Aluminum coprecipitates with Fe (hydr)oxides: Does isomorphous substitution of Al for Fe in goethite occur?

Iron (hydr)oxides are common in natural environments and typically contain large amounts of impurities, presumably the result of coprecipitation processes. Coprecipitation of Al with Fe (hydr)oxides occurs, for example, during alternating reduction-oxidation cycles that promote dissolution of Fe from Fe-containing phases and its re-precipitation as Fe-Al (hydr)oxides. We used chemical and spectroscopic analyses to study the formation and transformation of Al coprecipitates with Fe (hydr)oxides. In addition, periodic density functional theory (DFT) computations were performed to assess the structural and energetic effects of isolated or clustered Al atoms at 8 and 25 mol% Al substitution in the goethite structure. Coprecipitates were synthesized by raising the pH of dilute homogeneous solutions containing a range of Fe and Al concentrations (100% Fe to 100% Al) to 5. The formation of ferrihydrite in initial suspensions with ?20 mol% Al, and of ferrihydrite and gibbsite in initial suspensions with ?25 mol% Al was confirmed by infrared spectroscopic and synchrotron-based X-ray diffraction analyses. While base titrations showed a buffer region that corresponded to the hydrolysis of Fe in initial solutions with ?25 mol% Al, all of the Al present in these solutions was retained by the solid phases at pH 5, thus indicating Al coprecipitation with the primary Fe hydroxide precipitate. In contrast, two buffer regions were observed in solutions with ?30 mol% Al (at pH ∼2.25 for Fe³⁺ and at pH ∼4 for Al³⁺), suggesting the formation of Fe and Al (hydr)oxides as two separate phases. The Al content of initial coprecipitates influenced the extent of ferrihydrite transformation and of its transformation products as indicated by the presence of goethite, hematite and/or ferrihydrite in aged suspensions. DFT experiments showed that: (i) optimized unit cell parameters for Al-substituted goethites (8 and 25 mol% Al) in clustered arrangement (i.e., the formation of diaspore-like clusters) were in good agreement with available experimental data whereas optimized unit cell parameters for isolated Al atoms were not, and (ii) Al-substituted goethites with Al in diaspore-like clusters resulted in more energetically favored structures. Combined experimental and DFT results are consistent with the coprecipitation of Al with Fe (hydr)oxides and with the formation of diaspore-like clusters, whereas DFT results suggest isomorphous Al for Fe substitution within goethite is unlikely at ?8 mol% Al substitution.     

A Note on Turbulence Stationarity and Wind Persistence Within the Stable Planetary Boundary Layer

This note presents initial results of an analysis of the stationarity of turbulence kinetic energy and the persistence of winds within the stable boundary layer (SBL). Measurements were made at 1.5 and 11 m above ground level from 0100 to 0600 local time on five nights during the JORNADA field experiment. The average stationarity ranged from about 160 to about 570 s. Wind persistence ranged from about ± 40° (3-min average) to about ± 36° (30-min average) on a weakly stable night, and from about ± 40° (3-min average) to about ± 27° (30-min average) on an strongly stable night. It is shown that, at 1.5 m, which we take to be within the surface layer, the average duration of stationarity of turbulent kinetic energy tends to correlate with the kurtosis of the heat flux; however, at 11 m, which we take to be outside of the surface layer, this correlation is poorly approximated.     

