Cosmic‐ray exposure history of two Frontier Mountain H‐chondrite showers from spallation and neutron‐capture products

Abstract— We measured the concentrations of ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹⁴C in the metal and/or stone fractions of 27 Antarctic chondrites from Frontier Mountain (FRO), including two large H‐chondrite showers. To estimate the pre‐atmospheric size of the two showers, we determined the contribution of neutron‐capture produced ³⁶Cl (half‐life = 3.01 times 10⁵ years) and ⁴¹Ca (1.04 times 10⁵ years) in the stone fraction. The measured activities of neutron‐capture ³⁶Cl and ⁴¹Ca, as well as spallation produced ¹⁰Be and ²⁶Al, were compared with Monte Carlo‐based model calculations. The largest shower, FRO 90174, includes eight fragments with an average terrestrial age of (100 ± 30) × 10³ years; the neutron‐capture saturation activities extend to 27 dpm/kg stone for ³⁶Cl and 19 dpm/kg stone for ⁴¹Ca. The concentrations of spallation produced ¹⁰Be, ²⁶Al and ³⁶Cl constrain the radius (R) to 80–100 cm, while the neutron‐capture ⁴¹Ca activities indicate that the samples originated from the outer 25 cm. With a pre‐atmospheric radius of 80–100 cm, FRO 90174 is among the largest of the Antarctic stony meteorites. The large pre‐atmospheric size supports our hypothesis that at least 50 of the ～150 classified H5/H6‐chondrites from the Frontier Mountain stranding area belong to this single fall; this hypothesis does not entirely account for the high H/L ratio at Frontier Mountain. The smaller shower, FRO 90001, includes four fragments with an average terrestrial age of (40 ± 10) × 10³ years; they contain small contributions of neutron‐capture ³⁶Cl, but no excess of ⁴¹Ca. FRO 90001 experienced a complex exposure history with high shielding conditions in the first stage (150 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting ～1.0 million years (Ma) ago. Based on the measured ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne ratios, the cosmic‐ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90174 and 8 ± 1 Ma for FRO 90001. These ages coincide with the well‐established H‐chondrite peak and corroborate the observation that the exposure age distribution of FRO H‐chondrites is similar to that of non‐Antarctic falls. In addition, we found that corrections for neutron‐capture ³⁶Ar (from decay of ³⁶Cl) result in concordant ²¹Ne and ³⁸Ar exposure ages.     

Deep-sea suprabenthos assemblages (Crustacea) off the Balearic Islands (western Mediterranean): Mesoscale variability in diversity and production

The composition of suprabenthic crustacean assemblages, their diversity, production (P) and production/biomass (P/B) ratios, were analyzed at species level along two transects situated to the north (N) and south (S) of Mallorca (Balearic Islands, western Mediterranean) at depths between 134 m and 760 m, based on a ca. bi-monthly sampling performed between August 2003 and June 2004. Differences with depth and season in assemblage composition and diversity were analyzed as a function of the contrasting environmental features (e.g. water mass dynamics) of the two areas. We identified 187 species (18 decapods, 5 euphausiids, 16 mysids, 76 gammaridean amphipods, 13 hyperiids, 1 caprellid, 21 isopods and 37 cumaceans). Substantial mesoscale variability in the deep-sea suprabenthic assemblages coupled with diversity trends between the N and S transects were found. Seasonality was the most important gradient influencing the dynamics of suprabenthos over the upper (350 m) and middle (650–750 m) slope in the N area. Conversely, the S area appeared to be more stable temporally with depth as the main gradient inducing assemblage differences. Different depth-related patterns were observed both for diversity and P/B. To the north diversity was very low at the shelf-break, increasing on the upper-slope (H′ > 3.00) and then decreasing again on the middle-slope. To the south diversity increased smoothly downward, reaching the highest values on the middle-slope. Regarding productivity, P/B was highest at intermediate depths to the north (over ca. 450–500 m), while to the south highest P/Bs were found deeper (over ca. 600–650 m). The higher P/B at intermediate depths found along N are likely due to higher % of organic matter (OM) in sediments, a product of oceanographic frontal systems. In particular, P/B was higher along N among omnivores and detritus feeders (e.g. Andaniexis mimonectes, Lepechinella manco and combined cumaceans), coupled to enriched OM in sediments, while along S mesoplanktonic carnivores (Rhachotropis spp.) had higher P/Bs. We conclude that on the north slope the influence of frontal systems and more active flow dynamics of different water masses (WIW and LIW) increases natural disturbance in the area, increasing productivity and diversity of suprabenthic peracarids in the Benthic Boundary Layer. Also, species showed a displacement of their average distributions (their Centres of Gravity, CoG) to shallower depths along N, which is another indicator of more favorable habitat conditions for suprabenthos in the 400–500 m range at N.     

