Mixture models and atypical values

The resolution of a mixture of two or more populations into its component distributions may be markedly influenced by one or a few atypical values. The maximum likelihood solution effectively assigns (part of) each observation to one or another of the components via the posterior probabilities, even though the observation may be widely discrepant from all components. This paper presents a modification of the mixture problem, in which the typicality of each observation is considered, as well as the posterior probabilities, with the contribution of atypical observations being downweighted. The extension to the multivariate case is discussed.     

Absolute stellar photometry in the region 1900–3000 Å

Twin channel stellar photometers were flown in two sounding rockets in May 1967 and produced a number of absolute photoelectric measurements in the region 2000–3000 Å.The observations were made at wavelengths centred on 2150 Å and 2550 Å with passbands of 300 Å on 28 early-type stars in the range of spectral types B0-B9.5 down to a limiting visual magnitude of 4.8.A comparison of the experimental data with other observations and with the theoretically derived fluxes of Mihalas for unblanketed models, indicates a satisfactory agreement within the limits of the known photometric accuracies, although there are significant departures for those stars exhibiting emission characteristics and for the magnetic variable ₂ Canum Venaticorum.     

The difference between oceanic and continental tholeiites: a fluid dynamic explanation

Continental tholeiites have higher SiO₂, K₂O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.     

Barnacle-dominated limestone with giant cross-beds in a non-tropical, tide-swept, Pliocene forearc seaway, Hawke's Bay, New Zealand

Pliocene, non-tropical, widespread and locally thick (up to 100 m) limestones occur in Hawke's Bay, eastern North Island, where they are intimately associated with very thick ( > 5 km), terrigenous-dominated, Neogene sequences that formed in a tectonically active convergent margin setting. The non-tropical character of the limestones is shown unequivocally by (1) the complete dominance of skeletal calcarenites and calcirudites, (2) the occurrence of oyster banks as the only in situ organic structures, (3) the dominance of barnacles, epifaunal molluscs, bryozoans, echinoderms, foraminifers, brachiopods and calcareous red algae as skeletal components, and (4) the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements. The abundance of barnacle fragments in the limestones, and the related exclusive occurrence of only one major organic association, a barnacle-(epifaunal) bivalve-bryozoan assemblage, is striking and unusual given the extent of the limestones. Pecten and oyster valves acted as substrates for barnacle attachment, and their growth was promoted by strong tidal paleocurrents that swept the depositional setting: a long (450 km), narrow (30–50 km) forearc basin seaway, which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west. Synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current-swept, clastic-free submarine ridges that acted as the sites of carbonate production. Tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide-influenced, cross-bedded and horizontal structures. Most spectacular are occurrences of giant tabular cross-beds, with sets 10–40 m thick and foreset dips of 7–36°, some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms, and others analogous to modern linear sand ridges. The small- to large-scale planar and trough cross-beds, and the horizontal and lenticular beds that are invariably associated with the giant cross-beds and dominate most sections, represent mainly the deposits of sand waves and sand sheets at inner- to mid-shelf depths in the seaway.     

Terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest Ohio, USA

Lower slopes of the Sandia Mountains are characterized by granitic corestone topography and weathering-limited slopes with thin grusy colluvium and weakly developed soils. In contrast, thick soils with illuvial clay and pedogenic carbonate have developed below aplite outcrops. Aplite is resistant to chemical decomposition, but physically weathers to blocky clasts that enhance surface roughness and erosional resistance of colluvium, promoting accumulation of eolian fines. Thick B horizons on aplite slopes indicate limited erosion and prolonged periods of stability and soil development. Accretion of eolian material limits runoff and prevents attainment of a steady-state balance between soil production and downslope transport.     

A first estimate of organic carbon storage in Holocene lake sediments in Alberta, Canada

This paper reports a first estimate of the Holocene lake sediment carbon pool in Alberta, Canada. The organic matter content of lake sediment does not appear to depend strongly on lake size or other limnological parameters, allowing a simple first estimate in which we assume all Alberta lake sediment to have the same organic matter content. Alberta lake sediments sequester about 15 g C m^-2 yr^-1, for a provincial total of 0.23 Tg C yr^-1, or 2.3 Pg C over the Holocene. Alberta lakes may represent as much as 1/1700 of total global, annual permanent carbon sequestration.     

Natural Geological Weighing Lysimeters: Calibration Tools for Satellite and Ground Surface Gravity Monitoring of Subsurface Water-Mass Change

Increased accuracy in measuring temporal variations in the Earth's gravity field allow inprinciple the use of gravity observations to deduce subsurface water-mass changes. This canbe with respect to a small area, or as a larger spatial average of water mass change usinggravity observations from low-altitude satellites, such as the forthcoming GRACE mission.At both scales, there is a need to validate gravity-based estimates against field recordings ofactual subsurface water-mass variations. In practice, this could prove difficult because thespatial integral of all water-storage change components can be subject to considerable fieldmeasurement error. An alternative approach to the validation process is proposed by whichsuitable geological formations are utilized as giant weighing devices to directly measure area-integratedwater-mass changes. The existence of such natural geological weighing lysimetersis demonstrated using observations from a replicated experimental site in New Zealand. Sitesof this type could be used to verify water-storage change estimates derived from sensitiveground surface gravity instrumentation. In addition, geological lysimeters could be used tomake local checks on the accuracy of any estimated regional water-mass time series, whichis proposed for satellite calibration. The land area weighed by a geological lysimeter increaseswith formation depth and it is speculated that recordings made at oil well depth may allowdirect monitoring of subsurface water mass changes at the regional scale.     

Mountains,nations, parks,and conservation

Between 1985 and 1991, two new mountain protected areas (MTNPA) covering more than 35,000 km² and based on participatory management models — the Makalu-Barun National Park and Conservation Area, Nepal, and Qomolangma Nature Preserve, Tibet Autonomous Region — were successfully established through the collaborative efforts of Woodlands Mountain Institute and conservationists in China and Nepal. Characteristics common to both projects include the importance of establishing (1) effective rationales, (2) local support constituencies, (3) a senior advisory group, (4) a task force, (5) linkages between conservation and development, and (6) fund raising mechanisms. The lessons derived from the experiences of Woodlands Mountain Institute are of significant value to others in preserving MTNPA. Increased collaboration and communication between all interested in conservation, however, will remain a critical component for expanding mountain protected area coverage to throughout the world.