The ability of a field population of diatoms to discriminate between phosphate and arsenate

Dissolved inorganic arsenic and phosphate levels present in an enclosed experimental ecosystem have been studied during the course of a natural spring diatom bloom. Whilst phosphate levels dropped rapidly to almost zero as the bloom developed, arsenic levels remained constant even when arsenate levels exceeded the dissolved phosphate. Certain diatom species (Thalassiosira spp., Chaetoceras spp. and Skeletonema costatum) therefore appear to be capable of discriminating, either by uptake or excretion, between phosphate and arsenate. The implications of such findings are discussed.     

Grid cell distortion and MODFLOW's integrated finite-difference numerical solution

The ground water flow model MODFLOW inherently implements a nongeneralized integrated finite-difference (IFD) numerical scheme. The IFD numerical scheme allows for construction of finite-difference model grids with curvilinear (piecewise linear) rows. The resulting grid comprises model cells in the shape of trapezoids and is distorted in comparison to a traditional MODFLOW finite-difference grid. A version of MODFLOW-88 (herein referred to as MODFLOW IFD) with the code adapted to make the one-dimensional DELR and DELC arrays two dimensional, so that equivalent conductance between distorted grid cells can be calculated, is described. MODFLOW IFD is used to inspect the sensitivity of the numerical head and velocity solutions to the level of distortion in trapezoidal grid cells within a converging radial flow domain. A test problem designed for the analysis implements a grid oriented such that flow is parallel to columns with converging widths. The sensitivity analysis demonstrates MODFLOW IFD's capacity to numerically derive a head solution and resulting intercell volumetric flow when the internal calculation of equivalent conductance accounts for the distortion of the grid cells. The sensitivity of the velocity solution to grid cell distortion indicates criteria for distorted grid design. In the radial flow test problem described, the numerical head solution is not sensitive to grid cell distortion. The accuracy of the velocity solution is sensitive to cell distortion with error <1% if the angle between the nonparallel sides of trapezoidal cells is <12.5 degrees. The error of the velocity solution is related to the degree to which the spatial discretization of a curve is approximated with piecewise linear segments. Curvilinear finite-difference grid construction adds versatility to spatial discretization of the flow domain. MODFLOW-88's inherent IFD numerical scheme and the test problem results imply that more recent versions of MODFLOW 2000, with minor modifications, have the potential to make use of a curvilinear grid.     

Spatial modeling of annual minimum and maximum temperatures in Iceland

Bayesian hierarchical modeling is applied to the analysis of annual minimum and maximum temperatures. In both cases the generalized extreme value distribution is selected as the marginal distribution at each site due to its flexibility and theoretical basis. This distribution has three unknown parameters, a location parameter, a scale parameter and a shape parameter. The location and scale parameters are assumed to vary across sites while the shape parameter is assumed to be the same for all sites. The location and the scale parameters are modeled as two independent Gaussian spatial processes which are governed by the Matérn correlation function. These spatial processes play a central role in spatial predictions at unobserved sites. The location parameter is also modeled with a common time effect which includes a linear trend and a linear site effect with altitude, distance to open sea, latitude and longitude as covariates. This model allows for spatial predictions for any set of unobserved sites, the scale of the grid can be as fine as possible as long as the covariates are observed at each of the unobserved sites. Data on the annual minimum and maximum temperatures in Iceland from 1961 to 2009?at 72 sites are analyzed and used to predict the 2nd percentile of the minimum temperature for the 12-month period from July 2011 to June 2012 and the 98th percentile of maximum temperature for the year 2011 for a large set of unobserved sites across Iceland at which the covariates are available. The spatial predictions reveal that the 2nd percentile of minimum temperature for 2011 to 2012 reaches ?35°C in the central part of Iceland and ranges from ?12 to ?22°C around the coast of Iceland with the lowest coastal temperatures in the Northern part and the Northeastern part. The 98th percentile of maximum temperature in 2011 ranges from 10 to 30°C where the warmest areas are found in the Eastern part, the North-Eastern part and the Southern part of the country. The estimated increase in minimum and maximum temperatures over the years 1961 to 2009 is 0.71 and 0.47°C per decade, respectively, while the average annual temperature increased 0.24°C per decade over the same period.     

Origin of ice diapirism, true polar wander, subsurface ocean, and tiger stripes of Enceladus driven by compositional convection

We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an “ammonia feedback” mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer above. We then consider a large scale instability arising from the pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Enceladus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region.     

TNOs are Cool: A Survey of the Transneptunian Region

Over one thousand objects have so far been discovered orbiting beyond Neptune. These trans-Neptunian objects (TNOs) represent the primitive remnants of the planetesimal disk from which the planets formed and are perhaps analogous to the unseen dust parent-bodies in debris disks observed around other main-sequence stars. The dynamical and physical properties of these bodies provide unique and important constraints on formation and evolution models of the Solar System. While the dynamical architecture in this region (also known as the Kuiper Belt) is becoming relatively clear, the physical properties of the objects are still largely unexplored. In particular, fundamental parameters such as size, albedo, density and thermal properties are difficult to measure. Measurements of thermal emission, which peaks at far-IR wavelengths, offer the best means available to determine the physical properties. While Spitzer has provided some results, notably revealing a large albedo diversity in this population, the increased sensitivity of Herschel and its superior wavelength coverage should permit profound advances in the field. Within our accepted project we propose to perform radiometric measurements of 139 objects, including 25 known multiple systems. When combined with measurements of the dust population beyond Neptune (e.g. from the New Horizons mission to Pluto), our results will provide a benchmark for understanding the Solar debris disk, and extra-solar ones as well.     

Genetic techniques for livestock breeding: Restructuring institutional relationships in agriculture

This paper explores some of the key institutional transformations in livestock breeding associated with the increasing significance of genetic techniques, situating this within an assessment of an emerging agricultural bioeconomy. Focusing on beef cattle and sheep breeding in the United Kingdom, the paper examines how a move towards the involvement of international and corporate interests in livestock breeding is restructuring the network of institutional interests affecting the knowledge and decision making of individual breeders. The paper suggests that the structural transformation of beef cattle and sheep breeding is complicated by the need for negotiation between breeders’ ‘traditional’ knowledge-practices and the ‘geneticised’ techniques being made available to them. We are thus seeing the emergence of new and complex interactions between the major actors which are reconfiguring power relationships in the UK livestock breeding sector.