Holocene hydrological changes in south-western Mediterranean as recorded by lake-level fluctuations at Lago Preola,a coastal lake in southern Sicily,Italy

This paper presents a high-resolution lake-level record for the Holocene at Lago Preola (Sicily, southern Italy) based on a specific sedimentological approach, with a chronology derived from AMS radiocarbon dates. It gives evidence of three major successive palaeohydrological periods, with (1) a pronounced dryness during the early Holocene until ca 10300 cal BP, (2) a highstand from ca 10300 to 4500 cal BP, and (3) a marked lowstand from 4500 cal BP to present. Large amplitude lake-level fluctuations characterise two transition phases at ca 10300–9000 and 6400–4500 cal BP. Period 2 was interrupted between 8300 and 7000 cal BP by a dry phase that was punctuated to ca 7300 cal BP by the deposition of a tephra from neighbouring Pantelleria Island. Comparisons of the Preola record with other palaeohydrological records along north–south and west–east transects in the Mediterranean show contrasting patterns of hydrological changes: north (south) of around 40°N latitude, the records highlight a mid-Holocene period characterised by lake-level minima (maxima). Humid mid-Holocene conditions over the Mediterranean south of 40°N were probably linked to a strong weakening of the Hadley cell circulation and of monsoon winds. We suggest that the maximum of humidity in the Mediterranean during the mid-Holocene was characterised by humid winters to the north of 40°N and humid summers to the south. On a multi-centennial scale, the high-resolution palaeohydrological reconstructions in the central Mediterranean area reveal a strong climate reversal around 4500–4000 cal BP, with contrasting changes in the hydrological cycle. In addition to seasonal and inter-hemispherical changes related to orbital forcing, this major oscillation might be related to non-linear responses of the climatic system to the gradual decrease in summer insolation at northern latitudes. Another major climate oscillation around 7500–7000 cal BP may have resulted from the combined effects of (1) a strong rate of change in insolation, and (2) variations in solar activity. Finally, comparisons of the Preola lake-level record with Sicilian pollen records suggest a strong influence of moisture availability on vegetation development in Sicily. Very dry early Holocene conditions probably prevented the expansion of coastal evergreen forests, while decreasing moisture availability since the onset of the late Holocene may have exacerbated effects of intensive land-use.     

Magmatic cristobalite and quartz in the NWA 856 Martian meteorite

Abstract— Silica‐rich late‐stage crystallization pockets in the Martian meteorite Northwest Africa (NWA) 856 were investigated by transmission electron microscopy (TEM). The pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene. They consist of elongated grains of cristobalite and quartz embedded in a silica‐rich glass. Interstitial to the amorphous phase and silica minerals, a number of small accessory minerals have been identified, typical for late‐stage crystallization products. They are ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite, and chloroapatite. Cristobalite and quartz are shocked, as revealed by the occurrence of numerous amorphous lamellae. This assemblage suggests metastable dendritic crystallization under hydrous conditions. Cristobalite crystallization was probably facilitated by the presence of impurities such as Na or H₂O. Our observations show that silica minerals can be formed under magmatic conditions on Mars.     

Microstructural study of micron‐sized craters simulating Stardust impacts in aluminum 1100 targets

Abstract— Various microscopic techniques were used to characterize experimental microcraters in aluminum foils to prepare for the comprehensive analysis of the cometary and interstellar particle impacts in aluminum foils to be returned by the Stardust mission. First, scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS) were used to study the morphology of the impact craters and the bulk composition of the residues left by soda‐lime glass impactors. A more detailed structural and compositional study of impactor remnants was then performed using transmission electron microscopy (TEM), EDS, and electron diffraction methods. The TEM samples were prepared by focused ion beam (FIB) methods. This technique proved to be especially valuable in studying impact crater residues and impact crater morphology. Finally, we also showed that infrared microscopy (IR) can be a quick and reliable tool for such investigations. The combination of all of these tools enables a complete microscopic characterization of the craters.     

