Ground-water surface-water interactions and long-term change in riverine riparian vegetation in the southwestern United States

Riverine riparian vegetation has changed throughout the southwestern United States, prompting concern about losses of habitat and biodiversity. Woody riparian vegetation grows in a variety of geomorphic settings ranging from bedrock-lined channels to perennial streams crossing deep alluvium and is dependent on interaction between ground-water and surface-water resources. Historically, few reaches in Arizona, southern Utah, or eastern California below 1530 m elevation had closed gallery forests of cottonwood and willow; instead, many alluvial reaches that now support riparian gallery forests once had marshy grasslands and most bedrock canyons were essentially barren. Repeat photography using more than 3000 historical images of rivers indicates that riparian vegetation has increased over much of the region. These increases appear to be related to several factors, notably the reduction in beaver populations by trappers in the 19th century, downcutting of arroyos that drained alluvial aquifers between 1880 and 1910, the frequent recurrence of winter floods during discrete periods of the 20th century, an increased growing season, and stable ground-water levels. Reductions in riparian vegetation result from agricultural clearing, excessive ground-water use, complete flow diversion, and impoundment of reservoirs. Elimination of riparian vegetation occurs either where high ground-water use lowers the water table below the rooting depth of riparian species, where base flow is completely diverted, or both. We illustrate regional changes using case histories of the San Pedro and Santa Cruz Rivers, which are adjacent watersheds in southern Arizona with long histories of water development and different trajectories of change in riparian vegetation.     

Gated minds and gated lives as worlds of exclusion and fear

Three interrelated concepts about the “gatedness of life” are addressed in this essay: gated communities, gated lives, and gated minds. All are important components in understanding the places where gated communities are located within a city as well as those who live in them. Mapping these concepts is a further theme discussed.     

Great Lakes Hydrology Under Transposed Climates

Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrological response of the Great Lakes to warmer climates. The Great Lakes Environmental Research Laboratory used their conceptual models for simulating moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and timing within the existing climate; this is not true for General Circulation Model-generated corrections applied to existing historical data in many other impact studies. The transposed climates lead to higher and more variable over-land evapotranspiration and lower soil moisture and runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident in the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spreads more throughout the annual cycle. The response of runoff to temperature and precipitation changes is coherent among the lakes and varies quasi-linearly over a wide range of temperature changes, some well beyond the range of current GCM predictions for doubled CO₂ conditions.     

Evidence for dominant grain-boundary sliding deformation in greenschist- and amphibolite-grade polymineralic ultramylonites from the Redbank Deformed Zone, Central Australia

Microstructural and textural investigations by scanning (SEM) and transmission electron microscopy (TEM) techniques have been performed on samples taken across two quartzo-feldspathic mylonite zones from the Redbank Deformed Zone, Central Australia. One has been deformed at greenschist-facies (GS), the second at amphibolite-facies (Am), conditions. With increasing strain the rock type changes from protomylonite to mylonite to ultramylonite. The protomylonites and mylonites consist of alternating quartz and polymineralic quartz-feldspar bands. At the highest strains a homogeneous, fine-grained polymineralic ultramylonite occurs. Shear-zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites. TEM and SEM studies of the ultramylonites reveal a rectangular to square grain shape, a continuous alignment of grain and interphase boundaries across several grain diameters, a grain size (GS 0.5 μm; Am 5–11 μm) less than the equilibrium subgrain size, and open and void-containing grain and interphase boundaries. Analysis of local textures by electron back-scatter diffraction (EBSD) in the SEM showed a very weak crystallographic preferred orientation (CPO) for the quartz. The grain misorientation relationships are not consistent, with dislocation creep being the dominant deformation mechanism. All structures are of the type expected if grain-boundary sliding processes had contributed significantly to the deformation. Consequently, the deformation of such quartzo-feldspathic rocks, and by implication the rheology of the Redbank Deformed Zone, must have been controlled by the mechanical properties of these fine-grained polymineralic ultramylonites, deforming by grain-boundary sliding processes. This is in contrast to the pure quartz bands which deformed by dislocation-creep mechanisms and were less important in the rheology of the Redbank Deformed Zone.     

Petrologic hypothesis testing with Pearce element ratio diagrams: derivation of diagram axes

In petrology, Pearce element ratio (PER) diagrams have been used: i) to determine whether members of a rock suite are co-genetic, ii) to identify the minerals involved in differentiation processes, and iii) to evaluate the extent to which those mineral are involved. The axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends. Unfortunately, selection of the optimal combination of axis coefficients is a difficult task, especially if the system being investigated has a large number of phases or complicated solid solution minerals. Our work has established a formal set of rules and matrix operations which facilitate the determination of PER diagram axes coefficients. This methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral, given a set of axis coefficients. It can also be used to create PER diagrams on which minerals have predetermined vector displacements. By designating all vector displacements to be parallel, axis coefficients for assemblage test diagrams can be determined to test the following hypothesis: the observed chemical variation is due to the addition (or removal) of a specific set of minerals. Alternatively, by designating all vector displacements to be mutually perpendicular, phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation. Phase discrimination diagrams also provide a means to estimate the extent of that involvement. This methodology facilitates construction of powerful yet simple PER diagrams which provide an effective means of testing alternative differentiation hypotheses. Current address: Department of Geological Sciences, Queen's University Kingston, Ontario K7L 3N6, Canada     

