Compositions of post-Miocene basalts erupted in the Garibaldi and Central America volcanic arcs exhibit significant correlations with the age of the subducted plate. In general, SiO2, Al2O3, CaO, V, and (Sr/P)N decrease and FeO, MgO, TiO2 and Na2O increase as the age of the subducted plate decreases. Variations in CaO/Al2O3, SiO2, (Sr/P)N, and Ba are compatible with lesser slab input, and hence less hydrous melting conditions in the mantle wedge in segments of the arcs overlying the youngest oceanic lithosphere. This interpretation is supported by comparison with peridotite melting experiments, which suggest higher melt pressures and temperatures in the mantle wedge above very young oceanic lithosphere. These observations point to a model in which dehydration of the downgoing slab occurs at shallow depths in subduction systems involving oceanic lithosphere younger than about 20 Ma. Because young oceanic lithosphere is relatively warm, little post-subduction heating is required to produce metamorphic reactions that release slab volatiles. Geodynamic models indicate most volatile-liberating reactions will occur within the seismogenic zone in oceanic lithosphere younger than 20 Ma, thus limiting the volatile flux beneath the arc and encouraging drier, higher temperature and higher pressure melting conditions in the mantle wedge in comparison to typical arc systems. Liberation of volatiles in the downgoing plate is strongly dependant on the shear stress on the fault, but is predicted to occur within the seismogenic zone for shear stresses greater than 33 MPa. Similarly, early loss of volatiles is predicted over a wide range of convergence rates, plate dips, and convergence angles. These results are shown to be robust for realistic ranges of slab dip, convergence angle, and shear stress, suggesting that volatile-poor melt generation is a characteristic of modern and ancient arc systems that involve subduction of young oceanic lithosphere. 相似文献
Hydroelectric reservoirs generate energy without significant combustion of fossil fuels. However, these systems can, potentially,
emit greenhouse gases (GHG’s) at a rate which may be significant at the global scale, and, possible, co-equal, per kilowatt-hour,
to that from conventional coal or oil-fired systems. Although much of the new construction of hydroelectric reservoirs is
in the tropics, most of the data on GHG emissions comes from temperate regions. Further, much of the existing data on reservoir
gas emissions comes from single sites, usually near the terminal dams. Large tropical reservoirs often involve the impoundments
of river systems with complex morphology which in turn can cause spatial heterogeneity in gas flux. We evaluated spatial and
seasonal variability in CO2 concentrations and gas flux for five large (50–1,400 km2) reservoirs in the Cerrado region of Brazil. Most of data set (87% of all measurements) showed CO2 supersaturation and net efflux to the atmosphere. There was as much or more variation in pCO2 over space and among seasons. The large studied reservoirs showed different zones in terms of CO2 emission because those fluxes are dependent on flooded biomass, watershed input of organic matter and dam operation regime.
Here we demonstrate that the reservoirs in the Brazilian Cerrado have low rates of CO2 emissions compared to existing global comparisons. Our results suggest that ignoring the spatial variability can lead to
more than 25% error in total system gas flux. 相似文献
The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (Xi) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between Xi and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξss ≤ ξmax) that depends on input fluid composition (ξmax is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with Xi homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξss in all domains except upstream from the slowest moving front where ξ = ξss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξmax) are best reproduced by down-temperature infiltration of a disequilibrium fluid with X\textCO2 = 0. 1 9 6 X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 into a multi-domain medium with uniform X\textCO2 X_{{{\text{CO}}_{2} }} at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm2 rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature. 相似文献
Capturing the spatial and temporal correlation of multiple variables in a weather generator is challenging. A new massively multi-site, multivariate daily stochastic weather generator called IMAGE is presented here. It models temperature and precipitation variables as latent Gaussian variables with temporal behaviour governed by an auto-regressive model whose residuals and parameters are correlated through resampling of principle component time series of empirical orthogonal function modes. A case study using European climate data demonstrates the model’s ability to reproduce extreme events of temperature and precipitation. The ability to capture the spatial and temporal extent of extremes using a modified Climate Extremes Index is demonstrated. Importantly, the model generates events covering not observed temporal and spatial scales giving new insights for risk management purposes. 相似文献
Historical evidence of early salinity, vegetation and landuse changes, and pedological studies have been used in formulating a new model for salinity processes acting on the Dundas Tableland in southwestern Victoria. Contrary to previous assumptions, salinity in this area was a feature of the pre‐European landscape and was noted in the earliest surveys and journals. Analysis of historical records show an initial post‐settlement increase in the tree numbers, followed by a rapid decline much later than previously assumed. Accumulation of salts in the regolith may be attributed to marine incursions during the Miocene and Pliocene, the extensive weathering to develop a deep regolith, and wind‐blown and cyclic salt accumulation. A trend analysis of historical streamflow and bore hydrograph records does not indicate rising groundwater levels. The pedological features of duplex and sodic soil profiles support a history of prolonged seasonal waterlogging. A model with seasonal lateral flow of water through the upper regolith can better account for the spread of salinity than the rising groundwater hypothesis. By control of waterlogging, land managers could improve soil structure, enhance root growth and soil water use, as well as inhibit the spread of salinity. 相似文献
Climatic Change - Anomaly-diffusing energy balance models (AD-EBMs) are routinely employed to analyze and emulate the warming response of both observed and simulated Earth systems. We demonstrate a... 相似文献
HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE. 相似文献
Gravity, magnetic, and seismic surveys were conducted across the Wasatch fault zone east of Springville, Utah, near the mouth of Hobble Creek Canyon. The geophysical data were acquired, processed, and interpreted to determine possible locations of larger [total offset greater than 6 ft (1.8 m)], shallow normal faults within the fault zone. Interpretations of the individual data sets were integrated to help eliminate spurious readings and to strengthen the interpretations. Visual methods of integration, along with computer modeling, were chosen for this study. Furthermore, the geophysical data were correlated and integrated with available trench data and surface data. In addition to verifying locations of known faults, the geophysical surveys detected numerous possible additional faults not previously mapped. Of particular interest is a newly discovered graben structure near the southern end of the site, where building of new homes has recently been proposed.
New structural information about fault densities and styles was also determined from the surveys. The fault concentration for this site is 1.3 faults/100 ft (30.5 m), or one fault per 77 ft (23.5 m). Interpreted antithetic faults at the Hobble Creek site account for 65% of the total, while synthetic faults account for 35% with respect to the main fault strand.
Information derived from this study should be useful during planning and development of areas within the Wasatch fault zone. The characteristics of subsurface deformation can be used to gain a better understanding of the potential for surface rupture at a given site. This is also useful in planning appropriate site development and remedial measures to help mitigate hazards associated with large-magnitude earthquakes. 相似文献