Metal geochemistry in a mine-polluted estuarine system in Spain

The Rio Tinto and Rio Odiel drain the Iberian Pyrite Belt, an important metal-rich sulphide deposit. The rivers are highly acidic (pH 2.2–3.6) and have milli-molar SO4 and Fe concentrations and micro-molar Co, Cu, Mn and Zn concentrations. Observed dissolved metal levels were at a maximum during autumn and early winter surveys (e.g. Rio Tinto: 460–856 ? M Cu), and lower in late winter, spring and summer (121–175 ? M Cu). This variability is attributed to the production of concentrated acid mine drainage (AMD) during periods of enhanced microbial activity at higher temperatures in summer, and a subsequent run-off of the AMD into the rivers with the first rain in the autumn. Lower temperatures and dilution by winter floods resulted in a reduction of river metal concentration towards the end of the wet season. Metal distributions in the estuarine mixing zones of the Tinto and Odiel were governed by the acidity. The lack of metal transfer from the dissolved to the particulate phase in the low salinity region is attributed to the electrostatic repulsion between the metal cations and positive charges on particle surfaces, and/or to the protonation of adsorption sites at low pH. Dissolved Pb was an exception and showed marked removal in the low salinity zone at low pH (pH 2.5), due to its particle reactive nature. The gross metal fluxes from the Rio Tinto and Rio Odiel are important on a global scale, for example amounting to 8.1 and 1.6% of the estimated global riverine dissolved Zn and Cu fluxes. The fluxes of metals from the estuary contribute to enhanced dissolved metal concentrations observed in the Gulf of Cadiz

Activation and inhibition of retinal ganglion cells in response to epiretinal electrical stimulation: A computational modelling study

Objective. Retinal prosthetic devices aim to restore sight in visually impaired people by means of electrical stimulation of surviving retinal ganglion cells (RGCs). This modelling study aims to demonstrate that RGC inhibition caused by high-intensity cathodic pulses greatly influences their responses to epiretinal electrical stimulation and to investigate the impact of this inhibition on spatial activation profiles as well as their implications for retinal prosthetic device design. Another aim is to take advantage of this inhibition to reduce axonal activation in the nerve fibre layer. Approach. A three-dimensional finite-element model of epiretinal electrical stimulation was utilized to obtain RGC activation and inhibition threshold profiles for a range of parameters. Main results. RGC activation and inhibition thresholds were highly dependent on cell and stimulus parameters. Activation thresholds were 1.5, 3.4 and 11.3 mu A for monopolar electrodes with 5, 20 and 50 mu m radii, respectively. Inhibition to activation threshold ratios were mostly within the range 2-10. Inhibition significantly altered spatial patterns of RGC activation. With concentric electrodes and appropriately high levels of stimulus amplitudes, activation of passing axons was greatly reduced. Significance. RGC inhibition significantly impacts their spatial activation profiles, and therefore it most likely influences patterns of perceived phosphenes induced by retinal prosthetic devices. Thus this inhibition should be taken into account in future studies concerning retinal prosthesis development. It might be possible to utilize this inhibitory effect to bypass activation of passing axons and selectively stimulate RGCs near their somas and dendrites to achieve more localized phosphenes

Metal fluxes through the Strait of Gibraltar: the influence of the Tinto and Odiel rivers (SW Spain)

A large set of new data concerning dissolved metal concentrations has been acquired in the Gulf of Cadiz and in the Strait of Gibraltar from 1996 to 1999. These data, associated with models (hydrodynamic, tracer advection-dispersion and mixing), have been used to assess the influence of rivers draining the South Iberian Pyrite Belt on the Gulf of Cadiz and on the Atlantic inflow in the Strait of Gibraltar. Metal concentrations in surface waters from the Gulf of Cadiz are maximal near the mouth of the Tinto/Odiel rivers with values exceeding 50 nmol/kg (Mn), 5 nmol/kg (Ni), 30 nmol/kg (Cu), 100 nmol/kg (Zn), 0.9 nmol/kg (Cd) and 45 nmol/kg (As). From the Tinto/Odiel river, a plume of contamination follows the coast in the direction of the Strait of Gibraltar. The computation of a tracer advection-dispersion model confirms that the coastal currents carry the metals discharged from the Tinto and Odiel to the Strait of Gibraltar. From temperature-salinity and metal-salinity plots, four water masses can be recognised in the Gulf of Cadiz and in the Strait of Gibraltar: North Atlantic Surface Water (NASW), North Atlantic Central Water (NACW) and metal-enriched Spanish Shelf Waters from the Gulf of Cadiz (SSW). The Mediterranean Outflow Water (MOW) is also clearly seen at depths greater than 300 m. The chemical characteristics of these various water masses have been used in a mixing model to evaluate their relative contribution to the Atlantic inflow through the Strait of Gibraltar. These contributions are seasonally variable. in June 1997, the contribution was: 80 +/- 20%, 5 +/- 5% and 15 +/- 10% for NASW, NACW and SSW, respectively. In September, the SSW contribution was apparently negligible. Finally, these relative contributions allow the evaluation of the metal fluxes in the Strait of Gibraltar. The presence of SSW in the Strait increases the metal flux to the Mediterranean Sea by a factor of 2.3 (Cu), 2.4 (Cd), 3 (Zn) and 7 (Mn). It does not modify significantly As and Ni fluxes. (C) 2001 Published by Elsevier Science B.V

Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses

We present new 3.6 and 4.5 μm secondary eclipse measurements for five cool (T 1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We confirm and refine the orbital eccentricity of HAT-P-17b, which is also the only planet in our sample with a known outer companion. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to predictions from 1D atmosphere models for each planet. For planets with hydrogen-dominated atmospheres and equilibrium temperatures cooler than ~1000 K, this ratio should vary as a function of both atmospheric metallicity and the carbon-to-oxygen ratio. For HAT-P-26b, our observations are in good agreement with the low atmospheric metallicity inferred from transmission spectroscopy. We find that all four of the planets with detected eclipses are best matched by models with relatively efficient circulation of energy to the nightside. We see no evidence for a solar-system-like correlation between planet mass and atmospheric metallicity, but instead identify a potential (1.9σ) correlation between the inferred CH4/(CO + CO2) ratio and stellar metallicity. Our ability to characterize this potential trend is limited by the relatively large uncertainties in the stellar metallicity values. Our observations provide a first look at the brightness of these planets at wavelengths accessible to the James Webb Space Telescope, which will be able to resolve individual CH4, CO, and CO2 bands and provide much stronger constraints on their atmospheric compositions

Fluxes of Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn through the Strait of Dover into the southern North Sea

Measurements of dissolved and particulate Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn have been made on a regular basis over a fifteen month period at stations across the Strait of Dover, as a part of the Fluxmanche programme. These measurements have been integrated with data on fluxes of water and suspended particulate matter also obtained within the Fluxmanche programme to provide estimates of fluxes of these elements into the southern North Sea. For Fe, Mn, and Pb, particulate fluxes are greater than dissolved fluxes, whilst the reverse is true for Cd, Cu and Ni. Significant daily changes in magnitude of the modelled fluxes of water through the Strait when combined with averaged metal concentrations over longer timescales, result in large changes of fluxes of metals on a day to day basis. However, relatively small changes are evident for integrated autumn fluxes of metals for consecutive years using the available database. The annual fluxes of dissolved plus-leachable particulate metals through the Strait of Dover relative to the most recent estimates of riverine and atmospheric inputs to the North Sea were in the order Cu (44%) > Cd (39%) > Zn (16%) > Pb (11%)