EISCAT/CRRES observations: nightside ionospheric ion outflow and oxygen-rich substorm injections

We present combined observations made near midnight by the EISCAT radar, all-sky cameras and the combined released and radiation efects satellite (CRRES) shortly before and during a substorm. In particular, we study a discrete, equatorward-drifting auroral arc, seen several degrees poleward of the onset region. The arc passes through the field-aligned beam of the EISCAT radar and is seen to be associated with a considerable upflow of ionospheric plasma. During the substorm, the CRRES satellite observed two major injections, 17 min apart, the second of which was dominated by O⁺ ions. We show that the observed are was in a suitable location in both latitude and MLT to have fed O⁺ ions into the second injection and that the upward flux of ions associated with it was sufficient to explain the observed injection. We interpret these data as showing that arcs in the nightside plasma-sheet boundary layer could be the source of O⁺ ions energised by a dipolarisation of the mid- and near-Earth tail, as opposed to ions ejected from the dayside ionosphere in the cleft ion fountain.     

Micromechanics approach to poroelastic behavior of a jointed rock

The formulation of macroscopic poroelastic behavior of a jointed rock is investigated within the framework of a micro–macro approach. The joints are modeled as interfaces, and their behavior is modeled by means of generalized poroelastic state equations. Starting from Hill's lemma extended for a jointed medium and extending the concept of strain concentration to relate the joint displacement jump to macroscopic strain, the overall poroelastic constitutive equations for the jointed rock are formulated. The analysis emphasizes the main differences and similarities of the resulting behavior with respect to that characterizing ordinary porous media. It is shown that, unlike ordinary porous media, conditions on the poroelastic parameters of joints are required for the macroscopic drained stiffness to entirely define the poroelastic behavior. This is achieved, for instance, if the joint network is characterized by a unique Biot coefficient. Extension of the analysis to non‐linear poroelasticity is also outlined. Finally, the theoretical formulation is applied to two particular cases of jointed rock for which explicit expressions of the overall poroelastic parameters are derived. Copyright © 2011 John Wiley & Sons, Ltd.     

Anomalous optical variations in the grossular garnet from the Eden Mills,Belvidere Mountain (Vermont,USA)

The optical anomalies, and surface and lamellar textures of a birefringent grossular garnet crystal from the Eden Mills, Belvidere Mountain, Vermont, USA, have been investigated by optical polarizing microscope, electron-probe micro-analyzer, back-scattered electron imaging, infrared spectroscopy, and single-crystal X-ray diffractometer from the standpoint of crystal growth. This grossular shows one-to-one correlation between natural surface features and its internal textures under crossed polarizers. Electron-probe micro-analyzer (EPMA) gave average chemical composition in (110) thin section, of bright lamella {Ca_2.97Mn_0.06}_∑3.03 [Al_1.59Fe_0.37Ti_0.01]_∑1.97(Si_3.00)_∑3.00 (Gros_79.5And_18.9Sps_1.6) and of dark host {Ca_2.99Mn _0.06}_∑3.05 [Al_1.73Fe_0.26 Ti_0.01]_∑2.00(Si_2.97OH_0.03)_∑3.00 (Gros_85.4And₁₃Sps_1.6). The correspondence of surface features and the internal textures with spiral or pyramidal growth mechanism suggest that the internal textures of the Eden Mills grossular are formed during growth process. The optical vibrational orientations and the growth steps inclination along [001] and \( \left[\overset{-}{1}10\right] \) directions predict monoclinic symmetry. With X-ray diffractometer (XRD) method, pseudocubic parameters are a = 11.839(2) Å, b = 11.855(1) Å, and c = 11.868(2) Å with interaxial angles α = 90.00(1)°, β = 89.99(1)°, and γ = 90.02(2)° that show orthorhombic symmetry of this crystal. Lamellar texture of Al³⁺-rich host with Fe³⁺-rich lamella infers cation ordering at octahedral site of the garnet structure. IR data favors the non-cubic orientation of [(OH) ₄] at tetrahedral position in this grossular structure.     

