A densification mechanism to model the mechanical effect of methane hydrates in sandy sediments

Recent pore-scale observations and geomechanical investigations suggest the lack of true cohesion in methane hydrate-bearing sediments (MHBSs) and propose that their mechanical behavior is governed by kinematic constrictions at pore-scale. This paper presents a constitutive model for MHBS, which does not rely on physical bonding between hydrate crystals and sediment grains but on the densification effect that pore invasion with hydrate has on the sediment mechanical properties. The Hydrate-CASM extends the critical state model Clay and Sand Model (CASM) by implementing the subloading surface model and introducing the densification mechanism. The model suggests that the decrease of the sediment available void volume during hydrate formation stiffens its structure and has a similar mechanical effect as the increase of sediment density. In particular, the model attributes stress-strain changes observed in MHBS to the variations in sediment available void volume with hydrate saturation and its consequent effect on isotropic yield stress and swelling line slope. The model performance is examined against published experimental data from drained triaxial tests performed at different confining stress and with distinct hydrate saturation and morphology. Overall, the simulations capture the influence of hydrate saturation in both the magnitude and trend of the stiffness, shear strength, and volumetric response of synthetic MHBS. The results are validated against those obtained from previous mechanical models for MHBS that examine the same experimental data. The Hydrate-CASM performs similarly to previous models, but its formulation only requires one hydrate-related empirical parameter to express changes in the sediment elastic stiffness with hydrate saturation.     

Post-Last Glacial Maximum evolution of a “fjord-type” lake based on high-resolution seismic data: the Lago Roca/Acigami (southern Tierra del Fuego,Argentina/Chile)

Lago Roca/Acigami is a “fjord-type” lake located in the southernmost part of South America, in the proximity of the Beagle Channel. A high-resolution seismic survey was carried out to analyse the seismic stratigraphy of the lake and to shed some light on the post-Last Glacial Maximum history of the area. Six seismic units were recognised, and their nature and depositional context were interpreted using seismic stratigraphy and acoustic facies analysis. A buried large ridge was identified within the glacial unit (SU1), interpreted as a frontal moraine that indicates a stabilisation phase. After retreat of the glacier from the basin, the trough was flooded by meltwater and a lake developed (SU2). The seismic facies, from bottom to top, depict a transition from ice-contact (SU2; SU3) to ice-distal proglacial conditions (SU4). A thick draping unit (SU5) marks a marine transgressive event and the instauration of a fjord environment in the basin. The marine transgression was a rapid event preceded by a fall in the lake level that caused an erosional unconformity. During the fjord phase the sedimentation remained controlled by meltwater discharge. Sea level fall, and subsequent disconnection from the Beagle Channel was accompanied by a progradation of the glaciofluvial deltaic sediments and the occurrence of several mass-wasting deposits (SU6). © 2019 John Wiley & Sons, Ltd.     

The Ca II triplet lines in Starburst and AGN

In the frame of the Starburst Model for AGN, we show that the evolution of a massive stellar cluster in a high metallicity environment can reproduce the observed equivalent widths of Ca II triplet lines in absorption in Starburst and active galactic nuclei. In the case of Starburst galaxies, this strength, together with the emission-line ratio [O II]/[O III], can be used to determine the age of the cluster. The strength of the Ca II lines is mainly governed by the age of the stellar cluster through the presence of red supergiant stars.     

Statistical Study and Prediction of Variability of Erythemal Ultraviolet Irradiance Solar Values in Valencia,Spain

The goal of this study was to statistically analyse the variability of global irradiance and ultraviolet erythemal (UVER) irradiance and their interrelationships with global and UVER irradiance, global clearness indices and ozone. A prediction of short-term UVER solar irradiance values was also obtained. Extreme values of UVER irradiance were included in the data set, as well as a time series of ultraviolet irradiance variability (UIV). The study period was from 2005 to 2014 and approximately 250,000 readings were taken at 5-min intervals. The effect of the clearness indices on global irradiance variability (GIV) and UIV was also recorded and bi-dimensional distributions were used to gather information on the two measured variables. With regard to daily GIV and UIV, it is also shown that for global clearness index (k_t) values lower than 0.6 both global and UVER irradiance had greater variability and that UIV on cloud-free days (k_t higher than 0.65) exceeds GIV. To study the dependence between UIV and GIV the χ² statistical method was used. It can be concluded that there is a 95% probability of a clear dependency between the variabilities. A connection between high k_t (corresponding to cloudless days) and low variabilities was found in the analysis of bi-dimensional distributions. Extreme values of UVER irradiance were also analyzed and it was possible to calculate the probable future values of UVER irradiance by extrapolating the values of the adjustment curve obtained from the Gumbel distribution.     

