Foraminiferal biostratigraphy and paleoenvironmental reconstruction at the Yaxcopoil‐1 drill hole,Chicxulub crater,Yucatán Peninsula

Abstract The Yaxcopoil‐1 (Yax‐1) drill hole comprises Cretaceous limestones and calcarenites, the K/P boundary cocktail unit (including impact breccia), and a Danian marly clay layer overlain by calcareous marls. The biostratigraphy, paleobathymetry, and environmental turnover across the K/P interval were inferred after analyzing the planktic and benthic foraminiferal assemblages. The Cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle Campanian to Maastrichtian age, while low‐diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic, occasionally stressed environment. The impact breccia and the redeposited suevite are overlain by a 46 cm‐thick dolomitic calcareous sandstone unit that contains scarce, reworked planktic foraminiferal specimens. This unit probably represents the uppermost part of the initial infill of the crater. The uppermost centimeters of this unit are bioturbated, and its top represents a hiatus that spans at least the G. cretacea, Pv. eugubina, and part of the P. pseudobulloides biozones. This unit is overlain by a 3–4 cm‐thick marly clay layer that represents a condensed layer. Benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer. The increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the G. compressa biozone toward the A. uncinata (P2) biozone. The Danian planktic and benthic foraminiferal assemblages indicate a deeper, probably bathyal environment.     

The northern Patagonia Somuncura plateau basalts: a product of slab‐induced,shallow asthenospheric upwelling?

The Meseta de Somuncura forms the largest basaltic plateau (20 000 km²) of southern Argentina (extra-Andean domain). Most of these tholeiitic to alkaline rocks were extruded at ˜ 25 Ma (late Oligocene). The absence of rifting–thinning processes, plume activity, or slab-window phenomena leaves only one major possibility for the generation of Somuncura: asthenospheric ('OIB-like') corner flow leading to a transient thermal anomaly above the subducting plate. It is suggested herein that the intake of hot asthenosphere was forced into a favourable topography (concave-up) of the subducting plate, when a major plate reorganization event (Farallon to Nazca) was taking place in late Oligocene to early Miocene time. The fast and vigorous intake of asthenosphere would have been induced by slab roll-back, leading to decoupling of the subducting plate. The Somuncura volcanic episode can be regarded as a marker of the passage from the extremely oblique subduction of Farallon, to the birth of the Nazca plate and roughly perpendicular convergence between South America and Nazca.     

Reliability assessment of granular filters in embankment dams

Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Using bright ellipticals as dark energy cosmic clocks

The age of the Universe has been increasingly constrained by different techniques, such as the observations of type Ia supernovae (SNIa) at high redshift or dating the stellar populations of globular clusters. In this paper, we present a complementary approach using the colours of the brightest elliptical galaxies in clusters over a wide redshift range  ( z ≲ 1)  . We put new and independent bounds on the dark energy equation of state parametrized by a constant pressure-to-density ratio   w _Q  and by a parameter (ξ) which determines the scaling between the matter and dark energy densities. We find that accurate estimates of the metallicities of the stellar populations in moderate and high-redshift cluster galaxies can pose stringent constraints on the parameters that describe dark energy. Our results are in good agreement with the analysis of dark energy models using SNIa data as a constraint. Accurate estimates of the metallicities of stellar populations in cluster galaxies at   z ≲ 2  will make this approach a powerful complement to studies of cosmological parameters using high-redshift SNIa.     

Spatially resolved spectroscopic observations of Jupiter during the 1987 opposition

Spectroscopic observations in the spectral region 6000–6600 » of major belts and zones of Jupiter have been carried out at Calar Alto Observatory (Spain) during the 1987 opposition, in support to the International Jupiter Watch Program. More than a hundred long-slit, medium resolution spectra have been taken at the coudé focus of the 1.52 m telescope. The longitudinal and latitudinal distribution of the equivalent widths of the methane and ammonia absorption bands at 6190 » and 6450 » have been investigated. Unlike the CH₄-6190 » band, the longitudinal distribution in the NH₃-6450 » band shows asymmetries in some of the Jovian regions analyzed, being the absorption greater at West than at East limbs. These results are interpreted in terms of inhomogeneous scattering models by the simultaneous variation of the NH₃ cloud optical thickness and its altitude level. The north-south scans show little latitudinal dependence on absorption in both methane and ammonia bands, although some relationship could be established between the absorption and relative continuum reflectivity variation.

