Computer-simulation models of scoria cone degradation

Long-term erosional modifications of the relatively simple morphology of scoria (‘cinder') cones are ideally suited for study by field and computer-simulation methods. A series of temporally-distinct cones in the San Francisco and Springerville volcanic fields of Arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi-arid climate. Progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters, including scoria cone height, cone height/width ratio (H_co/W_co), crater depth/width ratio, and slope angle. For example, Holocene–latest Pleistocene cones in the San Francisco field have a mean H_co/W_co value of 0.178±0.041, a mean maximum slope angle of 29.7±4.2°, and a mean average slope angle of 26.4±7.3°, whereas the group of Pliocene cones have values of 0.077±0.024, 20.5±5.8°, and 8.7±2.7°, respectively. Comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields.In order to better understand the degradational modifications of these volcanic landforms, we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone. The simulation algorithm can apply either a linear diffusion-equation model or a model with a nonlinear transport law. Using a finite-difference formulation, the simulation operates upon a three-dimensional scoria cone input as a matrix of elevation values. Utilizing both field and model results, the correlation between changing H_co/W_co value, cone age, and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient (D_f) for both volcanic fields. For the San Francisco volcanic field, D_f had a calculated value of 21.4 m²/kyr for the linear model and 5.3 m/kyr for the nonlinear model, while for the Springerville volcanic field D_f had a calculated value of 24.4 m²/kyr for the linear model and 6.3 m/kyr for the nonlinear model.     

Kornerupine parageneses in whiteschists and other magnesian rocks: is kornerupine + talc a high-pressure assemblage equivalent to tourmaline + orthoamphibole?

Kornerupine, (□,Fe,Mg)(Mg,Fe,Al)₉(Si,Al,B)₅ (O,OH,F)₂₂, has been reported with talc in rocks from six localities worldwide, but only at Chilapila Hill in the Lufilian Arc, Zambia do textural relationships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at T≈ 640 °C, P≈ 13 kbar; a prograde Krn + Tlc assemblage has also been reported from Mautia Hill, Tanzania (P ≤ 13 kbar). In order to estimate possible constraints on the stability range for the kornerupine + talc paragenesis in nature, we constructed a P-T diagram in the model system MgO-Al₂O₃-SiO₂-H₂O (MASH) for seven phases quartz (Qtz), B-free kornerupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite (Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability, i.e., 6–8 kbar, at T≤ 780 °C. However, in the natural system, B₂O₃ and Na₂O are major constituents in Krn and orthoamphibole (Oam), respectively, and dravitic tourmaline (Tur) is widespread. The critical assemblage alternative to Krn + Tlc in nature is Tur + Oam. The upper pressure limit of Tur + Ath is determined by the upper pressure for anthophyllite: 7.7–10.5 kbar at 682–794 °C in the MgO-SiO₂-H₂O system (Chernosky et al. 1985, Am Mineral 70:223–236), and is undoubtedly higher in the presence of Na₂O, CaO, and Al₂O₃. At three of the six localities, talc is a retrograde phase; nonetheless, it possibly equilibrated with kornerupine on the retrograde path or during a later metamorphic event at P-T conditions appropriate for Ky + Tlc. At the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan), Krn is found in the same thin section as talc and kyanite and all three minerals formed during a prograde metamorphic event at T≥ 650 °C, P near 7 kbar. However, Krn is restricted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch either talc or kyanite. A reaction relating the Mulvoj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calculated from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath + 1.33H₂O + 0.27F = Krn + 2.16Tlc + 0.36B₂O₃ + 0.02Rutile + 0.19Na₂O + 0.17CaO. Given the difference in metamorphic pressures estimated for Mulvoj and Chilapila Hill, Krn + Tlc is inferred to be favored by increasing pressure as well as by low Na₂O and CaO contents. Some FeO, F, Fe₂O₃, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g., Krn contains 0.24–0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na₂O, CaO and B₂O₃. The Krn + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly at pressures where orthoamphibole is no longer stable. Received: 21 April 1997 / Accepted: 13 October 1997     

