Sedna and the Oort Cloud around a migrating Sun

Recent numerical simulations have demonstrated that the Sun’s dynamical history within the Milky Way may be much more complex than that suggested by its current low peculiar velocity (Sellwood, J.A., Binney, J.J. [2002]. Mon. Not. R. Astron. Soc. 336, 785-796; Roškar, R., Debattista, V.P., Quinn, T.R., Stinson, G.S., Wadsley, J. [2008]. Astrophys. J. 684, L79-L82). In particular, the Sun may have radially migrated through the galactic disk by up to 5-6 kpc (Roškar, R., Debattista, V.P., Quinn, T.R., Stinson, G.S., Wadsley, J. [2008]. Astrophys. J. 684, L79-L82). This has important ramifications for the structure of the Oort Cloud, as it means that the Solar System may have experienced tidal and stellar perturbations that were significantly different from its current local galactic environment. To characterize the effects of solar migration within the Milky Way, we use direct numerical simulations to model the formation of an Oort Cloud around stars that end up on solar-type orbits in a galactic-scale simulation of a Milky Way-like disk formation. Surprisingly, our simulations indicate that Sedna’s orbit may belong to the classical Oort Cloud. Contrary to previous understanding, we show that field star encounters play a pivotal role in setting the Oort Cloud’s extreme inner edge, and due to their stochastic nature this inner edge sometimes extends to Sedna’s orbit. The Sun’s galactic migration heightens the chance of powerful stellar passages, and Sedna production occurs around ∼20-30% of the solar-like stars we study. Considering the entire Oort Cloud, we find its median distance depends on the minimum galactocentric distance attained during the Sun’s orbital history. The inner edge also shows a similar dependence but with increased scatter due to the effects of powerful stellar encounters. Both of these Oort Cloud parameters can vary by an order of magnitude and are usually overestimated by an Oort Cloud formation model that assumes a fixed galactic environment. In addition, the amount of material trapped in outer Oort Cloud orbits (a > 20,000 AU) can be extremely low and may present difficulties for traditional models of Oort Cloud formation and long-period comet production.     

Heavy metal levels in New Zealand molluscs

Cadmium, lead, copper, mercury, zinc and iron were analysed by atomic absorption spectrophotometry in 13 species of edible molluscs from 199 sites around New Zealand; these included all molluscs commonly eaten in New Zealand. From each sampling site, a minimum of 14 specimens were combined and analysed together, so that the result would be representative of the population in that area. Wide variations are shown to occur in heavy metals from one location to another, and explanations for some of these variations are given.     

Improved moraine age interpretations through explicit matching of geomorphic process models to cosmogenic nuclide measurements from single landforms

The statistical distributions of cosmogenic nuclide measurements from moraine boulders contain previously unused information on moraine ages, and they help determine whether moraine degradation or inheritance is more important on individual moraines. Here, we present a method for extracting this information by fitting geomorphic process models to observed exposure ages from single moraines. We also apply this method to 94 ¹⁰Be apparent exposure ages from 11 moraines reported in four published studies. Our models represent ¹⁰Be accumulation in boulders that are exhumed over time by slope processes (moraine degradation), and the delivery of boulders with preexisting ¹⁰Be inventories to moraines (inheritance). For now, we neglect boulder erosion and snow cover, which are likely second-order processes. Given a highly scattered data set, we establish which model yields the better fit to the data, and estimate the age of the moraine from the better model fit. The process represented by the better-fitting model is probably responsible for most of the scatter among the apparent ages. Our methods should help resolve controversies in exposure dating; we reexamine the conclusions from two published studies based on our model fits.     

