New constraints on the late Cenozoic incision history of the New River, Virginia

The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late Cenozoic. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. We used cosmogenic ¹⁰Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. ¹⁰Be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield model cosmogenic exposure ages of 130, 600, and 600–950 ka, respectively, but uncertainties on these ages are not well constrained. These results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits. The exposure ages yield a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 100 m/my. Modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m/my, in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m/my rates of surface erosion from ancient terrace remnants. Fluctuations between aggradation and rapid incision operate on timescales of 10⁴− 10⁵ year, similar to those of late Cenozoic climate variations, though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events. These second-order fluctuations appear within a longer-term signal of dominant aggradation (until 2 Ma) followed by dominant incision. A similar signal is observed on other Appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late Cenozoic.     

An intercomparison between the GSWM, UARS, and ground based radar observations: a case-study in January 1993

The Global-Scale Wave Model (GSWM) is a steady-state two-dimensional linearized model capable of simulating the solar tides and planetary waves. In an effort to understand the capabilities and limitations of the GSWM throughout the upper mesosphere and thermosphere a comparative analysis with observational data is presented. A majority of the observational data used in this study was collected during the World Day campaign which ran from 20 January to 30 January 1993. During this campaign data from 18 ground-based observational sites across the globe and two instruments located on the UARS spacecraft were analyzed. Comparisons of these data with the simulations from the GSWM indicate that the GSWM results are in reasonable agreement with the observations. However, there are a number of cases where the agreement is not particularly good. One such instance is for the semidiurnal tide in the northern hemisphere, where the GSWM estimates may exceed observations by 50%. Through a number of numerical simulations, it appears that this discrepancy may be due to the eddy diffusivity profiles used by the GSWM. Other differences relating to the diurnal tide and the quasi-two-day wave are presented and discussed. Additionally, a discussion on the biases and aliasing difficulties which may arise in the observational data is alos presented.     

The effects of loess erosion on soil nutrients, plant diversity and plant quality in Negev desert wadis

Severe erosion, initiated by climatic changes during the Late Pleistocene-Early Holocene period and resultant declines in dust deposition, causes the formation of waterfalls during the winter floods in many wadi systems in the central Negev desert of Israel. In some areas, erosion of the original loess substrate has been complete, so that the underlying rock has been exposed. We examined the effects of this erosion in four wadis in the central Negev desert on soil nutrients, plant community structure and plant quality. We predicted that erosion has caused reductions in soil nutrients. Reductions in soil nutrients should result in reductions in plant cover. Furthermore, reduced soil nutrient availability should cause reductions in the nutrient status and quality of the plants growing there. In addition to the loss of biodiversity that may result, this erosion may result in economic hardship for the Bedouin peoples whose herds depend on these resources. In this study, there were significant negative effects of erosion on soil organic carbon, nitrate nitrogen and water-holding capacity, but not on soil phosphorus, conductivity or pH. Furthermore, there was a negative effect of soil erosion on an overall measure of soil quality derived from a principal components analysis in three of the four wadis we studied. Erosion resulted in an increase in plant species richness and significantly altered plant community structure in eroded areas of wadis. Increased plant species richness in eroded sites is consistent with the intermediate disturbance hypothesis of plant community structure. Plants growing in eroded areas did not differ in two quality indices (nitrogen content and digestibility), although plants typical of eroded areas had significantly lower levels of common digestion inhibitors (total polyphenols) and toxins (alkaloids) than plants from undisturbed sites. These last-mentioned results are contrary to our prediction and are consistent with the notion that plants growing in disturbed (e.g. eroded) sites maximize growth at the expense of investments in defense.     

Soil and stream sediment geochemical dispersion over the Bell Springs deposit, Hog Ranch Mine, Washoe County, Nevada

The Bell Springs deposit is a bulk-tonnage, low-grade gold deposit, formed in a hot-spring environment, that is hosted by middle Miocene weakly peralkaline, high-silica rhyolite ash flows and rheomorphic tuffs. Ore grade mineralization over about 460 × 460 m was controlled by northeast- and northwest-trending structures. Sixty-nine soil samples collected along a traverse across the Bell Springs deposit, screened into six mesh size fractions, +10, −10+35, −35+80, −80+120, −120+200, and −200, were treated by low-detection-limit acid digestion/organic extraction procedures, and analyzed by ICP or graphite furnace techniques for Au, Ag, As, Bi, Cd, Cu, Ga, Hg, Mo, Pb, Sb, Se, Te, Tl, and Zn. Twenty-five stream-sediment samples were collected from a drainage crossing mineralization and from a nearby tributary. Sediment size fractions of −200 mesh and −10+200 mesh were analyzed by the same procedures used for soils. In addition, analyses of bulk leachable gold via cyanide leach method (BLEG) was done on bulk −10 mesh sediments.Anomalous Au, Ag, As, Sb, and Mo in all soil size fractions revealed the underlying ore. Areas with anomalous Sb and As were about three times larger than areas with anomalous Au and Ag. Analyses of fine-fractions of stream sediments provided the best-defined Au anomalies in drainages. A multi-element factor calculated by multiplying Au, As, Sb, and Mo reinforced anomalous Au in drainages and correlated well with Bell Springs mineralization.     

Quasi-static propagator matrices: Creep on strike-slip faults

This paper presents a method for computing viscoelastic flow in a layered Earth by means of quasi-static propagator matrices. The method has advantages over approximate or purely numerical attacks in that exact, semi-analytical solutions are obtained. The procedure enables a more rapid calculation than is possible with finite elements, yet it does not sacrifice exactness as do analytical approximations. To illustrate the technique, I constructed a plausible model of the San Andreas fault and investigated the time and space behavior of displacement, displacement rate, shear-stress and shear-strain rate at depth as well as at the surface. Viscoelastic relaxation speeds the restressing of the fault. For events of magnitude 6.2 and 6.9, viscoelasticity reduces recharge time relative to the base strain rate by 15% and 50% respectively. For events of magnitude 6.2 and less, viscoelasticity has only a small influence and a linear extrapolation of stress accumulation will predict the time of recharge reasonably well. Relative plate velocities measured within 400 km of the fault are highly variable in space and time. Direct plate velocity measurements made as far as 100 km from a major fault could differ by a factor of two from the average rate. Features of the fault model at depth include: stresses and strain rates which exceed surface values by a factor of three; sign reversals in strain rates; and positive coseismic stress drops induced for limited periods in narrow thin zones. The latter feature could initiate and terminate aftershock sequences. Stress recharge does not occur simultaneously at all depths on the fault for all magnitude events. Recurrence times of earthquakes estimated from surface observations may thus be biased.     

An NMR survey of United States oil shales

Solid state nuclear magnetic resonance techniques were used to measure the fraction of aliphatic carbon in oil shales from United States deposits, ranging in age from Ordovician to Tertiary. The aliphatic carbon fraction was shown to correlate with the fraction of organic matter converted to oil during Fischer assay. The fraction of organic matter convertible to oil is a primary factor in evaluating the oil potential of an oil shale resource. Nuclear magnetic resonance determines this in a nondestructive, easily interpreted bulk analysis of the whole rock.