Response of Tidal Creek Fish Communities to Dredging and Coastal Development Pressures in a Shallow-Water Estuary

To investigate the effects of dredging and associated development pressures (i.e., shoreline armoring, developed land use) on fish, three sets of paired dredged and undredged tidal creeks were surveyed within Lynnhaven River, Virginia. Fish species diversity, community abundance, biomass, and size structure were compared among creeks and related to watershed, shoreline, and physicochemical characteristics. Mean fish community characteristics (e.g., abundance) were similar among creeks; however, species-specific analysis revealed subtle differences. Species biomass differed between dredged and undredged creeks, though species abundance was similar. Turbidity highly influenced differences in species abundance among creeks, while organic matter, dissolved oxygen, turbidity, and shoreline hardening may be influencing biomass patterns. The most recently dredged creek appeared to provide less suitable nursery habitat for some species than historically dredged creeks, suggesting initial adverse effects with eventual recovery. Protective measures, such as preservation of marshes, dredge depth, and time-of-year restrictions, may be moderating development and dredging pressures.     

Variability of groundwater nitrate concentrations over time in arid basin aquifers: sources, mechanisms of transport, and implications for conceptual models

In groundwater of the Trans-Pecos region of West Texas, unexpectedly high levels of nitrate (NO₃ ^?) are documented in four basins: Red Light Draw, Eagle Flats, Wild Horse and Michigan Flats, and Lobo and Ryan Flats. NO₃ ^? concentrations are changing over time in the majority (82.8 %) of wells and are increasing in most (69.8 %). The temporal change raises questions about the potential sources of NO₃ ^? and about flow dynamics in these basins. Presence of NO₃ ^? and temporal variability in concentration has implications beyond contamination risk because it indicates relatively rapid recharge (<60 years) to the basin groundwaters which was not expected based on previous estimates from chloride mass balance models and groundwater age-dating techniques. This research combines existing data ranging back to the 1940s with data collected in 2011 to document a multi-decadal trend of overall increasing NO₃ ^? concentration in deep basin groundwaters. Chlorofluorocarbon analyses of groundwater collected during 2011 indicate the presence of young (<70 years) water in the basins. The authors infer from these data that there are mechanism(s) by which relatively rapid and widespread recharge occurs on the basin floors; that recharge is spatially and temporally variable and that it results from both anthropogenic (irrigated agriculture) and natural (precipitation) sources. In light of these observations, fundamental conceptual models of flow in these basins should be re-evaluated.     

秦汉以来江西省名人分布及其成因

基于历代文献资料,结合历史文献追溯法、地理空间统计和GIS分析等方法,对江西省自秦汉以来的历史名人数量、分布特征、演变成因等方面进行研究。结果表明：从秦汉以来,江西省名人地域分布差异较大,赣中名人类型丰富,赣东北和赣南名人类型较为单一,赣江、抚河、信江中下游地区为名人集中区域。随着历史的演变,名人类型及规模逐渐丰富,名人重心轨迹由赣北向赣中、赣南转移;名人分布与发展不平衡是各地不同的自然条件、经济、教育、交通和人口迁移、宗族、科举家族和好学之风等因素共同作用的结果。     

广西北部山地沼泽20 ka以来的 碳同位素组成与气候变化

根据广西北部山地泥炭沼泽沉积的有机碳同位素组成、TOC和C/N等古气候指标对末次盛冰期以来该区植被与气候演变过程进行研究,结果发现：末次盛冰期（21―14 cal. ka B.P.）偏正的δ13Corg数据反映了山地植被面貌曾发生显著变化,指示冰期气候的冷干变化,但并未导致山地森林向地带性草地退化,C3植物仍占优势。沉积物岩性和TOC等数据表明：在14―10 cal. ka B.P.期间沼泽湿地逐渐形成,指示了东亚夏季风降雨的增强,与石笋指示的季风增强起始年代基本一致。冰消期碳同位素曲线负偏的变化要早于TOC指示的泥炭沼泽堆积,即10―9 cal. ka B.P.前后富有机质的泥炭堆积进入旺盛期。碳同位素组成在9―8 cal. ka B.P.存在一个正偏的变化波动,可能与8.2 ka降温事件或局地物源输入变化有关。7.0―2.8 cal. ka B.P.期间古田湿地的有机质输入较高,碳同位素偏负,指示气候暖湿。从1.7 cal. ka B.P.至今,δ13Corg值的再次正偏可能指示夏季风强度减弱,但考虑到该阶段为人类农业活动快速发展期,因此不排除人为因素的影响。此外,末次盛冰期广西山地与雷州半岛沿海低地的碳同位素值对比发现：沿海平原区与华南山地的沉积记录存在明显差别,其主要原因是山区因山地降雨效应使山区的湿度即便在盛冰期也能保持较高水平;而低地平原冰期阶段的降雨则显著减少,加上平原区较高的蒸发量使干旱程度增加,C4草本也相应增多。实际上,影响亚热带山地沼泽δ13Corg值变化的原因比较复杂,在利用δ13Corg值推论古气候时需要同时考虑气候因素和湿地沉积体的物源及其地貌环境等背景。     

Anomalous seismic crustal structure of oceanic fracture zones

Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment.     

Early agriculture in the Eastern Grand Canyon of Arizona,USA

Abandoned fields in Colorado River alluvium in the eastern Grand Canyon show signs of primitive agriculture. Presence of maize pollen in association with buried soils near Comanche Creek suggests that farming began prior to 3130 yr B.P. Cotton pollen, identified in buried soils near Nankoweap Creek, dates to 1310 yr B.P., approximately 500 years earlier than previously reported anywhere on the Colorado Plateau. Farming spanned three millennia in this reach of the canyon. Entrenchment, starting approximately 700 yr B.P., making water diversion to fields infeasible, was likely responsible for field abandonment. © 2000 John Wiley & Sons, Inc.     

Analyzing Changes in Urban Form and Commuting Time*

Urban work trips have changed in important ways during the last decades. In Québec City, a medium‐sized Canadian metropolitan area, commuting distances increased for both male and female workers between 1977 and 1996, while durations increased for male workers and decreased for female workers. This article seeks to identify spatial and social factors responsible for these changes. We develop a disaggregate model of trip duration estimated on the basis of large samples derived from travel surveys comparable through time. Using categorical variables to specify change, we are able to separate change effects from level effects attributable to various dimensions of urban form. Our analysis clearly indicates that, once travel mode and key social factors are controlled for, the shift from a monocentric to a dispersed city form is responsible, in the Québec metropolitan area, for increasing commuting time. This is contrary to findings in larger metropolitan areas, where, it has been argued, the suburbanization of jobs maintains stability in commuting duration.