Simulation of the interaction of galactic cosmic‐ray protons with meteoroids: On the production of radionuclides in thick gabbro and iron targets irradiated isotropically with 1.6 GeV protons

Abstract— Thick spherical targets made of gabbro (R = 25 cm) and of steel (R = 10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of galactic cosmic‐ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 × 10¹⁴ cm^?2 and 2.45 × 10¹⁴ cm^?2 were received by the gabbro and iron sphere, respectively, which corresponds to cosmic‐ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high‐purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by x‐ and γ‐spectrometry, by low‐level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, ～500 target‐product combinations were investigated of which the results for radionuclides are presented here. The experimental production rates show a wide range of depth profiles reflecting the differences between low‐, medium‐, and high‐energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least‐squares deconvolution techniques from experimental thick‐target production rates of up to five thick‐target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well as in lunar samples and terrestrial materials.     

Deep crustal structure and rheology of the Gascoyne volcanic margin,western Australia

Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (V_p > 7 kms^–1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.     

Construction of three-phase data to model multiphase flow in porous media: Comparing an optimization approach to the finite element approach

Multiphase flow modelling is a major issue in the assessment of groundwater pollution. Three-phase flows are commonly governed by mathematical models that associate a pressure equation with two saturation equations. These equations involve a number of secondary variables that reflect the fluid behaviour in a porous medium. To improve the computational efficiency of multiphase flow simulators, several simplified reformulations of three-phase flow equations have been proposed. However, they require the construction of new secondary variables adapted to the reformulated flow equations. In this article, two different approaches are compared to quantify these variables. A numerical example is given for a typical fine sand.     

Heat budget of the surface mixed layer south of Africa

ARGO hydrographic profiles, two hydrographic transects and satellite measurements of air–sea exchange parameters were used to characterize the properties and seasonal heat budget variations of the Surface Mixed Layer (SML) south of Africa. The analysis distinguishes the Subtropical domain (STZ) and the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) of the Antarctic Circumpolar Current. While no Subantarctic Mode Water forms in that region, occurrences of deep SML (up to ∼450 m) are observed in the SAZ in anticyclones detached from the Agulhas Current retroflection or Agulhas Return Current. These are present latitudinally throughout the SAZ, but preferentially at longitudes 10–20° E where, according to previous results, the Subtropical Front is interrupted. Likely owing to this exchange window and to transfers at the Subantarctic Front also enhanced by the anticyclones, the SAZ shows a wide range of properties largely encroaching upon those of the neighbouring domains. Heat budget computations in each zone reveal significant meridional changes of regime. While air–sea heat fluxes dictate the heat budget seasonal variability everywhere, heat is mostly brought through lateral geostrophic advection by the Agulhas Current in the STZ, through lateral diffusion in the SAZ and through air–sea fluxes in the PFZ and AZ. The cooling contributions are by Ekman advection everywhere, lateral diffusion in the STZ (also favoured by the ∼10° breach in the Subtropical Front) and geostrophic advection in the SAZ. The latter likely reflects an eastward draining of water warmed through mixing of the subtropical eddies.     

Quantum-thermodynamic treatment of intrinsic anharmonicity; Wallace’s theorem revisited

Wallace (in Thermodynamics of crystals, 1972) developed a theorem, rooted in rigid lattice dynamics, which incorporates intrinsic anharmonic effects in solids. The practical application of this theorem in mineral physics is computationally involved and this is the main reason for the theorem not getting the attention it deserves. Because intrinsic anharmonicity is an important issue at the extreme conditions in planetary mantles, we derived a method which removes the computational obstacles in applying this theorem. We extended the theorem to incorporate details of the phonon spectrum and tested our algorithm on forsterite (Mg₂SiO₄). Using a least squares inversion technique applied to all available experimental data, we show that it results in an accurate representation of thermodynamic properties and sound wave velocities of Mg₂SiO₄ in its complete pressure–temperature stability range. We also show that the accuracy of our results is not significantly affected by the use of a different equation of state.     

Response of littoral chironomid communities and organic matter to late glacial lake—level,vegetation and climate changes at Lago dell’Accesa (Tuscany,Italy)

This study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell’Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes.     

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif,NE China and its bearing on the evolution of the Yinshan–Yanshan orogenic belt: Part I: Structural analyses and geochronological constraints

With a cratonic nucleus, the North China Craton (NCC) experienced a complex tectonic evolution with multiphase compressional and extensional events during Mesozoic times. Along the northern part of the NCC, the Yinshan–Yanshan fold and thrust belt was a typical intraplate orogen. Jurassic and Cretaceous continental sedimentation, magmatism, widespread intraplate characterize the Yinshan–Yanshan orogenic belt. The geodynamic significance of these tectonic events is still in dispute. In the western part of the Liaoning province, the Yiwulüshan massif crops out at the eastern end of the Yinshan–Yanshan orogenic belt. The Yiwulüshan massif presents an elliptical domal shape with a NE–SW striking long axis. The structural evolution of this massif brings new insights for the understanding of the Mesozoic plutonic–tectonic history of the NCC. A multidisciplinary study involving structural geology, geochronology, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have been carried out. The presentation of the new results splits into two parts. Part I (this paper) deals with field and laboratory structural observations, and presents the main geochronological results. The AMS, gravity modeling data will be provided in a companion paper (Part II). The early compressional deformation (D₁) corresponds to a Late Jurassic to Early Cretaceous southward thrusting. The subsequent deformation is related to the Early Cretaceous exhumation of the Yiwulüshan massif. A detailed structural analysis allows us to distinguish several deformation events (D₂, D₃, and D₄). The Cretaceous extensional structures, such as syntectonic plutons bounded by ductile normal faults, metamorphic core complexes, and half-graben basins are recognized in many places in East Asia. These new data from the Yiwulüshan massif constitute a link between Transbaikalia, Mongolia, North China and South China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.     

The lateglacial Quercus expansion in the southern European Alps: rapid vegetation response to a late Allerød climate warming?

A pollen‐inferred vegetation shift, from pioneer birch–pine woodland to mixed pine–summergreen oak forests, in the southern Alpine forelands, is commonly attributed to a centennial‐scale warming that occurred between the Gerzensee Oscillation (GO) and the Younger Dryas. Two microtephra layers bracketing the Younger Dryas onset (the Laacher See Tephra and the Vedde Ash) improve the chronology at Lago Piccolo di Avigliana (northern Italy) and allowed accurate correlation with Central European records where the GO is clearly detected. We used pollen percentages, pollen accumulation rates (PARs) and plant macrofossils to assess the population dynamics of Quercus, and leaf‐cuticle analysis for a better taxonomic identification of Quercus. Our results indicate that the species that was locally present was probably Quercus robur. PARs suggest that the population expansion started as early as the Bølling and followed an exponential increase through time. We attribute this gradual shift to increasing summer temperatures and longer growing seasons which contrast with a gradually decreasing temperature trend as recorded in Greenland ice cores and in Central Europe. Breaks or set‐backs in the PAR record may indicate the biotic response to minor Lateglacial cooling events of different life‐history stages in the Quercus population. Copyright © 2011 John Wiley & Sons, Ltd.