Moisture History and Small Mammal Community Richness during the Latest Pleistocene and Holocene, Northern Bonneville Basin, Utah

Precipitation and net primary productivity are positively correlated in arid environments. Both variables are, in turn, correlated with mammal species richness, but this relationship is not necessarily positive. With increasing precipitation in arid areas of low to moderate productivity, mammal richness increases linearly; as rainfall and productivity increase beyond this point, mammal richness is known to decline in some areas, producing a relationship that has been termed “unimodal” or “humped.” In the Great Basin of the arid western United States, studies of the relationship between rodent species richness and precipitation have revealed only a positive relationship between these two variables. It has, however, been argued that if areas of higher precipitation were to be sampled within this region, the decline phase would become evident. When latest Pleistocene and Holocene small mammal assemblages from the northern Bonneville Basin (central Utah) are examined across a temporal moisture gradient, species richness declines as moisture declines. Since the Great Basin was significantly moister during the latest Pleistocene and Early Holocene than it has been since that time, the unimodal response model does not appear to apply to this region.     

Gold production history of the United States

Gold has played an important role in the settlement and economics of the United States. Commercial production from 24 states totaled more than 420 million troy ounces (13,000 metric tons) from 1804 through 1995. There were, no doubt, early undocumented discoveries by Native Americans, but the first records are those from the mid-1600's in the east and from the 1770's in the west. Commercial production began in 1804 in North Carolina and spread among several Appalachian states in the 1820's and 1830's as placer deposits were discovered and exploited. After peaking in the 1830's and 1840's, Appalachian production began to decline as deposits were worked out and as miners moved westward in response to the news of the discoveries in California in 1848. Appalachian production virtually ceased with the Civil War while production from California and adjacent states remained at several millions of ounces per year. As the California Gold Rush waned, new discoveries brought the Rocky Mountain states, South Dakota, and Alaska into prominence. Local, state and total production responded to time of discovery, extent of placers, labor availability, technology, government mandates and, of course, the price of gold. By 1900, the major gold rushes had occurred and most of the major producing districts were defined; however, the fixed price of gold (since 1837) provided a decreasingly attractive incentive for more exploration and exploitation. The rise in the price of gold, during the Depression provided a powerful incentive to increase gold production, but one that was cut short by War Production Board Order L-208 which closed the gold mines in 1942. In the post-war period, the fixed price, combined with the prohibition of American gold ownership, again proved to be a disincentive and gold production dropped. The era of free gold price that began in 1968, followed by the permission for Americans to again own gold, brought new interest in gold with prices rising briefly in 1980 to US$850 per ounce and remaining above US$350 per ounce through 1995. This higher price, combined with the technology for bulk mining and processing of low grade ores, provided incentive for new exploration and exploitation. Consequently, gold production soared above 10 million ounces per year in the early 1990's with 65% coming from Nevada. There has been a general decrease in the grade of gold ores since the early 1900's and significant changes in the methods of recovery. Despite the decreasing grade, the reserves of recoverable gold have more than doubled since the early 1970's.     

Electrogeochemical sampling with

Ionization in the polar atmosphere causes the formation of nitrate compounds, which are frozen out and incorporated into the layers of the polar ice sheets. From a 122-m ice core collected in 1992 on the central Greenland ice sheet, it has been possible to examine a solar signal in a ultra-high resolution record of nitrate concentrations. The sequence extends over a period of 415 years at a temporal resolution of no less than one analysis per month (total number of samples 7776 resulting from 1.5 cm sampling along the entire core). This type of measurement reveals major nitrate anomalies which are thought to result from the injection of individual solar proton events into the winter polar stratosphere. For this reason, the large nitrate anomalies provide the possibility to delineate a signal of solar activity well beyond the known geophysical records.     

