Geology and impact features of Vargeão Dome,southern Brazil

Abstract– Vargeão Dome (southern Brazil) is a circular feature formed in lava flows of the Lower Cretaceous Serra Geral Formation and in sandstones of the Paraná Basin. Even though its impact origin was already proposed in the 1980s, little information about its geological and impact features is available in the literature. The structure has a rim‐rim diameter of approximately 12 km and comprises several ring‐like concentric features with multiple concentric lineaments. The presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the Botucatu and Pirambóia formations. We present the morphological/structural characteristics of Vargeão Dome, characterize the different rock types that occur in its interior, mainly brecciated volcanic rocks (BVR) of the Serra Geral Formation, and discuss the deformation and shock features in the volcanic rocks and in sandstones. These features comprise shatter cones in sandstone and basalt, as well as planar microstructures in quartz. A geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the BVRs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies. No traces of meteoritic material have been found so far. The results confirm the impact origin of Vargeão Dome, making it one of the largest among the rare impact craters in basaltic targets known on Earth.     

Atlantic humpback dolphins Sousa teuszii in the Saloum Delta (Senegal): distribution,relative abundance and photo-identification

During October and November 2015, the first systematic survey of Sousa teuszii was carried out in the Saloum Delta (Senegal, West Africa), comprising 1 617.5 km of boat-based survey coverage. Thirty sightings were recorded in the Saloum and Diomboss rivers, and along the southern coastline. Dolphins were also observed entering the Bandiala and Djinack channels, and travelling across the border into Gambia. The initial sighting locations were 0.043–1.192 km from shore, and tracked dolphins did not move more than 2.082 km from shore. Groups comprised 1–29 animals (mean 9.3 animals), and at least three neonate calves were observed during November. The overall relative abundance was 0.018 sight. km^?1 and 0.175 ind. km^?1. Sightings were concentrated in the Diomboss where relative abundance reached 0.037 sight. km^?1 and 0.331 ind. km^?1. Non-intensive photo-identification produced a minimum population size of 103 animals, the highest recorded for S. teuszii anywhere in its range. Photo-identification also confirmed a movement of individuals between different parts of the Saloum Delta. Combined travel–forage dominated the behaviour. Dolphins were photographed capturing mullet (Mugil sp.) on three occasions. The distribution, population size and movements of S. teuszii are discussed in relation to management.     

Polar night vortex breakdown and large-scale stirring in the southern stratosphere

The present paper examines the vortex breakdown and large-scale stirring during the final warming of the Southern Hemisphere stratosphere during the spring of 2005. A unique set of in situ observations collected by 27 superpressure balloons (SPBs) is used. The balloons, which were launched from McMurdo, Antarctica, by the Stratéole/VORCORE project, drifted for several weeks on two different isopycnic levels in the lower stratosphere. We describe balloon trajectories and compare them with simulations obtained on the basis of the velocity field from the GEOS-5 and NCEP/NCAR reanalyses performed with and without VORCORE data. To gain insight on the mechanisms responsible for the horizontal transport of air inside and outside the well-isolated vortex we examine the balloon trajectories in the framework of the Lagrangian properties of the stratospheric flow. Coherent structures of the flow are visualized by computing finite-time Lyapunov exponents (FTLE). A combination of isentropic analysis and FTLE distributions reveals that air is stripped away from the vortex’s interior as stable manifolds eventually cross the vortex’s edge. It is shown that two SPBs escaped from the vortex within high potential vorticity tongues that developed in association with wave breaking at locations along the vortex’s edge where forward and backward FTLE maxima approximately intersect. The trajectories of three SPBs flying as a group at the same isopycnic level are examined and their behavior is interpreted in reference to the FTLE field. These results support the concept of stable and unstable manifolds governing transport of air masses across the periphery of the stratospheric polar vortex.     

