The Chandra view of NGC 1800 and the X-ray scaling properties of dwarf starbursts     

J. Rasmussen  I. R. Stevens  T. J. Ponman 《Monthly notices of the Royal Astronomical Society》2004,354(1):259-274

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A distant planetary-mass solar companion may have produced distant detached objects     

Rodney S. Gomes  Jack J. Lissauer 《Icarus》2006,184(2):589-601

Most known trans-neptunian objects (TNO's) are either on low eccentricity orbits or could have been perturbed to their current trajectories via gravitational interactions with known bodies. However, one or two recently-discovered TNO's are distant detached objects (DDO's) (perihelion, q>40 AU and semimajor axis, a>50 AU) whose origins are not as easily understood. We investigate the parameter space of a hypothetical distant planetary-mass solar companion which could detach the perihelion of a Neptune-dominated TNO into a DDO orbit. Perturbations of the giant planets are also included. The problem is analyzed using two models. In the first model, we start with a distribution of undetached, low-inclination TNO's having a wide range of semimajor axes. The planetary perturbations and the companion perturbation are treated in the adiabatic, secularly averaged tidal approximation. This provides a starting point for a more detailed analysis by providing insights as to the companion parameter space likely to create DDO's. The second model includes the companion and the planets and numerically integrates perturbations on a sampling that is based on the real population of scattered disk objects (SDO's). A single calculation is performed including the mutual interactions and migration of the planets. By comparing these models, we distinguish the distant detached population that can be attributable to the secular interaction from those that require additional planetary perturbations. We find that a DDO can be produced by a hypothetical Neptune-mass companion having semiminor axis, bc?2000 AU or a Jupiter-mass companion with bc?5000 AU. DDO's produced by such a companion are likely to have small inclinations to the ecliptic only if the companion's orbit is significantly inclined. We also discuss the possibility that the tilt of the planets' invariable plane relative to the solar equatorial plane has been produced by such a hypothetical distant planetary-mass companion. Perturbations of a companion on Oort cloud comets are also considered.  相似文献   

Sedimentology in metamorphic rocks,the Willyama Supergroup,Broken Hill,Australia     

B. P. J. Stevens  G. M. Bradley 《Australian Journal of Earth Sciences》2018,65(1):25-59

In the upper greenschist to granulite grade rocks of the Willyama Supergroup at Broken Hill, Australia, earlier recognition of metamorphosed graded bedding in siliciclastic metasedimentary rocks led to interpretations of these rocks as deep-water turbidites. However, graded beds can also be deposited in shelfal environments below storm wave base. This study identified other tempestite features including wave oscillation ripples, hummocky cross-stratification and swaley cross-stratification indicating that deposition took place above the wave base of the larger storms.

Albitised metasedimentary rocks of the upper Thackaringa Group show structures such as swaley cross-stratification typical of shallow-water conditions above fair-weather wave base. Deposition of the Broken Hill Group commenced with muddy Allendale Metasediments conformable on the Thackaringa Group. The Ettlewood Calc-Silicate Member, originally a dolomitic, siliceous sediment, is interpreted as coastal sabkha indicating onset of a marine transgression. The Parnell Gneiss represents a volcanic or volcaniclastic interruption, heralding gradually increasing input of sand in the Freyers Metasediments reaching a maximum in middle Freyers Metasediments, followed by an abrupt reversion to mud, still influenced by wave action. An open marine shelf is interpreted, possibly 30 m deep (no more than 100 m) in the final stage of a developing rift. The Broken Hill Group terminated with the massive Hores Gneiss volcanic unit.

Sedimentation of the siliciclastic Sundown Group took place in similar conditions, commencing with a muddy interval overlying the Hores Gneiss. The shallowing produced by ～90 m thickness of volcanic/volcaniclastic Hores Gneiss was compensated by subsidence.

Paragon Group deposition commenced with substantial black mud, resulting from isolation from the sand supply and probably isolation from the sea. A fresh connection to the sea led first to the deposition of dolomitic carbonate (King Gunnia Calc-Silicate Member), then to deposition of parallel-laminated fine sand below wave base (upper Cartwrights Creek Metasediments), followed by ripple cross-laminated sand above wave base (Bijerkerno Metasediments). The Dalnit Bore Metasediments show abundant very thin graded silt–mud units possibly deposited below storm wave base, and thicker units of stacked wave oscillation ripples deposited above the wave base of larger storms.

The Broken Hill orebody is hosted by altered Broken Hill Group metasedimentary rocks deposited at water depths of ～30 m. Unless the ore fluid temperature was less than 150°C, it is likely that the orebody formed below the seafloor: at such shallow-water depths, the confining pressure would be inadequate to suppress boiling of hotter rising hydrothermal fluids.  相似文献   

Development of a Multimetric Index for Integrated Assessment of Salt Marsh Ecosystem Condition     

Jessica L. Nagel  Hilary A. Neckles  Glenn R. Guntenspergen  Erika N. Rocks  Donald R. SchoolmasterJr  James B. Grace  Dennis Skidds  Sara Stevens 《Estuaries and Coasts》2018,41(2):334-348

Tools for assessing and communicating salt marsh condition are essential to guide decisions aimed at maintaining or restoring ecosystem integrity and services. Multimetric indices (MMIs) are increasingly used to provide integrated assessments of ecosystem condition. We employed a theory-based approach that considers the multivariate relationship of metrics with human disturbance to construct a salt marsh MMI for five National Parks in the northeastern USA. We quantified the degree of human disturbance for each marsh using the first principal component score from a principal components analysis of physical, chemical, and land use stressors. We then applied a metric selection algorithm to different combinations of about 45 vegetation and nekton metrics (e.g., species abundance, species richness, and ecological and functional classifications) derived from multi-year monitoring data. While MMIs derived from nekton or vegetation metrics alone were strongly correlated with human disturbance (r values from ?0.80 to ?0.93), an MMI derived from both vegetation and nekton metrics yielded an exceptionally strong correlation with disturbance (r = ?0.96). Individual MMIs included from one to five metrics. The metric-assembly algorithm yielded parsimonious MMIs that exhibit the greatest possible correlations with disturbance in a way that is objective, efficient, and reproducible.  相似文献   

Composite methodology for interpreting sediment transport pathways from spatial trends in grain size: A case study of the Lithuanian coast          下载免费PDF全文

Milda Kairytė  Rodney L. Stevens 《Sedimentology》2015,62(3):681-696

To achieve a more robust interpretation of sediment conditions and transport based on grain size, this study combines the perspectives of two different interpretation techniques that have been developed separately and applied in the literature: (i) ‘grain size trend analysis’ interprets changes between interrelated sampling sites assuming that sediment in the direction of transport should become either coarser, better sorted and more positively skewed, or finer, better sorted and more negatively skewed; and (ii) the ‘population anomalies’ method, which estimates the balance between erosion and accumulation processes at each sampling site based on individual site deviations compared with average values for grain‐size parameters in the area of interest. The simultaneous use of both approaches enables sedimentological conditions and transport alternatives to be distinguished. This improvement is necessary because site‐specific changes and general textural trends provide information that complements the strengths and weaknesses of each perspective. The methods are implemented here using two‐dimensional Geographic Information System tools and illustrated for the Lithuanian coast. Grain‐size characteristics of 712 surface samples of sandy sediments are used to interpret sediment transport pathways and predominant sedimentological conditions in the Lithuanian coastal zone. In general, coast‐parallel sediment transport dominates the entire investigation area, although wave‐induced movement perpendicular to the coastline is inferred in the shallow nearshore zone. The deepest areas offshore are characterized by sea floor erosion in the north, whereas an accumulation zone occurs in the relatively deep central part of the study area.  相似文献   

Spatial quick-clay predictions using multi-criteria evaluation in SW Sweden     

M. A. Persson  R. L. Stevens  Å. Lemoine 《Landslides》2014,11(2):263-279

The transformation of marine and glaciomarine clay deposits into high sensitive and quick clays is largely dependent on the influence of local and regional geologic history and the resulting stratigraphy. The general conditions that facilitate quick-clay development are well known from numerous laboratory investigations during the last century, but their local and regional in-field variation is less understood. In this study, the geographic distribution of quick clay in SW Sweden is predicted using a multicriteria evaluation model that incorporates both qualitative information (established theory and expert judgment concerning the influences on both quick-clay development and the stratigraphic and geomorphologic distribution of sediment types) and observational data (maps of surficial deposits, geotechnical records and digital elevation data). This information duality cannot be avoided if knowledge from different disciplines is utilized. Considering this, model transparency is important for improvements and for characterizing its reliability for risk analysis. The model was constructed stepwise by an initial parameterization with subsequent hierarchical structuring, weighting and standardization of criteria, before running the full analysis. Comparisons between regional model results and geotechnically documented localities have yielded promising results concerning the model’s ability to predict general trends. However, the large natural and site-specific variability of clay sensitivities is not always captured by the model. These deviations are examined and suggestions are given for minimizing their effect. Applications of model methodology and results are briefly discussed.  相似文献   

Small-scale Hf isotopic variability in the Peninsula pluton (South Africa): the processes that control inheritance of source 176Hf/177Hf diversity in S-type granites     

F. Farina  G. Stevens  A. Gerdes  D. Frei 《Contributions to Mineralogy and Petrology》2014,168(4):1-18

The ¹⁷⁶Hf/¹⁷⁷Hf composition of inherited and magmatic zircon in the 538 Ma S-type Peninsula pluton (South Africa) has been determined at different scales. In the smallest rock samples investigated (<0.5 dm³), as well as within individual thin sections, magmatic zircon crystals exhibit the same wide range in ε_Hf(538) as the pluton (8ε units). In addition, across a significant range of bulk-rock compositions, both the range and average of the magmatic zircon Hf isotopic composition do not vary significantly with compositional parameters that are expected to scale with the proportion of mantle-derived magma addition (e.g., Mg# and Ca). At all scales, the ε_Hf variability in the magmatic zircon fraction matches well with that portrayed by the time-evolved inherited zircon population [i.e., with the ε_Hf(538) range of the inherited zircon cores]. This evidence suggests that the ε_Hf heterogeneity of magmatic zircon is directly inherited from the source. However, the analysis of zircon core–rim pairs reveals that the ¹⁷⁶Hf/¹⁷⁷Hf composition of the inherited crystals does not directly transfer to their magmatic overgrowths. Small-scale modeling of zircon dissolution and re-precipitation in a static magma generates sub-mm melt domains having variable Zr content and Hf isotope composition. The composition of these domains is controlled by the size and isotope composition of the nearest dissolving zircon crystals and the cooling rate of the magma. These results suggest that in magma systems with a substantial inherited zircon load, zircon crystals within the same rock should record variable ¹⁷⁶Hf/¹⁷⁷Hf in the magmatic zircon fraction.  相似文献   

Fault-controlled sedimentation in a progressively opening extensional basin: the Palaeoproterozoic Vargfors basin, Skellefte mining district, Sweden     

Tobias E. Bauer  Pietari Skyttä  Rodney L. Allen  Pär Weihed 《International Journal of Earth Sciences》2013,102(2):385-400

The Vargfors basin in the central part of the Skellefte mining district is an inverted sedimentary basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The fault-segmented basin formed from upper-crustal extension and subsequent compression, following a period of intense sub-marine volcanism and VMS ore formation. New detailed mapping reveals variations in stratigraphy attributed to syn-extensional sedimentation, as well as provenance of conglomerate clasts associated with tectonic activity at the transition from extension to compression. The onset of fan delta to alluvial fan sedimentation associated with basin subsidence indicates that significant dip-slip displacement accommodating rapid uplift of the intrusive complex and/or subsidence of the adjacent volcano-sedimentary domain took place along a major fault zone at the southern margin of the intrusive complex. Subsidence of the Jörn intrusive complex and/or its burial by sedimentary units caused a break in erosion of the intrusion and favoured the deposition of a tonalite clast-barren conglomerate. Clast compositions of conglomerates show that the syn-extensional deposits become younger in the south-eastern parts of the basin, indicating that opening of the basin progressed from north-west to south-east. Subsequent basin inversion, associated with the accretion to the Karelian margin, involved reverse activation of the normal faults and development of related upright synclines. Progressive crustal shortening caused the formation of break-back faults accompanied by mafic volcanic activity that particularly affected the southern contact of the Jörn intrusive complex and the northern contact of the Vargfors basin.  相似文献   

On open boundary conditions for three dimensional primitive equation ocean circulation models     

David P. Stevens 《地球物理与天体物理流体动力学》2013,107(1-4):103-133

Abstract

An open boundary condition is constructed for three dimensional primitive equation ocean circulation models. The boundary condition utilises dominant balances in the governing equations to assist calculations of variables at the boundary. The boundary condition can be used in two forms. Firstly as a passive one in which there is no forcing at the boundary and phenomena generated within the domain of interest can propagate outwards without distorting the interior. Secondly as an active condition where a model is forced by the boundary condition. Three simple idealised tests are performed to verify the open boundary condition, (1) a passive condition to test the outflow of free Kelvin waves, (2) an active condition during the spin up phase of an ocean, (3) finally an example of the use of the condition in a tropical ocean.  相似文献   

The Life Cycle of Caravan Parks in Australia: the case of northern New South Wales   总被引：1，自引：0，他引：1  

Rodney W. Caldicott  Pascal Scherrer 《The Australian geographer》2013,44(1):63-80

The caravan park sub-sector of the Australian tourism accommodation industry provides at least half of the national tourism bed capacity, and in 2009 generated over A$1.1 billion in annual takings. However, the number of parks and park capacity is in decline nationally while both international and domestic demand for the drive-tourism experience is growing. This sets a trend towards an accommodation facilities shortage for the caravanning sector and exposes its vulnerability. This paper uses a case study of caravan parks in the Tweed Shire, New South Wales, Australia, to examine the life-cycle pattern of these parks as a discrete unit of tourist area development and to consider the sector's future. The sector's history is framed within Butler's (Canadian Geographer 24(1): 5–12 (1980)) concept of the tourist area life cycle (TALC). The historical data demonstrate the urban and market change that has occurred around and within caravan parks of this coastal region over almost two centuries. The pattern of caravan park development and evolution conformed to the involvement, exploration, development, consolidation and stagnation stages of the TALC. In 2011, caravan parks in the Tweed region were at a critical tipping point with potential for either decline or rejuvenation.  相似文献