An examination of the relationship between the Southern Annular Mode and Antarctic surface air temperatures in the CMIP5 historical runs

Climate Dynamics - Strong relationships exist between the Southern Annular Mode (SAM) and surface air temperature (SAT) across much of Antarctica. Changes in the SAM will have a profound influence...     

Axisymmetric annulus convection at unit Prandtl number

Abstract

We examine the role played in annulus flows by mechanisms dependent upon the Prandtl number, σ. Solutions are obtained at σ = 1 for both the real annulus system and for the hypothetical “free annulus” system (free slip lateral boundaries). These solutions are compared with previously obtained solutions at σ = 7.

In the free annulus, the solution at σ = 1 differs radically from that at σ = 7. The σ = 1 solution appears to be essentially a finite amplitude mode due to Solberg instability whereas the solution at σ = 7 manifests a flow caused by the diffusive overturning mechanism.

The variation with σ of the real annulus flow is not so fundamental but some differences in the dynamical structures are noted.     

Taphonomy and abundance of birds from the Lower Eocene Fur Formation of Denmark

The pattern, pace and extent of the evolutionary radiation of modern birds (Neornithes) by the end‐Cretaceous (65 Ma) has long been debated. Well‐dated, taphonomically understood and phylogenetically constrained fossil birds from both sides of the Cretaceous–Paleogene (K–Pg) boundary are required to quantify the shape of this radiation, but have largely been lacking. Here we report on a large collection of fossil birds from the Lower Eocene of Denmark (ca. 54 Ma) that includes three‐dimensionally preserved, articulated specimens from carbonate concretions as well as skeletal imprints and feathers. These birds are from a marine diatomite sequence (the Fur Formation), a low‐energy deep‐water preservational environment unique to the Cretaceous and Paleogene avian fossil record. We present taphonomic and palaeoecological information gleaned from these birds that in combination with phylogenetic data have implications for unravelling avian survivorship across the K–Pg boundary as well as for the pattern of the neornithine evolutionary radiation. Copyright © 2009 John Wiley & Sons, Ltd.     

Lineated valley fill (LVF) and lobate debris aprons (LDA) in the Deuteronilus Mensae northern dichotomy boundary region, Mars: Constraints on the extent, age and episodicity of Amazonian glacial events

In order to assess the nature, degradational processes and history of the dichotomy boundary on Mars, we conducted a detailed morphological analysis of a 70,000 km² region of its northern portion (north-central Deuteronilus Mensae, south of Lyot, in the vicinity of Sinton Crater). This region is characterized by the distinctive sinuous ∼2 km-high plateau scarp boundary, outlying massifs to the north, and extensive fretted valleys dissecting the plateau to the south. These features represent the first-order modification and retreat of the dichotomy boundary, and are further modified by processes that form lineated valley fill (LVF) in the fretted valleys, and lobate debris aprons (LDA) along the dichotomy scarp and surrounding the outlying massifs. We use new high-resolution image and topography data to examine the nature and origin of LVF and LDA and to investigate the climatic and accompanying degradational history of the escarpment. On the basis of our analysis, we conclude that: (1) LVF and LDA deposits within the study region are comprised of the same material, show integrated flow patterns, and originate as debris-covered valley glaciers; a significant amount of ice (hundreds of meters) is likely to remain today beneath a thin cover of sublimation till. (2) There is depositional evidence to suggest glacial highstands at least 800 m above the present level, implying previous conditions in which the distribution of ice was much more widespread; this is supported by similar deposits within many other areas across the dichotomy boundary. (3) The timing of the most recent large-scale activity of the LDA/LVF in this area is about 100-500 million years ago, similar to ages reported elsewhere along the dichotomy boundary. (4) There is evidence for a secondary, but significantly limited phase of glaciation; the deposits of which are limited to the vicinity of the alcoves; similar later phases have also been reported elsewhere along the dichotomy boundary. (5) Modification of the fretted valleys of the dichotomy boundary has been substantial locally, but we find no evidence that the Amazonian glacial epochs caused retreat of the dichotomy boundary of the scale of tens to hundreds of kilometers. Our findings support the results of an analysis just to the east of the study region and of studies carried out elsewhere along the dichotomy boundary that find further evidence for the remnants of debris-covered glaciers and extensive valley glacial land systems.     

A re-analysis of the marine bird Asiahesperornis from northern Kazakhstan

Fossil bird material is exceptionally rare in the Mesozoic of western Asia. With the notable exception of the Upper Cretaceous foot-propelled diving bird Asiahesperornis bazhanovi from northern Kazakhstan, the only bird fossils described from this region have been bone fragments from the Upper Cretaceous of Uzbekistan. In this paper, all material referred to date to the hesperornithiform Asiahesperornis is reviewed and re-described. New postcranial elements of this taxon from the holotype Kushmurun locality (Maastrichtian Zhuravlovskaya Svita) are presented, and the taxonomic history and likely affinities of this ocean-going bird are discussed.     

The spaces of modernisation: Outcomes, indicators and the local government modernisation agenda

The modernisation of local government has attempted to reinvent central-local government relations by offering freedoms and flexibilities to facilitate the governance of local issues. At the same time, a shift to outcome focussed targets as a new form of governmental rationality allows central government to delimit these opportunities. Drawing on aspects of governmentality and actor-network theory, the paper explores the tensions between these modes of government. It argues that outcome focussed targets circumscribe the limits of local governance by offering a despatialised technology of government. Using a case study of Local Public Service Agreements, the paper highlights the problems 10 English rural local authorities have experienced in their attempts to construct and negotiate a series of local policy targets with central government. The paper shows how the spatial limitations of statistical governance conspire against the construction of targets which reflect local policy priorities. In conclusion we consider the extent to which these limitations are a deliberate act of control and consider the implications for agency within networks of governmentality.     

Modeling damage and deformation in impact simulations

Abstract— Numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence. Quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters, particularly in cases where the final pristine crater morphology is ambiguous or unknown. In this paper, we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results. We demonstrate this with an example simulation of an impact into a terrestrial, granite target that produces a 10 km‐diameter transient crater. The results of the simulation are shown in terms of damage (a scalar quantity that reflects the totality of fragmentation) and plastic strain, both total plastic strain (the accumulated amount of permanent shear deformation, regardless of the sense of shear) and net plastic strain (the amount of permanent shear deformation where the sense of shear is accounted for). Damage and plastic strain are both greatest close to the impact site and decline with radial distance. However, the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains. Plastic strain in brittle rocks is very heterogeneous; however, continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock (large compared to the spacing between individual zones of sliding). This paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones. Furthermore, we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity.     

Hydrocode Simulations of Chicxulub Crater Collapse and Peak-Ring Formation

We use hydrocode modeling to investigate dynamic models for the collapse of the Chicxulub impact crater. Our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide-angle velocity data to further our understanding of the formation of large impact craters. In our simulations, we model the collapse of a 100-km diameter, bowl-shaped cavity formed in comprehensively fractured crustal material. To facilitate wholesale collapse, we require that the strength of the target be significantly weakened. In the present model, we achieve this using acoustic fluidization, where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target. At times and positions where the overburden pressure is sufficiently counteracted, the frictional resistance is reduced, enabling the rock debris to flow. Our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at Chicxulub. In particular, we observe a topographic peak ring, formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim. This model for peak-ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned.