Is Pre-Eruptive Null Point Reconnection Required for Triggering Eruptions?

We study the magnetic field evolution and topology of the active region NOAA 10486 before the 3B/X1.2 flare of October 26, 2003, using observational data from the French–Italian THEMIS telescope, the Michelson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (SOHO), the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observation Station (HSOS), and the Transition Region and Coronal Explorer (TRACE). Three dimensional (3D) extrapolation of photospheric magnetic field, assuming a potential field configuration, reveals the existence of two magnetic null points in the corona above the active region. We look at their role in the triggering of the main flare, by using the bright patches observed in TRACE 1600 Å images as tracers at the solar surface of energy release associated with magnetic reconnection at the null points. All the bright patches observed before the flare correspond to the low-altitude null point. They have no direct relationship with the X1.2 flare because the related separatrix is located far from the eruptive site. No bright patch corresponds to the high-altitude null point before the flare. We conclude that eruptions can be triggered without pre-eruptive coronal null point reconnection, and the presence of null points is not a sufficient condition for the occurrence of flares. We propose that this eruptive flare results from the loss of equilibrium due to persistent flux emergence, continuous photospheric motion and strong shear along the magnetic neutral line. The opening of the coronal field lines above the active region should be a byproduct of the large 3B/X1.2 flare rather than its trigger.     

Diffusion induced Li isotopic fractionation during the cooling of magmatic rocks: The case of pyroxene phenocrysts from nakhlite meteorites

Ion-microprobe was used to measure Li abundances and isotopic compositions in pyroxenes from three Martian meteorites belonging to the nakhlite family. The profiles performed across augite crystals from Northwest Africa 817 show a large isotopic zoning from crystal cores (δ⁷Li ∼ 0‰) to rims (δ⁷Li ∼ +20‰) while Li abundances are almost constant (∼9.2 μg/g). Unlike NWA 817, the pyroxene studied in the Miller Range 03346 nakhlite shows a zoning in Li abundance, with concentrations increasing from ∼2.5 μg/g in the core to ∼9 μg/g in the rim. The augite rim (δ⁷Li = +7‰) is slightly enriched in ⁷Li with regard to the core (δ⁷Li = +4‰), but most of the isotopic variations observed occur at an intermediate position along the profile, where δ⁷Li falls down to ∼−11‰. In the case of Nakhla, Li concentrations in augite increase from cores (∼3.5 μg/g) to rims (∼6.5 μg/g), while the δ⁷Li variation is restricted (i.e., between δ⁷Li = +6.0 and +12.6‰). For the three meteorites the Li abundances were also measured in the groundmass, which was found to be enriched in lithium (∼10 μg/g). Conventional magmatic and post-magmatic processes such as alteration and fractional crystallization, fail to explain the dataset obtained on nakhlites. Degassing processes, which were previously proposed to explain the Li distribution in shergottite crystals, cannot result in the strong decoupling between Li abundances and isotopic composition observed in nakhlites. We suggest that the original magmatic Li distributions (concentrations and isotopic compositions) in nakhlites have been modified by diffusion of Li from the Li-rich groundmass towards the pyroxene crystals during sub-solidus cooling. Diffusion appears to have been efficient for NWA 817 and MIL 03346 but, apparently, did not produce a significant migration of Li in Nakhla, possibly because of the lower abundance of groundmass in the latter. Diffusion induced Li redistributions may also affect terrestrial porphyric rocks but very specific cooling rates are required to quench the diffusion profiles as observed in two of the present nakhlites.     

The pelagic community of a gravel pit lake: Significance of Coleps hirtus viridis (Prostomatida) and its role as a scavenger