Tectonics of a fast spreading center: A Deep-Tow and sea beam survey on the East Pacific rise at 19°30′ S

Sea Beam and Deep-Tow were used in a tectonic investigation of the fast-spreading (151 mm yr^-1) East Pacific Rise (EPR) at 19°30 S. Detailed surveys were conducted at the EPR axis and at the Brunhes/Matuyama magnetic reversal boundary, while four long traverses (the longest 96 km) surveyed the rise flanks. Faulting accounts for the vast majority of the relief. Both inward and outward facing fault scarps appear in almost equal numbers, and they form the horsts and grabens which compose the abyssal hills. This mechanism for abyssal hill formation differs from that observed at slow and intermediate spreading rates where abyssal hills are formed by back-tilted inward facing normal faults or by volcanic bow-forms. At 19°30 S, systematic back tilting of fault blocks is not observed, and volcanic constructional relief is a short wavelength signal (less than a few hundred meters) superimposed upon the dominant faulted structure (wavelength 2–8 km). Active faulting is confined to within approximately 5–8 km of the rise axis. In terms of frequency, more faulting occurs at fast spreading rates than at slow. The half extension rate due to faulting is 4.1 mm yr^-1 at 19°30 S versus 1.6 mm yr^-1 in the FAMOUS area on the Mid-Atlantic Ridge (MAR). Both spreading and horizontal extension are asymmetric at 19°30 S, and both are greater on the east flank of the rise axis. The fault density observed at 19°30 S is not constant, and zones with very high fault density follow zones with very little faulting. Three mechanisms are proposed which might account for these observations. In the first, faults are buried episodically by massive eruptions which flow more than 5–8 km from the spreading axis, beyond the outer boundary of the active fault zone. This is the least favored mechanism as there is no evidence that lavas which flow that far off axis are sufficiently thick to bury 50–150 m high fault scarps. In the second mechanism, the rate of faulting is reduced during major episodes of volcanism due to changes in the near axis thermal structure associated with swelling of the axial magma chamber. Thus the variation in fault spacing is caused by alternate episodes of faulting and volcanism. In the third mechanism, the rate of faulting may be constant (down to a time scale of decades), but the locus of faulting shifts relative to the axis. A master fault forms near the axis and takes up most of the strain release until the fault or fault set is transported into lithosphere which is sufficiently thick so that the faults become locked. At this point, the locus of faulting shifts to the thinnest, weakest lithosphere near the axis, and the cycle repeats.     

Modeling the effects of riparian planting strategies on stream temperature: Increasing suitable habitat for endangered Formosan Landlocked Salmon in Shei‐Pa National Park,Taiwan

Reducing or stabilizing the stream temperature of ChiChiaWan Creek is a crucial work for Formosan Landlocked Salmon because ChiChiaWan Creek is the only one habitat for this endangered species. Planting trees in the riparian zone would be one of the alternatives. The purpose of this study was to evaluate the effects of several planting strategies on daily maximum stream temperature along the river. The results showed the effective vegetative shading angles should be more than 50° along ChiChiaWan Creek to reduce the direct solar radiation heating effectively. Upstream planting with 70° vegetative shading angle could be the most effective way among all the scenarios. However, this planting strategy could not improve the worst situations in summer because of the large solar elevation angles. The upstream planting in ChiChiaWan Creek was strongly recommended because the canopies could be easier to extend to totally cover the narrow width of river producing the most effective shades. Practicing the upstream planting with 90° vegetative shading angle can increase more than 1 km long suitable habitats for the endangered Salmon in summer. Alternatively, the west‐side planting scenario was the second effective way for temperature reduction. Our result provided a useful suggestion for the authorities in charge of saving the Formosan Landlocked Salmon, particularly under the stress of global warming. Copyright © 2011 John Wiley & Sons, Ltd.     

