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991.
本文分析了澜沧江-湄公河结合部次区域国际合作的现状、区域范围和合作前景。提出以交通先行,旅贸作导,推进次区域合作,开发资源,优化产业布局,加速交通网络和城市建设。 相似文献
992.
993.
Based on TM image data and other survey materials, this paper analyzed the spatiotemporal patterns of land use change in the
Bohai Rim during 1985–2005. The findings of this study are summarized as follows: (1) Land use pattern changed dramatically
during 1985–2005. Industrial and residential land in urban and rural areas increased by 643,946 hm2, of which urban construction land had the largest and fastest increase of 294,953 hm2 at an annual rate of 3.72%. (2) The outward migration of rural population did not prevent the expansion of residential land
in rural areas by 184,869 hm2. This increase reveals that construction of rural residences makes seriously wasteful and inefficient use of land. (3) Arable
land, woodland and grassland decreased at a rate of −0.02%, −0.12% and −1.32% annually, while unused land shrank by 157,444
hm2 at an annual rate of −1.69%. (4) The change of land use types showed marked fluctuations over the two stages (1985–1995 and
1995–2005). In particular, arable land, woodland and unused land experienced an inversed trend of change. (5) There was a
significant interaction between arable land and woodland. Industrial construction land in urban and rural areas showed a net
trend of increase during the earlier period, but only adjustment to its internal structure during the second period. The loss
of arable land to the construction of factories, mines and residences took place mainly in the fringe areas of large and medium-sized
cities, along the routes of major roads, as well as in the economically developed coastal areas in the east. Such changes
are closely related to the spatial differentiation of the level of urbanization and industrialization in the region. 相似文献
994.
995.
996.
The Gray Fossil Site (GFS) includes a small (<2 ha) paleosinkhole lake fill with an exceptionally well-preserved record of
sedimentation and fossils from the latest Miocene to earliest Pliocene. The uppermost lacustrine stratigraphy is characterized
by rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty clay (B)
laminae. Both the A and B components are almost exclusively comprised of exogenic sediment (including organic matter). Periodicities
of 24 and 4.4 are recorded within a continuous 96 interpreted year sequence of rhythmite sediment. In a small lake with a
poorly oxygenated bottom, the presence of laterally continuous laminated sediment that includes well-known periodicities in
rhythmite thickness is interpreted as representing annually generated varves that correspond to seasonal variations in sedimentation.
The distinctly larger fraction of medium sand-size quartz grains present within the A laminae, as well as the abrupt transitions
between A and B components suggest that the rhythmites represent deposition during alternating high-energy and lower-energy
seasons, which is consistent with a monsoonal precipitation pattern. The seasonal climate may relate to changes in the ocean
circulation pattern prior to 4.6 Ma that resulted in an increased temperature and atmospheric pressure gradient between the
east coast of North America and the Atlantic Ocean, but this climate phase seems to be only a temporary condition, as underlying
and overlying sediment are both consistent with drier conditions. The periodicity at 24 interpreted years is consistent with
the well-known Hale solar cycle. The 4.4 interpreted-year periodicity occurs within the ENSO frequency band, and if this documentation
of ENSO-like interannual climate change is correct, then it suggests that ENSO operated at times during the warm Earth conditions
characterizing the late Tertiary. 相似文献
997.
Jessica D. Tomkins Scott F. Lamoureux Dermot Antoniades Warwick F. Vincent 《Journal of Paleolimnology》2009,41(1):225-242
Sediment aggregates (“sedimentary pellets”) within the sedimentary record of Lake A (83°00′ N, 75°30′ W), Ellesmere Island,
Canada, are used to construct a 1000 year proxy record of ice-cover extent and dynamics on this perennially ice-covered, High
Arctic lake. These pellets are interpreted to form during fall or early winter when littoral sediment adheres to ice forming
around the lake’s periphery or during summer through the development of anchor ice. The sediment likely collects in ice interstices
and is concentrated in the upper ice layers through summer surface ice melt and winter basal ice growth. The pellets remain
frozen in the ice until a summer or series of summers with reduced ice cover allows for their deposition across the lake basin.
