Isostatic compensation on a continental scale: local versus regional mechanisms

Summary. Using the techniques of linear and quadratic programming, it can be shown that the isostatic response function for the continental United States, computed by Lewis & Dorman (1970), is incompatible with any local compensation model that involves only negative density contrasts beneath topographic loads. We interpret the need for positive densities as indicating that compensation is regional rather than local. The regional compensation model that we investigate treats the outer shell of the Earth as a thin elastic plate, floating on the surface of a liquid. The response of such a model can be inverted to yield the absolute density gradient in the plate, provided the flexural rigidity of the plate and the density contrast between mantle and topography are specified.
If only positive density gradients are allowed, such a regional model fits the United States response data provided the flexural rigidity of the plate lies between 10²¹ and 10²² N m. The fit of the model is insensitive to the mantle/ load density contrast, but certain bounds on the density structure can be established if the model is assumed correct. In particular, the maximum density increase within the plate at depths greater than 34 kin must not exceed 470 kg m⁻³; this can be regarded as an upper bound on the density contrast at the Mohorovicic discontinuity.
The permitted values of the flexural rigidity correspond to plate thicknesses in the range 5–10 km, yet deformations at depths greater than 20 km are indicated by other geophysical data. We conclude that the plate cannot be perfectly elastic; its effective elastic moduli must be much smaller than the seismically determined values. Estimates of the stress-differences produced in the earth by topographic loads, that use the elastic plate model, together with seismically determined elastic parameters, will be too large by a factor of four or more.     

The science process for selecting the landing site for the 2011 Mars Science Laboratory

The process of identifying the landing site for NASA’s 2011 Mars Science Laboratory (MSL) began in 2005 by defining science objectives, related to evaluating the potential habitability of a location on Mars, and engineering parameters, such as elevation, latitude, winds, and rock abundance, to determine acceptable surface and atmospheric characteristics. Nearly 60 candidate sites were considered at a series of open workshops in the years leading up to the launch. During that period, iteration between evolving engineering constraints and the relative science potential of candidate sites led to consensus on four final sites. The final site will be selected in the Spring of 2011 by NASA’s Associate Administrator for the Science Mission Directorate. This paper serves as a record of landing site selection activities related primarily to science, an inventory of the number and variety of sites proposed, and a summary of the science potential of the highest ranking sites.     

SPATIAL VARIATION OF INFLUENTS ON AVIAN DIVERSITY: A MULTIVARIATE APPROACH

Breeding bird censuses from 132 homogeneous sites representing a variety of habitats throughout North America are analyzed and a bird species diversity model based on environmental characteristics and sampling procedures is presented. Bivariate regression models relate the number of bird species detected in a site (NSPECIES) to plot size, latitude, and the number of bird pairs encountered during censusing. These models consistently overestimate NSPECIES in desert shrublands, grasslands, tundra, and scrub and underestimate NSPECIES in forested habitats, which indicates that the effects of the environmental and methodological variables on NSPECIES vary with respect to habitat. The expansion method of regression analysis is used to generate a multivariate model that accounts for this spatial variation in the influence of the independent variables on NSPECIES. With the exception of temperate wetlands, which have the third highest mean value for NSPECIES of nine habitat groups, forested sites and woodlands have higher mean NSPECIES values than more open habitats. Use of the expansion method to account for spatial variation in the effects exerted by independent variables introduces a geographically realistic element often lacking in broad-scale models. (Key words: avian diversity, sampling intensity, multiple regression analysis, vegetation structure, habitat types.)     

