Chemical and K–Ar systematics of a mylonitic mica after natural and experimental interactions with varied fluids

Flow-through and batch-leaching experiments combining mineralogical, chemical and K–Ar isotopic analyses of mica separates from a mylonitic sample of the Tyndrum faulting system in Scotland (U.K.) were conducted to evaluate and simulate the natural interaction with H₂SO₄-loaded river water at pHs of 3–4. The flow-through experiments with H₂SO₄-loaded and pure deionized water completed by a batch-leaching experiment with 1 M HCl at room temperature had varied effects on soluble mineral phases, such as Fe-sulfates and Ca/Mg-carbonates, that were present in the varied mica size fractions, but none caused the mineralogical and K–Ar characteristics of the mica to differ from separates of the natural environment.Despite the limited number of K–Ar ages, size fractionation of the mylonitic mica identified two generations of mica with different REE patterns. The coarser (2–8 μm) fraction yielded a high La/Yb ratio of 4.3 and crystallized at 359 ± 6 Ma at a temperature of 250–300 °C, probably during a major tectonic-thermal activity. The smaller (< 0.5 μm) fraction yielded a La/Yb ratio of 2.1, a younger K–Ar age at 315 ± 5 Ma and a lower crystallization temperature of about 200 °C.     

Evidence of tidal perturbations in the middle atmosphere over Southern Tropics as deduced from LIDAR data analyses

Systematic measurements of the middle-atmosphere temperature by a RAYLEIGH LIDAR located at La Réunion Island (20.8°S–55.5°E) has led to a preliminary study of the tidal effects in the height range of 30–70 km. Two analysis methods able to estimate the mean nighttime evolution of the temperature have been compared. Method 1 consists in averaging the temperature deviations from the nightly mean over several successive nights of measurement for given local solar times (LST); method 2 consists in averaging the raw data over the period of observation for given LST and in deriving afterwards the mean nighttime evolution of the temperature profiles. Some consistent LST-related structures have been observed with both methods, though better results have been obtained with method 1. One possible explanation for the differences between the two methods is the use of a series of data ranging from 4 to 8 h/night, depending on the meteorological conditions. In contrast to method 2, method 1 allows to correct the mean temperature for a given night, when the measurement window is different from night to night. Method 1 has been applied to two time series recorded in October and November 1995. The results clearly show the presence of tidal components with a downward phase propagation, specifically a warmer early night and a colder midnight in the stratosphere and the lower mesosphere. This behaviour is consistent with other LIDAR measurements made at similar latitudes in the Northern Hemisphere. In addition, a close comparison with the Global-Scale Wave Model (GSWM) tidal model predictions has also pointed out some similarities. Yet, large discrepancies in magnitude are also observed: as already reported in previous studies, the amplitudes predicted by GSWM are more than two times smaller compared to the corresponding values observed with the LIDAR.     

Diagnostic of the dayside ionosphere of Mars using the Total Electron Content measurement by the NEIGE/Netlander experiment: An assessment study

The NEtlander Ionosphere and Geodesy Experiment (NEIGE) of the Netlander mission to Mars will measure Doppler shifts affecting the radio links between ground stations and an orbiter. The experiment has two complementary scientific objectives which are the monitoring of the structure and dynamics of the ionosphere of Mars and the precise determination of Mars orientation parameters. The horizontal variation of the Total Electron Content (TEC) of the ionosphere will be derived from the so-called “geometric-free” combination of the Doppler shifts which affect radio links at two frequencies (in the UHF and S bands) between the Netlander microstations on the Mars surface and the data-relay orbiter. We describe a new method for retrieving the horizontal profile of the absolute value of the TEC. Simulations have allowed to evaluate the precision in the determination of the TEC using this method. We show that the daytime TEC can be retrieved with a precision of for a nominal accuracy of on the orbital pseudo-velocity, which represents a relative precision of a few percent. A preliminary analysis of the sensitivity of the TEC to the physical parameters which control the ionosphere has been performed. For this purpose, we have used a new one-dimensional ionospheric model based on the solution of coupled kinetic, fluid and MHD equations. This model describes the suprathermal electron component, the thermal plasma component as well as the induced horizontal magnetic field. The code which provides the vertical electron density profile has been used to study the variation of the TEC with the solar zenith angle and with the induced magnetic field at the top of the ionosphere. In particular, we show that NEIGE will allow to diagnose the penetration into the daytime ionosphere of an induced magnetic field.     

