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

Michael R. Waters  Lee C. Nordt 《Quaternary Research》1995,43(3)

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.  相似文献   

Ground Based FTIR Measurements of Stratospheric Species from Harestua, Norway During Sesame and Comparison with Models     

B. Galle  J. Mellqvist  D.W. Arlander  I. Fløisand  M.P. Chipperfield  A.M. Lee 《Journal of Atmospheric Chemistry》1999,32(1):147-164

Vertical columns of HF, HCl, HNO3, ClONO2, N2O, ClO and COF2 were measured at Harestua, Norway (60.22° N, 10.75° E, Elevation 600 a.s.l.) beginning on 24 November 1994 and concluding on 1 May 1995 during Phase-III of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) measurement campaign. The vertical columns of HCl, HNO3 and ClONO2 measured on 81 days were compared with columns calculated by the 3-D Cambridge model SLIMCAT. In addition the results were also interpreted by comparison with a photochemical trajectory model. Good agreement was seen for HCl while the nitrogen compounds showed larger discrepancies, especially for ClONO2. Evidence for chlorine activation was seen with 65% reduction of the chlorine reservoirs (HCl + ClONO2) while the levels of ClO were greatly enhanced. Interpretation of the loss with the trajectory model indicated condensation of chlorine on PSCs. The vertical column ratio of COF2 and HF was measured to 0.21 outside the vortex and a factor of two lower inside. The recovery of ClONO2 was seen to be much faster than that of HCl in the early spring.  相似文献   

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

John D. Greenough  Chi-Yu Lee  Brian J. Fryer 《Bulletin of Volcanology》1999,60(6):412-424

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  相似文献   

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

Richard Lee Armstrong  Steven M Hein 《Geochimica et cosmochimica acta》1973,37(1):1-18

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.  相似文献   

Integrated land-use systems: Assessment of promising agroforest and alternative land-use practices to enhance carbon conservation and sequestration   总被引：1，自引：1，他引：1  

Robert K. Dixon  Jack K. Winjum  Kenneth J. Andrasko  Jeffrey J. Lee  Paul E. Schroeder 《Climatic change》1994,27(1):71-92

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.  相似文献   

Using PETRIMES to estimate mercury deposits in California     

Lee  P. J.  Singer  Donald A. 《Natural Resources Research》1994,3(3):190-199

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).  相似文献   

A Method for the Exploratory Analysis of Airline Networks     

Jay Lee  Ligang Chen  Shih-Lung Shaw 《The Professional geographer》1994,46(4):468-477

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.  相似文献   

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

R. R. Lee  W. P. Staub 《Environmental Geology》1993,21(4):212-214

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.  相似文献   

Mineralogy and petrography of HAL,an isotopically-unusual Allende inclusion     

John M. Allen  Lawrence Grossman  Typhoon Lee  G.J. Wasserburg 《Geochimica et cosmochimica acta》1980,44(5):685-699

A detailed mineralogical study of HAL was initiated to elucidate the origin of this inclusion because Leeet al. (1979) had found large mass fractionation effects and small nuclear effects in its Ca isotopic composition, but no ²⁶Mg excesses in samples of it with very high ${}^{27}\text{Al}{}^{24}\text{Mg}$ ratios. HAL's 1.0 mm radius interior consists almost entirely of three hibonite crystals and is surrounded by a 2.0 mm thick, multi-layered rim. The first layer, called the black rim, is black and compact, resembles a devitrified glass and contains an anisotropic Al-Fe-oxide similar to hercynite in composition. This is followed by a friable rim sequence, layer I of which is predominantly feldspathoids with minor anorthite, Ti-Fe-oxide and Al-Fe-oxide. Layer II contains abundant perovskite, plus grossular, andradite and pyroxene in addition to the minerals of layer I. Layer III is mostly Ca-phosphate, possibly hydroxy-apatite and perovskite. Layer IV is rich in hibonite, Al-Fe-oxide, perovskite, nepheline and the two garnets, lacks Ca-phosphate but contains traces of a Ti-Sc-Zr-oxide. Layer V is rich in Al-Fe-oxide, pyroxene, nepheline, the two garnets and olivine whose crystals display peculiar rectangular cross-sections. The black rim does not completely surround the hibonite core. Sectors of the friable rim exist where layer I is missing and where the mineralogy of adjacent layers is no different from that of the same layers in other sectors. Pentlandite, nickel-iron and barrel-shaped olivine crystals, minerals typical of the matrix of Allende and found nowhere else in HAL, are found in layer V and increase in abundance toward its exterior, as if grains of these phases accreted together with the other minerals of layer V. This layer also contains alternating olivine-rich and garnet-, pyroxene-rich bands, resembling rhythmic layering. For these reasons, we conclude that each of the layers of the friable rim formed by the accretion of an assemblage of condensate grains rather than by complete reaction of a precursor to HAL with a nebular gas. Thus, the unusual isotopic characteristics of HAL are thought to have been inherited from a nebular reservoir which was isotopically distinct from that which gave rise to the bulk of the material in Allende. HAL's mineralogical peculiarities indicate that its formation reservoir was also chemically distinct from the latter one.  相似文献   

The city in an era of restricted car usage: Some potential responses and adjustments to future oil shortages     

T.R. Lee  L.J. Wood 《Geoforum》1980,11(1):17-29

This paper examines some of the possible changes in city form and function which may result from decreased personal mobility in the future due to oil shortages. The empirical dimension is provided by the results of studies of the observed response of the suburban population of Hobart, Tasmania, which suffered severe disruption in personal mobility and a marked reduction in car usage following the collapse of the Tasman Bridge in January 1975. In addition to a variety of relatively minor, or first-order, adjustments, a number of second-order adjustments were identified involving changes in job location and movement of residence. If undertaken at sufficient scale, such adjustments could lead to major changes in the social-spatial structure of cities, with gentrification of inner areas resulting in the displacement of the poor outwards to more inaccessible locations.  相似文献