Formation of accretionary dust mantles in the solar nebula: Evidence from preirradiated olivines in CM chondrites

Abstract— CM chondrites are regolith breccias consisting of lithic clasts embedded in a fine‐grained clastic matrix. The majority of these lithic clasts belongs to a texturally well‐defined rock type (primary rock) that can be described as an agglomerate of chondrules and other coarse‐grained components, most of which are surrounded by fine‐grained rims (dust mantles). Metzler et al. (1992) explain these textures as the result of accretionary processes in the solar nebula, while an alternative model explains them to be the result of regolith processes on the parent body (Sears et al. 1993). The main intention of the present study is to discern between both models by investigating the occurrence, frequency, spatial distribution, and textural setting of preirradiated (track‐rich) olivines in CM chondrites. Track‐rich olivines were studied in situ in six polished thin sections from 4 different CM chondrites (Cold Bokkeveld, Mighei, Murchison, Nogoya) by optical and scanning electron microscopy (SEM). It was found that their occurrence is restricted to the clastic matrix of these meteorites. The primary rock seems to have formed in an environment shielded from cosmic radiation, since fragments of this rock are free of track‐rich grains and solar noble gases. This finding supports the solar nebula model for the formation of dust mantles around chondrules and other coarse‐grained components, and points against a regolith origin. In Cold Bokkeveld, a small breccia‐in‐breccia clast was found, which has been irradiated as an entity within the uppermost millimeters to meters of its parent body for at least about 3 Ma. This clast seems to represent a compacted subsurface layer that was later excavated by impact and admixed to the host breccia. Furthermore, the results of this study may affect the interpretation of compaction ages obtained by fission track methods, since these ages may be mixtures of different contact ages between finegrained, U‐rich dust and U‐poor olivines. In some cases, they may date the formation of dust mantles in the solar nebula, while in other cases the lithification of the host breccias may be dated.     

Engineering characterisation of Norwegian glaciomarine silt

Guidance is provided for geotechnical engineers designing civil engineering works in silty soils based on a detailed characterisation of a glaciomarine silt from Os in western Norway. It was found that these soils are susceptible to disturbance by good quality fixed piston tube sampling and care needs to be taken when using laboratory derived design parameters, particularly for consolidation and shear strength properties. A technique for assessing sample disturbance using shear wave velocity and suction measurements proved promising. Conventional techniques for determining soil strength from triaxial tests in silt are inappropriate due to the dilational nature of the material and more reliable and logical strength estimates can be made from a limiting strain criterion. Field vane data should be used with caution as measured strength, particularly remoulded values, may be high and it seems more reliable parameters can be derived from CPTU tests. One dimensional consolidation and creep of these silts can be modelled successfully by the well-known Janbu formulation. The behaviour of the Os silts does not fit easily into classical soil mechanics and published frameworks for soft soils. It seems the material is of “transitional” type and this work adds to the database of such soils which includes other natural silts and gap graded soils. For future work it is recommended that larger sample tubes (say 75 mm) with a very sharp cutting edge should be used in parallel with in situ CPTU testing.     

A New method for identifying possible causal relationships between CO2, total solar irradiance and global temperature change

The study compares two formulas for calculating the daily evapotranspiration ET₀ for a reference crop. The first formula was proposed by Allen et al. (AL), while the second one was proposed by Katerji and Perrier with the addition of the carbon dioxide (CO₂) effect on evapotranspiration (KP). The study analyses the impact of the calculation by the two formulas on the irrigation requirement (IR). Both formulas are based on the Penman-Monteith equation but adopt different approaches for parameterising the canopy resistance r _c . In the AL formula, r _c is assumed constant and not sensitive to climate change, whereas in the KP formula, r _c is first parameterised as a function of climatic variables, then ET₀ is corrected for the air CO₂ concentration. The two formulas were compared in two periods. The first period involves data from two sites in the Mediterranean region within a measured climate change period (1981–2006) when all the input climatic variables were measured. The second period (2070–2100) involves data from a future climate change period at one site when the input climatic variables were forecasted for two future climate scenarios (A2 and B2). The annual cumulated values of ET₀ calculated by the AL formula are systematically lower than those determined by the KP formula. The differences between the ET₀ estimation with the AL and KP formulas have a strong impact on the determination of the IR for the reference crop. In fact, for the two periods, the annual values of IR when ET₀ is calculated by the AL formula are systematically lower than those calculated by the KP formula. For the actual measured climate change period, this reduction varied from 26 to 28 %, while for the future climate change period, it varied based on the scenario from 16 % (A2) to 20 % (B2).     

Dynamic properties of planar,coupled shear walls

The paper is a study of the natural frequencies and mode shapes of planar, coupled shear walls, a common lateral resistive element in building construction. The equations of motion are derived for the general case, and the eigenvalue problem associated with free vibrations of equal, constant shear walls is solved, both with and without the inclusion of the inertia of vertical motion. Explicit solutions are presented for the characteristic equation and the mode shapes and the results are illustrated with figures, including an example calculation based on the shear walls of the Mt. McKinley Building, damaged by the Alaskan earthquake of 1964. The results affirm the necessity of including vertical displacement of the shear walls in the analysis of such systems, and suggest that the inertia of vertical motion also must be considered in the analysis for certain ranges of the parameters.     

