Near-inertial motions in and around the Palamós submarine canyon (NW Mediterranean) generated by a severe storm

During 9–16 November 2001 the western Mediterranean Sea was lashed by one of the most extreme storms of the last decades. Current meter data from seven moorings in the Palamós submarine canyon (northwestern Mediterranean) are analyzed to understand the vertical propagation of near-inertial energy generated by the storm. The daily inertial rotary components are examined for evidence of free and forced near-inertial oscillations. Free near-inertial motions are increased during the storm, although they are larger outside the canyon than inside. Conversely, forced near-inertial motions are relatively large inside the canyon but are almost negligible outside. Based on the results of a three-dimensional ocean circulation model, these differences are shown to be caused by the presence of a storm-generated alongshore front. The mechanisms by which near-inertial energy propagates are distinct at each side of the front. On the onshore side of the front (inside the canyon), free near-inertial motions are rapidly carried away by normal inertial waves, and wave reflection off canyon wall is responsible for the dissipation of free near-inertial motions and enhancement of forced near-inertial motions. On the offshore side of the front (outside the canyon), on the other hand, free near-inertial motions propagates first downward with anomalously low frequency internal waves and are then advected southward and offshore by the mean flow.     

Parameterization of Entrainment in a Sheared Convective Boundary Layer Using a First-order Jump Model

Basic entrainment equations applicable to the sheared convective boundary layer (CBL) are derived by assuming an inversion layer with a finite depth, i.e., the first-order jump model. Large-eddy simulation data are used to determine the constants involved in the parameterizations of the entrainment equations. Based on the integrated turbulent kinetic energy budget from surface to the top of the CBL, the resulting entrainment heat flux normalized by surface heat flux is a function of the inversion layer depth, the velocity jumps across the inversion layer, the friction velocity, and the convection velocity. The developed first-order jump model is tested against large-eddy simulation data of two independent cases with different inversion strengths. In both cases, the model reproduces quite reasonably the evolution of the CBL height, virtual potential temperature, and velocity components in the mixed layer and in the inversion layer.The part of this work was done when the first author visited at NCAR.     

Field application of calcite Dispersed Alkaline Substrate (calcite-DAS) for passive treatment of acid mine drainage with high Al and metal concentrations

Passive treatment systems are widely used for remediation of acid mine drainage (AMD), but existing designs are prone to clogging or loss of reactivity due to Al- and Fe-precipitates when treating water with high Al and heavy metal concentrations. Dispersed alkaline substrate (DAS) mixed from a fine-grained alkaline reagent (e.g. calcite sand) and a coarse inert matrix (e.g. wood chips) had shown high reactivity and good hydraulic properties in previous laboratory column tests. In the present study, DAS was tested at pilot field scale in the Iberian Pyrite Belt (SW Spain) on metal mine drainage with pH near 3.3, net acidity 1400–1650 mg/L as CaCO₃, and mean concentrations of 317 mg/L Fe (95% Fe(II)), 311 mg/L Zn, 74 mg/L Al, 20 mg/L Mn, and 1.5–0.1 mg/L Cu, Co, Ni, Cd, As and Pb. The DAS-tank removed an average of 870 mg/L net acidity as CaCO₃ (56% of inflow), 25% Fe, 93% Al, 5% Zn, 95% Cu, 99% As, 98% Pb, and 14% Cd, but no Mn, Ni or Co. Average gross drain pipe alkalinity was 181 mg/L as CaCO₃, which increased total Fe removal to 153 mg/L (48%) in subsequent sedimentation ponds. Unfortunately, the tank suffered clogging problems due to the formation of a hardpan of Al-rich precipitates. DAS lifetime could probably be increased by lowering Al-loads.     

Global change revealed by palaeolimnological records from remote lakes: a review

Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.     