Coleps hirtus viridis was the dominant species of the planktonic ciliate community of Lake Fühlinger See (Germany) during the study in 1999 and 2000. Total ciliate densities ranged from 120 to 42,000 ind. l⁻¹ in 1999 and up to 8,000 ind. l⁻¹ in 2000. Coleps contributed up to 98% to both total ciliate abundance and biomass and made up an average of 64% of the total ciliate biomass. Oligotrichs (Rimostrombidium, Strobilidium) dominated the epilimnetic zone, whereas peritrich ciliates (Pelagovorticella, Vorticella) were predominantly located in the hypolimnion. The population maximum of Coleps changed locations from the epilimnion in early summer to the hypolimnion (up to 40,000 ind. l⁻¹) during stratification. High growth rates in the hypolimnion, presence of endosymbiontic algae and the ability to ingest detritus seem to be important for the success.Growth rates of Coleps in June were determined by Landry-Hassett dilution experiments in both the epilimnion and the hypolimnion. The instantaneous growth rates were similar in both layers (0.6 d⁻¹), but a distinctly higher instantaneous mortality was estimated for the epilimnion. These high loss rates may be due to grazing pressure by cladocerans.The significance of the histophagous feeding of Coleps was evaluated through an experiment using killed zooplankton. Parts of Daphnia magna were incorporated at rates of about 1,100 μm³ ind.⁻¹ h⁻¹ by Coleps without endosymbiotic algae and at rates of 500 μm³ ind.⁻¹ h⁻¹ by Coleps with endosymbionts. These high feeding rates support the conclusion that Coleps can use dead organic matter as an additional food source.     

Response of Nereis diversicolor population (Polychaeta,Nereididae) to the pollution impact – Authie and Seine estuaries (France)

A survey within the French National Programme of Ecotoxicology was carried out in 2002, 2003 and 2004 to study the response of Nereis diversicolor populations (Polychaeta, Nereididae) to the impact of pollution in the Authie estuary (non-contaminated site) and in the Seine estuary (contaminated site). In the period studied, the density varied from 672 ind. m⁻² to 3584 ind. m⁻² in the Authie estuary and from 80 ind. m⁻² to 920 ind. m⁻² in the Seine estuary. Biomass varied from 3.94 g m⁻² (dry weight) in February 2004 to 38.0 g m⁻² in August 2003 in the Authie estuary and from 3.4 g m⁻² in February 2002 to 0.6 g m⁻² in February 2004 in the Seine estuary. Density and biomass of the populations of N. diversicolor were consistently lower in the Seine estuary than in the Authie estuary. Size frequency histograms permit the analysis of the cohorts as well as the elaboration of the growth curves. For the individuals from the Authie estuary, the relation between dry weight (DW) and length L3 (prostomium, peristomium and chaetiger 1) was DW = 4.2205 L3^2.9832. For those from the Seine estuary, the relation between dry weight and L3 was DW = 0.4697e^1.7209L3. The individuals of N. diversicolor should belong to eight cohorts in Authie estuary (two cohorts each year) instead of six cohorts for those from the Seine estuary. These differences can be attributed to the effect of pollution on the population of N. diversicolor.     

Origin of EL3 chondrites: Evidence for variable C/O ratios during their course of formation—A state of the art scrutiny

Mineral inventories of enstatite chondrites; (EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen (fO₂) and sulfur (fS₂) fugacities. They are best preserved in the Almahata Sitta MS‐17, MS‐177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short‐lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre‐ and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be >0.83 and <1.03. These parameters not only induce unusual geochemical behavior of the elements inverting many nominally lithophile elements to chalcophile or even siderophile or anthracophile. Structure and mineral inventories in EL3 and EH3 chondrites are fundamentally different. Yet EH3 and EL3 members store crucial information relevant to eventual source regions and importantly possible variation in C/O ratio in the course of their evolution. EL3 and EH3 chondrites contain trichotomous lithologies (1) chondrules and their fragments, (2) polygonal enstatite‐dominated objects, and (3) multiphase metal‐rich nodules. Mineralogical and cosmochemical inventories of lithologies in the same EL3 indicate not only similarities (REE inventory and anomalies in oldhamite) but also distinct differences (sinoite‐enstatite‐graphite relationship). Oldhamite in chondrules and polygonal fragments in EL3 depict negative Eu anomaly attesting a common cosmochemical source. Metal‐dominated nodules in both EL3 and EH3 are conglomerates of metal clasts and sulfide fragments in EH3 and concentrically zoned C‐bearing metal micropebbles (≥25 μm ≤50 μm) in EL3 thus manifesting a frozen in unique primordial accretionary metal texture and composition. Sinoite‐enstatite‐diopside‐graphite textures reveal a nucleation and growth strongly suggestive of fluctuating C/O ratio during their nucleation and growth in the source regions. Mineral inventories, sulfide phase relations, sinoite‐enstatite‐graphite intergrowth, carbon and nitrogen isotopic compositions of graphite, spatial nitrogen abundance in graphite in metal nodules, and last but not least ¹²⁹I/¹²⁹Xe and ⁵³Mn/⁵³Cr systematics negate any previously suggested melting episode, pre‐accretionary or dynamic, in parental asteroids.     

