Orbit computation for transneptunian objects

Using statistical orbital ranging, we systematically study the orbit computation problem for transneptunian objects (TNOs). We have automated orbit computation for large numbers of objects, and, more importantly, we are able to obtain orbits even for the most sparsely observed objects (observational arcs of a few days). For such objects, the resulting orbit distributions include a large number of high-eccentricity orbits, in which TNOs can be perturbed by close encounters with Neptune. The stability of bodies on the computed orbits has therefore been ascertained by performing a study of close encounters with the major planets. We classify TNO orbit distributions statistically, and we study the evolution of their ephemeris uncertainties. We find that the orbital element distributions for the most numerous single-apparition TNOs do not support the existence of a postulated sharp edge to the belt beyond 50 AU. The technique of statistical ranging provides ephemeris predictions more generally than previously possible also for poorly observed TNOs.     

The role of bathymetry and directional wave conditions on observed crescentic bar dynamics

Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar–shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low–medium energetic waves with limited obliquity ( $\theta \lesssim 20$° at 10 m depth), while bar straightening occurred during medium–high energetic waves with strong oblique angles of incidence ( $\theta \gtrsim 15$°). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.     

Évolution de l'onde semi-diurne M2 de la marée à Brest de 1846 à 2005

The work of searching, recovering and quality control of ancient sea-level measurements at Brest is presented. This work enables us to complete a study carried out by Cartwright in 1972, which showed a decrease in the tidal M2 semi-diurnal amplitude of 1% per century. After including these ancient data, as well as the last four decades of observations in the analysis, our results show an increase of the amplitude of M2 after 1960 and a decrease before 1880, suggesting a long-period oscillation rather than a steady secular trend. To cite this article: N. Pouvreau et al., C. R. Geoscience 338 (2006).     

A methodology for regionalizing 3‐D effective porosity at watershed scale in crystalline aquifers

An innovative approach for regionalizing the 3‐D effective porosity field is presented and applied to two large, overexploited, and deeply weathered crystalline aquifers located in southern India. The method derives from earlier work on regionalizing a 2‐D effective porosity field in that part of an aquifer where the water table fluctuates, which is now extended over the entire aquifer using a 3‐D approach. A method based on geological and geophysical surveys has also been developed for mapping the weathering profile layers (saprolite and fractured layers). The method for regionalizing 3‐D effective porosity combines water table fluctuation and groundwater budget techniques at various cell sizes with the use of satellite‐based data (for groundwater abstraction), the structure of the weathering profile, and geostatistical techniques. The approach is presented in detail for the Kudaliar watershed (983 km²) and tested on the 730 km² Anantapur watershed. At watershed scale, the effective porosity of the aquifer ranges from 0.5% to 2% in Kudaliar and between 0.3% and 1% in Anantapur, which agrees with earlier works. Results show that (a) depending on the geology and on the structure of the weathering profile, the vertical distribution of effective porosity can be very different and that the fractured layers in crystalline aquifers are not necessarily characterized by a rapid decrease in effective porosity and (b) that the lateral variations in effective porosity can be larger than the vertical ones. These variations suggest that within a same weathering profile, the density of open fractures and/or degree of weathering in the fractured zone may significantly vary from a place to another. The proposed method provides information on the spatial distribution of effective porosity that is of prime interest in terms of flux and contaminant transport in crystalline aquifers. Implications for mapping groundwater storage and scarcity are also discussed, which should help in improving groundwater resource management strategies.     

Statistical forecast of the marine surge

Natural Hazards - This paper studies different machine learning methods for solving the regression problem of estimating the marine surge value given meteorological data. The marine surge is...     

State of fundamental knowledge inferred from the study of globular clusters

A critical review is presented of fundamental conclusions on the evolution of galaxies as derived from the information provided by studies of globular clusters. It is stressed that most of the essential results concerning our Galaxy — as well as extragalactic systems — depend strongly on effects involving the accuracies of the data and sample incompleteness. High-quality observations of a much larger group of objects than presently available are required before any definite conclusions can be drawn.     