Microfabric and rock properties of experimentally compressed silt-clay mixtures

Oedometric mechanical compaction tests were performed on brine-saturated synthetic samples consisting of silt-clay mixtures to study changes in microfabric and rock properties as a function of effective stress. The silt consisted of crushed quartz (∼100%) with grain size range between 4 and 40 μm, whereas the clay consisted of 81% kaolinite, 14% mica/illite and 5% microcline of grain size between 0.4 and 30 μm. Five sample pairs ranging in composition from pure silt to pure clay were compacted to 5 and 50 MPa effective stress respectively. SEM studies were carried out to investigate microfabric changes in the mechanically compacted silt-clay mixtures. The degree of alignment of the different minerals present (quartz, mica/illite and kaolinite) were computed by using an image analysis software. Experimental compaction have measured the changes in the rock properties such as porosity and velocity as a function of effective stress for different mixtures of clay and silt. Clay-rich samples showed a higher degree of mineral orientation and lower porosity compared to silt-dominated samples as a function of effective stress. Pure clay sample had 11% porosity at 50 MPa effective stress whereas the pure silt sample retained about 29% porosity at the same effective stress. The experiments showed that low porosity down to 11% is possible by mechanical compaction only. A systematic increase in strain was observed in the silt-clay mixtures with increasing clay content but the porosity values found for the 50:50 silt-clay mixture were lower than that of 25:75 silt-clay mixture. No preferential mineral orientation is expected before compaction owing to the high initial porosity suggesting that the final fabric is a direct result of the effective stress. Both P- and S-wave velocities increased in all silt-clay mixtures with increasing effective stress. The maximum P- and S-wave velocities were observed in the 25:75 silt-clay mixture whereas the minimum Vp and Vs were recorded in the pure silt mixture. At 50 MPa effective stress P- wave velocities as high as 3 km/s resulted from experimental mechanical compaction alone. The results show that fine-grained sediment porosity and velocity are dependent on microfabric, which in turn is a function of grain size distribution, particle shape, sediment composition and stress. At 5 MPa effective stress, quartz orientation increased as a function of the amount of clay indicating that clay facilitate rotation of angular quartz grains. Adding clay from 25% to 75% in the silt-clay mixtures at 50 MPa effective stress decreased the quartz alignment. The clay mineral orientation increased by increasing both the amount of clay and the effective stress, the mica/illite fabric alignment being systematically higher than that of kaolinite. Even small amount of silt (25%) added to pure clay reduced the degree of clay alignment significantly. This study demonstrates that experimental compaction of well characterized synthetic mudstones can be a useful tool to understand microfabric and rock properties of shallow natural mudstones where mechanical compaction is the dominant process.     

Role of effective permeability distribution in estimating overpressure using basin modelling

Overpressure generation is a function of the rates of sedimentation, compaction, fluid generation from kerogen and dehydration of minerals, and most importantly the lateral distribution of permeability within a basin as this controls lateral drainage. Sedimentary basins, however, are typically highly heterogeneous with respect to primary sedimentary facies, diagenesis and tectonic development. While fluid flow models based on idealised homogeneous basins may further our understanding of the processes that influence overpressure development, the results are very sensitive to the distribution of rock properties, particularly permeability. The absolute permeability of sedimentary rocks varies from more than 1 Darcy to less than 0.01 nanodarcy (nD) (10⁻¹¹ Darcy).     

Restructuring the Chinese seafood industry,global challenges and policy implications

This article discusses how the Chinese seafood industry will affect the rest of the world's fishing industries. The analysis is based on theories of economic comparative advantage, the international division of labour and the internationalization process related to trade activities. Given the increasing domestic demand for fish in China, the limited availability of domestic fish stocks and less success for farmed marine fish in China are considered some of the most important factors in restricting the growth in Chinese seafood production. The necessity of raw fish imports into China may increase pressure on global fish stocks and international fish prices, resulting in tighter supplies worldwide and higher seafood prices for the Chinese. Sustainable fish harvests and trade require stronger fishery management, in particular in the relations between seafood companies and governments along the entire international value chain supplying China. Such structural changes may allow new value-added possibilities for fish farming and the upgrading of certain fish species for human consumption that were previously utilized purely for fish feed.     

The Amoco Cadiz oil spill—At a glance

The tanker Amoco Cadiz grounded close to Aber Wrac'h in Brittany, France, on March 1978. Graphs are presented, showing the fate of the oil, the recovery of the biota and the social impacts after the grounding. Visual insepction in 1983 showed that there are still nuisance amounts of oil, and the biotic recovery is far from being complete with respect to species distribution.