Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum     

N.J. Tosca  S.M. McLennan  J.P. Grotzinger  A.H. Knoll  S.W. Squyres 《Earth and Planetary Science Letters》2005,240(1):122-148

New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe²⁺ and Fe³⁺ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop.  相似文献   

Controlling factors of gullying in the Maracujá Catchment,southeastern Brazil     

L. de A. P. Bacellar  A. L. Coelho Netto  W. A. Lacerda 《地球表面变化过程与地形》2005,30(11):1369-1385

Hundreds of gullies (‘voçorocas’) of huge dimensions (up to 400–500 m long, 150 m wide and 50 m deep) are very common in the small Maracujá Catchment in southeastern Brazil. These erosional features, which occur with an uneven intensity throughout the area, started due to bad soil management practices at the beginning of European settlement, at the end of the 17th century, and nowadays are still evolving, but at a slower rate. As surface soils are usually very resistant to erosion, the outcrop of the more erodible basement saprolites seems to be an essential condition for their beginning. An analysis of well known erosion controlling factors was performed, aiming to explain the beginning and evolution of these gullies and to understand the reasons for their spatial distribution. Data shows that geology and, mainly, geomorphology are the main controlling factors, since gullies tend to be concentrated in basement rock areas with lower relief (domain 2) of Maracujá Catchment, mainly at the fringes of broad and flat interfluves. At the detailed scale (1:10 000), gullies are more common in amphitheatre‐like headwater hollows that frequently represent upper Quaternary gullies (paleogullies), which demonstrate the recurrence of channel erosion. So, gullies occur in areas of thicker saprolites (domain 2), in places with a natural concentration of surface and underground water (hollows). Saprolites of the preserved, non‐eroded hollows are usually pressurized (confined aquifer) due to a thick seal of Quaternary clay layer, in a similar configuration to the ones found in hollows of mass movement (mudflow) sites in southeastern Brazil. Therefore, the erosion of the resistant soils by human activities, such as road cuts and trenches (‘valos’), or their mobilization by mudflow movements, seem to be likely mechanisms of gullying initiation. Afterwards, gullies evolve by a combination of surface and underground processes, such as wash and tunnel erosion and falls and slumps of gully walls. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

A simulation model of morphological,vegetation and sediment changes in ephemeral streams     

J. M. Hooke  C. J. Brookes  W. Duane  J. M. Mant 《地球表面变化过程与地形》2005,30(7):845-866

A model to simulate channel changes in ephemeral river channels and to test the effects of hydrological changes due to climate change and[sol ]or land use change was developed under the auspices of the EU funded MEDALUS programme (Mediterranean Desertification and Land Use). The model, CHANGISM (Channel Change GIS Simulation Model), is designed to simulate the effect of channel flow events and of climate conditions on morphology, sediment and vegetation, through sequences of events and conditions, over periods of up to several decades. The modelling is based on cellular automata but with calculations for water and sediment continuity. Process rules have both deterministic and stochastic elements. An important feature of the model is that it incorporates feedback elements between each event. The main aim of the model is to indicate the likely outcomes of events and combinations of conditions. It is linked to GIS for both input and output. The modelling is based on a channel reach and state is input as GIS layers of morphology (DEM), sediment and vegetation cover and state. Other initial conditions of soil moisture, groundwater level, and overall gradient are input. Parameters for processes are read from tables and can be easily changed for successive runs of the model. The bases for decisions on process specifications are discussed in this paper. Initial tests of the operation and sensitivity of the model were made on idealized reaches. The model was then tested using data from monitored sites in SE Spain. Simulations using clearwater flow worked well but initial simulations using events with sediment loads showed some tendency for excess deposition. Further tests and modifications are taking place. Overall, the model is one of the most sophisticated that simulates the interaction of flows with sediment and vegetation and the outcomes in terms of erosion, deposition, morphology, sediment cover, vegetation cover and plant survival over periods of up to 30 years for the scale of a channel reach. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d'Arolla,Switzerland     

Darrel A. Swift  Peter W. Nienow  Trevor B. Hoey 《地球表面变化过程与地形》2005,30(7):867-883

Proglacial suspended sediment transport was monitored at Haut Glacier d'Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub‐seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice‐bed separation leading to extra‐channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low‐pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less‐efficient drainage. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

A review of paired catchment studies for determining changes in water yield resulting from alterations in vegetation   总被引：21，自引：0，他引：21  

Alice E. Brown  Lu Zhang  Thomas A. McMahon  Andrew W. Western  Robert A. Vertessy   《Journal of Hydrology》2005,310(1-4):28-61