Site effects at Euroseistest—I. Determination of the valley structure and confrontation of observations with 1D analysis

This paper describes the process of construction of the 2D model of Volvi's geological structure and results of empirical and theoretical approaches to the evaluation of site response at Euroseistest. The construction of the 2D model is based on a re-interpretation of the available geophysical and geotechnical data in an effort to improve the definition of the subsoil structure at Euroseistest in terms of the most important parameters needed to model site response. The results of this re-interpretation are compared with a previous published 2D model of the same alluvial valley. Different analysis of the measurements and different criteria in the synthesis of data have led to a different model, even if both studies had access to the same field measurements. This underscores the fact that a model results of an interpretation and is not uniquely determined by the data, no matter how detailed they are. The well known subsoil structure opened the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from a detailed analysis of two events in frequency and time domains and 1D numerical modeling. The study of site response shows the important effect of the lateral variations on the ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In the case of Volvi's graben, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects in this basin.     

Towards more systematic perceptual model development: a case study using 3 Luxembourgish catchments

The synthesis of experimental understanding of catchment behaviour and its translation into qualitative perceptual models is an important objective of hydrological sciences. We explore this challenge by examining the cumulative understanding of the hydrology of three experimental catchments and how it evolves through the application of different investigation techniques. The case study considers the Huewelerbach, Weierbach and Wollefsbach headwater catchments of the Attert basin in Luxembourg. Subsurface investigations including bore holes and pits, analysis of soil samples and Electrical Resistivity Tomography measurements are presented and discussed. Streamflow and tracer data are used to gain further insights into the streamflow dynamics of the catchments, using end‐member mixing analysis and hydrograph separation based on dissolved silica and electrical conductivity. We show that the streamflow generating processes in all three catchments are controlled primarily by the subsolum and underlying bedrock. In the Huewelerbach, the permeable sandstone formation supports a stable groundwater component with little seasonality, which reaches the stream through a series of sources at the contact zone with the impermeable marls formation. In the Weierbach, the schist formation is relatively impermeable and supports a ‘fill and spill’‐type of flow mechanism; during wet conditions, it produces a delayed response dominated by pre‐event water. In the Wollefsbach, the impermeable marls formation is responsible for a saturation‐excess runoff generating process, producing a fast and highly seasonal response dominated by event water. The distinct streamflow generating processes of the three catchments are represented qualitatively using perceptual models. The perceptual models are in turn translated into quantitative conceptual models, which simulate the hydrological processes using networks of connected reservoirs and transfer functions. More generally, the paper illustrates the evolution of perceptual models based on experimental fieldwork data, the translation of perceptual models into conceptual models and the value of different types of data for processes understanding and model representation. Copyright © 2014 John Wiley & Sons, Ltd.     

THE SCATTERING OF SH-CHANNEL WAVES BY A FAULT IN A COAL SEAM*

The imaging of faults in coal seams by the in-seam seismic method has now become standard practice. In the UK over 300 surveys have been undertaken and the technique is now part of the exploration arsenal of colliery planners. From these users comes the pressure for two major improvements, namely an increase in range and target identification. This paper is directed towards the latter problem. It has long been recognized that the reflected channel waves must contain information on the fault structure that caused the reflection, and model experiments have been undertaken to investigate the reflection process. Only recently, however, have attempts been made to quantify the reflection process. Calculations using both the finite-difference and finite-element techniques have been carried out, and estimates of the reflection coefficient as a function of frequency have been obtained. The object of this paper is to extend these considerations by calculating analytically the scattering matrix of an SH-channel wave after interaction at a fault plane. The scattering matrix is calculated as a function of frequency, hade angle, and fault throw. The method employed is based on the decomposition of the incident SH-channel wave into Fourier components, the calculation of plane wave reflection and transmission coefficients within the constraints of geometrical acoustics, and finally the synthesis of the scattering matrix by application of the Helmholtz-Kirchhoff integral. The calculation throughout is restricted to normal modes.