin: Vertical structure and seasonality of the alongshore flows

We describe the seasonal cycle of the alongshore flows on the Northwestern Iberian Margin and explain what are the important mechanisms forcing the system. We used a 20-year model simulation at 2.3km horizontal resolution, with realistic atmospheric forcing and covering the whole Western Iberian Margin. The model results are compared with satellite data, with data measured at two moored buoys and with a compilation of current meter data available for the study region. We show that currents over the slope are divided in three different cores: the Iberian Poleward Current (IPC) occupying the top 350m, a deeper core at Mediterranean Water levels (∼600–1200m) and in between the two, an equatorward core centered just beneath the IPC core, the Upper Slope Countercurrent (USCC). The IPC is present almost yearlong, including in summer months, when it is close to the shelf-break and topped by the equatorward upwelling jet. After September, the IPC intensifies and its core surfaces. Heat and salinity budgets on the shelf and slope are dominated by advection, confirming the important role of the IPC in driving the temperature and salinity seasonal cycles. Analysis of the seasonal cycle of the barotropic vorticity equation on the slope, shows that the main forcing mechanism of northward flows is the “Joint Effect of Baroclinicity and Relief” (JEBAR), whose contribution is higher in summer than in winter. In December and January, when the IPC is stronger and surface intensified, the main contribution is from southerly winds. The cross-shore analysis of these terms shows that JEBAR decreases substantially at the core of the IPC because, as advection terms become important, the northward density flux diminishes the local meridional density gradients, resulting in the decrease of JEBAR in the months of strong IPC.

Wet to dry climatic trend in north-western Iberia within Heinrich events

The direct sea–land correlation applied to core MD99-2331 retrieved from the north-western Iberian margin shows a two-phase pattern within Heinrich events 4, 2 and 1 in the ocean and in the adjacent landmasses. Changes between wet/cold and dry/cool conditions in the Iberian Peninsula detected during these extreme events cannot be explained by a simple oceanographic mechanism related to changes in the strength of the Atlantic Meridional Overturning Circulation. Here we propose an additional atmospheric mechanism able to produce this scenario based on the comparison between the MD99-2331 record and other available palaeoclimate sequences from the North Atlantic region (18–75°N and 0–75°W). The climatic asymmetry observed between mid- and subtropical eastern North Atlantic latitudes (wet/dry) and the Blake Outer Ridge (dry/wet) during H4, H2 and H1 can be explained by changes in the position of the Atlantic jet-stream. During the first phase of H4, H2 and H1 the Atlantic jet-stream was located further south following the southward displacement of the oceanic thermal front as far south as 35°–37°N. On the contrary, during the second phase of H4, H2 and H1 the jet-stream was located further north following the northward displacement of this thermal front as far north as 42°N. From the atmospheric point of view, these two phases are reminiscent of the present-day negative and positive prevailing modes of the North Atlantic Oscillation (NAO), respectively, but high-resolution studies of additional North Atlantic key sites and climate simulations are needed to confirm the hypothesis of a NAO-like mechanism operating on millennial timescales.

Climatic variability of Marine Isotope Stage 7: direct land–sea–ice correlation from a multiproxy analysis of a north-western Iberian margin deep-sea core

The direct land–sea–ice correlation, performed in the north-western Iberian margin core MD01-2447, shows that Marine Isotope Stage 7 (MIS 7) is marked by an alternation of five warm and cold periods. This climatic variability is, in general, synchronously detected in marine and terrestrial realms by our record. Termination III is marked by three cold episodes. The last one interrupts the forest development in north-western Iberia related to the deglacial climatic amelioration. It begins in the middle of Termination III but the maximal cold conditions occur at the beginning of low ice volume conditions. MIS 7 is characterised by three periods of very low ice volume, associated with marine and continental high temperatures in and off north-western Iberia. The correlation of our record with ODP site 980 shows that the climatic variability detected in the north-western Iberian region has also an imprint in northern North Atlantic. It also highlights that most of the cold periods are associated with iceberg discharges in the North Atlantic. The comparison of the north-western Iberian margin sequence with the long European pollen records has allowed refining the correlation of the marine and continental stratigraphy for MIS 7.

