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排序方式: 共有256条查询结果,搜索用时 15 毫秒
1.
The history of water salinity in the Pearl River estuary,China, during the Late Quaternary 总被引:1,自引:0,他引:1
Yongqiang Zong Fengling Yu Guangqing Huang Jeremy M. Lloyd Wyss W.‐S. Yim 《地球表面变化过程与地形》2010,35(10):1221-1233
This research reconstructed the Late Quaternary salinity history of the Pearl River estuary, China, from diatom records of four sedimentary cores. The reconstruction was produced through the application of a diatom–salinity transfer function developed based on 77 modern surface sediment samples collected across the estuary from shallow marine environment to deltaic distributaries. The statistical analysis indicates that the majority of sediment samples from the cores has good modern analogues, thus the reconstructions are reliable. The reconstructed salinity history shows the older estuarine sequence formed during the last interglacial was deposited under similar salinity conditions to the younger estuarine sequence, which was formed during the present interglacial. Further analysis into the younger estuarine sequence reveals the interplays between sea level, monsoon‐driven freshwater discharge, and deltaic shoreline movement, key factors that have influenced water salinity in the estuary. In particular, a core from the delta plain shows the effects of sea‐level change and deltaic progradation, while cores from the mouth region of the estuary reveal changes of monsoon‐driven freshwater discharge. This study demonstrates the advantages of quantitative salinity reconstructions to improve the quality of reconstruction and allow direct comparison with other quantitative records and the instrumentally observed values of salinity. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
2.
B. Kruijt Y. Malhi J. Lloyd A. D. Norbre A. C. Miranda M. G. P. Pereira A. Culf J. Grace 《Boundary-Layer Meteorology》2000,94(2):297-331
The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth. 相似文献
3.
This study compares how humans and neural networks classify climate types. Human subjects were asked to classify climates from monthly temperature and precipitation patterns. To model their learning process, the same data were used to produce input vectors that trained a pattern associator neural network. Both human subjects and the neural network classified climates accurately after 10 rounds of supervised learning. The neural network successfully modeled the rate of human learning and the ability to learn specific climate categories. Moreover, the neural network weights used to classify climates correspond to distinct visual characteristics in temperature and precipitation. These results suggest that neural networks can model the formation of visual categories. 相似文献
4.
C. C. Ferguson P. K. Harvey G. E. Lloyd 《Contributions to Mineralogy and Petrology》1981,75(4):339-352
Whether or not a growing porphyroblast can displace its surrounding matrix is an important but contentious issue affecting the interpretation of metamorphic textures. As an alternative to treating the problem in terms of force of crystallization, this paper examines the mechanics of porphyroblast-matrix interaction using a different and much simpler conceptual framework. New microstructural evidence for matrix displacement is then presented and analysed in detail. This evidence, from a hornfelsed metagreywacke, consists of dome-shaped accumulations of muscovite and graphite, each dome being attached to (and concave towards) a rhombdodecahedral face of a garnet porphyroblast. Muscovite within the domes shows a dimensional preferred orientation subparallel to the dome outlines while, in the matrix away from the domes, there is no preferred orientation.Our model for the origin of the mica domes envisages muscovite and graphite being swept ahead of, and mechanically accumulated onto, the growing garnet faces as they advance through the matrix. Rigorous testing of this model provides strong evidential support for matrix displacement by growing porphyroblasts. A new analysis is provided of the conditions under which matrix grains are included or displaced by porphyroblasts. It is concluded that matrix grain displacement may be very common, although the special combination of circumstances necessary to produce a diagnostic microstructural pattern probably occurs very rarely. 相似文献
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7.
Alexander S. Lloyd Terry Plank Philipp Ruprecht Erik H. Hauri William Rose 《Contributions to Mineralogy and Petrology》2013,165(1):129-153
We have investigated the loss of H2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H2O contents (loss of ~1 wt% H2O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10?9 to 10?10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H2O concentrations in olivine-hosted MIs may reflect unrecognized H2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent. 相似文献
8.
The dramatic political and economic changes in Europe, Asia, and the Middle East during the early 1990s have had a dramatic impact on international tourism. Between 1974 and 1990 there have been significant shifts in international tourist originations. The purpose of this paper is to describe and analyze the changing patterns and to compare the two time periods. A key component of the study is the construction of a Population-Adjusted Travel Index which assesses the capability of a country to generate tourists relative to population characteristics. Findings indicate that Europe is still the leader in trips generated, even though the proportion from this region is declining over time. Asian countries exhibited the most marked improvement due primarily to the tremendous economic expansion taking place on the Pacific Rim. In contrast countries of the Middle East have declined in importance relative to international tourism. This is due to political and economic instability in the region. 相似文献
9.
Wafa M. Al Lawati Athanasios Rizoulis Elisabeth Eiche Christopher Boothman David A. Polya Jonathan R. Lloyd Michael Berg Patricio Vasquez-Aguilar Bart E. van Dongen 《Applied Geochemistry》2012
High concentrations of geogenic As in the groundwaters of south and SE Asia, which are used as drinking waters, are causing severe health impacts to the exposed human populations. It is widely accepted that As mobilisation from sediments into these shallow reducing groundwaters requires active metal-reducing microbes and electron donors such as organic matter (OM). Although OM in such Holocene aquifers has been characterised, there is a dearth of data on Pleistocene aquifers from the same areas. Reported here are preliminary studies of OM and microbial communities present in two aquifers, one of Pleistocene and one of Holocene age, with contrasting concentrations of As (viz. Pleistocene: low As <10 μg/L; Holocene: high As up to 600 μg/L) from Van Phuc village in the Red River Delta, Vietnam. Results revealed OM inputs from multiple sources, including potential contributions from naturally occurring petroleum seeping into the shallow aquifer sediments from deeper thermally mature source rocks. Although concentrations vary, no noticeable systematic differences in biomarker distribution patterns within the OM were observed between the two sites. Microbial analyses did not show a presence of microbial communities previously associated with As mobilisation. All clone libraries were dominated by α-, β-, and γ-Proteobacteria not known to be able to reduce Fe(III) or sorbed As(V). Furthermore, representatives of the Fe(III)-reducing genus Geobacter could only be detected at very low abundance by PCR, using highly selective 16S rRNA gene primers, supporting the hypothesis that metal reduction is not a dominant in situ process in these sediments. No correlation between As concentration in groundwater and OM composition nor microbial community in the host sediments was found. This suggests that either (i) As is not being significantly mobilised in situ in these sediments, instead As appears to be mobilised elsewhere and transported by groundwater flow to the sites or (ii) sorption/desorption processes, as implicated by geochemical data from the cores, play a critical role in controlling As concentrations at these sites. 相似文献
10.
Estuaries and Coasts - A coring system using a compressed air supply and a sediment shaking procedure using a soil dispersant are described which greatly simplify and shorten the task of... 相似文献