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41.
J. R. L. Allen 《Geological Journal》1982,17(1):31-47
On the southwest-facing slopes of a bedrock ridge lying between Cardigan Bay to the north and the Afon Teifi to the south stands a group of hills in which 30-35 m of cross-laminated and parallel-laminated sands with lenticular upward-fining gravel sequences are overlain by 10-12 m of gravel in a single foreset bed. The sediments mantle a surface of till sloping gently toward the southwest, were transported toward the southwest (across one margin of the Afon Teifi valley), and were cut by a system of densely arranged conjugate normal faults striking northwest-southeast. The lenticular gravels and fault system suggest that the deposits accumulated as a glaciofluvial outwash spread, and on top of an ice-lobe that became isolated in the Teifi valley during the downwasting of a glacier which had occupied Cardigan Bay and much of the country to the south. The large gravel foresets capping the succession are the only indication at Banc-y-Warren of the former existence of a lake, but neither a large nor deep body of water need be envisaged. 相似文献
42.
Gorka Merino Manuel Barange Julia L. Blanchard James Harle Robert Holmes Icarus Allen Edward H. Allison Marie Caroline Badjeck Nicholas K. Dulvy Jason Holt Simon Jennings Christian Mullon Lynda D. Rodwell 《Global Environmental Change》2012,22(4):795-806
Expansion in the world's human population and economic development will increase future demand for fish products. As global fisheries yield is constrained by ecosystems productivity and management effectiveness, per capita fish consumption can only be maintained or increased if aquaculture makes an increasing contribution to the volume and stability of global fish supplies. Here, we use predictions of changes in global and regional climate (according to IPCC emissions scenario A1B), marine ecosystem and fisheries production estimates from high resolution regional models, human population size estimates from United Nations prospects, fishmeal and oil price estimations, and projections of the technological development in aquaculture feed technology, to investigate the feasibility of sustaining current and increased per capita fish consumption rates in 2050. We conclude that meeting current and larger consumption rates is feasible, despite a growing population and the impacts of climate change on potential fisheries production, but only if fish resources are managed sustainably and the animal feeds industry reduces its reliance on wild fish. Ineffective fisheries management and rising fishmeal prices driven by greater demand could, however, compromise future aquaculture production and the availability of fish products. 相似文献
43.
A riparian ecosystem downstream of a small dam in central Texas was instrumented for sap flow, soil moisture content, and stream level from 2001. Stable isotopes in water (D and 18O) were analysed from rainfall, stream, lake, and cored sapwood cellulose from cedar elm (Ulmus crassifolia). The isotope signature of water source to cedar elm was identified by back calculation starting with the water isotopes in cellulose, and accounting for leaf‐water evaporation and biological fractionation during cellulose synthesis. The estimated mean isotope of the source water to cedar elm was enriched above rainfall in similarity to stream water during 2002. Flow paths that may have contributed to estimated variability from regional base flow and recharge water were identified using the variably saturated HYDRUS‐2D model. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
44.
45.
Allen F. Moench 《Ground water》1994,32(6):949-957
46.
Katrina Allen Kennedy 《Journal of Paleolimnology》1994,10(2):69-84
Medicine Lake is a highly saline, meromictic, magnesium sulfate, closed-basin lake in northeastern South Dakota. The geochemical, mineralogical, and magnetic stratigraphies of sediments deposited from about 10.8 to 4.5 ka B.P. document the evolution of the saline brine in response to climatic change in the early to mid-Holocene. During the spruce occupation of the Medicine Lake catchment (10.8–10.0 ka B.P.), dark-grey massive basal sediments with low total-sulfur and carbonate content, upwardly increasing organic-carbon content, and high magnetic susceptibility were deposited in a deep freshwater lake. As the vegetation in the area changed from spruce to birch to oak and elm and finally to prairie between 10.0 and 9.2 ka B.P., and as the lake became shallow and salinity increased from <2 to >10%, light-and dark-grey calcareous and organic-carbon-rich banded sediments with low total-sulfur content and low magnetic susceptibility were deposited. Previous studies have shown that during the forest/prairie transition the lake then changed abruptly from fresh to saline as it lost a substantial portion of its volume. During the early prairie period (9.2–5.5 ka B.P.), alternating sections of aragonite-rich laminae and grey massive sediments with high total-sulfur content and multiple gypsum layers were deposited in a meromictic environment under conditions of fluctuating lake levels and salinity. Continued aridity during the mid-Holocene (5.5–4.5 ka B.P.) probably maintained the lake at relatively low levels and high salinity as dark-grey generally massive sediments with moderate total-sulfur, carbonate, and organic-carbon content and no measurable magnetic susceptibility were deposited. 相似文献
47.
The quantitative significance of organic matter degradation in bringing about the early diagenetic mobility of anthropogenic trace metals (Cu, Zn, Pb) is assessed specifically in relation to the use of estuarine sediments as historical records of pollution. A 1,500 mm salt-marsh sediment depth profile from Tites Point, Severn Estuary, England, was sampled at 10-mm intervals. Organic carbon determinations were carried out by a wet oxidation technique, and ‘organic fraction’ metals were separated by sequential leaching. Results demonstrated that organic phase metals are quantitatively significant in Severn Estuary sediments, particularly Cu and Zn (Cu>Zn), and that metals are probably released from this fraction during early diagenesis. The degree of release, and the apparent loss of the released trace metals from the sediment, would suggest that the use of estuarine sediments as historical records of pollution requires qualification. 相似文献
48.
Boundary-layer depth and entrainment zone characterization with a boundary-layer profiler 总被引:13,自引:0,他引:13
A technique for determining the height of the convective atmospheric boundary layer (CBL) with a 915 MHz boundary-layer profiler is discussed. The results are compared with CBL heights determined from radiosonde measurements. The profiler provides continuous CBL height measurements with very good time resolution (30 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In addition, the profiler provides information about the degree of definition of the CBL top and the thickness of the entrainment zone. The measurements discussed were taken during the Rural Oxidants in the Southern Environment II (ROSE II) experiment. 相似文献
49.
A regional fluid inclusion study of Cu-Au (+Zn-Pb) mineralisation in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralisation at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High salinity fluids, normally characteristic of potassic alteration, are confined to the host rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late-Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralising fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localisation of economic concentrations of gold in veins at the level of the Clogau Formation is ascribed to a destabilisation of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO2. Furthermore the Gold-belt fluids have a distinctive low CO2/CH4+N2+H2 ratio. 相似文献
50.
Dan H. Allen 《Ground water》1985,23(1):85-89
Abstract. WMPLUME is an analytic dispersion model based on the Wilson and Miller equation presented here in a program for the HP-41CV handheld, programmable calculator, the HP-41CX, or the HP-41C with four memory modules. The user specifies the spacing and extent of the field of observation points. The calculator can handle up to 36 fully penetrating, continuous injection well sources or a lesser number of discontinuous wells. The time of observation can be changed easily and sources can be added or deleted readily. The code is prepared for use with or without a printer attached to the calculator. 相似文献