Ultra-High Resolution Nitrate in Polar Ice as Indicator of Past Solar Activity

Ionization in the polar atmosphere causes the formation of nitrate compounds, which are frozen out and incorporated into the layers of the polar ice sheets. From a 122-m ice core collected in 1992 on the central Greenland ice sheet, it has been possible to examine a solar signal in a ultra-high resolution record of nitrate concentrations. The sequence extends over a period of 415 years at a temporal resolution of no less than one analysis per month (total number of samples 7776 resulting from 1.5 cm sampling along the entire core). This type of measurement reveals major nitrate anomalies which are thought to result from the injection of individual solar proton events into the winter polar stratosphere. For this reason, the large nitrate anomalies provide the possibility to delineate a signal of solar activity well beyond the known geophysical records.     

Instability and Diffusion in the Elliptic Restricted Three-body Problem

The importance of the stability characteristics of the planar elliptic restricted three-body problem is that they offer insight about the general dynamical mechanisms causing instability in celestial mechanics. To analyze these concerns, elliptic–elliptic and hyperbolic–elliptic resonance orbits (periodic solutions with lower period) are numerically discovered by use of Newton's differential correction method. We find indications of stability for the elliptic–elliptic resonance orbits because slightly perturbed orbits define a corresponding two-dimensional invariant manifold on the Poincaré surface-section. For the resonance orbit of the hyperbolic–elliptic type, we show numerically that its stable and unstable manifolds intersect transversally in phase-space to induce instability. Then, we find indications that there are orbits which jump from one resonance zone to the next before escaping to infinity. This phenomenon is related to the so-called Arnold diffusion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measuring crop residue cover using remote sensing techniques

Summary Crop residues are managed under conservation tillage programs to leave as much as possible on the surface for minimization soil erosion and for improving water quality. Because current methods for measuring crop residue cover are tediuous and somewhat subjective, there is a need for new methods to measure residue cover that are rapid, accurate, and objective. We discuss the potential for discriminating crop residues from soils using reflectance and fluorescence techniques and examine experimentally the changes in wheat residue fluorescence during weathering. The fluorescence of crop residue was a board band phenomenon with emissions extending from 420 to 600 nm for excitation of 350–420 nm. Soils had low intensity broad band emissions over the 400–690 nm region for excitations of 300–600 nm. We found that the fluorescence intensities for the crop residues were much greater than the fluorescence of the soils, but as the crop residues decompose, their blue-green fluorescence intensities approach the fluorescence of the soils. We conclude that fluorescence techniques are less ambiguous and better suited for discriminating crop residues from soils than the reflectance methods. However, the potential problems, that must be addressed to implement the fluorescence technique, are (i) adequate excitation energy must be supplied to induce fluorescence and (ii) the fluorescence signal is small relative to normal, ambient sunlight. Nevertheless, if properly implemented, we believe that the fluorescence techniques can be used to quantify crop residue cover in the field.With 7 Figures     

Use of obliquely rotated principal component analysis to identify coherent structures

Principal component analysis (PCA) with oblique rotation is applied to Large Eddy Simulation (LES) results to discern and quantify coherent structures within the convective boundary layer (CBL). Sensitivity tests are first conducted on a moderately convective LES run. Once the ability of PCA to generate robust results is verified, the method is applied to LES runs spanning a range of stability regimes. Interregime similarities and differences in the coherent structures are discussed. For the moderately convective LES run, three-dimensional convective cells are arrayed in two-dimensional bands aligned with the geostrophic wind. The resulting gravity waves in the free atmosphere and convective inflow and outflow in the boundary layer are also captured by the PCA. Convective modes are more sensitive to the ratio of w _* to u _* than are the dynamic modes.PCA has demonstrated advantages over previous analysis methods. PCA score maps provide information on the spatial distribution of phenomena that has not been available from traditional conditional sampling studies. Principal components provide information on the vertical structures of phenomena that would be obscured by life-cycle effects or erratic tilts from the vertical in the conventional approaches to either conditional sampling or composite analysis. Future work includes application of this technique to multi-level observational time series from a surface-layer tower for the Risø Air/Sea Experiment (RASEX).     

COARSE-PARTICLE TRANSPORT IN A GRAVEL-BED RIVER

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