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991.
Zhizhong Zhao Yansong Qiao Shubing Wang Haitao Yao Yan Wang Jianli Fu Chaozhu Li Zongxiu Liu Fuchu Jiang 《Frontiers of Earth Science》2008,2(2):209-216
The eastern edge of the Qinghai-Tibetan plateau developed an integrated series of late Cenozoic lacustrine, loess, red and
moraines deposits. Various genetic sediments recorded rich information of Quaternary palaeoenvironment changes. Xigeda Pliocene
lacustrine deposits, formed during 4.2 Ma B.P.–2.6 Ma B.P., experienced nine periodic warm-cold stages. Eolian deposition
in western Sichuan began at 1.15 Ma B.P., and the loess-soil sequences successively record fourteen palaeomonsoon change cycles.
Red clay in the Chengdu plain record five stages of paleoclimatic change stages since 1.13 Ma B.P.. There was an old glacial
period of 4.3 Ma B.P. in the eastern Qinghai-Tibetan plateau. During the Quaternary, there were five extreme paleoclimatic
events corresponding to five glaciations.
__________
Translated from Geological Bulletin of China, 2007, 26(12): 1620–1626 [译自: 地质通报] 相似文献
992.
993.
The paper describes a model of an agricultural society in which agents live in a single settlement and use the surrounding area to produce essential and non-essential goods. Agents make, and attempt to fulfil, consumption and production plans but markets do not always clear and goods can change hands at different prices between different pairs of agents. The model generates a wide range of agricultural landscapes, including those of a classical von Thünen economy. Demographically, it produces outcomes varying from logistic growth to periodic collapse caused by cyclical famines. 相似文献
994.
Ti-in-zircon thermometry: applications and limitations 总被引:16,自引:5,他引:11
Bin Fu F. Zeb Page Aaron J. Cavosie John Fournelle Noriko T. Kita Jade Star Lackey Simon A. Wilde John W. Valley 《Contributions to Mineralogy and Petrology》2008,156(2):197-215
The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come
from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons
(32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most
of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon
thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon
saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue
for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous
rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually,
the effects of reduced or , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low
temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous
melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While
some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included
within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in
Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once.
However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio,
or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide
overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock
SiO2 and HfO2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks
average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures
of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons
have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons
from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent
Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration
in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
995.
Origin and Evolution of Silicic Magmatism at Yellowstone Based on Ion Microprobe Analysis of Isotopically Zoned Zircons 总被引:5,自引:0,他引:5
The origin of large-volume Yellowstone ignimbrites and smaller-volumeintra-caldera lavas requires shallow remelting of enormous volumesof variably 18O-depleted volcanic and sub-volcanic rocks alteredby hydrothermal activity. Zircons provide probes of these processesas they preserve older ages and inherited 18O values. This studypresents a high-resolution, oxygen isotope examination of volcanismat Yellowstone using ion microprobe analysis with an averageprecision of ± 0·2 and a 10 µm spot size.We report 357 analyses of cores and rims of zircons, and isotopeprofiles of 142 single zircons in 11 units that represent majorYellowstone ignimbrites, and post-caldera lavas. Many zirconsfrom these samples were previously dated in the same spots bysensitive high-resolution ion microprobe (SHRIMP), and all zirconswere analyzed for oxygen isotope ratios in bulk as a functionof grain size by laser fluorination. We additionally reportoxygen isotope analyses of quartz crystals in three units. Theresults of this work provide the following new observations.