Increasing vegetation and climate gradient in Western Europe over the Last Glacial Inception (122–110 ka): data-model comparison

High-resolution terrestrial (pollen) and marine (planktic and benthic isotopes, coarse fraction, and N. pachyderma (s)) analyses have been performed in the Marine Isotope Stage (MIS) 5 interval of IMAGES core MD99-2331 retrieved in the northwestern Iberian margin. This study shows the occurrence of a Zeifen Interstadial/Stadial succession on land and in the ocean during the first part of MIS 5e. In northwestern Iberia, the Eemian is marked from 126 to 122 ka by the development of deciduous Quercus forest at the same time as Mediterranean forest colonised southern Iberia, and deciduous Quercus-Corylus forest occupied northernmost European regions. From 121 to 115 ka Carpinus betulus forest developed in NW Iberia indicating a winter cooling by 2 °C on land and an increase in annual precipitation by 100–200 mm along with a Sea Surface Temperature (SST) decreasing trend off Iberia. A similar cooling has been documented at the same time in northern Germany (52° N) by the replacement of deciduous forest by coniferous ( Abies-Picea) formations, implying a southward displacement of the deciduous tree line between ∼60° N and 50° N as early as 120 ka. The southward migration of the tree line between 72° N and 58° N simulated by the Earth Model of Intermediate Complexity MoBidiC from 122 and 120 ka and considered as a major process to initiate the last glaciation is, therefore, compatible with data. Between 115 and 110 ka, the substantial ice accumulation in northern high latitudes (MIS 5e/5d transition) was synchronous with successive drops, C26 and C25, in northeastern Atlantic SST. In northwestern Iberia Abies-Pinus trees developed at the expense of Quercus-Carpinus forest. A tundra-like environment occupied northern Germany, marking the end of the interglacial in northwestern Europe at 115 ka, and boreal forest likely colonised northeastern France. The first displacement of the vegetation belts at 121 ka was enhanced at 115 ka indicating an amplification of the vegetation and climate gradients in northeastern Atlantic and European borderlands probably related with the well-developed ice caps at that time. The comparison between the general trend in the estimated and simulated MoBidiC winter and summer temperatures for latitudes between 35 and 45° N, shows that both follow quite straightforwardly the precession signal although the simulated and reconstructed temperatures agree better in the South than North of 40° N. Annual precipitation is exhibiting opposite trend in the data and in the model. This contradiction is likely the fact that the zonal climate simulated by the model may not accurately represent the regional climate features, as reconstructed from the pollen.

Particle residence times in surface waters over the north-western Iberian Margin: comparison of pre-upwelling and winter periods

Thorium-234 is used over the north-western Iberian Margin as a tracer of sinking particle fluxes on time scales of days to weeks. As a part of Ocean Margin Exchange (OMEX) project (II-II), samples were collected during pre-upwelling (June 1997) and winter (January 1998) periods. During the pre-upwelling situation, the distribution of total 234Th is rather uniform, with a mean deficiency of 24%. In contrast, particulate 234Th activities increase from the shelf to open ocean. As a result, particle residence times derived from 234Th show a considerable range between 4 and 45 days, with a mean value of about 26 days. In the shelf waters, there is no clear trend in particle residence time, whereas the offshore stations present a more obvious signal with a gradual increase from about 10 days over the slope to nearly 30 days in the open ocean. In winter, total 234Th activities are more variable, and both deficit and equilibrium situations are observed. The mean deficit of total 234Th is low, about 10%, which may reflect reduced export fluxes at this time. Moreover, in the shelf waters during winter, particulate 234Th presents higher levels when compared with the pre-upwelling situation, which may be due to more intense resuspension of shallow sediments. As a consequence, average near-surface particle residence times are significantly longer in winter, with a mean value of 113 days. Estimates of export flux of particulate organic carbon (POC) from the upper ocean, using the ratio of POC and 234Th of particles, range between 1 and 56 mmol C m −2 day −1 in June 1997, and ca. 0 and 25 mmol C m −2 day −1 in January 1998. These variations reflect the large differences in hydrographic conditions and primary production levels between the two seasonal situations.