NURSE PLANT RETATIONSHIPS OF COLUMNAR CACTI IN ARIZONA

Patterns of association of three columnar cactus species with potential nurse plants and rocks were examined in Organ Pipe Cactus National Monument, Arizona. Close spatial associates of cacti are often called “nurses” because they provide a less extreme microclimate for cactus seedlings, which improves survival rates. Of the three cacti, Carnegiea gigantea had the greatest frequency of association with nurse plants. Stenocereus thurberi, which is common on steep hillslopes where rocks and crevices offer numerous protected microsites for seedling establishment, was associated with nurse rocks more frequently than were the other two species. Lophocereus schottii showed the lowest frequency of association with nurses. In the Monument, L. schottii reproduces primarily through vegetative propagation, which reduces the need of small cacti for protection from nurses. Species most frequently associated with the three cacti were among the dominant species where each cactus occurred except that sparsely foliated species were associated with cacti less frequently than expected. This pattern supports previous claims that structural characteristics are more important than species identification in providing cacti with enhanced chances for survival. It also suggests that at a landscape scale, population dynamics of columnar cacti are more closely linked to patterns of overall vegetation change than to population fluctuations of a particular species that acts as a nurse. [Key words: northern Sonoran Desert, cactus establishment, nurse plants.]     

EDAPHIC PROPERTIES AND FOLIAR ELEMENTAL CONCENTRATIONS FROM SAND PINE (PINUS CLAUSA) POPULATIONS THROUGHOUT FLORIDA

Soil physical and chemical properties, plus foliar elemental concentrations, are reported for surface (5–10 cm) and subsurface (35–40 cm) soil and foliar tissue samples from 20 sand pine (Pinus clausa) populations, 9 from the Choctawhatchee variety on the Florida panhandle and 11 from the Ocala variety on the Florida peninsula. Sand pine occurred exclusively on sandy soils, with mean sand content >97%. Soils generally were low in organic carbon and soil elemental concentrations. Both K and P were below analytic detection limits, whereas Ca and Mg were present at only modest levels. Only soil Al and Fe were relatively abundant. Foliar elemental concentrations indicated pronounced bioconcentration of nutrients by sand pine. Macronutrients (Ca, K, Mg, and P) approached or exceeded levels typical of other southern pine species. Some contrasts in edaphic and foliar tissue properties were apparent between varieties, but contrasts were more pronounced between groups of populations based on both soil color (yellow-sand group) and variety (Choctawhatchee white-sand and Ocala white-sand groups). The yellow-sand group, dominated by inland Choctawhatchee sites on sediments of mixed marine/fluvial origin, possessed less well sorted sand grains, higher clay content, lower pH, and higher levels of soil Al and Fe. The Choctawhatchee white-sand group consisted primarily of coastal dune sites, with well-sorted sands that reflect an eolian substrate. The Ocala white-sand group ranged primarily along the interior sand ridges of central Florida and was intermediate in sedimentological character. Foliar elemental concentrations did not reflect soil elemental patterns. Whereas Ocala samples were significantly higher in P and Mg, Choctawhatchee samples were higher in K, and there were no signficant varietal differences in foliar Ca. [Key words: sand pine, soil elemental concentrations, foliar elemental concentrations, Florida.]     

Resilience Theory and Thomas Vale's Plants and People: A Partial Consilience of Ecological and Geographic Concepts of Succession

Geography has discovered resilience theory, a body of thought about ecological change that initiated with C.S. Holling in the 1970s. We describe the similarities and differences between resilience theory and a geographical treatise, Thomas Vale's (1982) book Plants and People. Vale's work draws more from the tradition of field botany and plant succession than from the theoretical and mathematical ecology that prompted Holling's ideas. Yet like resilience theory, Vale's model of ecological change emphasized multiple states, the threshold transitions between them, and their irreversibility. Each described how forests and rangelands can flip between stability domains in response to altered fire regimes, modified grazing pressures, and climate change. Plants and People also recognized the dual nature of stability encapsulated in Holling's formalization of engineering and ecological resilience. Although resilience theory predates Vale's work and retains primacy through its citation record, we show how their partial consilience promotes a more critical understanding of resilience theory and the ways in which models, scale, and human values influence our comprehension of ecological change.     