Benthic Carbon Mineralization and Nutrient Turnover in a Scottish Sea Loch: An Integrative In Situ Study

Based on in situ microprofiles, chamber incubations and eddy covariance measurements, we investigated the benthic carbon mineralization and nutrient regeneration in a ~65-m-deep sedimentation basin of Loch Etive, UK. The sediment hosted a considerable amount of infauna that was dominated by the brittle star A. filiformis. The numerous burrows were intensively irrigated enhancing the benthic in situ O₂ uptake by ~50 %, and inducing highly variable redox conditions and O₂ distribution in the surface sediment as also documented by complementary laboratory-based planar optode measurements. The average benthic O₂ exchange as derived by chamber incubations and the eddy covariance approach were similar (14.9 ± 2.5 and 13.1 ± 9.0 mmol m^?2 day^?1) providing confidence in the two measuring approaches. Moreover, the non-invasive eddy approach revealed a flow-dependent benthic O₂ flux that was partly ascribed to enhanced ventilation of infauna burrows during periods of elevated flow rates. The ratio in exchange rates of ΣCO₂ and O₂ was close to unity, confirming that the O₂ uptake was a good proxy for the benthic carbon mineralization in this setting. The infauna activity resulted in highly dynamic redox conditions that presumably facilitated an efficient degradation of both terrestrial and marine-derived organic material. The complex O₂ dynamics of the burrow environment also concurrently stimulated nitrification and coupled denitrification rates making the sediment an efficient sink for bioavailable nitrogen. Furthermore, bioturbation mediated a high efflux of dissolved phosphorus and silicate. The study documents a high spatial and temporal variation in benthic solute exchange with important implications for benthic turnover of organic carbon and nutrients. However, more long-term in situ investigations with like approaches are required to fully understand how environmental events and spatio-temporal variations interrelate to the overall biogeochemical functioning of coastal sediments.     

Effect of sampling and linkage on fault length and length–displacement relationship

The power-law exponent (n) in the equation: D=cL ⁿ , with D = maximum displacement and L = fault length, would be affected by deviations of fault trace length. (1) Assuming n=1, numerical simulations on the effect of sampling and linkage on fault length and length–displacement relationship are done in this paper. The results show that: (a) uniform relative deviations, which means all faults within a dataset have the same relative deviation, do not affect the value of n; (b) deviations of the fault length due to unresolved fault tip decrease the values of n and the deviations of n increase with the increasing length deviations; (c) fault linkage and observed dimensions either increase or decrease the value of n depending on the distribution of deviations within a dataset; (d) mixed deviations of the fault lengths are either negative or positive and cause the values of n to either decrease or increase; (e) a dataset combined from two or more datasets with different values of c and orders of magnitude also cause the values of n to deviate. (2) Data including 19 datasets and spanning more than eight orders of fault length magnitudes (10⁻²–10⁵ m) collected from the published literature indicate that the values of n range from 0.55 to 1.5, the average value being 1.0813, and the peak value of n _d (double regression) is 1.0–1.1. Based on above results from the simulations and published data, we propose that the relationship between the maximum displacement and fault length in a single tectonic environment with uniform mechanical properties is linear, and the value of n deviated from 1 is mainly caused by the sampling and linkage effects.     

A surface complexation model for cadmium and lead adsorption onto diatom surface

This work is devoted to the physico-chemical study of cadmium and lead interaction with diatom–water interfaces for two marine planktonic (Thalassiosira weissflogii = TW, Skeletonema costatum = SC) and two freshwater periphytic species (Achnanthidium minutissimum = AMIN, Navicula minima = NMIN) by combining adsorption measurements with surface complexation modeling. Reversible adsorption experiments were performed at 20 °C after 3 h of exposure as a function of pH, metal concentration in solution, and ionic strength. While the shape of pH-dependent adsorption edge is similar among all four diatom species, the constant-pH adsorption isotherm and maximal binding capacities differ. These observations allowed us to construct a surface complexation model for cadmium and lead binding by diatom surfaces that postulates the constant capacitance of the electric double layer and considers Cd and Pb complexation with mainly carboxylic and, partially, silanol groups. Parameters of this model are in agreement with previous acid–base titration results and allow quantitative reproduction of all adsorption experiments.