A microtome for sectioning lake sediment cores at a very high resolution

A microtome device for sectioning sediment cores at a very high resolution down to 100 m is described. It is suitable for a variety of applications, but it is described in this paper specifically for diatom analysis. The aim is to facilitate the study of annual or even seasonal variations. The instrument is of simple design and is easy to manipulate.     

Permeability Evolution During Non-linear Viscous Creep of Calcite Rocks

Permeability, storage capacity and volumetric strain were measured in situ during deformation of hot-pressed calcite aggregates containing 10, 20, and 30 wt% quartz. Both isostatic and conventional triaxial loading conditions were used. The tests were performed at confining pressure of 300 MPa, pore pressures between 50 to 290 MPa, temperatures from 673 to 873 K and strain rates of 3 × 10⁻⁵ s⁻¹. Argon gas was used as the pore fluid. The initial porosities of the starting samples varied from 5% to 9%, with higher porosity correlated to higher quartz content. Microstructural observations after the experiment indicate two kinds of pores are present: 1) Angular, crack-like pores along boundaries between quartz grains or between quartz and calcite grains and 2) equant and tubular voids within the calcite matrix. Under isostatic loading conditions, the compaction rate covaries with porosity and increases with increasing effective pressure. Most of the permeability reduction induced during compaction is irreversible and probably owes to plastic processes. As has been found in previous studies on hot-pressed calcite aggregates, permeability, k, is nonlinearly related to porosity, ϕ. Over small changes in porosity, the two parameters are approximately related as k ∝ ϕⁿ. The exponent n strongly increases as porosity decreases to a finite value (from about 4 to 6% depending on quartz content), suggesting a porosity percolation threshold. When subjected to triaxial deformation, the calcite-quartz aggregates exhibit shear-enhanced compaction, but permeability does not decrease as rapidly as it does under isostatic conditions. During triaxial compaction the exponent n only varies between 2 and 3. Non-isostatic deformation seems to reduce the percolation threshold, and, in fact, enhances the permeability relative to that at the same porosity during isostatic compaction. Our data provide constraints on the governing parameters of the compaction theory which describes fluid flow through a viscous matrix, and may have important implications for expulsion of sedimentary fluids, for fluid flow during deformation and metamorphism, and melt extraction from partially molten rocks.     

Relative performance of future altimeter systems and tide gauges in constraining a model of North Sea high-frequency barotropic dynamics

We evaluate in this paper the ability of several altimeter systems, considered separately as well as together with tide gauges, to control the time evolution of a barotropic model of the North Sea shelf. This evaluation is performed in the framework of the particular model errors due to uncertainties in bathymetry. An Ensemble Kalman Filter (EnKF) data assimilation approach is adopted, and observing-systems simulation experiments (OSSEs) are carried out using ensemble spread statistics. The skill criterion for the comparison of observing networks is, therefore, not based on the misfit between two simulations, as done in classic twin experiments, but on the reduction of ensemble variance occurring as a consequence of the assimilation. Future altimeter systems, such as the Wide Swath Ocean Altimeter (WSOA) and satellite constellations, are considered in this work. A single WSOA exhibits, for instance, similar performance as two-nadir satellites in terms of sea-level correction, and is better than three satellites in terms of model velocity control. Generally speaking, the temporal resolution of observations is shown to be of major importance for controlling the model error in these experiments. This result is clearly related to the focus adopted in this study on the specific high-frequency response of the ocean to meteorological forcing. Altimeter systems lack adequate temporal sampling for properly correcting the major part of model error in this context, whereas tide gauges, which provide a much finer time resolution, lead to better global statistical performance. When looking into further detail, tide gauges and altimetry are demonstrated to exhibit an interesting complementary character over the whole shelf, as tide gauge networks make it possible to properly control model error in a ∼100-km coastal band, while high-resolution altimeter systems are more efficient farther from the coast.