The Intertidal Burrowing Crab <Emphasis Type="Italic">Neohelice</Emphasis> (=<Emphasis Type="Italic">Chasmagnathus</Emphasis>) <Emphasis Type="Italic">granulata</Emphasis> Positively Affects Foraging of Rodents in South Western Atlantic Salt Marshes

The role of positive and indirect interactions is often crucial in communities with intense abiotic stress such as salt marshes. The burrowing crab, Neohelice (=Chasmagnathus) granulata, is the dominant benthic macroinvertebrate of southwest Atlantic marshes (southern Brazil to Northern Argentinean Patagonia), having strong direct and indirect effects on marsh soil and, in consequence, on marsh vegetation and primary consumers. In this work, we investigate if this crab indirectly modifies habitat use by the granivorous rodents, Akodon azarae and Oligoryzomys flavescens, by increasing nutrient availability and thus enhancing seed production by the marsh plant Spartina densiflora. The study was conducted at the Mar Chiquita Coastal Lagoon, Argentina (37°32′ S). Rodent frequencies in S. densiflora were positively correlated with crab densities throughout the low and middle marsh. Additionally, the highest quality of S. densiflora and inflorescence density was recorded at the highest crab densities. Experimental manipulation of crab densities shows that N. granulata indirectly enhances the performance of S. densiflora (e.g., decreased fiber content and C/N ratios) and increases density of seeds. Moreover, N. granulata also facilitates S. densiflora seed availability to rodents by concentrating them in sediment mound at their burrows entrances. Experimental rodent exclusions showed that rodent species used S. densiflora seeds, a variable positively related to crab burrow density. Thus, our results show that N. granulata drives the granivorous rodent distribution and the intensity of seeds–rodent interaction trough facilitative and indirect interactions in marsh community.     

Morphometric analysis of Colangüil river basin and flash flood hazard,San Juan,Argentina

This work analyzes various morphometric characteristics of the Colangüil river basin in order to evaluate flash flood hazards. Such high-water events pose a risk to the similarly named small village located at the basin’s foot area. For this purpose, the basin is divided into seven sub-basins and some basic measurements (surface, perimeter, basin length, river beds, elevations and slope of the main river bed, and of a number of minor river beds) are calculated. These measurements permit to predict approximately the behavior of the basin in the presence of a series of theoretical rainstorms that may generate unusual runoff volumes that make up such flash floods.     

Environmental evaluation of MODIS-derived land units

A land stratification of the French territory had been previously performed based on time series of vegetation and texture indices. This stratification led to 300 radiometrically homogenous regions that were considered as land units (LUs). In this paper, we present a quantitative analysis of the LUs, with the aim of testing if these LUs are linked to landscape. In this sense, an evaluation of their thematic meaning in terms of environmental variables and land cover was performed. In order to achieve this, we first conducted a statistical analysis at national scale using a set of environmental variables and land cover by means of Moran’s autocorrelation index and Spearman rank correlation index. Second, to analyze the quality of the boundaries between neighboring LUs, we developed a method based on the Spearman rank correlation index calculated on test areas across the boundaries. The first analyses showed that the most explanatory variables of the LUs were land cover, topography and parent material. The boundaries analysis was applied at a regional scale (Pyrenean region), and showed that 89% of the boundaries were well explained by the land cover compositions. The results obtained support the hypothesis that time series of broad resolution remote-sensing images can capture landscape identities and produce LUs maps that have an environmental and land occupation sense.     

Green areas and water management in residential developments in the European Western Mediterranean. A case study of Alicante,Spain

The intense urbanization of the Mediterranean coast as a result of the development of residential-tourist activities since the 1960s, especially from the mid-1990s to 2008, has generated new urban environments associated with extensive urban typologies. These include gardens linked to houses with outdoor spaces that are shared between all of the residents of the residential developments. The aim of this study is to determine the main characteristics of these new urban green spaces and their effect on domestic water consumption in Alicante (Spain). To do this, we analysed the characteristics that define these types of gardens (surface area, density, plant species and irrigation system) and determined their water needs using the Water Use Classifications of Landscape Species method. The main conclusions are that the increase in paved areas is one of the strategies to reduce water consumption in spaces where turf grass is the most common plant species. This is due to increased water prices and lack of water resources.