A large shock vein in L chondrite Roosevelt County 106: Evidence for a long‐duration shock pulse on the L chondrite parent body

A large shock‐induced melt vein in L6 ordinary chondrite Roosevelt County 106 contains abundant high‐pressure minerals, including olivine, enstatite, and plagioclase fragments that have been transformed to polycrystalline ringwoodite, majorite, lingunite, and jadeite. The host chondrite at the melt‐vein margins contains olivines that are partially transformed to ringwoodite. The quenched silicate melt in the shock veins consists of majoritic garnets, up to 25 μm in size, magnetite, maghemite, and phyllosilicates. The magnetite, maghemite, and phyllosilicates are the terrestrial alteration products of magnesiowüstite and quenched glass. This assemblage indicates crystallization of the silicate melt at approximately 20–25 GPa and 2000 °C. Coarse majorite garnets in the centers of shock veins grade into increasingly finer grained dendritic garnets toward the vein margins, indicating increasing quench rates toward the margins as a result of thermal conduction to the surrounding chondrite host. Nanocrystalline boundary zones, that contain wadsleyite, ringwoodite, majorite, and magnesiowüstite, occur along shock‐vein margins. These zones represent rapid quench of a boundary melt that contains less metal‐sulfide than the bulk shock vein. One‐dimensional finite element heat‐flow calculations were performed to estimate a quench time of 750–1900 ms for a 1.6‐mm thick shock vein. Because the vein crystallized as a single high‐pressure assemblage, the shock pulse duration was at least as long as the quench time and therefore the sample remained at 20–25 GPa for at least 750 ms. This relatively long shock pulse, combined with a modest shock pressure, implies that this sample came from deep in the L chondrite parent body during a collision with a large impacting body, such as the impact event that disrupted the L chondrite parent body 470 Myr ago.     

ETHNIC NEIGHBORHOODS AND URBAN REVITALIZATION: CAN EUROPE USE THE AMERICAN MODEL?

ABSTRACT. Ethnic neighborhoods have long been used to facilitate urban revitalization in older, inner‐city neighborhoods in American cities, but this strategy is much less common in European cities. This is especially surprising because immigrants make up a significant percentage of the population in a large number of those cities. This article explores the role that a largely invented Little Italy has played in revitalizing a section of downtown San Diego in contrast to the difficulty of creating such districts in European cities. The question posed here is, Will this contrast in approaches and outcomes remain indefinitely, or will the use of ethnically themed revitalization strategies become more alike as globalization and the expansion of the European Union serve to lessen historic ethnic tensions and increase the number of distinctive immigrant districts in Europe?     

Mesophotic coral buildups in a prodelta setting (Late Eocene,southern Pyrenees,Spain): a mixed carbonate–siliciclastic system

Lower Priabonian coral bioherms and biostromes, encased in prodelta marls/clays, occur in the Aínsa‐Jaca piggyback basin, in the South Central Pyrenean zone. Detailed mapping of lithofacies and bounding surfaces onto photomosaics reveals the architecture of coral buildups. Coral lithosomes occur either isolated or amalgamated in larger buildups. Isolated lithosomes are 1 to 8 m thick and a few hundred metres wide; clay content within coral colonies is significant. Stacked bioherms form low‐relief buildups, commonly 20 to 30 m thick, locally up to 50 m. These bioherms are progressively younger to the west, following progradation of the deltaic complex. The lowermost skeletal‐rich beds consist of bryozoan floatstone with wackestone to packstone matrix, in which planktonic foraminifera are abundant and light‐related organisms absent. Basal coral biostromes, and the base of many bioherms, consist of platy‐coral colonies ‘floating’ in a fine‐grained matrix rich in branches of red algae. Corals with domal or massive shape, locally mixed with branching corals and phaceloid coral colonies, dominate buildup cores. These corals are surrounded by matrix and lack organic framework. The matrix consists of wackestone to packstone, locally floatstone, with conspicuous red algal and coral fragments, along with bryozoans, planktonic and benthonic foraminifera and locally sponges. Coral rudstone and skeletal packstone, with wackestone to packstone matrix, also occur as wedges abutting the buildup margins. Integrative analysis of rock textures, skeletal components, buildup anatomy and facies architecture clearly reveal that these coral buildups developed in a prodelta setting where shifting of delta lobes or rainfall cycles episodically resulted in water transparency that allowed zooxanthellate coral growth. The bathymetric position of the buildups has been constrained from the light‐dependent communities and lithofacies distribution within the buildups. The process‐product analysis used here reinforces the hypothesis that zooxanthellate corals thrived in mesophotic conditions at least during the Late Eocene and until the Late Miocene. Comparative analysis with some selected Upper Eocene coral buildups of the north Mediterranean area show similarities in facies, components and textures, and suggest that they also grew in relatively low light (mesophotic) and low hydrodynamic conditions.     

Long-term Arctic peatland dynamics, vegetation and climate history of the Pur-Taz region, Western Siberia

Stratigraphic analyses of peat composition, LOI, pollen, spores, macrofossils, charcoal and AMS ages are used to reconstruct the peatland. vegetation and climatic dynamics in the Pur-Taz region of western Siberia over 5000 years (9300-4500 BP). Section stratigraphy shows many changes from shallow lake sediment to different combinations of forestcd or open sedge, moss, and Equisetum fen and peatland environments. Macrofossil and pollen data indicate that Larix sibirica and Beth pubescens trees were the first to arrive, followed by Picea obovata . The dominance of Picea macrofossils 6000-5000 BP in the Pur-Taz peatland along with regional Picea pollen maxima indicate warmer conditions and movement of the spruce treeline northward at this time. The decline of pollen and macrofossils from all of these tree species in uppermost peats suggests a change in the environment less favorable for their growth, perhaps cooler tempratures and/or less moisture. Of major significance is the evidence for old ages of the uppermost peats in this area of Siberia, suggesting a real lack of peat accumulation in recent millennia or recent oxidation of uppermost peat.