Late Quaternary Floodplain History of the Brazos River in East-Central Texas

The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.     

Evidence for volatile-influenced differentiation in a layered alkali basalt flow, Penghu Islands, Taiwan

An approximately 20-m-thick alkali basalt flow on the Penghu Islands contains ∼20 cm thick, horizontally continuous (>50 m), vesicular layers separated by ∼1.5 m of massive basalt in its upper 8.5 m. The three layers contain ocelli-like "vesicles" filled with nepheline and igneous carbonate. They are coarse grained and enriched in incompatible elements relative to the massive basalt with which they form sharp contacts. These vesicular layers (segregation veins) formed when residual liquid in the underlying crystal mush was forced (gas filter pressing) or siphoned into three thermally induced horizontal cracks that opened successively in the advancing crystal mush of the flow's upper crust. Most vesicular layer trace elements can be modelled by residual melt extraction after 25–40% fractional crystallization of massive basalt underlying each layer. Sulphur, Cl, As, Zn, Pb, K, Na, Rb, and Sr show large concentration changes between the top, middle, and bottom layers, with each vesicular and underlying massive basalt forming a chemically distinct "pair." The large changes between layers are difficult to account for by crystal fractionation alone, because other incompatible elements (e.g., La, Sm, Yb, Zr, Nb) and the major elements change little. The association of these elements (S, Cl, etc.) with "fluids" in various geologic environments suggests that volatiles influenced differentiation, perhaps by moving alkali, alkaline earth, and chalcophile elements as magma-dissolved volatile complexes. Volatiles may have also led to large grain sizes in the segregation veins by lowering melt viscosities and raising diffusion rates. The chemical variability between layers indicates that a convection and concentration mechanism acted within the flow. The specific process cannot be determined, but different rates of vesicle plume rise (through the flow) and/or accumulation in the upper crust's crystal mush might account for the chemical pairing and extreme variations in Cl, S, As, and C. This study emphasizes the importance of sampling vesicular rocks in flows. It also suggests that volatiles play important physical and chemical roles in rapidly differentiating mafic magmas in processes decoupled from crystal fractionation. Received: 11 November 1996 / Accepted: 20 September 1998     

Uncertainty assessment in river flow projections for Ethiopia’s Upper Awash Basin using multiple GCMs and hydrological models

ABSTRACT

Uncertainty in climate change impacts on river discharge in the Upper Awash Basin, Ethiopia, is assessed using five MIKE SHE hydrological models, six CMIP5 general circulation models (GCMs) and two representative concentration pathways (RCP) scenarios for the period 2071–2100. Hydrological models vary in their spatial distribution and process representations of unsaturated and saturated zones. Very good performance is achieved for 1975–1999 (NSE: 0.65–0.8; r: 0.79–0.93). GCM-related uncertainty dominates variability in projections of high and mean discharges (mean: –34% to +55% for RCP4.5, – 2% to +195% for RCP8.5). Although GCMs dominate uncertainty in projected low flows, inter-hydrological model uncertainty is considerable (RCP4.5: –60% to +228%, RCP8.5: –86% to +337%). Analysis of variance uncertainty attribution reveals that GCM-related uncertainty occupies, on average, 68% of total uncertainty for median and high flows and hydrological models no more than 1%. For low flows, hydrological model uncertainty occupies, on average, 18% of total uncertainty; GCM-related uncertainty remains substantial (average: 28%).     

Computer simulation of Pb and Sr isotope evolution of the Earth's crust and upper mantle

Isotope evolution in a differentiated (crust and upper mantle) and chemically heterogeneous Earth has been computed for a model with isotopic exchange between crust and mantle at an exponentially decreasing rate. To simulate the effects of subduction, cells of crust and mantle are selected at random, their contents mixed and then redistributed into the cells from which they came—to give new chemically heterogeneous but momentarily isotopically homogeneous systems. Daughter isotopes in each cell grow according to equations appropriate for closed chemical systems. Rock age distributions and isotopic data created by the computer calculation mimic nature. Pb isotope data changes through geologic time are illustrated to demonstrate that two-stage interpretations applied to Pb data for rocks with complex histories may be misleading. The intercept of the Pb ore growth curve and regressions fit to Pb data gives minimum values only for the duration of heterogeneous $\text{U}\text{Pb}$ systems, not the time when heterogeneous distribution first occurred. An intercept derived time of about 3 b.y., in the Pb system is shown to be quantitatively compatible with an average $\text{Rb}\text{Sr}$ age of crustal rocks of 1.5 b.y. and with a constant degree of chemical heterogeneity for all of Earth history.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