(1) Most zircons from post-caldera low-18O lavas are zoned,with higher 18O values and highly variable U–Pb ages inthe cores that suggest inheritance from pre-caldera rocks exposedon the surface. (2) Many of the higher-18O zircon cores in theselavas have U–Pb zircon crystallization ages that postdatecaldera formation, but pre-date the eruption age by 10–20kyr, and represent inheritance of unexposed post-caldera sub-volcanicunits that have 18O similar to the Lava Creek Tuff. (3) Youngand voluminous 0·25–0·1 Ma intra-calderalavas, which represent the latest volcanic activity at Yellowstone,contain zircons with both high-18O and low-18O cores surroundedby an intermediate-18O rim. This implies inheritance of a varietyof rocks from high-18O pre-caldera and low-18O post-calderaunits, followed by residence in a common intermediate-18O meltprior to eruption. (4) Major ignimbrites of Huckleberry Ridge,and to a lesser extent the Lava Creek and Mesa Falls Tuffs,contain zoned zircons with lower-18O zircon cores, suggestingthat melting and zircon inheritance from the low-18O hydrothermallyaltered carapace was an important process during formation ofthese large magma bodies prior to caldera collapse. (5) The18O zoning in the majority of zircon core–rim interfacesis step-like rather than smoothly inflected, suggesting thatprocesses of solution–reprecipitation were more importantthan intra-crystalline oxygen diffusion. Concave-downward zirconcrystal size distributions support dissolution of the smallercrystals and growth of rims on larger crystals. This study suggeststhat silicic magmatism at Yellowstone proceeded via rapid, shallow-levelremelting of earlier erupted and hydrothermally altered Yellowstonesource rocks and that pulses of basaltic magma provided theheat for melting. Each post-caldera Yellowstone lava representsan independent homogenized magma batch that was generated rapidlyby remelting of source rocks of various ages and 18O values.The commonly held model of a single, large-volume, super-solidus,mushy-state magma chamber that is periodically reactivated andproduces rhyolitic offspring is not supported by our data. Rather,the source rocks for the Yellowstone volcanism were cooled belowthe solidus, hydrothermally altered by heated meteoric watersthat caused low 18O values, and then remelted in distinct pocketsby intrusion of basic magmas. Each packet of new melt inheritedzircons that retained older age and 18O values. This interpretationmay have significance for interpreting seismic data for crustallow-velocity zones in which magma mush and solidified areasexperiencing hydrothermal circulation occur side by side. Newbasalt intrusions into this solidifying batholith are requiredto form the youngest volcanic rocks that erupted as independentrhyolitic magmas. We also suggest that the Lava Creek Tuff magmawas already an uneruptable mush by the time of the first post-calderaeruption after 0·1 Myr of the climactic caldera-formingeruption. KEY WORDS: Yellowstone; oxygen isotopes; geochronology; isotope zoning; zircon; U–Pb dating; caldera; rhyolite; ion microprobe 相似文献
996.
The 1.78 Ga Xiong'er Volcanic Province (XVP) and coeval North China giant mafic Dyke Swarm (NCDS) are the most important magmatic events occurring after the amalgamation of the North China craton (NCC). The XVP consists of 3–7 km of extrusive volcanics and some feeder dykes/sills located along the southern margin of the NCC and extending over an area > 0.06 M km2. Compositions vary from basalt to rhyolite, but are predominantly intermediate in terms of silica content. There are also minor sedimentary intercalations and pyroclastic units. The sedimentary interlayers indicate an environment changing from continental-facies to oceanic-facies up-section. The XVP is characterized by fractional crystallization from an EM I type mantle source, and both continental arc (Andean-type) and rift environments have been proposed. The NCDS is widespread in the central NCC with an outcrop area > 0.1 M km2, and are exposed at variable depths up to 20 km (deepest in the north). Dyke compositions vary from basalt to andesite and dacite, but are dominantly mafic, and comprise two series of magmatism. Previous studies revealed that the NCDS recorded assimilation and fractional crystallization of an EM I type magma source, with a minor DM contribution in the younger magmas. Both syn-collisional and intra-continental anorogenic environments have been proposed. Spatial and petrogenic correlations suggest a cogenetic relationship between the NCDS and XVP, and considered together, they define a Large Igneous Province (LIP) of > 0.1 M km2 in area and > 0.1 M km3 in volume, which is also notable for its continuous compositional range from mafic to felsic (with no gap at intermediate compositions). The petrology is explained by a common magma source that undergoes a silica-poor and iron-enriched fractionation trend at depth followed by a silica-rich and iron-poor fractionation trend in shallow-level magma conduits (dykes) and surface lavas. A mantle plume is favored as the cause of this 1.78 Ga North China LIP. 相似文献
997.
