Rheological control on the tectonic evolution of a continental suture zone: the Variscan example from NW Iberia (Spain)

The Variscan continental suture zone exposed in NW Iberia is examined to uncover the long-lived rheological control exerted by the strata deposited over the external parts of Gondwana on its geodynamic evolution. The suture occurs within a set of allochthonous terranes whose limits were taken as domain boundaries to interpret the Variscan stacking of Paleozoic continental domains and retrodeform the resulting nappe pile. The suture zone formed due to closure of ocean basins located between Gondwana and Laurussia during the Late Paleozoic and consists of relics of oceanic and transitional crust. The suture zone exhibits a tabular to lens shape due to repeated tectonic events dominated by non-coaxial deformation (thrusts and low-angle normal faults). Thrusting and normal faulting also involved the margins of the continents bounding the suture. The structure of the continental blocks, however, is dominated by folds, particularly large nappe folds with pronounced superimposed flattening. The upper part of the basal allochthonous units comprises a rheologically incompetent domain below the suture zone. This domain is typified by the carbonaceous-rich strata, which are probably Ordovician–Silurian sediments based on U–Pb detrital zircon populations. The rheology of this layer determined the location of the first accretionary thrust that initiated the Late Devonian subduction of the Gondwana margin below the suture zone. By favoring fault development, the upper sequence of the basal allochthonous units as a whole influenced the exhumation of deep-seated continental crust, the transference of the suture zone over Gondwana, and the re-equilibration of the resulting overthickened crust.     

Molecular investigations into pollutant impact on roundnose grenadier (C. rupestris) and transplanted common mussel (M. edulis) in Skagerrak, the North Sea

The use of a suite of different biochemical markers in the deep sea fish Roundnose grenadier and caged common mussels, and analyses of contaminant levels in the mussels, indicate that Skagerrak are more polluted than waters around the Faroe Islands. In Roundnose, induction of ethoxyresorufin-O-deethylase (EROD) is consistent with increased benzo(a)pyrene hydroxylase (BPH) in mussels and elevated levels of polynuclear aromatic hydrocarbons (PAHs) in mussels and sediment from Skagerrak. This evidence of a large-scale environmental impact of pollutants should lead to further efforts to decrease transport into, and deposition of, waste compounds in the Skagerrak area.     

Investigating young water fractions in a small Mediterranean mountain catchment: Both precipitation forcing and sampling frequency matter

The proportion of water younger than 2–3 months (young water fraction, F_yw) has become increasingly investigated in catchment hydrology. F_yw is typically estimated by comparing seasonal tracer cycles in precipitation and streamflow, through water sampling. However, some open research questions remain, such as: (i) whether part of the summer precipitation should be discarded because the high evapotranspiration demand, (ii) how well F_yw serves as a metric to compare catchments, and (iii) how sampling frequency affects F_yw estimates. To address these questions, we investigated F_yw in soil-, ground- and stream waters for the small Mediterranean Can Vila catchment. Rainfall was sampled at 5-mm intervals. Mobile soil water and groundwater were sampled fortnightly. Stream water was sampled depending on flow at variable time intervals (30 min to 1 week). Over 58 months, this sampling provided 1,529 δ¹⁸O determinations. Isotopic analyses results led us to include summer precipitation in the input signal. We found the highest F_yw in mobile soil waters (34%), while this was almost zero for groundwater except during wet periods. For stream waters, F_yw depended on the discharge variations, so that the flow-weighted young water fraction () was 22.6%, whereas the time-weighted F_yw was just 6.2%. Both and its discharge sensitivity (S_d) varied when different 12-month sampling periods were investigated. The young water fraction that would be obtained from a virtual thorough sampling () was estimated from the S_d and the observed stream flow. This showed an underestimation of by 25% for the frequent dynamic sampling and 66% for weekly sampling, due to missing high flows. Our results confirm that F_yw and its discharge sensitivity are metrics very sensitive to meteorological forcing during the analysed period. Thus, comparisons between catchments need long-term mean annual values and their variability. Our findings also support the dependence of F_yw estimates on the sampling rate and show the advantages of flow-weighted analysis. Finally, catchment water turnover investigations should be accompanied by the analysis of flow duration curves.     

Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain)

Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5?hm³ of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contamination, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may affect the precious ecosystems of the Doñana National Park and the Guadalquivir marshlands. This report describes the evaluation of the risk of mobilisation of the waste sludge through (1) detailed geomorphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynamics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calculations to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposited within the Doñana National Park during future flood events.     

Stratigraphy of the Middle Devonian boundary: Formal definition of the susceptibility magnetostratotype in Germany with comparisons to sections in the Czech Republic, Morocco and Spain

One major difficulty in geology is high-resolution correlation among widely separated sections, especially in the Paleozoic where magnetostratigraphy polarity is not well established because rocks are often remagnetized, where critical biostratigraphic zonation may be poor or lacking, or where structural complexities make correlations very difficult. To address this problem, we have been using magnetostratigraphy susceptibility measurements. Here, we report our work from the Middle Devonian in Europe and North Africa. The Middle Devonian (Emsian–Eifelian) global boundary stratotype section and point (GSSP), located in the Eifel Hills, western Germany, was ratified by the International Subcommission on Stratigraphy in 1985, after careful evaluation of the biostratigraphy for this and many other sections. The boundary interval has been characterized using biostratigraphy, and the beginning of the Eifelian stage has been specifically defined by the first occurrence of the conodont Polygnathus costatus partitus. We have collected the Eifel Hills section for magnetic susceptibility (MS) measurement and here we establish it as the magnetostratotype for the Emsian–Eifelian stage boundary, by formally defining the magnetostratigraphy susceptibility for the section. We then collected, measured and compared the magnetostratotype to four other sections for which conodont biostratigraphy has been studied and where P. costatus partitus is present; two Emsian–Eifelian sections in Morocco and two sections in the Czech Republic (including the Emsian–Eifelian parastratotype). Finally, we have measured the MS for the El Puerto Creek section in the Cantabrian Mountains of Spain and identified the location of the Emsian–Eifelian boundary within the section based on MS comparison to the GSSP in conjunction with excellent biostratigraphic indicators, primarily brachiopods. While the conodont zonation in the El Puerto Creek section is poorly defined, we believe that the correspondence between the MS and biostratigraphy in the section allows the identification of the Emsian–Eifelian boundary. These results indicate that this method can be successfully applied to marine sequences where ambiguities in correlation exist.     