The impact of land use change and check‐dams on catchment sediment yield

Extensive land use changes have occurred in many areas of SE Spain as a result of reforestation and the abandonment of agricultural activities. Parallel to this the Spanish Administration spends large funds on hydrological control works to reduce erosion and sediment transport. However, it remains untested how these large land use changes affect the erosion processes at the catchment scale and if the hydrological control works efficiently reduce sediment export. A combination of field work, mapping and modelling was used to test the influence of land use scenarios with and without sediment control structures (check‐dams) on sediment yield at the catchment scale. The study catchment is located in SE Spain and suffered important land use changes, increasing the forest cover 3‐fold and decreasing the agricultural land 2·5‐fold from 1956 to 1997. In addition 58 check‐dams were constructed in the catchment in the 1970s accompanying reforestation works. The erosion model WATEM‐SEDEM was applied using six land use scenarios: land use in 1956, 1981 and 1997, each with and without check‐dams. Calibration of the model provided a model efficiency of 0·84 for absolute sediment yield. Model application showed that in a scenario without check dams, the land use changes between 1956 and 1997 caused a progressive decrease in sediment yield of 54%. In a scenario without land use changes but with check‐dams, about 77% of the sediment yield was retained behind the dams. Check‐dams can be efficient sediment control measures, but with a short‐lived effect. They have important side‐effects, such as inducing channel erosion downstream. While also having side‐effects, land use changes can have important long‐term effects on sediment yield. The application of either land use changes (i.e. reforestation) or check‐dams to control sediment yield depends on the objective of the management and the specific environmental conditions of each area. Copyright © 2008 John Wiley & Sons, Ltd.     

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

We look for high‐amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS‐3 Photometric V ‐band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (ε Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with Hα emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known σ Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Archean to Recent aeolian sand systems and their sedimentary record: Current understanding and future prospects

The sedimentary record of aeolian sand systems extends from the Archean to the Quaternary, yet current understanding of aeolian sedimentary processes and product remains limited. Most preserved aeolian successions represent inland sand‐sea or dunefield (erg) deposits, whereas coastal systems are primarily known from the Cenozoic. The complexity of aeolian sedimentary processes and facies variability are under‐represented and excessively simplified in current facies models, which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour, and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes. Archean and Neoproterozoic aeolian successions remain poorly constrained. Palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts. During the Triassic, widespread desert conditions prevailed across much of Europe. During the Jurassic, extensive ergs developed in North America and gave rise to anomalously thick aeolian successions. Cretaceous aeolian successions are widespread in South America, Africa, Asia, and locally in Europe (Spain) and the USA. Several Eocene to Pliocene successions represent the direct precursors to the present‐day systems. Quaternary systems include major sand seas (ergs) in low‐lattitude and mid‐latitude arid regions, Pleistocene carbonate and Holocene–Modern siliciclastic coastal systems. The sedimentary record of most modern aeolian systems remains largely unknown. The majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes, whereas successions representing linear and star dunes remain under‐recognized. Research questions that remain to be answered include: (i) what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology; (ii) what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation; (iii) can sophisticated four‐dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation; and (iv) is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of Earth history when large‐scale aeolian systems were active, or has the evidence to support the existence of other major desert basins been lost for many periods throughout Earth history?     

Age and impacts of the caldera-forming Aniakchak II eruption in western Alaska

The mid-Holocene eruption of Aniakchak volcano (Aniakchak II) in southwest Alaska was among the largest eruptions globally in the last 10,000 years (VEI-6). Despite evidence for possible impacts on global climate, the precise age of the eruption is not well-constrained and little is known about regional environmental impacts. A closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90–120 yr following tephra deposition. During this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a Cyperaceae-dominated assemblage to a Poaceae-dominated vegetation cover, suggesting a drier and/or more nutrient-rich ecosystem. Oribatid mites are extremely abundant in the peat at the depth of the ash, and show a longer-term, increasingly wet peat surface across the tephra layer. The radiocarbon sample immediately below the tephra gave a date of 1636–1446 cal yr BC suggesting that the eruption might be younger than previously thought. Our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long-lasting.