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41.
Environmental Changes at the Desert Margin: An Assessment of Recent Paleolimnological Records in Lake Qarun, Middle Egypt 总被引:1,自引:0,他引:1
R. J. Flower C Stickley N. L. Rose S Peglar A. A. Fathi P. G. Appleby 《Journal of Paleolimnology》2006,35(1):1-24
Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years.
Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater.
Sediment cores (c. 1 m long) were collected from this shallow (Zmax 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing
the recent environmental history of the site. According to 137Cs and 210Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year−1 during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were
present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning
around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed
landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected
by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s.
Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar
abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater
diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater
inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of
lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but
inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial
sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate
and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking,
the relationships between diatom records and past water quality changes require careful interpretation, especially in the
upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well
supported by sediment records, a change that is probably linked to ingress of saline ground water 相似文献
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43.
相对湿度对静电收集型连续测氡仪检定结果的影响 总被引:1,自引:0,他引:1
NR-667A(Ⅲ)型快速连续测氧仪是目前环境氡测量时间响应最短的一种仪器。该仪器在核工业第六研究所标准氡室进行检定时发现,环境相对相对度对仪器检定结果有很大影响。在相对湿度为80%-99%的标准氡室内分别对仪器进行检定,结果在相对湿度为80%时仪器检定合格,而在相对湿度为99%时仪器检定不合格。本从仪器工作原理对检定结果进行。并提出有效的解决方法,当相对湿度太大时,可在仪器进气口安装干燥器对进气进行除湿。 相似文献
44.
Introduction The tidal variation signals of the solid Earth recorded with the modern measuring techniques can be used to provide effectively the means to cognize the Earths motion, deformation and structure character, especially to monitor the material motion in the Earths interior. In the recent 20 years, the distribution character of the global gravity field has made great progress since the continuous and deep theoretical studies in geophysics and geodesy domains. The successful manufactur… 相似文献
45.
Florence Sylvestre 《Journal of Paleolimnology》2002,27(1):45-57
A diatom study of lacustrine sediments in the southern part of the Bolivian Altiplano (Salar of Coipasa) provides a continuous record of the period 21,000–17,500 14C yr BP. Constrained by seven AMS 14C dates, this record provides evidence that the Coipasa basin was filled by a shallow body of water during this time. Diatom/salinity and diatom/ionic composition transfer functions indicate that the lake was saline, dominated by sodium-chloride throughout all the period.A comparison with regional data shows that Lake Titicaca could not have overflowed towards the southern Bolivian Altiplano at that time. As this dry phase was not registered in Lake Coipasa, this lake was probably supplied by winter precipitation originating from the Pacific. But, recent data from the deep basin of Lake Titicaca show that the lake-level was higher during this time interval, and the question arises whether precipitation from Atlantic and Amazonia sources could have played an important role on the Altiplano during the Last Glacial Maximum (LGM). This hypothesis needs to match other available paleoclimatic data from the lowlands of tropical South America, where there is evidence that during the LGM, conditions were drier than today. Global climate simulations suggest a positive P-E on the Altiplano, due to a strong cooling, reducing evaporative demand more than any increase in precipitations. An increase of winter precipitation from the Pacific is in agreement with data from the Chilean coast showing a northward locations of the Westerlies during the LGM. This paleoclimatic hypothesis is also in agreement with a coupled ocean-atmosphere model, which indicates weaker summer precipitation and stronger winter precipitation in the tropical areas. 相似文献
46.
Exploration for volcanogenic massive sulfide deposits of the kuroko-type is underway in many places. Clarifying the spatial patterns of the metals in kuroko deposits will be useful for understanding their genetic mechanisms and for future exploration of such types of deposits. This study represents a spatial distribution analysis on the contents of principal metals of kuroko deposits: Cu, Pb, and Zn, in the Hokuroku district, northern Japan, by a feedforward neural network and 1917 sample data at 143 drillhole sites. The network, which consists of three layers, was trained by the principle of SLANS in which the numbers of neurons in the middle layer and training data are changed to improve estimation accuracy. Using the weight coefficients connecting adjacent neurons, sensitivity analysis of the neural network was carried out to identify factors influencing spatial distributions of the three metals. The coordinates depth (z) direction, Bouguer gravity, and specific lithology such as dacite were determined to be influencing factors. The high frequency of the z coordinate signifies that the metal contents differ to a large extent by depth. The sensitivity vector was defined using sensitivity coefficients for x, y, and z coordinates of an estimation point. We determined that the directions of large vectors were different inside and outside of the Hanawa-Ohdate area. This characteristic is considered to originate from the differences in the permeability of fractures that became the paths for rising ore solutions, and the depths that the solutions mixed with sea water. 相似文献
47.
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49.
The Influence of Vegetation Cover on Summer Precipitation in China: a Statistical Analysis of NDVI and Climate Data 总被引:18,自引:0,他引:18
This study provides new evidence for the feedback effects of vegetation cover on summer precipitation in different regions of China by calculating immediate (same season), and one-and two-season lagged correlations between the normalized difference vegetation index (NDVI) and summer precipitation. The results show that the correlation coefficients between NDVI in spring and the previous winter and precipitation in summer are positive in most regions of China, and they show significant difference between regions. The stronger one-and two-season lagged correlations occur in the eastern arid/semi-arid region, Central China,and Southwest China out of the eight climatic regions of China, and this implies that vegetation cover change has more sensitive feedback effects on summer precipitation in the three regions. The three regions are defined as sensitive regions. Spatial analyses of correlations between spring NDVI averaged over each sensitive region and summer precipitation of 160 stations suggest that the vegetation cover strongly affects summer precipitation not only over the sensitive region itself but also over other regions, especially the downstream region. 相似文献
50.
变形体的稳定性及其定量分析方法初探 总被引:3,自引:0,他引:3
变形体的稳定性是变形体性质的重要内容,它是工程实际中极为关注的问题,但从变形监测的角度上来分析变形的稳定性目前在国内外还未见任何实质性的报导。本文讨论了从变形监测角度上分析变形体稳定性的一般方法,进行变形稳定性的分类,初步研究了变形体稳定性的定量分析方法,定义了一系列量化指标,如稳定度、因素的影响域与因素的稳定域等。这些有利于把稳定性分析推向应用和深入,从而提高变形监测在变形体定性分析中的地位,增 相似文献