Astrometry and Photometry of Open Clusters: NGC 1746, NGC 1750 and NGC 1758

We present an astrometric and photometric analysis of a 5 square degree region around NGC 1746, NGC 1750 and NGC 1758, close to the Galactic anti-center (l = 179°, b = -10°), in Taurus. We describe the acquisition, reduction and analysis of the observational data, including the development of innovative data analysis tools. The data comprise CCD UBVRI photometry, photographic BVR photometry and proper motions. The clusters NGC 1750 and 1758 are poor and loose. We determine several physical parameters (distance, age, colour excess, positions, density profiles). The mutual separation, their relative velocity and their age difference allows us to conclude that these two clusters are physically independent. No real object in the area can be identified as NGC 1746. This revised version was published online in July 2006 with corrections to the Cover Date.     

OSIRIS Imaging and Spectroscopy for the GTC

The characteristics of OSIRIS (Optical System for Imaging and low-Resolution Integrated Spectroscopy), an instrument for imaging and low resolution spectroscopy proposed for the GTC 10m telescope, are described. OSIRIS is a multi-purpose instrument that incorporates in a "classical design" several innovations, already tested in 4m-class telescopes, that will provide a powerful first light instrument for the GTC. This revised version was published online in July 2006 with corrections to the Cover Date.     

General patterns of circulation, sediment fluxes and ecology of the Palamós (La Fonera) submarine canyon, northwestern Mediterranean

Currents, particle fluxes and ecology were studied in the Palamós submarine canyon (also known as the Fonera canyon), located in the northwestern Mediterranean. Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope. Additionally, local and regional hydrographic cruises were carried out. Current data showed that mean near surface and mid-depth currents were oriented along the mean flow direction (NE–SW), although at 400 and 1200 m depth within the canyon current reversals were significant, indicating a more closed circulation inside the canyon. Mean near-bottom currents were constrained by the local bathymetry, especially at the canyon head. The most significant frequency at all levels was the inertial frequency. A second frequency of about three days, attributed to a topographic wave, was observed at all depths, suggesting that this wave was probably not trapped near the bottom. The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow.In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher up-canyon from about 1200 m depth than offshore, defining, along with the different hydrodynamics, two canyon domains: one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf-slope front. The higher near-bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows. During the deployment period, the increase in downward particle fluxes occurred by mid-November, when a severe storm took place. On the canyon walls at 1200 m depth, suspended sediment concentrations, downward particle fluxes, chlorophyll and particulate C and N were higher on the southern wall than on the northern wall inversely to the current’s energy. This could be caused by an upward water supply on the southern canyon wall and/or the mean flow interacting with the canyon bathymetry. In the swimmers collected by the sediment traps, the dominant species was an elasipod holothurian, which has not been recorded in other canyons or elsewhere in the Mediterranean, indicating particular speciation.     

Shelf-slope exchanges by frontal variability in a steep submarine canyon

We study the dynamics of a frontal jet and its short-timescale variability generated by the interaction with a submarine canyon using a limited-area fine-resolution three-dimensional coastal ocean model. The focus is on the steep and narrow Palamós Canyon located off the northeast Catalan coast (northwestern Mediterranean) that is characterized by the presence of a permanent along-slope density-driven current. First, we analyse the stationary circulation induced with different jet locations and show a deflection of the flow in the vicinity of the canyon. Significant vertical motions develop as a result of these current adjustments; the general pattern such as downwelling upstream of the canyon and upwelling downstream are always observed. Second, we analyse the circulation and exchanges associated with an onshore displacement of the jet; thus produces a meander propagating with the flow that interacts with the canyon. We find that the resulting three-dimensional patterns present an oscillation characterized by an intense downwelling followed by upwelling. As a result of this interaction, shelf-slope exchanges and vertical motions are enhanced in the area compared with the passing of a meander above a shelf that is not indented by a submarine canyon. The resulting horizontal transports through the Palamós canyon represent up to 10% of the along-shore fluxes on the shelf and appear to be sufficient to exchange the shelf water of the Gulf of Lions and Catalan sea in 2.5 years. Considering the number of canyons existing in the area, we can estimate an exchange of all the shelf waters in less than 3 months.