小波变换及其在影像表示中的应用和其方向选择性算法的改进

小波理论是研究多分辨率信号处理的理论,目前被公认为有前途的新理论。金字塔结构的影像处理、影像理解,现代GIS处理多比例尺的要求以及地形、场论中的逼近均需有关这一理论的支持。本文利用小波变换分解原始影像以实现影像的小波表示,并发展为任意方向的小波变换、逆变换,利用它检测航片中建筑物及其阴影的公共边缘。这种表示方式可为某些影像特征或识别目标提供较为经济的分辨率,该表示方式使影像中具有一定层次的结构更易于发现和利用。实验结果证明了此方法是有效的。     

Integrated land-use systems: Assessment of promising agroforest and alternative land-use practices to enhance carbon conservation and sequestration

Degraded or sub-standard soils and marginal lands occupy a significant proportion of boreal, temperate and tropical biomes. Management of these lands with a wide range of existing, site-specific, integrated, agroforest systems represents a significant global opportunity to reduce the accumulation of greenhouse gases in the atmosphere. Establishment of extensive agricultural, agroforest, and alternative land-use systems on marginal or degraded lands could sequester 0.82–2.2 Pg carbon (C) per year, globally, over a 50-year time-frame. Moreover, slowing soil degradation by alternative grassland management and by impeding desertification could conserve up to 0.5–1.5 Pg C annually. A global analysis of biologic and economic data from 94 nations representing diverse climatic and edaphic conditions reveals a range of integrated land-use systems which could be used to establish and manage vegetation on marginal or degraded lands. Promising land-use systems and practices identified to conserve and temporarily store C include agroforestry systems, fuelwood and fiber plantations, bioreserves, intercropping systems, and shelterbelts/windbreaks. For example, successful establishment of low-intensity agroforestry systems can store up to 70 Mg C/ha in boreal, temperate and tropical ecoregions. The mean initial cost of soil rehabilitation and revegetation ranges from $500–3,000/ha for the 94 nations surveyed. Natural regeneration of woody vegetation or agro-afforestation establishment costs were less than $1000/ha in temperate and tropical regions. The costs of C sequestration in soil and vegetation systems range from $1-69/Mg C, which compares favorably with other options to reduce greenhouse gas emissions to the atmosphere. Although agroforestry system projects were recently established to conserve and sequester C in Guatemala and Malaysia, constraints to wide-spread implementation include social conditions (demographic factors, land tenure issues, market conditions, lack of infrastructure), economic obstacles (difficulty of demonstrating benefits of alternative systems, capital requirements, lack of financial incentives) and, ecologic considerations (limited knowledge of impacts and sustainability of some systems).The information in this document has been funded by the U.S. Environmental Protection Agency. It has been subject to the Agency's peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement for use.     

Using PETRIMES to estimate mercury deposits in California

In this article, we examine the use of an unconventional procedure, PETRIMES, to estimate mineral resources of mercury deposits in California. The study, which is based on the nonparametric discovery process model and Q-Q plots, suggests that a lognormal distribution is appropriate for the mercury deposits in California. The results of the assessment are summarized as follows: (1) the total number of mercury deposits in the population is approximately 165; (2) the median value of the largest undiscovered deposit size is 487 flasks; (3) the mean of the remaining mercury potential is 2,500 flasks; and (4) the population resource ranges from 1,040,000 to 4,300,000 flasks (at a 0.9 probability level).     

A Method for the Exploratory Analysis of Airline Networks

The airline industry in the United States has experienced significant changes in the spatial configuration of networks since the Airline Deregulation Act of 1978. Attempts have been made to study the structure of the hub-and-spoke network in the airline industry, but no efficient methodology has been available. This paper describes an exploratory approach to analyzing the spatial configuration of airline networks. Flight frequencies and numbers of passengers from published schedules of six U.S. domestic airlines (American, Continental, Delta, Northwest, United, and USAir) are used in this study. The analytic components in the exploratory system include the examination of network connectivity and network autocorrelation in a dynamic mapping environment. The results of the analyses show both the overall spatial patterns of airline networks and the hierarchical hubbing structures.     

Role of slope stability in cumulative impact assessment of hydropower development: North Cascades, Washington

Two environmental assessments considered the potential cumulative environmental impacts resulting from the development of eight proposed hydropower projects in the Nooksack River Basin and 11 proposed projects in the Skagit River Basin, North Cascades, Washington, respectively. While not identified as a target resource, slope stability and the alteration of sediment supply to creeks and river mainstems significantly affect other resources. The slope stability assessment emphasized the potential for cumulative impacts under disturbed conditions (e.g., road construction and timber harvesting) and a landslide-induced pipeline rupture scenario. In the case of smallscale slides, the sluicing action of ruptured pipeline water on the fresh landslide scarp was found to be capable of eroding significantly more material than the original landslide. For large-scale landslides, sluiced material was found to be a small increment of the original landslide. These results predicted that hypothetical accidental pipeline rupture by small-scale landslides may result in potential cumulative impacts for 12 of the 19 projects with pending license applications in both river basins.