Field‐Scale Relationships Among Soil Properties and Shallow Groundwater Quality

It is important to understand the link between land surface/soil properties and shallow groundwater quality. To that end, soil properties and near‐water‐table groundwater chemistry of a shallow, unconfined aquifer were measured on a 100‐m grid on a 64‐ha irrigated field in southeastern North Dakota. Soil properties and hydrochemistry were compared via multivariate analysis that included product‐moment correlations and factor analysis/principal component analysis. Topographic low areas where the water table was in close proximity to the soil surface generally had higher apparent electrical conductivity (EC_a) and higher percent silt and clay than higher positions on the landscape. The majority of the groundwater was characterized by Ca‐ and Mg‐HCO₃ type water and was associated with topographic high areas with lower EC_a and net groundwater recharge. Small topographic depressions were areas of higher EC_a (net groundwater discharge) where salts that precipitated via evapotranspiration and evaporative discharge dissolved and leached to the groundwater during short‐term depression‐focused recharge events. At this site, groundwater quality and soil EC_a were related to surface topography. High‐resolution topography and EC_a measurements are necessary to characterize the land surface/soil properties and surficial groundwater quality at the field‐scale and to delineate areas where the shallow groundwater is most susceptible to contamination.     

An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice‐flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice‐flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.     

Evaluation of a surface energy balance method based on optical and thermal satellite imagery to estimate root‐zone soil moisture

Various remote‐sensing methods are available to estimate soil moisture, but few address the fine spatial resolutions (e.g. 30‐m grid cells) and root‐zone depth requirements of agricultural and other similar applications. One approach that has been previously proposed to estimate fine‐resolution soil moisture is to first estimate the evaporative fraction from an energy balance that is inferred from optical and thermal remote‐sensing images [e.g. using the Remote Sensing of Evapotranspiration (ReSET) algorithm] and then estimate soil moisture through an empirical relationship to evaporative fraction. A similar approach has also been proposed to estimate the degree of saturation. The primary objective of this study is to evaluate these methods for estimating soil moisture and degree of saturation, particularly for a semi‐arid grassland with relatively dry conditions. Soil moisture was monitored at 28 field locations in south‐eastern Colorado with herbaceous vegetation during the summer months of 3 years. In situ soil moisture and degree of saturation observations are compared with estimates calculated from Landsat imagery using the ReSET algorithm. The in situ observations suggest that the empirical relationships with evaporative fraction that have been proposed in previous studies typically provide overestimates of soil moisture and degree of saturation in this region. However, calibrated functions produce estimates with an accuracy that may be adequate for various applications. The estimates produced by this approach are more reliable for degree of saturation than for soil moisture, and the method is more successful at identifying temporal variability than spatial variability in degree of saturation for this region. Copyright © 2015 John Wiley & Sons, Ltd.     

Climate teleconnections influence on West Africa's terrestrial water storage

There is some evidence of rapid changes in the global atmosphere and hydrological cycle caused by the influence of climate variability. In West Africa, such changes impact directly on water resources leading to incessant extreme hydro‐meteorological conditions. This study examines the association of three global climate teleconnections—El‐Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Multi‐decadal Oscillation (AMO) with changes in terrestrial water storage (TWS) derived from both Modern‐Era Retrospective Analysis for Research and Applications (MERRA, 1980–2015) and Gravity Recovery and Climate Experiment (GRACE, 2002–2014). In the Sahel region, positive phase of AMO coincided with above‐normal rainfall (wet conditions) and the negative phase with drought conditions and confirms the observed statistically significant association (r = 0.62) between AMO and the temporal evolutions of standardised precipitation index. This relationship corroborates the observed presence of AMO‐driven TWS in much of the Sahel region (though considerably weak in some areas). Although ENSO appears to be more associated with GRACE‐derived TWS over the Volta basin (r =?0.40), this study also shows a strong presence of AMO‐ and ENSO‐induced TWS derived from MERRA reanalysis data in the coastal West African countries and most of the regions below latitude 10°N. The observed presence of ENSO‐ and AMO‐driven TWS is noticeable in tropical areas with relatively high annual/bimodal rainfall and strong inter‐annual variations in surface water. The AMO has a wider footprint and sphere of influence on the region's TWS and suggests the important role of North Atlantic Ocean. IOD‐related TWS also exists in West Africa and its influence on the region's hydrology maybe secondary and somewhat complementary. Nonetheless, presumptive evidence from the study indicates that ENSO and AMO are the two major climatic indices more likely to impact on West Africa's TWS.     

