On effects of trawling, benthos and sampling design

The evidence for the wider effects of fishing on the marine ecosystem demands that we incorporate these considerations into our management of human activities. The consequences of the direct physical disturbance of the seabed caused by towed bottom-fishing gear have been studied extensively with over 100 manipulations reported in the peer-reviewed literature. The outcome of these studies varies according to the gear used and the habitat in which it was deployed. This variability in the response of different benthic systems concurs with established theoretical models of the response of community metrics to disturbance. Despite this powerful evidence, a recent FAO report wrongly concludes that the variability in the reported responses to fishing disturbance mean that no firm conclusion as to the effects of fishing disturbance can be made. This thesis is further supported (incorrectly) by the supposition that current benthic sampling methodologies are inadequate to demonstrate the effects of fishing disturbance on benthic systems. The present article addresses these two erroneous conclusions which may confuse non-experts and in particular policy-makers.     

Effect of ionic strength on the Mg content of calcite: Toward a physical basis for minor element uptake during step growth

This experimental study determined the effect of ionic strength (IS) on the uptake of Mg into calcites that grew by the classical step propagation process. Using flow-through AFM and defined solution chemistry, calcite was grown in NaCl and KCl solutions of known supersaturation state while measuring the corresponding growth kinetics. Analysis of the resulting calcite compositions by SIMS shows that Mg content is inversely correlated with IS for both electrolytes. A sixfold increase in IS decreases the Mg-content by up to 40%. Overgrowths that developed in NaCl solutions contain more Mg than samples that grew in KCl solutions. The corresponding kinetic measurements reveal that step propagation rates are independent of IS within experimental error but are electrolyte-specific. In NaCl solutions, steps with the obtuse geometry move significantly faster than acute steps, but in KCl solutions, the acute and obtuse steps move at similar rates.Analysis of the data suggests that the decrease in Mg content with increasing IS arises from the interplay of ion-kink interactions between the background cations (Na⁺ or K⁺), the primary solute cation (Ca²⁺), and the impurity (Mg²⁺). A simple physical model proposes that increasing levels of electrolytes block the attachment of the strongly hydrated Mg²⁺ ion relative to Ca²⁺ but the effects are step-specific for each type of electrolyte. Whereas K⁺ interacts weakly with kink sites along both step directions, Na⁺ interacts preferentially with acute steps and, consequently, slows their rate of step propagation relative to obtuse steps. Because Na⁺ increases the fraction of the surface that develops from acute steps and because Mg is preferentially incorporated into the kink sites of acute steps, calcite overgrowths developed in NaCl solutions contain more Mg than those in grown in KCl. Thus, the salt-specific Mg contents measured in this study can be explained by shifts in the distribution of step types and the ability of each step type to incorporate Mg. The findings reconcile apparent discrepancies regarding the effect of IS on calcite kinetics and Mg incorporation observed in laboratory-based studies.     

The inherent displacive flexibility of the hexacelsian tetrahedral framework and its relationship to polymorphism in Ba-hexacelsian

 The results of a rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the tetrahedral framework of the ideal hexacelsian structure type are presented. One of the three types of RUM found to exist is characterized by modulation wave vectors perpendicular to <110> and atomic displacement patterns involving tetrahedral rotation around the parent c axis while a second type of RUM is found to be soft at any modulation wave vector and to involve tetrahedral rotation about in-plane rotation axes. It is shown how a combination of these two types of RUM motion associated with the same q=1/2<1ˉ101>^* modulation wave vector enables the outstanding crystal chemical problems and apparently mutually contradictory results as regards polymorphism in Ba-hexacelsian to be resolved. Received: 30 December 1999 / Accepted: 16 May 2000     

Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulations

The distributions of Al ³⁺/Mg ²⁺ and Al ³⁺/ Fe ³⁺ were studied in the octahedral sheet of illites and smectites. Cation exchange interaction parameters J _i, as first, second, third and fourth neighbours were calculated by means of empirical interatomic potentials. Several compositions with different interlayer cations and tetrahedral charge were studied in both Al/Mg and Al/Fe systems. The values of J _i parameters were similar in all Al/Mg samples. From these J _i values, a strong trend to form AlMg pairs was observed in the Al/Mg system. In the Al/Fe system, the values of J _i are very small, indicating no preference for Al/Fe mixing. From these J _i parameters, Monte Carlo simulations of octahedral cation ordering were performed. In the Al/Mg system, an order/disorder phase transition was observed obtaining a fully ordered distribution without presence of an MgMg pair, according to experimental data. Similar phase transitions were observed for the octahedral compositions Al/Mg 1/1 and 3/1. In the Al/Fe system an order/disorder phase transition was also detected but at very low temperature for illite and smectite. Complete Al/Fe mixing is observed in the most stable ordered distribution. This is consistent with experimental results for synthetic Fe/Al smectites.     

