Using a pore‐scale model to quantify the effect of particle re‐arrangement on pore structure and hydraulic properties

A pore‐scale model based on measured particle size distributions has been used to quantify the changes in pore space geometry of packed soil columns resulting from a dilution in electrolyte concentration from 500 to 1 mmol l^?1 NaCl during leaching. This was applied to examine the effects of particle release and re‐deposition on pore structure and hydraulic properties. Two different soils, an agricultural soil and a mining residue, were investigated with respect to the change in hydraulic properties. The mining residue was much more affected by this process with the water saturated hydraulic conductivity decreasing to 0·4% of the initial value and the air‐entry value changing from 20 to 50 cm. For agricultural soil, there was little detectable shift in the water retention curve but the saturated hydraulic conductivity decreased to 8·5% of the initial value. This was attributed to localized pore clogging (similar to a surface seal) affecting hydraulic conductivity, but not the microscopically measured pore‐size distribution or water retention. We modelled the soil structure at the pore scale to explain the different responses of the two soils to the experimental conditions. The size of the pores was determined as a function of deposited clay particles. The modal pore size of the agricultural soil as indicated by the constant water retention curve was 45 µm and was not affected by the leaching process. In the case of the mining residue, the mode changed from 75 to 45 µm. This reduction of pore size corresponds to an increase of capillary forces that is related to the measured shift of the water retention curve. Copyright © 2007 John Wiley & Sons, Ltd.     

Late Quaternary stratigraphy of the northern Rockall trough and Faeroe-Shetland channel,northeast Atlantic Ocean

A late Quaternary deep-water stratigraphic framework has been established for the deep-water areas (>450m) of the northern Rockall Trough and Faeroe-Shetland Channel. Four stratigraphic units (1–4) are identified; these are primarily biostratigraphic units based on dinoflagellate cyst evidence. Unit 1 represents the late Weichselian glacial (pre-13 000 yr BP); unit 2 the Late Glacial Interstadial (11 000-13 000 yr BP); unit 3 is of Younger Dryas age (10 000-11 000 yr BP); and unit 4 represents the Holocene interglacial (post-10 000 yr BP). This stratigraphy is supported by the discovery of the mixed Vedde Ash (10 600 yr BP) and North Atlantic Ash zone 1, and the Saksunarvatn Ash (9000–9100 yr BP), concentrated in units 3 and 4 respectively. The sedimentology indicates that the oceanographic regime underwent a major change between the glacial and interglacial stages. This is marked by the onset of strong bottom current activity, allied to the restoration of overflow of the Norwegian Sea Deep Water into the North Atlantic, towards the end of the Younger Dryas Stadial. Despite intense bioturbation and bottom-current reworking the basic stratigraphic framework is maintained. Recognition of two volcanic ash markers enables correlation with established onshore and offshore sequences of marine and non-marine environments.     

The expansion of the giant filamentary shell, DEM 171, in the Magellanic Bridge

The giant filamentary shell, DEM 171, is found to be expanding at approximately 37 km s⁻¹. A supernova and stellar wind origin are both explored as possible causes for the expanding shell. A stellar wind origin would imply a mass-loss rate of the order of 10⁻⁵ M_⊙ yr⁻¹, indicating that it could be caused by a Wolf–Rayet star. A number of blue stars are found to lie within the shell and one is identified as a Wolf–Rayet candidate.     

The Tuxtuac,Mexico, Meteorite,an LL5 Chondrite Fall

Abstract— The Tuxtuac meteorite fell in Zacatecas state, Mexico, on 16 October 1975, at 1820 hours. Two partly crusted masses, weighing 1924 g and 2340 g, were recovered. The stone is an ordinary chondrite, LL5, with olivine Fa₃₀ and 19.22 weight % total iron. The silicates contain numerous voids and a froth-like mesostasis is present within some chondrules. Metal phases present are kamacite (5.7–6.4% Ni, 6–7% Co) and high nickel metal (taenite 37–41% Ni, 1.7 ± 0.3% Co; tetrataenite 47–52% Ni, 0.8–1.4% Co). The stone is unusual for an LL-group chondrite in that it exhibits neither large-scale brecciation features nor dark veins.     

Comparison of Archean structural styles in two belts of the Canadian Superior Province

In the Wabigoon granite-greenstone belt, volcanic strata are folded isoclinally about vertical axes, and the strata and folds wrap around mushrooming intrusions of tonalitic gneiss. The vertical fold plunges of the volcanic strata may result from diapirism deflecting the pre-existing ESE verging and facing recumbent folds, Steep, post-tectonic metamorphic gradients cross-cut the axial surfaces. There is no consistent tectonic trend on the regional scale.In the adjacent Quetico slate and gneiss belt tectonic trends are consistently east-west. Metagreywackes are isoclinally folded with vertical, east-west axial planes. The folds have strongly curving periclinal axes so that the ‘structural facing’ of the strata varies from up-through sideways- to down-facing. Isograds are parallel to the east-west axial traces.These structural contrasts may be valid for much of the length of the Quetico and Wabigoon belts along their interface.     

`Molar-tooth microspar': a chemical explanation for its disappearance ∼ 750 Ma

Molar-tooth structures are intricately crumpled, microsparry calcite fissure fills that formed during the Precambrian. Strontium isotope stratigraphy constrains the last occurrence of volumetrically significant molar-tooth structure (MT) in the geological record to ∼ 750 Ma. Although the disappearance of MT is commonly ascribed to the influence of metazoans on sediment cohesion, this now seems less plausible because there is no evidence for significant sediment disruption by metazoans before ∼ 550 Ma. It is proposed here that the most likely alternative explanation for MT disappearance is a change in ocean chemistry. A decrease in CaCO₃ saturation and/or an increase in the concentration of precipitation inhibitors in mid-Neoproterozoic seawater may have contributed to MT disappearance, and might also help to explain the approximately contemporaneous decline in stromatolite diversity.