Paired catchment studies have been widely used as a means of determining the magnitude of water yield changes resulting from changes in vegetation. This review focuses on the use of paired catchment studies for determining the changes in water yield at various time scales resulting from permanent changes in vegetation. The review considers long term annual changes, adjustment time scales, the seasonal pattern of flows and changes in both annual and seasonal flow duration curves. The paired catchment studies reported in the literature have been divided into four broad categories: afforestation experiments, deforestation experiments, regrowth experiments and forest conversion experiments. Comparisons between paired catchment results and a mean annual water balance model are presented and show good agreement between the two methodologies. The results highlight the potential underestimation of water yield changes if regrowth experiments are used to predict the likely impact of permanent alterations to a catchment's vegetation. An analysis of annual water yield changes from afforestation, deforestation and regrowth experiments demonstrates that the time taken to reach a new equilibrium under permanent land use change varies considerably. Deforestation experiments reach a new equilibrium more quickly than afforestation experiments. The review of papers reporting seasonal changes in water yield highlights the proportionally larger impact on low flows. Flow duration curve comparison provides a potential means of gaining a greater understanding of the impact of vegetation on the distribution of daily flows.  相似文献   

Spatial and Temporal Cusp Structures Observed by Multiple Spacecraft and Ground Based Observations     

K. J. Trattner  S. A. Fuselier  T. K. Yeoman  C. Carlson  W. K. Peterson  A. Korth  H. Reme  J. A. Sauvaud  N. Dubouloz 《Surveys in Geophysics》2005,26(1-3):281-305

Downward precipitating ions in the cusp regularly exhibit sudden changes in ion energy distributions, forming distinctive structures that can be used to study the temporal/spatial nature of reconnection at the magnetopause. When observed simultaneously with the Polar, FAST, and Interball satellites, such cusp structures revealed remarkably similar features. These similar features could be observed for up to several hours during stable solar wind conditions. Their similarities led to the conclusion that large-scale cusp structures are spatial structures related to global ionospheric convection patterns created by magnetic merging and not the result of temporal variations in reconnection parameters. The launch of the Cluster fleet allows cusp structures to be studied in great detail and during changing solar wind conditions using three spacecraft with identical plasma and field instrumentation. In addition, Cluster cusp measurements are linked with ionospheric convection cells by combining the satellite observations with SuperDARN radar observations that are used to derive the convection patterns in the ionosphere. The combination of satellite observations with ground-based observations during variable solar wind conditions shows that large-scale cusp structures can be either spatial or temporal. Cusp structures can be described as spatial features observed by satellites crossing into spatially separated flux tubes. Cusp structures can also be observed as poleward-traveling (temporal) features within the same convection cell, most probably caused by variations in the reconnection rate at the magnetopause.  相似文献   

The inadequacy of the fish-bearing criterion for stream management     

Kenneth W. Cummins  Margaret A. Wilzbach 《Aquatic Sciences - Research Across Boundaries》2005,67(4):486-491

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Ground water nitrate-nitrogen trends in relation to urban development, Helena, Montana, 1971–2003     

Vivian M. Drake  James W. Bauder 《Ground Water Monitoring & Remediation》2005,25(2):118-130

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Construction of detrital mineral populations: insights from mixing of U–Pb zircon ages in Himalayan rivers     

William H. Amidon  Douglas W. Burbank  George E. Gehrels† 《Basin Research》2005,17(4):463-485

Fission‐track, U–Pb and Pb–Pb analyses of detrital heavy mineral populations in depositional basins and modern river sediments are widely used to infer the exhumational history of mountain belts. However, relatively few studies address the underlying assumption that detrital mineral populations provide an accurate representation of their entire source region. Implicit in this assumption is the idea that all units have equal potential to contribute heavy minerals in proportion to their exposure area in the source region. In reality, the detrital mineral population may be biased by variable concentrations of minerals in bedrock and differential erosion rates within the source region. This study evaluates the relative importance of these two variables by using mixing of U–Pb zircon ages to trace zircon populations from source units, through the fluvial system, and into the foreland. The first part of the study focuses on the Marsyandi drainage in central Nepal, using tributaries that drain single formations to define the U–Pb age distributions of individual units and using trunk river samples to evaluate the relative contributions from each lithology. Observed mixing proportions are compared with proportions predicted by a simple model incorporating lithologic exposure area and zircon concentration. The relative erosion rates that account for the discrepancy between the observed and predicted mixing proportions are then modelled and compared with independent erosional proxies. The study also compares U–Pb age distributions from four adjacent drainages spanning ～250 km along the Himalayan front using the Kolmogorov–Smirnov statistic and statistical estimates of the proportion of zircon derived from each upstream lithology. Results show that, along this broad swath of rugged mountains, the U–Pb age distributions are remarkably similar, thereby allowing data from more localized sources to be extrapolated along strike.  相似文献   

Crustal structure and earthquake focal depths beneath northeastern India and southern Tibet   总被引：3，自引：0，他引：3  

Supriyo Mitra  Keith Priestley  Anjan Kr Bhattacharyya  V. K. Gaur 《Geophysical Journal International》2005,160(1):227-248

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