Numerical studies of shear banding in interface shear tests using a new strain calculation method

Strain localization is closely associated with the stress–strain behaviour of an interphase system subject to quasi‐static direct interface shear, especially after peak stress state is reached. This behaviour is important because it is closely related to deformations experienced by geotechnical composite structures. This paper presents a study using two‐dimensional discrete element method (DEM) simulations on the strain localization of an idealized interphase system composed of densely packed spherical particles in contact with rough manufactured surfaces. The manufactured surface is made up of regular or irregular triangular asperities with varying slopes. A new simple method of strain calculation is used in this study to generate strain field inside a simulated direct interface shear box. This method accounts for particle rotation and captures strain localization features at high resolution. Results show that strain localization begins with the onset of non‐linear stress–strain behaviour. A distinct but discontinuous shear band emerges above the rough surface just before the peak stress state, which becomes more expansive and coherent with post‐peak strain softening. It is found that the shear bands developed by surfaces with smaller roughness are much thinner than those developed by surfaces with greater roughness. The maximum thickness of the intense shear zone is observed to be about 8–10 median particle diameters. The shear band orientations, which are mainly dominated by the rough boundary surface, are parallel with the zero extension direction, which are horizontally oriented. Published in 2007 by John Wiley & Sons, Ltd.     

Theoretical modelling of cis -vacant and trans -vacant configurations in the octahedral sheet of illites and smectites

 The cis-vacant configurations of smectites and illites have been studied theoretically by using transferable empirical interatomic potentials. A wide range of compositions of octahedral and tetrahedral cation and interlayer charge has been considered. All results have been compared with the trans-vacant configurations in each sample. The calculated values reproduce the differences in the lattice parameters between the cis- and trans-vacant configurations of experimental studies. Taking into account the cis-/trans-vacant proportion, the calculated structures agree with experiment for the main structural features of the crystal lattice. The effect of the cation substitution in the octahedral and tetrahedral sheets on the cell parameters has been also studied, finding good linear relationships. The calculated cation substitution effects are consistent with experimental results. Although the energy difference between the cis- and trans-vacant configurations is small, the cis-vacant is more stable when the composition of clays is more smectitic, like the experimental behaviour. Similar trends of the cation substitution effect on the cis-/trans-vacant proportion to the experimental results are found. The structure of the hydroxy groups has also been analysed. The OH bond length, the orientation of the O–H bond with respect to the (001) plane and the non-bonding H...O distances have been studied. Received: 4 September 2000 / Accepted: 29 January 2001     

Neutron diffraction study of the tricritical orientational order/disorder phase transition in calcite at 1260 K

Neutron powder diffraction measurements of the temperature dependence of superlattice reflections in calcite have shown that there is a continuous phase transition at 1260 K. The change in space group symmetry and the halving of the unit cell size on heating indicate that this transition is an orientational order/disorder transition. The intensities of the superlattice reflections show that the temperature dependence of the order parameter, Q, is of the form (T _c –T), where is 0.25, indicating that the transition is tricritical. The transition is accompanied by a large contraction along the c axis on cooling, defining a spontaneous strain e₃ which is related to the order parameter (and hence temperature) via e₃ Q ². No evidence for critical lowering of the value of was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO₃.     

Quasielastic incoherent neutron scattering study of the rotational dynamics of the water molecules in analcime

The rotational motion of the water molecules in analcime has been investigated by quasielastic incoherent neutron scattering. Reorientational times vary from 65 ± 20 ps at 260 K to 26 ± 3 ps at 370 K, and the temperature dependence suggests an activation energy for reorientations of 780 ± 200 K. Measurements of the temperature dependence of the intensity of the elastic incoherent scattering indicate that there is no cooperative ordering at low temperatures, and that instead there is a gradual freezing-in of the motion of individual water molecules.