Endogenous and nonimpact origin of the Arkenu circular structures (al‐Kufrah basin—SE Libya)

Abstract– The twin Arkenu circular structures (ACS), located in the al‐Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica‐impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind‐erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper‐gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic‐phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.     

An integrated approach to environmental quality assessment in a coastal setting in Campania (Southern Italy)

The coastal region of Southern Italy’s Caserta province, known as the Litorale Domitio (Domitia coast) has been subjected to increasing pressure from unsustainably fast economic and urban growth in the last century, that resulted in a induced serious land degradation. To obtain a comprehensive picture of the ecological status of the Domitia coastal zone (Campania, Southern Italy), a holistic methodology has been applied. Sedimentological, geochemical, and biological analyses of the surface sediments and water samples were performed along the submerged beach. The data were integrated using a geographical information system, together with information on past and present land use and human activities along the coast and in inland regions. Heavy metal concentrations in sediments plus As and Se (Cd, Co, Cr, Cu, Hg, Ni, Pb, V, Zn) and their enrichment factors, which are important for identifying polluted areas in countries like Italy (where environmental legislation has not yet established intervention limits), indicate excesses Cd, Cr, Pb, and V of probable anthropogenic origin. A microbiological contamination of marine waters has been detected by the Regional Environmental Protection Agency of Campania. The Bacteriological Quality Index map shows that bathing areas must be classified as “contaminated” and “highly contaminated”. High levels of chemical and biological contamination are particularly common in both bathing seawater and sediments along the coastal zone south of the Volturno River, where the existing wastewater treatment plant is not properly working, to date. Factory farming, which is widespread in the area, is likely to be an additional important source of contamination. In accordance with these conclusions, analyses of the benthic diatom community revealed dominance of eutrophic species.     

Predictive assessment of the asbestos content in the Western Italian Alps: an essential tool for an effective approach to risk analysis and management in tunneling operations and muck reuse

The modern approach to the design and management of tunnel excavation, and muck reuse, can be influenced to a great extent by the possible presence of rock formations containing asbestos minerals. This situation creates problems concerning the protection of the workers’ health and the expectable environmental criticalities, while a drastic re-consideration of the muck destination is necessary. Since, in the case of carcinogens, corrective action following exposure or dispersion is not acceptable, detailed knowledge of the characteristics of the material to be excavated is all the more essential. Only on this basis it is possible to design the tunneling operations in a prevention through design approach, so that the tunnel driving techniques, fittings and technologies, and special equipment and modus operandi, can grant, where necessary, minimized risk conditions, and make a correct decision on whether, how and when these measures must be fully activated, since such an approach involves a remarkable impact on costs and work organization. In the Western Alps a detailed identification and quantification of the asbestos content in rocks is a difficult task, due to the fact that the asbestos in the host rocks, mainly serpentinite, shows a highly variable distribution, typical of ophiolitic belts throughout the world, as it is mostly associated with joints and shear zones. The possibilities of a predictive assessment of the asbestos content in the formations to be excavated are here discussed, with special reference to the reliability of the achievable results.     

On the estimation of the maximum depth of investigation of transient electromagnetic soundings: the case of the Vizcaino transect,Mexico

We test an approach proposed in the literature for estimating the maximum depth of investigation (MDI) of in-loop transient electromagnetic soundings (TEM) with data from the Vizcaino transect, a profile of 38 TEM soundings crossing the Baja California peninsula. The validity of this approach is confirmed by comparing the MDI with the deepest interface of 16 stratified models. In these soundings, all located over the Vizcaino basin, the measured data are not affected by induced polarization. The models indicate the presence of a dipping conductor interpreted as a zone of seawater intrusion with a large lateral extension of over 70 km. The remaining 22 soundings, located over igneous and metamorphic rocks, show reversals in the voltage polarity, indicating the presence of induced polarization effects. The layered Cole-Cole models for these soundings suggest a significant decrease in the MDI. This is confirmed by analyzing the depth behavior of the subsurface current densities. We further analyze the noise level of a data set comprising close to 2000 late-time voltages of about 400 TEM sites acquired in northwestern Mexico. No difference was found between the stationary noise levels of winter and summer, presumably because near thunderstorms are practically absent in this part of Mexico.     

On the probability of exceeding a given river flow threshold

ABSTRACT

To obtain estimates of the probability that a river flow will exceed a given threshold at time t + 1, given the flow value at time t, two stochastic models are considered: a filtered Poisson process and a diffusion process with jumps. Estimates derived from linear regression are also considered. The model parameters are assumed to depend on the flow value. An application to the Delaware River is presented.

