Models of maar volcanoes,Lunar Crater (Nevada,USA)

Maar volcanoes are generally understood to be the result of highly energetic, explosive interaction between magma and water (groundwater or surface water). Two end-member conceptual models have been proposed to explain the dimensions (diameter, depth) of maar craters: (1) an incremental growth model, where a crater grows due to subsidence and ejection of debris over the course of many explosions, and the final size is an integrated result of multiple explosive events; (2) a model in which the dimensions of a maar crater are the result of the largest single explosion during the lifetime of the maar (major-explosion dominated model). In the latter case, the maar size can be used to estimate the energy and depth of the largest explosion, which in turn allows estimation of the magma mass involved. This paper describes Lunar Crater maar (Nevada, USA) and tests the two models as explanations for the characteristics of the volcano, in particular the major-explosion dominated model. This model implies magma mass and supply rates that are unrealistic, and the tephra at the maar do not contain key features observed in the ejecta at large single-explosion craters. The incremental growth model seems most suitable based upon geological evidence.     

Exposure dating of precariously balanced rocks

Precariously balanced rocks (PBRs) are freestanding boulders that are precarious or fragile in the sense that they could be toppled by relatively low-amplitude earthquake ground motion. They are important in paleoseismology because their continued existence limits the amplitude of ground motion experienced at their location during their lifetime. In order to make quantitative use of PBRs for seismic hazard studies, one must determine when they attained their present state of fragility, that is, the point in time when the contact between the rocks and the pedestals on which they rest was exhumed from surrounding soil and the rock became vulnerable to earthquake ground motions. Cosmogenic-nuclide exposure dating can be used for this purpose, but is complicated because nuclide production occurs throughout exhumation of the PBR, so the apparent exposure age of any part of the rock surface exceeds the time that the rock has actually been precariously balanced. Here we describe a method for determining the length of time that a PBR has been fragile by measuring cosmogenic-nuclide concentrations at several locations on the PBR surface, and linking them together with a forward model that accounts for nuclide production before, during, and after exhumation of the PBR. Fitting model to data yields the rate and timing of rock exhumation and thus the length of time the rock has been fragile. We use this method to show that an example PBR in southern California has been fragile for 18.7 ± 2.8 ka.     

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T _s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr₂O₃ at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.     

Methods for exploring management options to reduce greenhouse gas emissions from tropical grazing systems

Increasing atmospheric concentrations of greenhouse gases are expected to result in global climatic changes over the next decades. Means of evaluating and reducing greenhouse gas emissions are being sought. In this study an existing simulation model of a tropical savanna woodland grazing system was adapted to account for greenhouse gas emissions. This approach may be able to be used in identifying ways to assess and limit emissions from other rangeland, agricultural and natural ecosystems.GRASSMAN, an agricultural decision-support model, was modified to include sources, sinks and storages of greenhouse gases in the tropical and sub-tropical savanna woodlands of northern Australia. The modified model was then used to predict the changes in emissions and productivity resulting from changes in stock and burning management in a hypothetical grazing system in tropical northeastern Queensland. The sensitivity of these results to different Global Warming Potentials (GWPs) and emission definitions was then tested.Management options to reduce greenhouse gas emissions from the tropical grazing system investigated were highly sensitive to the GWPs used, and to the emission definition adopted. A recommendation to reduce emissions by changing burning management would be toreduce fire frequency if both direct and indirect GWPs of CO₂, CH₄, N₂O, CO and NO are used in evaluating emissions, but toincrease fire frequency if only direct GWPs of CO₂, CH₄ and N₂O are used. The ability to reduce greenhouse gas emissions from these systems by reducing stocking rates was also sensitive to the GWPs used. In heavily grazed systems, the relatively small reductions in stocking rate needed to reduce emissions significantly should also reduce the degradation of soils and vegetation, thereby improving the sustainability of these enterprises.The simulation studies indicate that it is possible to alter management to maximise beef cattle production per unit greenhouse gases or per unit methane emitted, but that this is also dependent upon the emission definition used. High ratios of liveweight gain per unit net greenhouse gas emission were found in a broadly defined band covering the entire range of stocking rates likely to be used. In contrast, high values of liveweight gain per unit anthropogenic greenhouse gas emission were found only at very low stocking rates that are unlikely to be economically viable.These results suggest that policy initiatives to reduce greenhouse gas emissions from tropical grazing systems should be evaluated cautiously until the GWPs have been further developed and the implications of emission definitions more rigorously determined.     

Cottagers in an Urban Field

The use of rural areas by the urban population is an important organizing principle in the study of the rural hinterland of urban centers. One of the important land uses identified within the urban field is recreation. This paper focuses on cottage property ownership in one part of the urban field, the rural-recreational countryside. Using three case studies from Canada, it was found that cottage properties account for a large proportion of local residential property. Although most cottage properties are owned by urban residents, up to one-quarter are owned by local residents. Using a questionnaire survey, it was also found that cottage owners are more likely to be older, better educated, and have higher incomes than rural residents.     

Predicting gully erosion using landform evolution models: Insights from mining landforms

Incision as a result of fluvial erosion is an important process to model when simulating landform evolution. For gullies, it is apparent that coupled with the processes that cause incision there must be a range of processes that stop incision. Once started, rills and gullies will grow infinitely without a reduction in support area and/or being arrested by deposition and armouring. Some of these processes have been well studied under the heading of inter-rill erosion. Other limiting processes are related to the shape of the landform and how downstream deposition areas are linked geomorphically to the upstream gullies. Armouring is also an important process that reduces gully incision and extension, where the gully erodes to bedrock and the resistant base limits further development. Post-mining landscapes are new surfaces with new materials and provide the opportunity to examine gully initiation, extension and stabilization. The work presented here has largely been driven by the mining industry, where there has been a need to assess erosion over hazardous wastes like mine tailings and low-level nuclear waste. We demonstrate the usefulness of computer-based landscape evolution models and the more recent soilscape models (that include both surface and subsurface processes) to understand both fluvial and diffusive processes as well as armouring in a digital elevation model framework (as well as landscape evolution). Landscape evolution models provide insights into complex non-linear systems such as gullies. A key need is that of field data to parameterize and validate the models. It is argued that current models have more capability than field data available for parameterization and importantly the validation of model outputs.     