Hydrocarbon contamination in sediments from urban stormwater runoff

This investigation showed that urban stormwater runoff provides a significant amount of petrogenic material to receiving waters and sediments. A characteristic hydrocarbon ‘fingerprint’ for sediments and particulate matter in the Hillsborough Reservoir, Hillsborough River and upper Hillsborough Bay was provided. Determination of source material for petroleum contamination in stormwater runoff and river sediment indicated that crankcase oil was a primary contributor to sediment hydrocarbon contamination. A comparison of sediment hydrocarbons with hydrocarbons from stormwater runoff showed that the most probable source of crankcase oil-like petrochemicals found in sediment was the stormwater runoff.A comparison of hydrocarbon composition in suspended particulate matter with that of accumulated bottom sediments in the reservoir, river and bay, during a non-storm period and rising tide showed no resuspension and upriver transport of petroleum contaminated bay sediment. No special influence of the bay upon the lower river was observed relative to hydrocarbon tracers, indicating that most contaminated sediment transport was downriver during storm events. Additional studies should be performed over various tidal cycles and storm events incorporating sediment cores, sediment grain size analysis and hydrocarbon characterization at more closely spaced stations near the river mouth to address adequately the question of specific hydrocarbon pollution sources, rate of petroleum influx and persistence of petrochemical contaminants in the sediment.     

Effects of non-conserved denominators on pearce element ratio diagrams

Pearce element ratios (PER's) have conserved denominators which have not participated in the material transfer processes that cause chemical variations in rocks. Theoretically, there is no truly conserved element (constituent) which can be used as a PER denominator because in every material transfer process all constituents have non-zero concentrations in the phases that are being transferred. Thus, constituents used as denominators of PERs may have undergone at least a small amount of material transfer. This communication investigates the degree to which a non-conserved PER denominator changes the trend of data produced by a material transfer process from that produced by the same process but plotted on a PER diagram with a truly conserved denominator. An equation is developed that utilizes the partition coefficient as the measure of the degree of involvement of the denominator constituent in the phase undergoing transfer. This equation is examined to determine how the magnitude and direction of a PER diagram data trend change with increasing involvement of the denominator constituent in the transferring phase. A set of plagioclase fractionation examples are presented which use different elements as PER denominators and consider the effects that small amounts of these elements in the plagioclase structure will have on the data trend, as a function of the element partition coefficient between crystal and melt. Results demonstrate that the direction of change in slope of a material transfer data trend is a function of the initial relative magnitudes of the numerator constituents on the PER diagram. Additionally, if the amount of involvement of a PER denominator in a separating phase is very small relative to the amount of the numerator constituents in the separating phase, there is no significant change in the data trend caused by material transfer on a PER diagram. Moreover, if the denominator constituent substitutes for a numerator constituent in the phase undergoing transfer, the intercept of the trend of the data may not converge to zero when there is a large partition coefficient, as would be expected from theory. Thus, statistical tests to determine if a PER denominator is conserved, which evaluate whether the intercept is significantly different from zero, may not be very powerful because a large amount of denominator variation is necessary before the intercept of a data trend is forced through the origin, if at all.     

Definition and detailed characterization of Lunar surface units using remote observations

Remote sensing techniques and data may be subdivided into three principal types according to how they are used: (1) defining techniques help to define unit boundaries and extent; (2) characterizing techniques allow classification and characterization of physical features, lithology, or chemical composition; (3) supporting techniques provide additional useful information but are not fundamental to the definition or characterization of units. Defined units represent a fundamental subdivision of the rocks in a planetary crust and thus represent processes and sequences of events. The definition and characterization of units provides a framework for the interpretation of planetary processes and history. Detailed consideration of unit definition and characterization is presented using the mare deposits of the Imbrium basin as an example. This example provides guidelines for the utilization of remote sensing techniques in geologic mapping of the Moon and other planets.     

Structure and evolution of solar radio bursts at 26.4 MHz

Two dimensional source brightness distributions at 26.4 MHz for solar bursts of spectral type II, III, IV, and V are derived from observations with a multiple-baseline, time-sharing interferometer system. It was designed explicitly to study the large angle (40 halo) component of low frequency solar bursts first reported by Weiss and Sheridan (1962). Thirty-two bursts occurring in the interval of June–August, 1975, were fit with a circular gaussian core and an elliptical gaussian halo component. Half-power halo diameters (E-W×N-S) averaged 30×28 for type III bursts and 42×27, 28×37, 30×25 for type V, II and IV bursts respectively. Typical core sizes fell in the range of 10±4 giving 31 halo to core size ratio. All burst types were found to have some large angle structure: the specific intensity was 10% compared to the core but the total power in each component was comparable. Two processes for producing the core-halo structure of type III bursts are compared: scattering and refraction of a point source and refraction from many sources over an extended region. It is concluded that the core can be explained by either model but the halo is more consistent with emission from an extended source region of 40° in longitude.