Stepwise mass extinctions: the case for the Late Cenomanian event

The various extinctions of marine taxa in the Late Cenomanian can be shown as conforming to the 'step-wise extinction' model. The order of such extinctions is, however, compatible with the presence of an expanded oxygen minimum zone in the world ocean. The occurrence of iridium anomalies at this level in the succession may, however, indicate a volcanogenic control on the global environment. Detailed time-analysis of the Late Cenomanian event has been used to produce a correlation of some key successions.     

A three-dimensional numerical model of particulate transport for coastal waters

A Lagrangian marker particle in Eulerian finite difference cell solution to the three-dimensional incompressible mass transport equation was developed for predicting particulate transport in coastal and estuarine waters. Special features of the solution procedure include a finite difference grid network which translates horizontally and vertically with the mean particle motion and expands with the dispersive growth of the marker particle cloud. The cartesian vertical coordinate of the three-dimensional mass transport equation has been transformed, using instantaneous water column depth to allow adaptation to flow situations with a temporally and spatially varying bottom topography and free surface. Results from this model for turbulent diffusion and advection of a uniform plug flow of sediment in an unbounded uniform flow field with various sediment settling velocities were in excellent agreement with the corresponding analytic solutions. Using current information from a two-dimensional vertically averaged hydrodynamic's model, the model was utilized to predict the long term diffusion and advection of dilute neutrally and negatively buoyant suspended sediment clouds resulting from a hypothetical instantaneous release of dredge spoil waste at Brown's Ledge in Rhode Island Sound.     

A Model of Conductive Heat Flow in Forest Edges and Fragmented Landscapes

Although the creation of edges during forest fragmentation can have important abiotic and biotic impacts, especially under conditions of future climate change, mechanistic models of edge effects have not been forthcoming. A simple numerical model of two-dimensional heat flow is developed and applied to a vertical forest/clearcut edge profile and to simulated fragmented landscapes. Height-specific thermal diffusivity and conductivity in the forest were assumed to vary in proportion to foliage densities measured in the central Amazon. In the edge profile, the clearcut that abutted the edge served as a heat source and its temperature was maintained at a constant value higher than in the initially cooler forest. In the fragmented landscapes, simulated treefall gaps were heat sources whose temperature varied with sun movements during the day. Gap frequency was varied so as to approximate the gap coverage observed in selectively logged forests. In one set of simulations, temperature in the openings was systematically varied; in another, thermal diffusivity of the forest was varied. Along the edge profile, high temperatures in the clearcut were rapidly transmitted into the upper canopy due to additive edge effects. Temperatures in the forest understory were also very sensitive to clearcut temperatures due to relatively sparse understory foliage. An overall increase in forest diffusivity led to markedly higher temperatures close to the edge and a more even temperature distribution among height strata. In fragmented landscapes, total gap coverage and additivity from neighboring gaps strongly influenced forest temperatures. At low conductivities, heat flowed only into the forest close to the gaps and hence forest temperature increased almost linearly with gap area. However, at high conductivities, heat flowed far into the forest and forest temperature varied as a function of gap density in the surrounding neighborhood. Because of these additive effects, slight increases in total gap area led to disproportionate changes in the thermal profile of the landscape. These results have important implications for the conservation of forest ecosystems.     

Some methods for statistical analysis of multimodal distributions and their application to grain-size data

Some of the methods used in the resolution of mixed normal distributions are discussed under three headings: analytical, graphical, and numerical methods. Attention is given to their applicability in the analysis of grain-size data as derived from sieving. Comparisons are made by applying several methods to published data. It is concluded that the numerical methods offer most scope, especially the method of nonlinear least squares. Some analyses of beach sediments, using this method, are presented. The adoption of a convention for the number of individuals in the sample increases ease of interpretation.     

On the use of X-ray and γ-ray telescopes for identifying the origin of electrons and positrons observed by ATIC, Fermi, and PAMELA

X-ray and γ-ray observations can help understand the origin of the electron and positron signals reported by ATIC, PAMELA, PPB-BETS, and Fermi. It remains unclear whether the observed high-energy electrons and positrons are produced by relic particles, or by some astrophysical sources. To distinguish between the two possibilities, one can compare the electron population in the local neighborhood with that in the dwarf spheroidal galaxies, which are not expected to host as many pulsars and other astrophysical sources. This can be accomplished using X-ray and γ-ray observations of dwarf spheroidal galaxies. Assuming the signal detected by Fermi and ATIC comes from dark matter and using the inferred dark matter profile of the Draco dwarf spheroidal galaxy as an example, we calculate the photon spectrum produced by electrons via inverse Compton scattering. Since little is known about the magnetic fields in dwarf spheroidal galaxies, we consider the propagation of charged particles with and without diffusion. Extending the analysis of Fermi collaboration for Draco, we find that for a halo mass ∼10⁹ M_⊙, even in the absence of diffusion, the γ-ray signal would be above the upper limits. This conclusion is subject to uncertainties associated with the halo mass. If dwarf spheroidal galaxies host local magnetic fields, the diffusion of the electrons can result in a signal detectable by future X-ray telescopes.     

The geology of Australian Mars analogue sites

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.