Model Averaging Techniques for Quantifying Conceptual Model Uncertainty

In recent years a growing understanding has emerged regarding the need to expand the modeling paradigm to include conceptual model uncertainty for groundwater models. Conceptual model uncertainty is typically addressed by formulating alternative model conceptualizations and assessing their relative likelihoods using statistical model averaging approaches. Several model averaging techniques and likelihood measures have been proposed in the recent literature for this purpose with two broad categories—Monte Carlo-based techniques such as Generalized Likelihood Uncertainty Estimation or GLUE (Beven and Binley 1992) and criterion-based techniques that use metrics such as the Bayesian and Kashyap Information Criteria (e.g., the Maximum Likelihood Bayesian Model Averaging or MLBMA approach proposed by Neuman 2003) and Akaike Information Criterion-based model averaging (AICMA) (Poeter and Anderson 2005). These different techniques can often lead to significantly different relative model weights and ranks because of differences in the underlying statistical assumptions about the nature of model uncertainty. This paper provides a comparative assessment of the four model averaging techniques (GLUE, MLBMA with KIC, MLBMA with BIC, and AIC-based model averaging) mentioned above for the purpose of quantifying the impacts of model uncertainty on groundwater model predictions. Pros and cons of each model averaging technique are examined from a practitioner's perspective using two groundwater modeling case studies. Recommendations are provided regarding the use of these techniques in groundwater modeling practice.     

The origin of the acidic and basic rocks of the Tichka Massif,Morocco, based on rare earth elements

The Tichka Massif is one of many late-Paleozoic massifs emplaced in northwestern Africa. It consists of granitic pods surrounded by a complex assemblage of dioritic and gabbroic rocks. Previous workers have suggested an independent origin for the granitic and gabbroic rocks and this study evaluates this suggestion using REE data.The Ce/Yb ratio versus Ce plot for the gabbro shows a large Ce/Yb variation for small changes in Ce, whereas the granites show a linear increase of Ce/Yb with Ce. In the granites, the samples with the highest Ce/Yb ratio have the lowest SiO₂, whereas in the gabbros the Ce/Yb ratio is lowest in the low SiO₂ rocks. The diorites show a tightly clustered pattern.Based on the REE data the granite-diorite-gabbro sequence cannot be related by fractional crystallization. Partial melting of a garnet bearing mantle is consistent with the gabbroic trends. Partial melting of a crustal source with small amounts of apatite and abundant hornblende can produce the granitic rocks.     

Evaluating a new landform evolution model: A case study using a proposed mine rehabilitation landform

Landform evolution models are powerful tools for determining long-term erosional stability and denudation rates spanning geological timescales. SIBERIA, CAESAR and CHILD are examples of these model. The newly developed State Space Soil Production and Assessment Model (SSSPAM) coupled soilscape-landform evolution model has the ability to assess overall erosion rates of catchment scale landforms either using short-term precipitation events, variable precipitation or time-averaged precipitation (annual average). In addition, SSSPAM has the capability of developing the subsurface soil profile through weathering and armouring. In SSSPAM, physical processes of pedogenesis such as erosion and armouring, diffusion, sediment deposition and weathering are modelled using a state space matrix approach. In this article we simulate the short-term evolution (100 years) of a proposed post-mining landform using both SIBERIA and SSSPAM and compare the erosion and sediment output results. For the short-term simulations SSSPAM's armouring capability was disabled. The models were then used to simulate the evolution of the catchment for 10,000 years. Results demonstrate that the short-term SSSPAM simulation results compare well with the results from the established landform evolution model SIBERIA. The long-term armouring disabled SSSPAM simulations produces simulated erosion rates comparable with SIBERIA simulations both of which are similar to upper limit of field measured denudation rates. The SSSPAM simulation using armouring demonstrated that armouring reduced the erosion rate of the catchment by a factor of 4 which is comparable with the lower limit of field measured denudation rates. This observation emphasizes the importance of armouring in long-term evolution of landforms. Soil profile cross-sections developed from the same results show that SSSPAM can also reproduce subsurface soil evolution and stratification and spatial variability of soil profile characteristics typically observed in the field.     

Use of rhodamine WT with XAD-7 resin for determining groundwater flow paths

A passive sampling system for use with rhodamine WT (RWT) in groundwater tracing experiments was developed to assist in the characterisation of groundwater flow paths. Amberlite XAD-7 resin was found to be suitable for adsorption of RWT, which can then be extracted using an ethanol/water mix and analysed fluorometrically. Batch and column experiments showed that XAD-7 resin has a high RWT capacity. The adsorption was slightly dependent on pH, but was always above 75% under batch conditions. The resin had a high percentage mass recovery at flow velocities around 1.5 m/day, but this decreased with increasing flow velocities. Desorption of RWT off the resin in water is dependent on the flow velocity of water and the time after the peak RWT has passed. The mass of RWT extracted from the resin bags correlated very well with both the RWT mass flux in the water and the peak concentrations observed in the monitoring wells in a field experiment. The results of resin bags were reproducible in the field with a mean coefficient of variation equal to 16%. This method has been successfully applied to two field situations with different flow velocities to indicate groundwater flow paths. Electronic Publication