Lacan on urban development and national identity in a global city: Integrated Resorts in Singapore

Lacanian psychoanalysis has been used in film, literature and other areas of social thought, but rarely in the domain of urban studies and human/cultural geography. Following its introduction by urban planners Michael Gunder and Jean Hillier, I apply the theory of the four discourses and the mirror stage of development to Singapore's urban development of the two Integrated Resorts at Marina Bay and Sentosa. The decision to allow gambling and build casinos was a contentious one and provides a point of departure for insight into the identity issues and planning decision making processes in Singapore. I critically analyse the rhetoric of the public debate from 2004–2005 to draw conclusions about the government's self‐perception of Singapore as a city‐state and the manifestation of this identity through the creation of cosmopolitan spaces as an attempt to project that identity onto its citizens. The aim of Lacanian psychoanalysis is to provide an understanding and recognition by analysis to enable a change of signifiers, values and ideology among the masters and the subjects to better represent the true needs and wants of the community. This reflective position enables a movement toward postcolonial urban studies and planning.     

Surface Heterogeneity Effects on Regional-Scale Fluxes in the Stable Boundary Layer: Aerodynamic Roughness Length Transitions

The effects of abrupt streamwise transitions of the aerodynamic roughness length ( $z_\mathrm{o}$ z o ) on the stable atmospheric boundary layer are evaluated using a series of large-eddy simulations based on the first Global Energy and Water Cycle Experiment Atmospheric Boundary Layer intercomparison study (GABLS1). Four $z_\mathrm{o}$ z o values spanning three orders of magnitude are used to create all possible binary distributions with each arranged into patches of characteristic length scales equal to roughly one-half, one, and two times the equivalent homogeneous boundary-layer height. The impact of the heterogeneity on mean profiles of wind speed and temperature, on surface fluxes of heat and momentum, and on internal boundary-layer dynamics are considered. It is found that $z_\mathrm{o}$ z o transitions do not significantly alter the functional relationship between the average surface fluxes and the mean profiles of wind speed and potential temperature. Although this suggests that bulk similarity theory is applicable for modelling the stable boundary layer over $z_\mathrm{o}$ z o heterogeneity, effective surface parameters must still be specified. Existing models that solve for effective roughness lengths of momentum and heat are evaluated and compared to values derived from the simulation data. The existing models are unable to accurately reproduce both the values of the effective aerodynamic roughness lengths and their trends as functions of patch length scale and stability. A new model for the effective aerodynamic roughness length is developed to exploit the benefits of the other models tested. It accurately accounts for the effects of the heterogeneity and stratification on the blending height and effective aerodynamic roughness length. The new model provides improved average surface fluxes when used with bulk similarity.     

High-resolution observations of aggregate flux during a sub-polar North Atlantic spring bloom

An aggregate flux event was observed by ship and by four underwater gliders during the 2008 sub-polar North Atlantic spring bloom experiment (NAB08). At the height of the diatom bloom, aggregates were observed as spikes in measurements of both particulate backscattering coefficient (b_bp) and chlorophyll a fluorescence. Optical sensors on the ship and gliders were cross-calibrated through a series of simultaneous profiles, and b_bp was converted to particulate organic carbon. The aggregates sank as a discrete pulse, with an average sinking rate of ∼75 m d⁻¹; 65% of aggregate backscattering and 90% of chlorophyll fluorescence content was lost between 100 m and 900 m. Mean aggregate organic carbon flux at 100 m in mid-May was estimated at 514 mg C m⁻² d⁻¹, consistent with independent flux estimates. The use of optical spikes observed from gliders provides unprecedented coupled vertical and temporal resolution measurements of an aggregate flux event.