Magmatic diversity of western Mexico as a function of metamorphic transformations in the subducted oceanic plate

This geochemical study of the Mexican subduction zone elucidates how metamorphic and dehydration reactions affecting the subducted oceanic plate at different depths can influence magmatic diversity. In the western Trans-Mexican Volcanic Belt, there is a narrow potassic volcanic front running in parallel to the Middle American Trench that becomes replaced by intraplate-like high-Nb rocks to the north, and by more typical calc-alkaline products to the southeast. Potassic rocks have high MgO and are enriched in incompatible trace elements, but have lower heavy rare earth element contents than more evolved calc-alkaline and high-Nb magmas, and slightly more enriched Sr, Nd and Pb isotopes. Potassic magmas also have higher Rb/Cs and Ba/Cs ratios than the calc-alkaline and high-Nb suites, and extend to unusually high Nb/Ta ratios that correlate positively with Rb/Ta, Zr/Ta, La/Ta and Gd/Yb. These chemical variations are inconsistent with different extents of melting of a peridotitic source, but are also incompatible with melting of a phlogopite-rich mantle (vein-plus-wall-rock relationship), unless mica is totally consumed during melting, and a titaniferous phase such as rutile remains in the residue together with garnet. This assemblage is unlikely in the source region of primitive hydrous magmas, but it is what would be expected during dissolution of phengite and monazite/allanite in the subducted slab, with the concurrent formation of an anhydrous rutile-bearing eclogite. The magmatic diversity of western Mexico can thus be explained by invoking contributions of chemically different subduction agents as a function of slab depth and residual mineralogy: a low-pressure/temperature aqueous fluid would induce melting of the peridotitic mantle wedge and form typical calc-alkaline volcanoes, whereas a deeper and hotter slab-derived melt (or supercritical liquid) would contribute to the formation of potassic magmas due to phengite/monazite/allanite disintegration. In this context, intraplate-like magmas derive from decompression melting of the upper mantle as a natural consequence of subduction geodynamics.     

Geostatistical simulation of spatial variability of convective storms in Mexico City Valley

Precipitation is one of the main components of the hydrological cycle and knowledge of its spatial distribution is fundamental for the prediction of other closely related environmental variables, for example, runoff, flooding and aquifer recharge. Most of the precipitation in Mexico City is due to convective storms characterized by a high spatial variability, implying that modeling its behavior is very complex. In this work stochastic simulation techniques with a geostatistical approach were applied to model the spatial variability of the rainfall of three convective storms. The analysis of the results shows that using the proposed methodology spatial distributions of rain are obtained that reproduce the statistical characteristics present in the available information.     

The Mesolithic–Neolithic transition in southern Iberia

New data and a review of historiographic information from Neolithic sites of the Malaga and Algarve coasts (southern Iberian Peninsula) and from the Maghreb (North Africa) reveal the existence of a Neolithic settlement at least from 7.5 cal ka BP. The agricultural and pastoralist food producing economy of that population rapidly replaced the coastal economies of the Mesolithic populations. The timing of this population and economic turnover coincided with major changes in the continental and marine ecosystems, including upwelling intensity, sea-level changes and increased aridity in the Sahara and along the Iberian coast. These changes likely impacted the subsistence strategies of the Mesolithic populations along the Iberian seascapes and resulted in abandonments manifested as sedimentary hiatuses in some areas during the Mesolithic–Neolithic transition. The rapid expansion and area of dispersal of the early Neolithic traits suggest the use of marine technology. Different evidences for a Maghrebian origin for the first colonists have been summarized. The recognition of an early North-African Neolithic influence in Southern Iberia and the Maghreb is vital for understanding the appearance and development of the Neolithic in Western Europe. Our review suggests links between climate change, resource allocation, and population turnover.     

An improved approach for assigning pumping rates to heterogeneous aquifer models

A common assumption of ground water models formulated using a block-centered finite-difference method is that a well is located at the center of a cell regardless of its actual location. Due to this assumption, errors are introduced in the spatial distribution of simulated heads. This paper presents an alternative approach for assigning the pumping rates of wells that are located off cell centers. This approach consists of assigning the pumping rate not only to the cell in which the well is located but also to adjacent cells, taking into account the length of the well screen, the hydraulic conductivity, and the distance from the well to the center of its cell. The advantage of this alternative approach over the conventional one is illustrated with a test problem of a synthetic aquifer. Statistical measures of error indicate a much better model fit when pumping rates of wells are distributed over several cells.     

