M1, M2 site populations and distortion parameters in synthetic Mg-Fe orthopyroxenes from Mössbauer spectra and X-ray structure refinements

Synthetic orthopyroxenes free of trivalent cations Mg _x Fe _2-x Si₂O₆ (x=0; 1; 1.6) were investigated using both Mössbauer spectroscopy (MS) and single-crystal X-ray diffraction (XRD) in order to compare the results obtained on the same samples by the two techniques. The Mg-Fe distribution over M1 and M2 octahedral sites was determined on compounds with x = 1 and x=1.6, unheated and annealed at 650, 850 and 1040° C for 29 to 42 hours to obtain different degrees of order. The data obtained by the two techniques are in very good agreement, thus suggesting, for these compositions, the absence of systematic errors. The investigation of a synthetic compound with x=0 (ferrosilite) confirmed that the accuracy of both methods is very good. The observed small discrepancies between the results are not significant if compared with the relevant standard deviations which, for both methods, were derived by leastsquares calculations. The best fit is obtained for Fe(M1)/ Fe(tot) parameters that, in both methods, are directly calculated without using data from chemical analysis. For the unheated samples the values by MS and XRD are respectively: 0.185(9) and 0.180(4) for x = 1.6; 0.282(8) and 0.285(2) for x=1; 0.503(7) and 0.502(1) for x=0. For all the investigated samples the correlation between the geometrical distortion parameters, calculated by X-ray refinement, and the quadrupole splitting values, measured by Mössbauer spectroscopy, agrees with the theoretical model of Ingalls (1964).     

Microbial photosynthesis in coral reef sediments (Heron Reef, Australia)

We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 μm in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concentrations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O₂ m⁻² h⁻¹ and was strongly correlated with Chl a content, which lay between 31 and 84 mg m⁻². An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2–1.0 mmol Ca²⁺ m⁻² h⁻¹). The sediments of at least three stations were net calcifying. Sedimentary N₂-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N₂ m⁻² h⁻¹ and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N₂-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.     

High connectivity and directional gene flow in European Atlantic and Mediterranean populations of Ciona intestinalis sp. A

Understanding the factors that cause population divergence has long been of interest to marine biologists in their attempts to interpret the effect of human‐mediated vectors. Broadcast‐spawning species with limited dispersal capability are excellent candidates to measure the present‐day patterns of genetic diversity. The tunicate Ciona intestinalis (Ascidiacea) is comprised of a complex of morphologically cryptic species that form vigorous aggregates in eutrophic habitats (harbors, gulfs and lagoons) where they can compete with the epibenthic community and cause biofouling problems. This study investigated biogeographic variability and migration patterns of C. intestinalis sp. A along Northeast Atlantic and Mediterranean coasts using microsatellite markers. Data presented here on 371 specimens collected from 17 populations reveal high genetic polymorphism, but with a deficit of heterozygote deficiency. Absence of evidence for isolation by distance suggests that the genetic patterns do not reflect the geographic distribution of sampled populations. Substantial gene flow and artificial potential for dispersal boost high levels of within‐population genetic variability and prevent genetic differentiation within and between seas. A predominant eastward migration pattern was revealed by the data set, with very limited opportunity for C. intestinalis sp. A to travel westward. This directional movement indicates that other properties (e.g. habitat quality, genetic traits, mating system, life cycle) may cause adaptive divergence at a large biogeographic scale.     

A Fe K-edge XAS study of amethyst

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO₂) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe²⁺ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell:  = 1.78(2) Å, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe³⁺ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe⁴⁺ is stabilised by the favourable local structural arrangement.     

Lateral variations in tectonic style across cross-strike discontinuities: an example from the Central Apennines belt (Italy)

In foreland thrust belts, abrupt lateral changes in tectonic style, structural–stratigraphic features, and topography usually occur across cross-strike faults. The Central Apennines of Italy offer an exceptional scenario of lateral variations in tectonic setting. Here, the Sangro Volturno oblique thrust ramp (SVOTR) represents the outer thrust front of the Pliocene–Quaternary foreland thrust system, confining southward the axial culmination of the orogen that occurs in the Central Apennines. We present an interpretation of the Pliocene–Quaternary evolution of this cross-strike fault through an integrated dataset including structural-geological mapping and subsurface onshore seismic reflection profiles. The interpretation of the structural framework is augmented by the analysis of low-temperature thermochronometers from 32 new sites extending across the subsurface transverse structure. As evidenced by seismic line interpretation, the localization and development of the SVOTR have been influenced by inherited extensional faults within a positive inversion tectonics context. The regional distribution of the maximum paleotemperature values across the SVOTR constrains the original extent of the allochthonous thrust sheet over all its hanging-wall and footwall blocks. The Pliocene–Quaternary thrusting and inversion of SVOTR caused the strong hanging-wall uplift, which brought to the complete erosion of the allochthonous units and the exhumation of the Adria units. The integrated analysis of low-temperature thermochronometers and structural evidence as applied in the study case can define the role of major cross-strike discontinuities in foreland thrust belts, by constraining and verifying their tectonics inversion significance and the amount of related exhumation.     

