Spatial Characteristics of Regional Wildfire Frequencies in Intermountain West Grass-Dominated Communities

Understanding the environmental factors that influence spatial patterns of rangeland wildfires facilitates both fire and land management decisions. I used discriminant analysis to examine ignition frequency of lightning-initiated fires in grass-dominated communities in the Intermountain West between 1980 and 1994, then mapped regional fire frequencies to illustrate spatial patterns. Two canonical discriminant functions effectively separate groups of high, medium, and low ignition frequencies, based on climate conditions and fuel characteristics. Regions of highest frequency tend to have large elevational differences, more mesic climates, and less annual grass cover. Spatial patterns of ignition frequencies tend to reflect local topography, with higher frequencies west of 119° W. Expansion of exotic annual grasses throughout the region may be reducing differences in ignition frequency between regions of highest and lowest frequency.     

Migration velocity analysis using a genetic algorithm

Migration velocity analysis, a method for determining long wavelength velocity structure, is a critical step in prestack imaging. Solution of this inverse problem is made difficult by a multimodal objective function; a parameter space often vast in extent; and an evaluation procedure for candidate solutions, involving the calculation of depth-migrated image gathers, that can be prohibitively expensive. Recognizing the global nature of the problem, we employ a genetic algorithm (GA) in the search for the optimum velocity model. In order to describe a model efficiently, regions of smooth variation are identified and sparsely parametrized. Region boundaries are obtained via map migration of events picked on the zero-offset time section. Within a region, which may contain several reflectors, separate components describe long and short wavelength variations, eliminating from the parameter space, models with large velocity fluctuations. Vital to the success of the method is rapid model evaluation, achieved by generating image gathers only in the neighbourhood of specific reflectors. Probability of a model, which we seek to maximize, is derived from the flatness of imaged events. Except for an initial interpretation of the zero-offset time section, our method is automatic in that it requires no picking of residual moveout on migrated gathers. Using an example data set from the North Sea, we show that it is feasible to solve for all velocity parameters in the model simultaneously: the method is global in this respect also.     

Changes in Surface Morphology of Calcite Exposed to the Oceanic Water Column

Reactions occurring on the surfaces of biogenic carbonate minerals can have important consequences for the biogeochemical cycle of carbon. In this study, carbonate mineral surface reactions with ambient seawater were investigated by atomic force microscopy (AFM). A sampling method was developed in which calcite surfaces were hung at discrete depths on a sediment trap array line for a three-day deployment period in subtropical North Pacific waters. Changes in surface morphologies were examined at nanometer resolution and evaluated using as a constraint the depth profile of calcite saturation in these waters. Evidence suggests that: (1) organic films which develop on carbonate surfaces exposed to shallow seawater may be responsible for the oversaturated state of the upper oceanic water column, (2) dissolution of carbonate minerals within the shallow warm layer of the ocean could be responsible for part of the alkalinity anomaly observed in the North Pacific.     

A Raman spectroscopic study of shocked single crystalline quartz

We have carried out a Raman Spectroscopic study of single crystalline quartz samples shocked to peak pressures up to 31.4GPa. Samples shocked to above 22 GPa show shifts in peak positions consistent with the quartz being under tensile stress, and new broad bands associated with the formation of high density SiO₂ glass appear in the spectra. These changes are accompanied by an increase in the lattice parameters of the quartz. Formation of the diaplectic glass could be due to a metastable melting event, or spinodal lattice collapse on attainment of a mechanical stability limit of crystalline quartz, as suggested by previous studies of pressure-induced amorphization in static pressurization experiments on SiO₂ and GeO₂ polymorphs.     

Alkaline and calc-alkaline lavas near Los Volcanes,Jalisco, Mexico: geochemical diversity and its significance in volcanic arcs

A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K₂O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO₂, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO₂ contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle.     

Avalanche Climatology of the Western United States,with an Emphasis on Alta,Utah*

Climate and snowpack characteristics of avalanches vary spatially across the western United States, distinguishing three regions. The coastal mountain renges have warmer, denser snow; interior (continental) ranges have colder, less-dense snow; and intermountain ranges have intermediate characteristics. Avalanche character of Alta, Utah, is related to eastern Pacific 700-mb height anomalies for December, January, and March, but not for February. Avalanche conditions around Alta do not always relate to localized pressure gradient winds for December and February.     

Investigation of Possible Contamination of Shallow Ground Water by Deeply Injected Liquid Industrial Wastes

The objective of this study was to assess the possible impact of deep well disposal operations, conducted between 1958 and 1974, on the ground water quality in a shallow fresh water aquifer beneath Sarnia, Ontario, Canada. Because of the breakout of formation fluids in Sarnia and Port Huron, Michigan, in the early 1970s, it had been hypothesized that liquid waste from the disposal zone in bedrock had leaked through numerous abandoned oil, gas, and salt wells in the area up to the shallow fresh water aquifer and from there to the surface.
A monitoring well network of 29 5cm (2 inch) diameter piezometers was established in the thin sand and shale aquifer system, which exists between 30 and 70m (100 and 230 feet) below ground surface. In addition, a 300m (1000 foot) deep borehole was drilled and instrumented with a Westbay multilevel casing, which permitted sampling of the disposal zone.
Ground water samples from the shallow monitoring wells and the Westbay multilevel casing were analyzed for volatiles by GC/MS. Those volatile aromatics that were conspicuously present in the deep disposal zone, e.g., ethyl toluenes and trimethyl benzene, were not detected in the shallow monitoring wells. Thus, if contaminants from the disposal zone did indeed migrate to the shallow aquifer, contamination was not widespread and probably consisted mostly of displaced chloride-rich formation waters.     

Small-Scale Retrospective Ground Water Monitoring Study for Simazine in Different Hydrogeological Settings

A ground water monitoring study was conducted for the triazine herbicide simazine at 11 sites in the United States. The study used carefully selected, small-scale sites (average size: about 33 acres) with documented product use and sensitive hydrogeological settings. The sites selected were Tulare County, California (two sites); Fresno County, California; Sussex County, Delaware; Hardee and Palm Beach counties, Florida; Winnebago County, Illinois; Jackson County, Indiana; Van Buren and Berrien counties, Michigan; and Jefferson County, West Virginia. These sites satisfied the following criteria: a history of simazine use, including the year prior to the start of the study; permeable soil and vadose zone; shallow depth to water; no restrictive soil layers above the water table; and gentle slopes not exceeding 2 percent. A variety of crop types, climates, and irrigation practices were included. Monitoring well clusters (shallow and deep) were installed at each site except in California and West Virginia, where only shallow wells were installed. Simazine was monitored at these sites at quarterly intervals for a two-year period during 1986–1988.
The results of the study showed that out of 153 samples analyzed, 45 samples showed simazine detections. A substantial majority of the detections (32 out of 45) occurred in Tulare, Fresno, and Jefferson counties. The detections in these areas were attributed to mechanisms other than leaching, such as drainage wells, karst areas, surface water recharge, or point source problems. An additional 11 detections in Van Buren County were apparently due to an unknown upgradient source. Only one detection (in Palm Beach County, Florida) near the screening level of 0.1 ppb was attributed to possible leaching. The results of this investigation support the hypothesis that simazine does not leach significantly under field use conditions.