Fine‐grained material encased in microtracks of Stardust samples

Dust from comet 81P/Wild 2 was captured at high speed in silica aerogel collectors during the Stardust mission. Studies of deceleration tracks in aerogel showed that a number of cometary particles were poorly cohesive and fragmented during impact. Fragments are now scattered along the walls of impact cavities. Here, we report a transmission electron microscope study of a piece of aerogel extracted from the wall of track 10. We focused on micron‐sized secondary tracks along which fragments of a fine‐grained material are disseminated. Two populations of fragments were identified. The first is made of polycrystalline silicate assemblages (olivine, pyroxene, and spinel) that appear to be chemically related to each other. The second corresponds to silica‐rich glassy clumps characteristic of a mixture of melted cometary material and aerogel. A significant number of fragments have been found with a composition close to chondritic CI for the major elements Fe‐Mg‐S at a submicron scale. These fragments have thus never been chemically differentiated by high‐temperature processes prior to the accretion on the comet, in contrast to terminal particles, which are dominated by larger, denser, and frequently monomineralic components.     

Dendrogeomorphic reconstruction of past debris‐flow activity using injured broad‐leaved trees

Tree‐ring records from conifers have been regularly used over the last few decades to date debris‐flow events. The reconstruction of past debris‐flow activity was, in contrast, only very rarely based on growth anomalies in broad‐leaved trees. Consequently, this study aimed at dating the occurrence of former debris flows from growth series of broad‐leaved trees and at determining their suitability for dendrogeomorphic research. Results were obtained from gray alder (Alnus incana (L.) Moench), silver birch and pubescent birch (Betula pendula Roth and Betula pubescens Ehrh.), aspen (Populus tremula L.), white poplar, black poplar and gray poplar (Populus alba L., Populus nigra L. and Populus x canescens (Ait.) Sm.), goat willow (Salix caprea L.) and black elder (Sambucus nigra L.) injured by debris‐flow activity at Illgraben (Valais, Swiss Alps). Tree‐ring analysis of 104 increment cores, 118 wedges and 93 cross‐sections from 154 injured broad‐leaved trees allowed the reconstruction of 14 debris‐flow events between AD 1965 and 2007. These events were compared with archival records on debris‐flow activity at Illgraben. It appears that debris flows are very common at Illgraben, but only very rarely left the channel over the period AD 1965–2007. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of six patterns of debris‐flow routing and led to the determination of preferential breakout locations of events. The results of this study demonstrate the high potential of broad‐leaved trees for dendrogeomorphic research and for the assessment of the travel distance and lateral spread of debris‐flow surges. Copyright © 2010 John Wiley & Sons, Ltd.     

Pyroxenes microstructure in comet 81P/Wild 2 terminal Stardust particles

Abstract— We report the examination by transmission electron microscopy (TEM) of four Stardust terminal particles extracted from two neighboring tracks (32 an 69). The particles are made of well‐preserved crystalline grains dominated by low‐Ca pyroxene ranging from nearly pure enstatite to pigeonite. Some olivine grains are also found, in chemical equilibrium with the surrounding pyroxenes. Various microstructures are observed, as a function of the composition of the grains. They include (100)‐twinned pigeonite, clino/ortho domains in enstatite and exsolution in a Ca‐rich grain. The microstructures are mostly consistent with a formation by cooling from high‐temperature phases, which could be associated to igneous processes. Some dislocations in glide configuration are also present, probably attesting for small intensity shocks. Possible effects of the rapid heating/cooling stage and thermal shock associated to the collect are discussed. It appears that most of the microstructural features reported here are plausibly pristine.     

A transmission electron microscopy (TEM) investigation of opaque phases in shocked chondrites

Abstract— Shock defects in the most common silicate minerals of chondrites (olivine, pyroxenes and feldspars) have been investigated in detail, but there have been almost no studies of the shock defects in other components, like metal and sulfide. This probably stems from the fact that these latter phases are opaque in the optical microscope. The same reason explains why veins and melt pockets, which are constituted of microcrystalline or glassy phases (i.e., isotropic) are also poorly documented. We have investigated such phases by analytical transmission electron microscopy (ATEM) in two shocked chondrites, Tenham (L6) and Gaines County (H5). We have characterized shock defects in troilite very similar to those occurring in silicates (i.e., a mosaic texture and sets of straight and very narrow, ?10 nm, lamellae of amorphized FeS). There are many small regions in shocked chondrites that are composed of very fine grained (?1 μm) mixtures of metal and sulfide or of various silicates. They must result from local melting followed by a rapid cooling that prevented grain growth. We have determined the chemical compositions and the volume proportions of the tiny grains in these veins and melt pockets, which has allowed their temperature and pressure (T, P) history to be partially deciphered. Finally, we have observed a dense network of very narrow fractures (down to 10 nm) in the olivine and enstatite grains. These fractures are systematically filled with an amorphous (or cryptocrystalline) material that stems from the melt pockets and was injected when the fractures were opened by the rarefaction wave. This material was then quenched at the contact with the colder crystalline rims.     