---

Resumen Se presentan los resultados obtenidos de las observaciones espectroscópicas de las principales zonas y cinturones de Júpiter realizadas durante la oposición de 1987 como una contribución al International Jupiter Watch Program. Las observaciones fueron llevadas a cabo haciendo uso del foco coudé del telescopio de 1.52 m del observatorio hispanoalemán de Calar Alto (España), habiéndose obtenido algo más de cien espectros, de resolución media, en la región espectral de 6000–6600 ». Se exponen los resultados de las anchuras equivalentes de las bandas de absorción del metano en 6190 » y del amoniaco en 6450 », estudiándose, al mismo tiempo, la distribución tanto longitudinal como latitudinal de las mismas. Mientras que los resultados para la banda del metano presentan un comportamiento simétrico en todas las regiones analizadas, las anchuras equivalentes para la banda NH₃-6450 » presentan un comportamiento asimétrico en algunas regiones del planeta, con una mayor absorción hacia el limbo del oeste que hacia el del este. Los resultados son interpretados en base a un modelo de scattering no homogéneo, variando, simultáneamente, el espesor óptico y la altitud de la nube de amoniaco. Los espectros tomados en la dirección norte-sur, aunque presentan una dependencia latitudinal pequeña en la absorción de las bandas mencionadas, sugieren una relación entre la absorción y las variaciones de reflectividad en el continuo.

---

---

Also at Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Campus Universitario Fuentenueva, Granada, Spain.     

Bayesian estimation of seismic hazard for two sites in Switzerland

Seismic hazard analysis is based on data and models, which both are imprecise and uncertain. Especially the interpretation of historical information into earthquake parameters, e.g. earthquake size and location, yields ambiguous and imprecise data. Models based on probability distributions have been developed in order to quantify and represent these uncertainties. Nevertheless, the majority of the procedures applied in seismic hazard assessment do not take into account these uncertainties, nor do they show the variance of the results. Therefore, a procedure based on Bayesian statistics was developed to estimate return periods for different ground motion intensities (MSK scale).Bayesian techniques provide a mathematical model to estimate the distribution of random variables in presence of uncertainties. The developed method estimates the probability distribution of the number of occurrences in a Poisson process described by the parameter . The input data are the historical occurrences of intensities for a particular site, represented by a discrete probability distribution for each earthquake. The calculation of these historical occurrences requires a careful preparation of all input parameters, i.e. a modelling of their uncertainties. The obtained results show that the variance of the recurrence rate is smaller in regions with higher seismic activity than in less active regions. It can also be demonstrated that long return periods cannot be estimated with confidence, because the time period of observation is too short. This indicates that the long return periods obtained by seismic source methods only reflects the delineated seismic sources and the chosen earthquake size distribution law.     

Red dust rain within the Spanish Mediterranean area

Dust rain belongs to the climatological conditions of the Iberian Peninsula's Mediterranean seaboard. Traditionally known as muddy' or bloody rains, red dust rain has become a topical issue as a result of more frequent rainfall of this kind in recent years. In spite of the difficulties that studying this phenomenon involves, owing to the lack of systematically kept records at meteorological observatories, details are provided of the high frequency of such phenomena within the Spanish Mediterranean area, using records from other phenological sources. The study analyzes the chemical composition of red dust rain and the atmospheric factors giving rise to such phenomena. Whether there is a relationship betwen the higher frequency of dust rain and possible changes in global atmospheric circulation is as yet unclear.     

On the structure of Saturn's rings and the ‘real’ rotational period for the planet

An analysis of the Lowell Observatory photographic plates of Saturn gave the following results: (1) ring A and B show peculiar brightness distributions around the planet, from which we conclude that both are composed of particles in synchronous rotation. (2) The leading side of the particles in ring A is brighter than the trailing side by about 4%, which may indicate an interaction between such particles and the interplanetary medium. (3) Scans of the rings across the major axis show a small (～0.3″) region of enhanced brightness, from which we derive a value ofT _s =10^h13 _. ^m 8±5 _. ^m 4 for the actual planetary rotational period of Saturn. (4) In order to explain the synchronous rotation, the particles in ring A have to be at least 42 m in diameter.     

Groundwater-flow modeling in the Yucatan karstic aquifer, Mexico

The current conceptual model of the unconfined karstic aquifer in the Yucatan Peninsula, Mexico, is that a fresh-water lens floats above denser saline water that penetrates more than 40 km inland. The transmissivity of the aquifer is very high so the hydraulic gradient is very low, ranging from 7–10 mm/km through most of the northern part of the peninsula. The computer modeling program AQUIFER was used to investigate the regional groundwater flow in the aquifer. The karstified zone was modeled using the assumption that it acts hydraulically similar to a granular, porous medium. As part of the calibration, the following hypotheses were tested: (1) karstic features play an important role in the groundwater-flow system; (2) a ring or belt of sinkholes in the area is a manifestation of a zone of high transmissivity that facilitates the channeling of groundwater toward the Gulf of Mexico; and (3) the geologic features in the southern part of Yucatan influence the groundwater-flow system. The model shows that the Sierrita de Ticul fault, in the southwestern part of the study area, acts as a flow barrier and head values decline toward the northeast. The modeling also shows that the regional flow-system dynamics have not been altered despite the large number of pumping wells because the volume of water pumped is small compared with the volume of recharge, and the well-developed karst system of the region has a very high hydraulic conductivity. Electronic Publication