Sediment diffusive fluxes of Fe,Mn, and P in a eutrophic lake: Contribution from lateral vs bottom sediments

Water column data and porewater profiles are used to study the chemical evolution with time and with depth of a eutrophic lake. By using different approaches, diffusion fluxes for dissolved iron, manganese and phosphate are calculated and used to describe the processes occurring at the sediment-water interface as well as in the hypolimnion of the lake. These data are used in the elaboration of a qualitative model to describe the chemical behaviour of the sedimentary interface of an anoxic lake with emphasis on the Fe/P/S system. Acorona model is proposed to explain the evolution with time of the diffusion process by estimating the relative contribution of bottom and lateral sediment surfaces to the total fluxes of dissolved elements diffusing from the sediment to the overlying water. As the hypolimnion becomes more anoxic, it has been observed that lateral sediment surfaces (16 to 10 meters in depth) represents a larger supplier of diffusing dissolved components than the bottom sediment portion (bottom to 18 meters).     

Computer simulation of the low-molecular-weight inorganic species distribution of antimony(III) and antimony(V) in natural waters

Computer speciation models of antimony in aqueous, multicomponent solutions of environmental and biological interest have been developed. Antimony is an element of increasing environmental significance but one whose chemical speciation has not previously been modelled in any comprehensive way. The available thermodynamic data have been critically evaluated and entered into a thermodynamic database. The JESS suite of computer programs has been used to develop the thermodynamically-consistent mass balance equations used for modelling purposes. The calculated speciation distributions successfully account for some but not all key known properties of antimony in natural waters. It is believed that the main difference is most likely due to kinetic factors.     

Particle fluxes dynamics in Blanes submarine canyon (Northwestern Mediterranean)

Within the framework of the multidisciplinary RECS project and with the aim of describing the particle flux transfer from the continental shelf to the deep basin, an array of five mooring lines equipped with a total of five pairs of PPS3/3 sequential-sampling sediment traps and RCM-7/8 current meters were deployed 30 m above the bottom from March 2003 to March 2004 inside and outside the Blanes Canyon. One mooring line was located in the upper canyon at 600 m depth, one in the canyon axis at 1700 m depth and other two close to the canyon walls at 900 m depth. A fifth mooring line was deployed in the continental open slope at 1500 m water depth.The highest near-bottom downward particle flux (14.50 g m⁻² d⁻¹) was recorded at the trap located in the upper canyon (M1), where continental inputs associated with the presence of the Tordera River are most relevant. On the other hand, the downward fluxes (4.35 g m⁻² d⁻¹) in the canyon axis (M2) were of the same order as those found in the western flank (M3) of the canyon. Both values were clearly higher than the value (1.95 g m⁻² d⁻¹) recorded at the eastern canyon wall (M4). The open slope (M5) mass flux (5.42 mg m⁻² d⁻¹) recorded by the sediment trap located outside the canyon system was three orders of magnitude lower than the other values registered by the inner canyon stations. The relevance of our data is that it explains how the transport pathway in the canyon occurs through its western flank, where a more active and persistent current toward the open ocean was recorded over the entire year of the experiment.Off-shelf sediment transport along the canyon axis showed clear differences during the period of the study, with some important events leading to strong intensifications of the current coupled with large transport of particle fluxes to the deepest parts of the canyon. Such events are primarily related to increases in river discharge and the occurrence of strong storms and cascading events during the winter.In summary, in this study it is shown that the dynamics of the water masses and the currents in the study area convert the sharp western flank of the Blanes Canyon in a more active region that favors erosion processes than the eastern flank, which has a smoother topography and where the absence of erosional conditions yields to steadier sedimentary processes.     

Sediment segregation by biodiffusing bivalves

The selective processing of sediment fractions (sand and mud; >63 μm and ≤63 μm median grain size) by macrofauna was assessed using two size classes of inert, UV-fluorescent sediment fraction tracers (luminophores). The luminophores were applied to the sediment surface in 16 m² replicated plots, defaunated and control, and left to be reworked by infauna for 32 days. As the macrofaunal assemblage in the ambient sediment and the control plots was dominated by the common cockle Cerastoderma edule, this species was used in an additional mesocosm experiment. The diversity, abundance and biomass of the defaunated macrobenthic assemblage did not return to control values within the experimental period. Both erosion threshold and bed elevation increased in the defaunated plots as a response to the absence of macrofauna and an increase in microphytobenthos growth. In the absence of macrobenthos, we observed an accretion of 7 mm sediment, containing ca. 60% mud. Image analysis of the vertical distribution of the different luminophore size classes showed that the cockles preferentially mobilised fine material from the sediment, thereby rendering it less muddy and effectively increasing the sand:mud ratio. Luminophore profiles and budgets of the mesocosm experiment under “no waves–no current” conditions support the field data very well.