Experimental study and TEM characterization of dusty olivines in chondrites: Evidence for formation by in situ reduction

Abstract— An analytical transmission electron microscopy (ATEM) study was undertaken in order to better understand the formation conditions of dusty olivines (i.e., olivines containing abundant tiny inclusions of Fe‐Ni metal) in primitive meteorites. Dusty olivines from type I chondrules in the Bishunpur chondrite (LL3.1) and from synthetic samples obtained by reduction of San Carlos olivines were examined. In both natural and experimental samples, micron size metal blebs observed in the dusty region often show preferential alignments along crystallographic directions of the olivine grains, have low Ni contents (typically <2 wt%), and are frequently surrounded by a silica‐rich glass layer. These features suggest that dusty olivines are formed by a sub‐solidus reduction of initially fayalitic olivines according to the following reaction: Some volatilization of SiO_gas may account for the apparent excess of metal relative to silica‐rich glass observed in both experimental and natural samples. Comparison with experimentally produced dusty olivines suggests that time scales of the order of minutes usually inferred for chondrule formation are also adequate for the formation of dusty olivines. These observations are in agreement with the hypothesis that at least part of the metal phase in chondrites originated from reduction during chondrule formation.     

Time-lapse resistivity investigations for imaging saltwater transport in glaciofluvial deposits

Five intersecting resistivity sections have been measured in glaciofluvial deposits hosting an aquifer of regional importance situated along a heavy traffic highway in Sweden. The winter salt spreading has caused a regular salinity increase through the years. For imaging the transport of saltwater in the aquifer, the sections were measured exactly in the same location before and after winter, and interpreted using a time-lapse inverse procedure. Some auger drilling and RCPT data were available for correlation. After winter, the resistivity had generally decreased under the water table and increased above it. The decrease in resistivity in the saturated zone is interpreted as a plume of more saline groundwater created by de-icing salt from the road. The increase in the upper layer can be explained by changes in temperature and soil moisture. The study shows that time-lapse resistivity investigations has potential for imaging hydraulic pathways in complex hydrogeological environments.     

Constraining magma sources using primitive olivine-hosted melt inclusions from Puñalica and Sangay volcanoes (Ecuador)

Constraining arc magma sources at continental arc settings is a delicate task, because chemical signatures from crustal processes obscure the slab and mantle signatures. Here, we present major, trace, and volatile element compositions of olivine-hosted melt inclusions (Fo_82–89) selected from the most primitive lavas (Mg# >?60) from two Ecuadorian volcanoes (Puñalica and Sangay) situated at the southern termination of the Andean Northern Volcanic Zone. Melt inclusions (MI) from Puñalica are nepheline normative and have basaltic-to-basaltic-andesite compositions (45–56 wt% SiO₂) similar to peridotite-derived melts. Sangay MI is also nepheline normative, with high CaO (up to 16 wt% and CaO/Al₂O₃?<?1) and low silica contents (41.9–44.5 wt%) pointing out an amphibole-bearing clinopyroxenite source. Both volcanoes display volatile-rich compositions (up to 6100 ppm Cl, 2200 ppm F, and 6700 ppm S). These MI cannot be related to their host lavas by fractional crystallization, implying that they represent true primitive liquids. The source of Puñalica MI was metasomatized by slab-derived melts that imprints its low Ba/Th, Sr/Th, and high Th/La (average values of 66, 129, and 0.22, respectively). On the contrary, the slab component added to the source of Sangay MI has a higher Ba/Th, Sr/Th, and low Th/La (average values of 261, 517, and 0.11, respectively) which could suggest a relative contribution of aqueous fluids. This dichotomy is related to the presence of the Grijalva Fracture Zone that separates a younger and hotter oceanic crust to the north (below Puñalica) from a colder and older oceanic crust to the south (below Sangay).