Distributed shear of subglacial till due to Coulomb slip

In most models of the flow of glaciers on till beds, it has been assumed that till behaves as a viscoplastic fluid, despite contradictory evidence from laboratory studies. In accord with this assumption, displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution. Here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by Coulomb friction. Motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress. These fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing. Newton\u27s second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed. Consequent displacement profiles are convex upward, similar to those measured by Boulton and colleagues at Breidamerkurjökull, Iceland. The model results, when considered together with the long-term and widespread empirical support for Coulomb models in soils engineering, indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided

Preexisting fractures and the formation of an iconic American landscape: Tuolumne Meadows, Yosemite National Park, USA

Tuolumne Meadows, in Yosemite National Park (USA), is a large sub-alpine meadow in the Sierra Nevada Mountains. Immediately adjacent to Tuolumne Meadows—and underlain by the same bedrock lithology (Cathedral Peak Granodiorite)—are vertical rock faces that provide exceptional opportunities to climbers. While the presence of a broad meadow suggests bedrock erodibility, the vertical rock walls indicate bedrock durability. We propose that the Tuolumne Meadows’s landscape is the result of variable glacial erosion due to the presence or absence of pre-existing bedrock fractures. The meadows and valleys formed because of concentrated tabular fracture clusters—a distinctive and locally pervasive type of fracturing—that were particularly susceptible to glacial erosion. In contrast, the vertical rock walls consist of sparsely fractured bedrock that was originally bounded by zones of pervasive tabular fracture clusters. Glacial erosion preferentially removed the highly fractured rock, forming prominent ridges in the upland surrounding Tuolumne Meadows. The orientation and spacing of the tabular fracture clusters, relative to ice flow, has exerted a fundamental control on the geomorphology of the area. The erosional variability exhibited by a single lithology indicates that the degree of fracturing can be more important than the host lithology in controlling landscape evolution

Volcano collapse promoted by progressive strength reduction: new data from Mount St. Helens

Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices

Acute Sensitivity of Landslide Rates to Initial Soil Porosity

Some landslides move imperceptibly downslope, whereas others accelerate catastrophically. Experimental landslides triggered by rising pore water pressure moved at sharply contrasting rates due to small differences in initial porosity. Wet sandy soil with porosity of about 0.5 contracted during slope failure, partially liquefied, and accelerated within 1 second to speeds over 1 meter per second. The same soil with porosity of about 0.4 dilated during failure and slipped episodically at rates averaging 0.002 meter per second. Repeated slip episodes were induced by gradually rising pore water pressure and were arrested by pore dilation and attendant pore pressure decline

Effects of Transgenic Bacillus thuringiensis Maize Grain on B. thuringiensis- Susceptible Plodia interpunctella (Lepidoptera: Pyralidae)

Percentage survivorship, developmental time, adult body length, and sex ratio of Plodia interpunctella (Hu¨ bner) reared on Þeld-produced grain from sixteen cultivars of maize, Zea mays L., including several transgenic Bacillus thuringiensis (Bt) Berliner hybrids and selected non-Bt isolines, were evaluated under laboratory conditions. Compared with isolines, development was delayed and survivorship reduced for P. interpunctella reared on grain from transgenic hybrids with the CaMV/35s promoter that express Cry1Ab protein. Similarly, compared with non-Bt hybrids, a transgenic hybrid with the CaMV/35s promoter that expresses Cry9C protein delayed development, decreased survivorship, and caused reductions in adult body length of P. interpunctella. In contrast, no signiÞcant differences in P. interpunctella developmental times or survivorship were observed between transgenic hybrids with the PEPC promoter expressing Cry1Ab and their isolines. Additionally, developmental time, survivorship, and adult body length were similar between P. interpunctella reared on a transgenic hybrid with the CaMV/35s promoter expressing Cry1Ac and non-Bt hybrids. Our data demonstrate that transgenic Bt maize grain, especially grain from hybrids with the CaMV/35s promoter expressing Cry1Ab or Cry9C, can signiÞcantly affect B. thuringiensis-susceptible P. interpunctella populations up to 4 or 5 mo after harvest

Changes in Phytoplankton and Bacterioplankton Biomass and Rate Processes in Apalachicola Bay, Florida, in Response to Reduction in River Discharge

