Linking shock textures revealed by BSE,CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure,Brazil

A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.     

Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps

The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale mega-shear, as their model suggests.     

SciMeasure Wavefront Sensor Cameras and their Application in the Palomar Adaptive Optics System

SciMeasure, in collaboration with Emory University and the Jet Propulsion Laboratory, has developed a very versatile CCD controller for use in adaptive optics, optical interferometry, and other applications requiring high-speed readout rates and/or low read noise. The overall architecture of this controller system will be discussed and its performance using both EEV CCD39 and MIT/LL CCID-19 detectors will be presented. This controller is used in the adaptive optics system, developed by JPL, for the 200′′ Hale telescope at Palomar Mountain. Early diffraction-limited science results, recently achieved by the AO system, are presented. We gratefully acknowledge the financial support of NASA through SBIR contracts NAS8–97195 and NAS8–98081.     

The Cretaceous–Palaeogene boundary at Gorgonilla Island,Colombia, South America

The discovery of a new Cretaceous/Palaeogene (K/Pg) bathyal marine sequence on Gorgonilla Island, SW Colombia, extends the presence of Chicxulub impact spherule deposits to the Pacific region of northern South America and to the Eastern Pacific Ocean. The Gorgonilla spherule layer is approximately 20 mm thick and consists of extraordinarily well‐preserved glass spherules up to 1.1 mm in diameter. About 70–90% of the spherules are vitrified, and their chemical composition is consistent with Haiti (Beloc) impact glass spherules. Normal size‐grading, delicate spherule textures, welded melt components and an absence of bioturbation or traction transport suggest that the Gorgonilla spherule layer represents an almost undisturbed settling deposit.     

Characterization of defoliation conditions within a boreal Montane forest ecosystem

Landsat Thematic Mapper (TM) digital data acquired in the fall of 1988 and digital elevation model (DEM) data were evaluated for use in characterizing the spatial distribution of defoliation conditions within the boreal montane spruce‐fir ecosystem in the Black Mountains (Mtns.) of North Carolina. Correlation coefficients between the TM waveband data and field estimates of defoliation taken from 22 one hectare field plots were low (0.10 to ‐0.54). TM band four (near infrared) was the only waveband found to be significantly correlated with needle loss. Defoliation was also shown to be significantly correlated with digital elevation and aspect data. A multivariate linear regression model predicting percent defoliation was developed from the TM, elevation, and aspect data for the 21 field plots. In addition to having a high R2(0.85) the model was shown to reliably predict defoliation conditions throughout the ecosystem. A color‐coded classified image depicting the spatial distribution of defoliation conditions within the study site was generated by applying the model to the TM and DEM data.     

Laboratory studies on zooplankton‐cyanobacteria interactions

Laboratory studies on cyanobacteria‐zooplankton interactions have largely focused on the inadequacy of cyanobacteria as a food source. Some features of cyanobacteria can be regarded as anti‐herbivore defences. Large colonies of Aphanizomenon, Anabaena and Microcystis cannot be handled by zooplankton, but do not interfere seriously with the filtering process. Small colonies and filaments, however, may cause severe inhibition of the feeding process by mechanical interference. This reduces zooplankton growth, reproduction, and survival. Copepods, rotifers, and Bosmina are less affected by mechanical disturbance than cladocerans. If ingested, some cyanobacteria may be poorly digested or may not provide essential nutrients. Some cyanobacteria are reported to be toxic to zooplankton. Several strains of Microcystis produce an endotoxin, but the amount of toxin produced differs among strains and with the condition of Microcystis. Zooplankton encountering toxic cells cease feeding. Some evidence of an extra‐cellular herbivore deterrent was found for Anabaena. Thus, cyanobacteria may either be individually protected or may create an environment that is unfavourable for efficient grazers (e.g., Daphnia).     

Biological control of short-term variations in the concentration of DMSP and DMS during a Phaeocystis spring bloom

In the spring of 1995, short-term variations in the concentration of particulate and dissolved dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) were monitored in the western Wadden Sea, a shallow coastal region in open connection with the North Sea. Significant correlations were found between abundance of Phaeocystis globosa and particulate DMSP; concentrations increased rapidly from 100 to 1650 nM in the middle of April. Highest DMS concentrations were found during the initial phase of the exponential growth of the bloom. DMS production and loss rates of DMSP and DMS were estimated experimentally during various phases of the bloom. DMS production and consumption were roughly in balance, with production only slightly exceeding consumption at the start of the bloom. Rates of production and consumption were highest during the exponential growth phase of Phaeocystis and declined in the course of the bloom (from 300–375 to less than 5 nmol dm⁻³ d⁻¹). Demethylation of DMSP increased during the bloom (from 11 to 1300 nmol dm⁻³ d⁻¹); it accounted for up to 100% of the DMSP loss at the end of the bloom. The shift from DMSP cleavage to demethylation in the course of a Phaeocystis bloom implies that DMS concentrations are not necessarily highest at the peak or towards the end of blooms.     

Absolute photometric calibration of large aperture optical systems

A method of absolute calibration for the air shower fluorescence detectors of the Pierre Auger Observatory is presented, along with preliminary results from prototype equipment. A 2.5 m diameter light source uniformly illuminated by ultra-violet light emitting diodes is calibrated and mounted at the detector aperture. The resulting end-to-end measurement provides a 7% absolute photon calibration at a wavelength of 375 nm.     

Homochirality as the signature of life: the SETH Cigar

A characteristic hallmark of life is its homochirality: all biomolecules are usually of one hand, e.g. on Earth life uses only L-amino acids for protein synthesis and not their D mirror images. It is therefore suggested that a search for extra-terrestrial life can be approached as a Search for Extra-Terrestrial Homochirality (SETH). A novel miniaturized space polarimeter, called the SETH Cigar, is described which could he used to detect optical rotation as the homochiral signature of life on other planets. Moving parts are avoided by replacing the normal rotating polarizer by multiple fixed polarizers at different angles as in the eye of the bee. It is believed that homochirality will be found in the subsurface layers on Mars as a relic of extinct life.