Upper-ocean heat budget and ocean eddy transport in the south-east Pacific in a high-resolution coupled model

We present an analysis of the oceanic heat advection and its variability in the upper 500 m in the southeastern tropical Pacific (100W–75W, 25S–10S) as simulated by the global coupled model HiGEM, which has one of the highest resolutions currently used in long-term integrations. The simulated climatology represents a temperature advection field arising from transient small-scale (<450 km) features, with structures and transport that appear consistent with estimates based on available observational data for the mooring at 20S, 85W. The transient structures are very persistent (>4 months), and in specific locations they generate an important contribution to the local upper-ocean heat budget, characterised by scales of a few hundred kilometres, and periods of over a year. The contribution from such structures to the local, long-term oceanic heat budget however can be of either sign, or vanishing, depending on the location; and, although there appears some organisation in preferential areas of activity, the average over the entire region is small. While several different mechanisms may be responsible for the temperature advection by transients, we find that a significant, and possibly dominant, component is associated with vortices embedded in the large-scale, climatological salinity gradient associated with the fresh intrusion of mid-latitude intermediate water which penetrates north-westward beneath the tropical thermocline.     

Buoyancy reversal, decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers

The transition in a marine boundary layer (MBL) from stratocumulus topped to shallow cumulus topped is investigated by using a large eddy simulation (LES) model. The experiments performed aim to examine the influence on the transition of (1) the probability of buoyancy reversal at the MBL top (i.e. situations in which the mixture of two air parcels becomes denser than either of the original parcels due to phase change or other nonlinear processes involved in the mixing), and (2) the degree of decoupling in the MBL (i.e. the strength of a shallow stably stratified layer near cloud base). Our results suggest that a stratocumulus-topped MBL is most likely to transit to a cumulus-topped one when (1) there exists high probability of buoyancy reversal at the MBL top, and (2) the MBL is decoupled due to large surface evaporation. We argue that a parameterization that includes representation of those two effects combined has the potential to provide a simple way of predicting the MBL transition in climate models.     

A treatment for the stratocumulus-to-cumulus transition in GCMs

Numerical models of climate have great difficulties with the simulation of marine low clouds in the subtropical Pacific and Atlantic Oceans. It has been especially difficult to reproduce the observed geographical distributions of the different cloud regimes in those regions. The present study discusses mechanisms proposed in previous works for changing one regime into another. One criterion is based on the theory of stratocumulus destruction through cloud top entrainment instability due to buoyancy reversal—situations in which the mixture of two air parcels becomes denser than either of the original parcels due to evaporation of cloud water. Another criterion is based on the existence of decoupling in the boundary layer. When decoupled, the stratocumulus regime changes to another in which these clouds can still exist together with cumulus. In a LES study, the authors have suggested that a combination of those two criteria can be used to diagnose whether, at a location, the cloud regime corresponds to a well-mixed stratocumulus regime, a shallow cumulus regime, or to a transitional regime where the boundary layer is decoupled. The concept is tested in the framework of an atmospheric general circulation model (GCM). It is found that several outstanding features of disagreement between simulation and observation can be interpreted as misrepresentations of the cloud regimes by the GCM. A novel criterion for switching among regimes is proposed to alleviate the effects of these misrepresentations.     

Diagnosis of the marine low cloud simulation in the NCAR community earth system model (CESM) and the NCEP global forecast system (GFS)-modular ocean model v4 (MOM4) coupled model

We present a diagnostic analysis of the marine low cloud climatology simulated by two state-of-the-art coupled atmosphere–ocean models: the National Center for Atmospheric Research community earth system model version 1 (CESM1) and the National Center for Environmental Predictions global forecasting system-modular ocean model version 4 (GFS-MOM4) coupled model. In the CESM1, the coastal stratocumulus (Sc)-topped planetary boundary layers (PBLs) in the subtropical Eastern Pacific are well-simulated but the climatological transition from Sc to shallow cumulus (Cu) is too abrupt and occurs too close to the coast. By contrast, in the GFS-MOM4 the coastal Sc amount and PBL depth are severely underestimated while the transition from Sc to shallow Cu is “delayed” and offshore Sc cover is too extensive in the subtropical Eastern Pacific. We discuss the possible connections between these differences in the simulations and differences in the parameterizations of shallow convection and boundary layer turbulence in the two models.     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。