Rainfall simulations have been conducted to study the soil erosion process of purple soil in two cultivation practices---contour cultivation and downslope cultivation. Results showed that under the two cultivation practices, the surface runoff can be described by the logarithmic function formula. In the initial period of rainfall, the amount of runoff increased with the rainfall duration and 20 minutes later it became relatively constant. The changing process of soil erosion rate may be described by the logarithmic function formula. The erosion rate increased with the rainfall duration and 20 minutes later it also became constant. Under downslope cultivation condition, the soil erosion rate increased more significantly than that under contour cultivation condition in the case of gentle rainfall intensity, and there is no obvious difference in erosion rate for downslope cultivation and contour cultivation practices. However, with increasing rainfall intensity the soil erosion rate under the downslope cultivation condition could be more than 30 times that under the contour cultivation condition. But this kind of difference would be reduced to some extent in the case of heavy rain. 相似文献
998.
Zhenlong Li Fei Hu John L. Schnase Daniel Q. Duffy Tsengdar Lee Michael K. Bowen 《International journal of geographical information science》2017,31(1):17-35
Climate observations and model simulations are producing vast amounts of array-based spatiotemporal data. Efficient processing of these data is essential for assessing global challenges such as climate change, natural disasters, and diseases. This is challenging not only because of the large data volume, but also because of the intrinsic high-dimensional nature of geoscience data. To tackle this challenge, we propose a spatiotemporal indexing approach to efficiently manage and process big climate data with MapReduce in a highly scalable environment. Using this approach, big climate data are directly stored in a Hadoop Distributed File System in its original, native file format. A spatiotemporal index is built to bridge the logical array-based data model and the physical data layout, which enables fast data retrieval when performing spatiotemporal queries. Based on the index, a data-partitioning algorithm is applied to enable MapReduce to achieve high data locality, as well as balancing the workload. The proposed indexing approach is evaluated using the National Aeronautics and Space Administration (NASA) Modern-Era Retrospective Analysis for Research and Applications (MERRA) climate reanalysis dataset. The experimental results show that the index can significantly accelerate querying and processing (~10× speedup compared to the baseline test using the same computing cluster), while keeping the index-to-data ratio small (0.0328%). The applicability of the indexing approach is demonstrated by a climate anomaly detection deployed on a NASA Hadoop cluster. This approach is also able to support efficient processing of general array-based spatiotemporal data in various geoscience domains without special configuration on a Hadoop cluster. 相似文献
999.
北极航道管理与北极渔业管理都是由气候变化所催生的北极重要事务。由于同处北极,两者面临着一些共性的管理问题,使两者之间具有了一定的可比性。两者的对比研究显示,北极航道管理机制构建过程体现了国际合作的精神,航道管理制度体系在形成之初始终处于动态发展以期得到完善,依托国际组织制定航行规则也有利于未来北极全面通航后执行统一协调的航道管理制度,这些北极航道管理实践中所体现的管理理念和路径对北极渔业管理具有启示作用。对比研究也揭示了北极渔业所面临的管理瓶颈,具体表现为北极5国谋求北冰洋公海"领导者"地位的单边主义行为,以及北极渔业现阶段所呈现出的"碎片式"管理格局。为实现北极渔业可持续性发展,北极5国之间、北极5国与北极域外国家之间应加强交流与合作,同时适时成立相应的区域性渔业管理组织以协调并开展国际交流与合作。 相似文献
1000.
产业转型背景下创业区位选择和集群空间演化 总被引:5,自引:4,他引:1
以广州软件和信息技术产业集群为案例,分析产业范式转变背景下地方创业区位选择对产业集群空间演化的影响。结果显示:初始资源富集和政策环境导向使特定空间成为创业的优势区位,早期第一代创业企业在其中集聚发展形成集群雏形,并在关系网络和区域认同作用下形成产业集群空间核心区。第二代创业企业在产业转型契机下选择开拓缝隙市场,新的区位诉求由此产生并作用在集群的空间演化上。总的来说,产业转型期同时也是集群空间转变的关键时期,创业企业通过区位选择可以改变集群的空间形态和层次。 相似文献