Editor D. Koutsoyiannis; Associate editor S. Grimaldi     

Multitemporal Landsat-MODIS fusion for cropland drought monitoring in El Salvador

Abstract

This study aims to develop an approach to characterize cropland drought conditions in El Salvador, Central America. The data were processed for 2016–2017 through three main steps: (1) reconstructing MODIS land-surface temperature (LST), (2) Landsat-MODIS data fusion and (3) drought delineation using the temperature vegetation dryness index (TVDI). The results of LST reconstruction using the random forests (RF) indicated the median RMSE value of 0.5?°C. The fusion results achieved from the STARFM compared with the reference Landsat data revealed close agreement with the correlation coefficient (r) values higher than 0.84. The TVDI results verified with that from the reference Landsat data indicated r values of 0.85 and 0.75 for 2016 and 2017, respectively. The larger very dry area was observed for the 2016 primera season due to prolonged droughts. Approximately 11.5% and 10.7% of croplands were, respectively, associated with very dry moisture condition in the 2016 and 2017 primera seasons.     

Sponge spicules indicate Holocene environmental changes on the Nabileque River floodplain, southern Pantanal, Brazil

Sponge spicules are siliceous microfossils that are especially useful for analysis of sandy fluvio-lacustrine sediments. Sponge spicules in a long sediment core (~550 cm below surface), consisting of fine sand, sandy silt, and organic-rich mud, recovered from the floodplain of the Nabileque River, southern Pantanal, Brazil (S20°16′38.3″/W57°33′00.0″), form the basis of a novel paleoenvironmental interpretation for this region. Optically stimulated luminescence dates constrain the timing of deposition to the middle-late Holocene and all spicules identified are typical of the Brazilian cerrado biome. The base of the section is dominated by Oncosclera navicella Carter 1881, Metania spinata Carter 1881, and Corvospongilla seckti Bonetto and Ezcurra de Drago 1966, which indicate a lotic to semi-lotic environment strongly influenced by an actively meandering river channel at ~6.7–5.7 ka BP. The appearance of Heterorotula fistula Volkmer-Ribeiro and Motta 1995, Dosilia pydanieli Volkmer-Ribeiro 1992 and Radiospongilla amazonensis Volkmer-Ribeiro and Maciel 1983 at ~340 cm downcore suggests a reduction in flowing water and a more stable lentic environment, consistent with deposition in an oxbow lake. This oxbow lake environment existed during an interval of regional aridity between ~4.5 and 3.9 ka BP. Spicules, as well as phytoliths and diatoms, are highly variable moving up-section, with species from both lotic and lentic ecosystems present. Above ~193 cm, the total abundance of spicules declines, consistent with wetter climate conditions and development of an underfit river similar to the modern floodplain. Results support hypotheses related to migration of the Paraguay River inferred from geomorphological studies and add a key southern-region dataset to the emerging Holocene database of paleoenvironmental records from the Pantanal wetlands.     

40Ar/39Ar dating and cosmic‐ray exposure time of desert meteorites: Dhofar 300 and Dhofar 007 eucrites and anomalous achondrite NWA 011

Abstract— We performed high‐resolution ⁴⁰Ar‐³⁹Ar dating of mineral separates and whole‐rock samples from the desert meteorites Dhofar 300, Dhofar 007, and Northwest Africa (NWA) 011. The chronological information of all samples is dominated by plagioclase of varying grain size. The last total reset age of the eucrites Dhofar 300 and Dhofar 007 is 3.9 ± 0.1 Ga, coeval with the intense cratering period on the Moon. Some large plagioclase grains of Dhofar 007 possibly inherited Ar from a 4.5 Ga event characteristic for other cumulate eucrites. Due to disturbances of the age spectrum of NWA 011, only an estimate of 3.2–3.9 Ga can be given for its last total reset age. Secondary events causing partial ⁴⁰Ar loss ≤3.4 Ga ago are indicated by all age spectra. Furthermore, Ar extractions from distinct low temperature phases define apparent isochrons for all samples. These isochron ages are chronologically irrelevant and most probably caused by desert alterations, in which radiogenic ⁴⁰Ar and K from the meteorite and occasionally K induced by weathering are mixed, accompanied by incorporation of atmospheric Ar. Additional uptake of atmospheric Ar by the alteration phase(s) was observed during mineral separation (i.e., crushing and cleaning in ultrasonic baths). Consistent cosmic‐ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of Dhofar 300 (25 ± 1 Ma) and Dhofar 007 (13 ± 1 Ma) using the mineral's specific target element chemistry and corresponding ³⁸Ar production rates.     

Discussion on sea level fluctuations along the Adriatic coasts coupling to climate indices forced by solar activity: Insights into the future of Venice

The North Atlantic Oscillation (NAO), which is a dominant circulation pattern in Northern Hemisphere winter, is known to affect sea-level variability in the Mediterranean Sea mainly through the hydrostatic response of water masses to pressure anomalies and changes in evaporation/precipitation budgets. In this study the influence of the NAO on sea levels along the Adriatic coasts is re-assessed in the attempt to uncover the potential causes of the observed high sensitivity of the northern basin to NAO fluctuations. The investigation is focused on the role of the NAO as forcing factor of the winds blowing in the area and of the freshwaters input from the Po River, both of which influence the hydrodynamics of the Northern Adriatic. In addition, some insights into the future of Venice are discussed on the basis of the hypothesis that NAO phases are modulated by the varying solar activity through the intensity of the Earth's geomagnetic activity.