Bacterioplankton abundance and chlorophyll concentration and the factors that control them (temperature, nutrient concentrations, and rates of growth, grazing, and export) were studied in Apalachicola Bay, FL, during two summers with contrasting river discharge. A reduction in river discharge from the summer of 2003 (S03) to the summer of 2004 (S04) led to a reduction in estuarine concentrations of dissolved inorganic nitrogen and rates of export, phytoplankton and bacterioplankton growth, and microzooplankton grazing on phytoplankton and bacterioplankton. Bacterioplankton abundance and chlorophyll concentration during S03 were not significantly different from those during S04. Neither the growth rates and abundances of ciliates nor the egg production rates and abundances of Acartia tonsa were significantly affected by the reduction in rate processes of bacterioplankton or phytoplankton. The proposed diversion of freshwater from the Apalachicola River during summer months may lead to substantial changes in the rate processes of bacterioplankton and phytoplankton, but little change in the standing stocks of bacterioplankton, phytoplankton, ciliates, A. tonsa, or ciliate growth rates and A. tonsa egg production rates

Podocnemis sextuberculata

Number of pages: 24Geological SciencesIntegrative Biolog

A systems approach to pelagic ecosystem dynamics in an estuarine environment

Phytoplankton dynamics in Auke Bay, Alaska, were studied during summer, 1969. Nitrate, chlorophyll a, particulate organic carbon, phytoplankton and zooplankton species composition and hydrographic and meteorological data were collected and analyzed. Nitrate input into Auke Bay through freshwater runoff was negligible. A bloom of Thalassiosira aestivalis formed the spring bloom in Auke Bay in 1969. No phytoplankton bloom occurred during a June period when winds were light and variable. Two major blooms of Skeletonema costatum occurred after periods when Auke Bay surface layers were mixed by wind-induced turbulence. The water column became nearly isothermal after periods of high wind mixing although a pronounced density and halal structure persisted, a consequence of input of freshwater to the bay from Auke Creek. Examination of fecal pellets collected from the bay and results of laboratory grazing experiments suggested that Skeletonema costatum was not grazed by zooplankton living in Auke Bay. Nonlinear ordinary differential equations were written to describe phytoplankton and nitrate dynamics in Auke Bay. The phytoplankton dynamics equation included formulations for time-varying insolation and for time-varying wind mixing coefficients. Formulations for effects of nitrate concentration on the photosynthetic assimilation number and for effects of phytoplankton standing crop on the extinction coefficient of light in the water column were included. The nitrate dynamics equation included a formulation for effects of wind-mixing of nitrate-rich water into the euphotic zone from deeper layers of the water column, as well as a formulation for utilization of nitrate in phytoplankton growth. Computer simulation response of the equations reproduced the bloom pattern observed in the field data with some discrepancies caused by assumptions used in model development. The phytoplankton and nitrate model response was strongly coupled to the pattern of the wind-mixing coefficient, as required by the field data. Variations in model parameters had little effect on phase relations between model response and field data but strongly affected model response magnitude

The efficacy of various coniferous stock types planted on brushy sites in the Oregon Coast Range

The selection of planting stock is a critical step in the reforestation planning process. In addition to selecting the proper seed source and species for the planting site, consideration must be made for the type of planting stock which will be used. The survival and growth potential of various classes of seedlings is basic to the evaluation of the specific properties of each type in relation to adaptation to local site conditions. This study provides both specific and general information for a range of seedling types. Several stock types were evaluated in two separate investigations. Comparisons of first-year survival were made for nine different stock types including wildlings on two differently-prepared sites near Eddyville, Oregon. A five-year analysis of growth and survival was conducted for transplant, seedling, and container-grown Douglas-fir seedlings on many different sites on International Paper Company's Hinman Tree Farm between Gardiner and Lorane, Oregon. An interpretation of the observed survival and growth response of seedlings on the Hinman plots was facilitated by characterizing each plot according to potential incoming solar radiation and recording seedling damage and estimates of vegetative competition around each seedling. Survival of the wildlings at the Eddyville site was considerably less than the survival of the nursery stock and the overall survival was less on the brushy site than the burned site. In general, the pattern of survival between the stock types was the same for both sites. An analysis of survival and growth for the seedlings on the Hinman plots showed that both survival and growth are related to the initial height of the seedling. Larger seedlings are vulnerable o fewer adverse site factors and are, therefore, capable of better survival. Seedling growth is a function of size, almost independent of stock type. This relationship is consistent among vegetation types with the effect of size being particularly crucial for seedlings in heavy brush or areas of soil movement. The greater survival and faster growth of large seedlings makes them a better economic investment in the Oregon Coast Range than plugs or 2-0s of the small size classes. Greater survival decreases costs per surviving tree and risks of nonstocked areas; increased growth means reduced expenses for pest control because of a shorter exposure to adverse site factors. Any option that reduces vulnerability to pests in combination with innately shorter periods to maturity increases present net worth. Increasing the size of Douglas-fir stock meets these criteria with a favorable ratio of benefit to cost