Spatial modelling and uncertainty assessment of pyroclastic cover thickness in the Sorrento Peninsula

Assessing spatial variability of soil thickness is a critical issue for understanding and predicting slope processes. The present work was aimed at estimating the spatial scales at which the variation of pyroclastic cover thickness occurs in a sample area in the Sorrento Peninsula (Italy). Stochastic simulation was used to understand the spatial variability of pyroclastic cover thickness on Mount Pendolo and to assess its spatial uncertainty. In the study area, covering about 0.7 km², thickness measurements were collected using electrical resistivity tomography profiles, continuous core drillings and steel rod penetrometric tests. Variographic analysis revealed the occurrence of an anisotropic behaviour along the N50 and N140 directions. In the latter anisotropic direction, a nested variogram was fitted including (1) a long-range component which could be related to large-scale factors, like the curvature of the slope and contributing area and (2) a shorter scale variation which is probably associated with the occurrence of denudation processes or to the articulate cover/bedrock interface. To assess the spatial variability and uncertainty of pyroclastic cover thickness, a stochastic simulation algorithm was used and 500 equally probable images of cover thickness were yielded. The results showed that a better thickness distribution map can be drawn by simulating the data collected on the slope and at the footslope separately. The approach also allowed delineating the areas characterized by greater uncertainty, suggesting supplementary measurements to further improve the cover thickness distribution model, thus reducing the uncertainty.     

The response of nematodes to deep-sea CO2 sequestration: A quantile regression approach

One proposed approach to ameliorate the effects of global warming is sequestration of the greenhouse gas CO₂ in the deep sea. To evaluate the environmental impact of this approach, we exposed the sediment-dwelling fauna at the mouth of the Monterey Submarine Canyon (3262 m) and a site on the nearby continental rise (3607 m) to CO₂-rich water. We measured meiobenthic nematode population and community metrics after ～30-day exposures along a distance gradient from the CO₂ source and with sediment depth to infer the patterns of mortality. We also compared the nematode response with that of harpacticoid copepods. Nematode abundance, average sediment depth, tail-group composition, and length: width ratio did not vary with distance from the CO₂ source. However, quantile regression showed that nematode length and diameter increased in close proximity to the CO₂ source in both experiments. Further, the effects of CO₂ exposure and sediment depth (nematodes became more slender at one site, but larger at the other, with increasing depth in the sediment) varied with body size. For example, the response of the longest nematodes differed from those of average length. We propose that nematode body length and diameter increases were induced by lethal exposure to CO₂-rich water and that nematodes experienced a high rate of mortality in both experiments. In contrast, copepods experienced high mortality rates in only one experiment suggesting that CO₂ sequestration effects are taxon specific.     

Thermoluminescence dating of the Kamil impact crater (Egypt)

Thermoluminescence (TL) dating has been used to determine the age of the meteorite impact crater at Gebel Kamil (Egyptian Sahara). Previous studies suggested that the 45 m diameter structure was produced by a fall in recent times (less than 5000 years ago) of an iron meteorite impactor into quartz‐arenites and siltstones belonging to the Lower Cretaceous Gilf Kebir Formation. The impact caused the complete fragmentation of the impactor, and the formation of a variety of impactites (e.g., partially vitrified dark and light materials) present as ejecta within the crater and in the surrounding area. After a series of tests to evaluate the TL properties of different materials including shocked intra‐crater target rocks and different types of ejecta, we selected a suite of light‐colored ejecta that showed evidence of strong thermal shock effects (e.g., partial vitrification and the presence of high‐temperature and ‐pressure silica phases). The abundance of quartz in the target rocks, including the vitrified impactites, allowed TL dating to be undertaken. The variability of radioactivity of the intracrateric target rocks and the lack of direct in situ dosimetric evaluations prevented precise dating; it was, however, possible to constrain the impact in the 2000 BC–500 AD range. If, as we believe, the radioactivity measured in the fallback deposits is a reliable estimate of the mean radioactivity of the site, the narrower range 1600–400 BC (at the 2σ confidence level) can be realistically proposed.