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.     

Coleoptera and microbe biomass in Antarctic Dry Valley paleosols adjacent to the Inland Ice: Implications for Mars

Bulk paleosol samples collected from a Middle to Early Miocene moraine in the New Mountain area of the Dry Valleys, Antarctica, yielded Coleoptera exoskeletons and occasional endoskeletons showing considerable diagenetic effects along with several species of bacteria, all lodged in a dry-frozen but salt-rich horizon at shallow depth to the land surface. The till is at the older end of a chronologic sequence of glacial deposits, thought to have been deposited before the transition from wet-based to cold-based ice (～15 Ma), and hence, entirely weathered in contact with the subaerial atmosphere. It is possible, though not absolutely verifiable, that the skeletons date from this early stage of emplacement having undergone modifications whenever light snowmelt occurred or salt concentrations lowered the freezing temperature to maintain water as liquid. Correlation of the Coleoptera species with cultured bacteria in the sample and the likelihood of co-habitation with Beauveria bassiani found in two adjacent, although younger paleosols, leads to new questions about the antiquity of the Coleoptera and the source of N and glucose from chitinase derived from the insects. The skeletons in the 831 section may date close to the oldest preserved chitin (Oligocene) yet found on Earth. While harsh Martian conditions make it seemingly intolerable for complex, multicellular organisms such as insects to exist in the near-surface and subaerially, life within similar cold, dry paleosol microenvironments (Cryosols) of Antarctica point to life potential for the Red Planet, especially when considering the relatively diverse microbe (bacteria and fungi) population.     

Sedimentary signatures of cyclic growth and destruction of stratovolcanoes: A case study from Mt. Taranaki, New Zealand

The long-term behaviour of andesite stratovolcanoes is characterised by a repetition of edifice growth phases followed by collapse. This cyclic pattern represents a natural frequency at varying timescales in the growth dynamics of stratovolcanoes worldwide. Around the > 130 ka Mt. Taranaki (Egmont volcano), New Zealand, coastal–cliff successions at 20–40 km distance comprise repeating packages of lithologically and sedimentologically distinctive mass-flow deposits. Varying depositional mechanisms and source properties of these units record growth and collapse cycles of the central edifice. These are used to construct a model for cyclic volcaniclastic sedimentation in the surrounds of stratovolcanoes. During edifice-construction phases, thick packages of tabular, predominantly monolithologic, hyperconcentrated-flow and debris-flow deposits accumulate with intercalated tephra beds. The mass-flow units commonly contain large proportions of fresh pumice or juvenile-lithic andesite. Intervals of quiescence separating eruptive periods are characterised by landscape re-adjustment, accompanied by deposition of fluvial and aeolian sediments, along with steady accretion of medial ash. In contrast, brief episodes of destruction are marked by wide-spread, distinctively clay-rich, polylithologic debris-avalanche deposits and related marginal debris flow units. The growth stages can be terminated by an eruption-triggered sector collapse, or by external forces once the edifice exceeds a critical stable height or profile (dependent on eruptive style and local geo-tectonic conditions). Once the edifice becomes metastable, regional tectonic earthquakes or shallow-level intrusion events are likely triggers for collapse. Although the resulting debris avalanches represent the greatest individual hazard from such andesite stratovolcanoes, their frequency is relatively low compared with other types of mass-flows generated during edifice-growth phases. Accurate forecasts of future hazard from mass-flow events are therefore dependent on recognition of both the frequency of a stratovolcano's growth cycle and its current position in that cycle.     

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.     

Louth crater: Evolution of a layered water ice mound

We report on observations made of the ∼36 km diameter crater, Louth, in the north polar region of Mars (at 70° N, 103.2° E). High-resolution imagery from the instruments on the Mars Reconnaissance Orbiter (MRO) spacecraft has been used to map a 15 km diameter water ice deposit in the center of the crater. The water ice mound has surface features that include roughened ice textures and layering similar to that found in the North Polar Layered Deposits. Features we interpret as sastrugi and sand dunes show consistent wind patterns within Louth over recent time. CRISM spectra of the ice mound were modeled to derive quantitative estimates of water ice and contaminant abundance, and associated ice grain size information. These morphologic and spectral results are used to propose a stratigraphy for this deposit and adjoining sand dunes. Our results suggest the edge of the water ice mound is currently in retreat.     

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.     

Carbon sequestration and environmental effects of afforestation with Pinus radiata D. Don in the Western Cape, South Africa

A three-step methodology to assess the carbon sequestration and the environmental impact of afforestation projects in the framework of the Flexible Mechanisms of the Kyoto Protocol (Joint Implementation and Clean Development Mechanism) was developed and tested using a dataset collected from the Jonkershoek forest plantation, Western Cape, South Africa, which was established with Pinus radiata in former native fynbos vegetation and indigenous forest. The impact of a change in land use was evaluated for a multifunctional, a production and a non-conversion scenario. First, the carbon balance was modelled with GORCAM and was expressed as (1) C sequestration in tC ha⁻¹ year⁻¹ in soil, litter, and living biomass according to the rules of the first commitment period of the Kyoto Protocol, and (2) CO₂ emission reductions in tC ha⁻¹ year⁻¹, which includes carbon sequestered in the above-mentioned pools and additionally in wood products, as well as emission reductions due to fossil fuel substitution. To estimate forest growth, three data sources were used: (1) inventory data, (2) growth simulation with a process-based model, and (3) yield tables. Second, the effects of land use change were assessed for different project scenarios using a method related to Life Cycle Assessment (LCA). The method uses 17 quantitative indicators to describe the impact of project activities on water, soil, vegetation cover and biodiversity. Indicator scores were calculated by comparing indicator values with reference values, estimated for the climax vegetation. The climax vegetation is the site-specific ecosystem phase with the highest exergy content and the highest exergy flow dissipation capacity. Third, the land use impact per functional unit of 1 tC sequestered was calculated by combining the results of step 1 and step 2. The average baselines to obtain carbon additionality are 476 tC ha⁻¹ for indigenous forest and 32 tC ha⁻¹ for fynbos. Results show that the influence of the growth assessment method on the magnitude of C sequestration and hence on the environmental impact per functional unit is large. When growth rate is assessed with the mechanistic model and with the yield table, it is overestimated in the early years and underestimated in the long term. The main conclusion of the scenario analysis is that the production forest scenario causes higher impacts per functional unit than the multifunctional scenario, but with the latter being less efficient in avoiding CO₂ emissions. The proposed method to assess impacts on diverse components of the ecosystem is able to estimate the general tendency of the adverse and positive effects of each scenario. However, some indicators, more specifically about biodiversity and water balance, could be improved or reinterpreted in light of specific local data about threat to biodiversity and water status.