Spectrum of the Extreme Ultraviolet Nightglow as Measured by EURD

After 2 years of operation onboard the Spanish satellite MINISAT, EURDhas achieved an unprecedented success in the observation of the terrestrialnightglow in the EUV, covering a range of 350-1100 Å EURD hasprovided a total of more than 543 hours ofintegration in the long wavelength spectrometer and more than898 hours in the short wavelength one, allowing the achievement of the most detailed atlas of the terrestrial EUV nightglow ever obtained. We present here the spectra obtained, together with the identification of the lines, some of themdetected for the first time in the nightglow. These spectrarepresent an improvement in sensitivity of several orders of magnitude with respect to previous observations. It has beenpossible, for the first time, to identify the complete Lymanseries of atomic hydrogen, resolving up to Lyman-. It hasalso been possible to identify the helium Lyman- line at 537 Å andto detect other lines of the blended Lyman series of helium, at 515 and522 Å. The spectra clearly show the presenceof the OII lines at 617, 644, 673, and 718 Å, previouslyobserved in the dayglow but seen here for the first time in thenightglow. In addition to the recombination continuum of the atomicoxygen at 911 Å, two features of OI have been detected in thenightglow: the 3s ³D_o transition at 989 Å, previouslyobserved by Chakrabarti (1984) and the 2p³4s³S^o transition at 1040 Å, partially overlapped with Lyman , but clearly distinguishable from it. This feature has been seen for the first timein the terrestrial nightglow. The radiative recombination continuum of atomic oxygen at 911 Å, that wasabsent in the observations of the first year of operation of EURD,is now clearly visible. The reasons of the absence of the OIfeature during the first year of operation are still unknown.Anderson et al. (1976) also noticed a strong variation with time of this 911 Å emission.     

Factors Controlling In Situ Biogeochemical Transformation of Trichloroethene: Field Survey

We have previously defined in situ biogeochemical transformation as the biogenic formation of reactive minerals that are capable of abiotically degrading chlorinated solvents such as trichloroethene without accumulation of degradation products such as vinyl chloride (AFCEE et al. 2008 ). This process has been implemented in biowalls used to intercept contaminated groundwater. Abiotic patterns of contaminant degradation were observed at Altus Air Force Base (AFB) and in an associated column study, but not at other sites including Dover AFB. These abiotic patterns were associated with biogenic formation of reactive iron sulfide minerals. Iron sulfides in the form of small individual grains, coatings on magnetite, and sulfur‐deficient pyrite framboids were observed in samples collected from the Altus AFB biowalls and one of the EPA columns. Larger iron sulfide grains coated with oxide layers were observed in samples collected from Dover AFB. Altus AFB and the EPA column differed from Dover AFB in that groundwater flow at Dover AFB was relatively slow and potentially reversing. High volumetric sulfate consumption rates, an abiotic pattern of trichloroethene (TCE) degradation, and the formation of small, high surface area iron sulfide particles were associated with relatively high rates of TCE removal via an abiotic pattern. Geochemical modeling demonstrated that iron monosulfides such as mackinawite were near saturation, and iron disulfides such as pyrite were supersaturated at all sites. This environmental condition can be supportive of nucleation of small particles rather than crystal growth leading to larger particles. When nucleation is dominant, small, high surface area, and reactive particles result. When crystal growth dominates the crystals are larger and have lower specific surface area and reactivity. These results taken together suggest that creation of a dynamic environment can promote biogeochemical transformation based on generation of reactive iron sulfides.     

Radiative habitable zones in martian polar environments

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO2 ice cover and a permanent H2O ice layer. It was found that, though the CO2 ice is insufficient by itself to screen the UV radiation, at approximately 1 m depth within the perennial H2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties of the H2O ice layers (for instance snow-like layers). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the northern and southern polar Radiative Habitable Zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that photosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each martian polar spring and summer between approximately 1.5 and 2.4 m for H2O ice-like layers, and a few centimeters for snow-like covers. These martian Earth-like radiative habitable environments may be primary targets for future martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes.     

Effects of sewage pollution and bacterial load on growth and oxidative balance in the freshwater mussel Diplodon chilensis

The effect of chronic exposure to Escherichia coli on morphometrical parameters, different antioxidant defenses, lipid peroxidation and protein oxidation has been studied in digestive gland of the freshwater mussel Diplodon chilensis in the laboratory during 6 weeks. In, a parallel field study, we evaluated morphometrical and oxidative stress parameters in digestive glands of mussels collected from a sewage polluted area (SMA) and from a pristine area (control). Both sites were characterized by analyzing bacteriological and physic-chemical parameters. In the laboratory D. chilensis was able to filter bacteria at high concentrations with a clearing rate of 0.510 ± 0.036 L/h per gram of dry soft tissue mass. The digestive gland mass to shell length ratio (DGM/SL), reduced glutathione (GSH), lipid peroxidation, as concentration of thiobartituric acid reactive substances (TBARS), and glutathione-S-transferase (GST) activity of mussels fed with bacteria were significantly higher than those of control mussels after the fourth week. Fecal bacteria in lake water samples were undetectable in the control, and higher than 24,000 MPN (most probable number)/100 mL in SMA. DGM/SL was higher in SMA. No differences between sites were observed in total lipid and protein content, neither in superoxide dismutase activity. GSH content was higher in SMA, with no difference in the oxidized form. GST activity and MDA were significantly higher in SMA but protein oxidation was not affected.     

Reconstructions of the Mediterranean Outflow Water during the quaternary based on the study of changes in buried mounded drift stacking pattern in the Gulf of Cadiz

Contourite deposits in the central sector of the middle slope of the Gulf of Cadiz have been studied using a comprehensive acoustic, seismic and core database. Buried, mounded, elongated and separated drifts developed under the influence of the lower core of the Mediterranean Outflow Water are preserved in the sedimentary record. These are characterised by depositional features in an area where strong tectonic and erosive processes are now dominant. The general stacking pattern of the depositional system is mainly influenced by climatic changes through the Quaternary, whereas changes in the depositional style observed in two, buried, mounded drifts, the Guadalquivir and Huelva Drifts, are evidence of a tectonic control. In the western Guadalquivir Drift, the onset of the sheeted drift construction (aggrading QII unit) above a mounded drift (prograding QI unit) resulted from a new Lower Mediterranean Core Water hydrodynamic regime. This change is correlated with a tectonic event coeval with the Mid Pleistocene Revolution (MPR) discontinuity that produced new irregularities of the seafloor during the Mid- to Late-Pleistocene. Changes in the Huelva Drift from a mounded to a sheeted drift geometry during the Late-Pleistocene, and from a prograding drift (QI and most part of QII) to an aggrading one (upper seismic unit of QII), highlight a new change in oceanographic conditions. This depositional and then oceanographic change is associated with a tectonic event, coeval with the Marine Isotope Stage (MIS) 6 discontinuity, in which a redistribution of the diapiric ridges led to the development of new local gateways, three principal branches of the Mediterranean Lower Core Water, and associated contourite channels. As a result, these buried contourite drifts hold a key palaeoceanographic record of the evolution of Mediterranean Lower Core Water, influenced by both neotectonic activity and climatic changes during the Quaternary. This study is an example of how contourite deposits and erosive elements in the marine environment can provide evidence for the reconstruction of palaeoceanographic and recent tectonic changes.     

Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.     

New discoveries of mud volcanoes on the Moroccan Atlantic continental margin (Gulf of Cádiz): morpho-structural characterization

During the MVSEIS-08 cruise of 2008, ten new mud volcanoes (MVs) were discovered on the offshore Moroccan continental margin (Gulf of Cádiz) at water depths between 750 and 1,600?m, using multibeam bathymetry, backscatter imagery, high-resolution seismic and gravity core data. Mud breccias were recovered in all cases, attesting to the nature of extrusion of these cones. The mud volcanoes are located in two fields: the MVSEIS, Moundforce, Pixie, Las Negras, Madrid, Guadix, Almanzor and El Cid MVs in the western Moroccan field, where mud volcanoes have long been suspected but to date not identified, and the Boabdil and Al Gacel MVs in the middle Moroccan field. Three main morphologies were observed: asymmetric, sub-circular and flat-topped cone-shaped types, this being the first report of asymmetric morphologies in the Gulf of Cádiz. Based on morpho-structural analysis, the features are interpreted to result from (1) repeated constructive (expulsion of fluid mud mixtures) and destructive (gravity-induced collapse and submarine landsliding) episodes and (2) interaction with bottom currents.