The climatology of tornadoes and waterspouts in Italy

In this work 10 years of reports collected by weather amateurs are used to define a preliminary climatology of tornadoes and waterspouts in Italy. The results show behaviors different from those observed in other countries. Generally, tornadoes and waterspouts are more frequent in late summer and autumn than in the other seasons. The seasonality of tornadoes and waterspouts appears different for different Italian zones, in particular in the Po Valley and Friulian plain and coast (south to the Alps) tornadoes and waterspouts are more frequent in spring and early summer while in the Tirrenian and Ionian coasts (western and southern Italy), tornadoes and waterspouts are more frequent in late summer and autumn. As observed in other studies (Brooks, H., E. and Doswell, C. A. III, 2001. Some aspects of the international climatology of tornadoes by damage classification. Atmos. Res., 56, 191–201.) Italian tornadoes and waterspouts are statistically weaker than in other countries but this difference cannot be completely ascribed to the presence of waterspouts. The “CAPE Storm-Relative-Helicity diagrams” and “Shear Magnitude diagrams” obtained for Italian tornadoes and waterspouts show different characteristics than those obtained for US. The cause of these differences is still unknown, it can rely in the sample selection (problems with the concept of proximity sounding) or in a real climatic effect.     

Structure of the rift basins in the central Gulf of California: Kinematic implications for oblique rifting

We use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central Gulf of California. Our analysis reveals that oblique strain in the central Gulf formed two tectono–sedimentary domains during distinct deformation stages. The eastern domain, offshore Sonora, is bounded by the East and West Pedro Nolasco faults that may constitute the southernmost segments of the Tiburón Fault System. Within this domain, the dip-slip Yaqui Fault controlled deposition of 3.9 km of sediments in the half-graben Yaqui Basin. The western domain, offshore Baja California, is bounded by the Guaymas Transform Fault, which controlled the accumulation of 1.45 km of sediments within a half-graben that formed the early Guaymas Basin. The tectono–sedimentary activity offshore Sonoran likely ranges from Late Miocene–Pliocene to Late Pliocene time, while activity in the Guaymas Basin commenced in Late Pliocene time. Extinction of the main faults offshore Sonora was nearly coeval to the initiation of the Guaymas Transform Fault. Our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central Gulf. The Guaymas Basin is now a nascent spreading center, but prior to this, it evolved as a half-graben controlled by the Guaymas Transform Fault; such drastic transition is not constrained, but likely occurred during the Pleistocene time and must be localized < 30 km north of the axial troughs. The faults within the central Gulf transpose the Miocene N–S oriented grabens of Basin and Range style preserved onshore in the conjugate rifted margins.     

Changes in the Wetlands of Andalusia (Doñana Natural Park, SW Spain) at the End of the Little Ice Age

The Little Ice Age is a climatic period still insufficiently known. This ignorance is more marked in the case of Andalusia (southern Spain), where only recent works throw any light on the topic. Our studies, primarily from the perspective of botany and changes in the plant landscape, reveal the effects that this period had in these latitudes, and in particular on the Doñana Natural Park (SW Spain). This work shows an aridization in climatic conditions coinciding with the end of the Little Ice Age. The results corroborate the most recent theses on this period in Spain. Furthermore, the effects on the marsh areas studied lead us to conclude that the interpretation of the Little Ice Age in these southern latitudes should be different to that normally given for more northerly latitudes – much better known and more studied than the Mediterranean regions.     

Hydrolysis of neptunium(V) at variable temperatures (10-85°C)

Neptunium is one of the few radioactive elements that are of great concern in the disposal of nuclear wastes in the geological repository, due to its hazards and the long half-life of the isotope, ²³⁷Np (t_1/2 = 2.14 × 10⁶ years). To understand and predict the migration behavior of neptunium in the geological media, it is of importance to study its hydrolysis at elevated temperatures, because the temperature in the waste package and the vicinity of the repository could be high. Moreover, the chemical analogy between neptunium(V) and plutonium(V) adds even greater value to this investigation, because the latter could exist at tracer levels in neutral and slightly oxidizing waters but is difficult to study due to its rather labile redox behavior.In this work, the hydrolysis of neptunium(V) was studied at variable temperatures (10 to 85°C) in tetramethylammonium chloride (1.12 mol kg⁻¹). Two hydrolyzed species of neptunium(V), NpO₂OH(aq) and NpO₂(OH)₂⁻, were identified by potentiometry and Near-IR absorption spectroscopy. The hydrolysis constants (*β_n) and enthalpy of hydrolysis (ΔH_n) for the reaction NpO₂⁺ + nH₂O = NpO₂(OH)_n⁽¹⁻ⁿ⁾⁺ + nH⁺ (n = 1 and 2) were determined by titration potentiometry and microcalorimetry. The hydrolysis constants, *β₁ and *β₂, increased by 0.8 and 3.4 orders of magnitude, respectively, as the temperature was increased from 10 to 85°C. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature is increased. The hydrolysis reactions are endothermic but become less endothermic as the temperature is increased. The heat capacities of hydrolysis, ΔC_p1 and ΔC_p2, are calculated to be −(71 ± 17) J K⁻¹ mol⁻¹ and −(127 ± 17) J K⁻¹ mol⁻¹, respectively. Approximation approaches to predict the effect of temperature, including the constant enthalpy approach, the constant heat capacity approach and the DQUANT equation, have been tested with the data.