Spontaneous tangential discontinuities and the optical analogy for static magnetic fields. VI. Topology of current sheets

Abstract

The electric surface current in a tangential discontinuity in a force-free magnetic field is conserved. The direction of the current is halfway between the direction of the continuous fields on either side of the surface of discontinuity. Hence the current sheets, i.e. the surface of tangential discontinuity, have a topology that is distinct from the lines of force of the field. The precise nature of the topology of the current sheet depends upon the form of the winding patterns in the field. Hence, invariant winding patterns and random winding patterns are treated separately. Current sheets may have edges, at the junction of two or more topological separatrices. The current lines may, in special cases, be closed on themselves. The lines of force that lie on either side of a current sheet somewhere pass off the sheet across a junction onto another sheet. In most cases the current sheets extending along a field make an irregular honeycomb.

The honeycomb pattern varies along the field if the winding pattern of the field varies. The surface current density in a tangential discontinuity declines inversely, or faster, with distance from its region of origin. The edges of weaker tangential discontinuities (originating in more distant regions) are bounded by the stronger tangential discontinuities (of nearby origin).

An examination of the force-free field equations in a small neighborhood of the line of intersection of two tangential discontinuities shows that the lines of force twist around to cross the line of intersection at right angles. If the angle between the tangential discontinuities exceeds π/2, there is also the possibilitity that the lines twist around so as to come tangent to the line of intersection as they cross it.     

Time‐Elevation Head Sections: Improved Visualization of Data from Multilevels

Hydrogeologic investigations of fractured rock are evolving toward increasing spatial and temporal resolution with increasing use of multilevel systems with 10 or more intervals in a single borehole, each with auto‐sampling sensors monitoring pressure, temperature or chemistry for weeks or months, creating large quantities of densely sampled data (time and space). These data are typically displayed as hydrographs for analysis of site‐specific controls on groundwater flow. We present a method for presentation of high density pressure head data from multilevel installations referred to as time‐elevation head (TEH) sections that improves visualization of spatial and temporal responses of the hydrogeologic system to external stresses. Data collected from two multilevel installations, each with 13 functioning pressure transducers monitoring the upper 40 m of a dolostone aquifer, over a period of 83 d, prior to, during and after a pumping test are used to present TEH sections and examples of data processing. TEH sections are produced using commercially available software designed for geophysical data collected at closely spaced intervals along sub‐parallel lines. These algorithms perform calculations orthogonally either in time (“X” axis) or elevation (“Y” axis) to interpolate a regular grid of head and subsequently when filtering is used to identify subtle trends within the data. The base and filtered TEH sections are used to interpret response of the system to transients and infer hydrogeologic characteristics of the site. The utility of the process is dependent on the precision and accuracy of the head data as well as an informed user to avoid introducing spurious features into the sections.     

Hydraulic testing using a versatile straddle packer system for improved transmissivity estimation in fractured-rock boreholes

Equipment has been developed for straddle packer testing in fractured-rock boreholes to conduct four types of tests (constant-head step tests, slug tests, constant-rate pumping tests, and recovery tests) without deflating the packers or adjusting equipment in the hole between tests. The goal is to achieve improved accuracy and precision in the determination of transmissivity (T). Water-pressure measurements are recorded using pressure transducers positioned above and below the test interval to identify connections from the test interval to the open borehole. Insights concerning the nature of test conditions are gathered with this equipment to assess errors related to deviations from assumptions inherent in the mathematical models used to determine T, including validation of the Darcian flow assumption, validation of slug test assumptions, cross-connection to the open borehole, inadequate borehole development, and dual permeability, thereby giving greater confidence in the calculated T values. When the errors indicated above are minimized, the constant-head step tests, slug tests and constant-rate pumping/recovery tests give nearly identical values. This multiple-test approach to fractured-rock studies increases confidence in test results, which is important when the goal is characterization of fracture networks for contaminant transport and fate assessment.