Sedimentary pellet frequency within multiple sediment cores is used to develop a chronology of ice-cover fluctuations. This
proxy ice-cover record is largely corroborated by a record of unusual sedimentation in Lake A involving iron-rich, dark-orange
to red laminae overlying more diffuse laminae with a lighter hue. This sediment sequence is hypothesized to represent years
with reduced ice cover through increased chemocline ventilation and iron deposition. During the past millennium, the most
notable period of inferred reduced ice cover is ca. 1891 AD to present. Another period of ice cover mobility is suggested
ca. 1582–1774 AD, while persistent ice cover is inferred during the 1800s and prior to 1582 AD. The proxy ice-cover record
corresponds well with most regional melt-season proxy temperature and paleoecological records, especially during the 1800s
and 1900s.
相似文献
Jessica D. TomkinsEmail: |
998.
Lithological and geochemical record of anthropogenic changes in recent sediments of a small and shallow lake (Lake Pusty Staw,northern Poland) 总被引:4,自引:0,他引:4
Wojciech Tylmann 《Journal of Paleolimnology》2005,33(3):313-325
The article presents the results of lithological and geochemical investigations of recent sediments of Lake Pusty Staw. The analysed sediments document about 300 years of the history of this lake. Historical materials indicate that significant changes in the catchment of the lake took place from the beginning of the 18th century (deforestation and afforestation) followed by 19th century tourist development, and 20th century industrialisation. The sediments were dated using the 210Pb and 137Cs method and core lithology. These made it possible to establish a reliable chronology to the year 1730. The calculated sedimentation rates ranged between 0.17 and 0.83 cm year–1. There was a period of intensified erosion caused by land clearance between 1734 and 1810, which resulted in an acceleration in sedimentation rate (0.36 cm year–1) and a change of lithological type of sediment from detritus gyttja to clayey gyttja. On the basis of Cu, Zn, Cd and Pb content in the sediments, it was determined that the beginning of pollution of the lake dates back to the 19th century and was caused probably by the existence of a health resort. A systematic increase in pollution occurred in the 20th century as a result of industrial plants. Normalised with respect to Al, the content of heavy metals in polluted sediments was from several to twenty times higher than in sediments of the preindustrial period, and a comparison of historical materials with changes of sediments in most cases made it possible to identify the direct causes of the increase in pollution. 相似文献
999.
我国西部生态退化的社会经济分析--以川西为例 总被引:9,自引:11,他引:9
导致川西生态退化主要原因是经济发展水平低,人口超过环境容量,农村生产方式和生活方式落后,以柴、粪为薪、天然放牧,陡坡垦殖,砍伐森林等导致植被破坏,草场退化,水土流失,据此,对川西的区域资源优势,川西较低密度人口,川西资源开发和支柱产业选择应重新评价和定位。进行川西生态建设,实施可持续发展战略,必须营造开放的社会-经济-自然复合系统的耗散结构,构造社会、经济发展与生态建设协调的有序结构和机制。同时,较彻底地改造农林牧生态系统,开创与生态协调的农业结构,生产模式,种植模式和管理模式,进行农牧区社区建设,根除刀耕火种,陡坡垦殖,游牧的社会基础。 相似文献
1000.
实施农业名牌战略,创建梅州特色农业品牌的构想 总被引:1,自引:0,他引:1
实施农业名牌战略,既是适应社会经济发展和提高农业综合效益的需要,也是实现区域农业可持续发展的必然选择,必须遵循“市场导向、特色优势、绿色发展”3大原则。梅州市具有发展特色农业,构建名牌的有利条件,应在制定创名牌规划、开发无公害优质农产品、创造培育农业名牌的制度环境和经营机制3方面寻求对策。 相似文献