On the significance of aragonite occurrences in the Western Alps

Aragonite occurrences from two areas of the Western Alps are described. It is shown that aragonite has been formed under blueschist metamorphic conditions in the Western Vanoise, while it has been precipitated under sub-surface conditions in the Queyras region. An uplift (P, T) path of the aragonite-bearing rocks of the Western Vanoise is constructed using two independent methods: (1) temperature estimates using the Sr⁺⁺ content of aragonite in the successive veins and (2) the kinetics of the aragonitecalcite solid-state transition. The uplift (P, T) path has an unusual shape with an important temperature decrease (100° to 150° C for a pressure decrease of 0.2 GPa) following blueschists metamorphism (P=0.7 GPa, T=300° C). Thermal models show that this unusual (P, T) path of the Western Vanoise can be explained if one maintains a low temperature (between 125° and 175° C) at the base of the tectonic unit containing the aragonite-bearing rocks during part of its burial history, followed by the whole of its exhumation. A tectonic scenario is proposed to account for the observed and modelled (P, T) path.     

Chronostratigraphy and paleoclimatic meaning of cryogenic deformations in the Central European loess

Cryoturbations in loess reflect cold and wet environments but not necessarily permafrost. Only flat bottomed cryoturbations are indicative of it. Outside the permafrost zone they occur during the wet period of the last glaciation: during the early Weichselian, the lower pleniglacial, the end of the middle pleniglacial and the late glacial. Their morphology ist related to the frost susceptibility of the superimposed layers, expressed in form of differential frost heaving, ruled by the drainage quality and the frost susceptibility gradient. Their diagenesis is discussed.     

A theoretical approach to the kinetics of calcite ⇌ aragonite transition: application to laboratory experiments

A theoretical approach to the calcite ? aragonite transformation is presented for the case of a dislocation glide mechanism obeying first-order kinetics. The kinetic parameters are determined from recent experimental data from the literature; the sensitivity of the model to these and other parameters like the grain size and the deviatoric stresses are discussed.The model results in the definition of what could be the actual transformation domains of the CaCO₃ polymorphs in the (P,T) plane. These domains are different from the classical ones, i.e. different from the stability fields defined with respect to Clapeyron's curve; they can evolve in the (P,T) plane when a deviatoric stress is applied to the system. In this case, we can define a domain in which the two senses of the calcite ? aragonite transition are activated simultaneously.The application of the model to laboratory experiments needs the integration of the kinetic differential equation (K varying withP andT). Using this method, we demonstrate the role of (P,T,t) path on reaction rate and the role of experimental uncertainties on the reaction parametersK andn.Finally, we discuss the problem of the synthesis of polymorphs out of their stability field and of their formation by grinding; these problems, difficult to tackle in terms of diffusion, can be approached with the proposed dislocation transformation kinetic model.     

Structures caused by repeated freezing and thawing in various loamy sediments: A comparison of active,fossil and experimental data

In this paper, the authors present the results of both macroscopic and microscopic investigations on structure development created by repeated ice lensing in various loamy experiments. Experimental data are compared with observations performed on active forms in High Arctic and Alpine Mountain environments. Those observations are also compared with phenomena observed in fossil periglacial formations of Western Europe. Platy and short prismatic structure formation is bonded to the hydraulic and thermal conditions during ice segregation. When a long series of alternating freezing and thawing affects platy structures, the fabric evolves, also being influenced by slope and drainage conditions: cryoturbations, frostcreep, and gelifluction can appear. They are characterized by specific microfabrics which are better developed with an increasing number of cycles: this is clear in experiments where hydraulic and thermal parameters are better controlled. Vesicles are also a prominent characteristic of the surface horizon in experiments and arctic soils. The genesis of vesicles is discussed on the basis of new observations and is related to the mechanical collapse of frost-created aggregates under the mechanical work of soil air escape during soil saturation by water at thaw.