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排序方式: 共有396条查询结果,搜索用时 62 毫秒
391.
Hans W. Paerl Lexia M. Valdes Alan R. Joyner Benjamin L. Peierls Michael F. Piehler Stanley R. Riggs Robert R. Christian Lisa A. Eby Larry B. Crowder Joseph S. Ramus Erika J. Clesceri Christopher P. Buzzelli Richard A. Luettich 《Estuaries and Coasts》2006,29(6):1033-1045
Since the mid 1990s, the Atlantic and Gulf Coast regions have experienced a dramatic increase in the number of hurricane landfalls. In eastern North Carolina alone, eight hurricances have affected the coast in the past 9 years. These storms have exhibited individualistic hydrologic, nutrient, and sediment loading effects and represent a formidable challenge to nutrient management aimed at reducing eutrophication in the Pamlico Sound and its estuarine tributaries. Different rainfall amounts among hurricanes lead to variable freshwater and nutrient discharge and variable nutrient, organic matter, and sediment enrichment. These enrichments differentially affected physical and chemical properties (salinity, water residence time, transparency, stratification, dissolved oxygen), phytoplankton primary production, and phytoplankton community composition. Contrasting ecological responses were accompanied, by changes in nutrient and oxygen cycling, habitat, and higher trophic levels, including different direct effects on fish populations. Floodwaters from the two largest hurricances, Fran (1996) and Floyd (1999), exerted, multi-month to multi-annual effects on hydrology, nutrient loads, productivity, and biotic composition. Relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical mixing and storm surges, but relatively minor hydrologic and nutrient effects. Both hydrologic loading and wind forcing are important drivers and must be integrated with nutrient loading in assessing short-term and long-term ecological effects of these storms. These climatic forcings cannot be managed but should be considered in the development of water quality management strategies for these and other large estuarine ecosystems faced with increasing frequencies and intensities of hurricane activity. 相似文献
392.
Serial sampling of tooth enamel growth increments for carbon and oxygen isotopic analyses of Macropus (kangaroo) teeth was performed to assess the potential for reconstructing paleoseasonality. The carbon isotope composition of tooth enamel apatite carbonate reflects the proportional intake of C3 and C4 vegetation. The oxygen isotopic composition of enamel reflects that of ingested and metabolic water. Tooth enamel forms sequentially from the tip of the crown to the base, so dietary and environmental changes during the tooth's formation can be detected. δ13C and δ18O values were determined for a series of enamel samples drilled from the 3rd and 4th molars of kangaroos that were collected along a 900 km north–south transect in southern Australia. The serial sampling method did not yield pronounced seasonal isotopic variation patterns in Macropus enamel. The full extent of dietary isotopic variation may be obscured by attenuation of the isotopic signal during enamel mineralisation. Brachydont (low-crowned) Macropus teeth may be less sensitive to seasonal variation in isotopic composition due to time-averaging during mineralisation. However, geographic variations observed suggest that there may be potential for tracking latitudinal shifts in vegetation zones and seasonal environmental patterns in response to climate change. 相似文献
393.
Wenbin Wang Jiuhou Lei Alan G. Burns Liying Qian Stanley C. Solomon Michael Wiltberger Jiyao Xu 《Solar physics》2011,274(1-2):457-472
Ionospheric F2 peak electron densities (NmF2) measured at ten ionosonde stations have been analyzed to investigate ionospheric day-to-day variability around the Whole Heliosphere Interval (WHI) in 2008 (Day of Year (DOY) 50?–?140). The ionosonde data showed that there was significant global day-to-day variability in NmF2. This variability had 5-, 7-, 9-, 11-, 13.5-, and 16?–?21-day periodicities. At middle latitudes, the ionosphere appeared to respond directly to the solar-wind and interplanetary-magnetic-field (IMF) induced geomagnetic-activity forcing, with the day-to-day variability having the same periods as those in the solar-wind/IMF and geomagnetic activity. At the geomagnetic Equator, the ionosphere had a strong 7-day periodicity, corresponding to the same periodicity in the IMF B z component. In the equatorial anomaly region, the ionosphere showed more complicated day-to-day variability, dominated by the 9-day periodicity. In addition, there were also periodicities of 11 days and 16?–?21 days in the ionosonde data at some stations. The ionosonde data were compared with the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) simulations that were driven by the observed solar-wind and IMF data during the WHI. The CMIT simulations showed similar ionospheric daily variability seen in the data. They captured the positive and negative responses of the ionosphere at middle latitudes during the first corotating interaction region (CIR) event in the WHI. The response of the model to the second CIR event, however, was relatively weak. 相似文献
394.
Stanley A. Changnon 《Climatic change》1993,23(3):213-230
This study concerns effects along the Illinois shoreline resulting from the record-low levels of Lake Michigan during 1964–65, and the potential impacts of future low water levels at Chicago resulting from potential climate changes. The low lake levels of the 1960s did not cause major impacts at Chicago when they occurred. The two major impacts discerned (damage to shoreline protection structures and the encroachment of structures onto areas too close to the lake) resulted from a sequence of low-water levels followed by high-water levels a few years later. Climate scenarios, derived from general circulation models (GCMs) and extreme historical precipitation values, were used in a basin hydrologic model to determine potential future lake levels. Possible impacts were then estimated by 29 lakeshore experts. If the average lake level were reduced 1.0 m during the next 50 yr, shoreline impacts would not be severe and cost about $ 100 million. Many of the adjustment costs could be offset by normal maintenance and replacement costs. If the average lake level was reduced more than 1.5 m during the next 50 yr, very sizable economic impacts would occur, costing between $3 and $35 billion (1988 dollars). Some of the adjustment cost could be offset by normal replacement costs, particularly if a master plan for changing affected facilities were implemented in advance. The type of climate change and degree of lakelevel change will greatly affect the severity of the effects at Chicago. 相似文献
395.
O. Yu. Plotinskaya V. A. Kovalenker R. Seltmann C. J. Stanley 《Mineralogy and Petrology》2006,87(3-4):187-207
Summary The Late Paleozoic Kochbulak and Kairagach deposits are located on the northern slope of the Kurama Ridge, Middle Tien Shan,
in the same volcanic structure and the same ore-forming system. Au–Ag–Cu–Bi–Te–Se mineralization is confined to veins and
dissemination zones accompanied by quartz-sericite wall-rock alteration. The tellurides, calaverite, altaite, hessite, and
tetradymite are widespread at both deposits; at Kairagach selenides and sulfoselenides of Bi and Pb are common, while at Kochbulak
Bi and Pb telluroselenides and sulfotelluroselenides are typical. The paragenetic sequence of telluride assemblages are similar
for both deposits and change from calaverite + altaite + native Au to sylvanite + Bi tellurides + native Te, Bi tellurides
+ native Au, and, finally, to Au + Ag tellurides with time. These mineralogical changes are accompanied by an increase in
the Ag content of native gold that correlates with a decrease in temperature, fTe2 and fO2 and an increase in pH. 相似文献
396.
This study records that Pompeii, long before its final devastation by the 79 A.D. Vesuvius eruption in southern Italy, was damaged by several mass gravity flows. Composition of the deposits indicates that they were derived from volcaniclastic cover of carbonate highlands positioned 14 km NE of the city. Stratigraphic and petrologic analyses of sections in excavations and drill cores in and near Pompeii record the presence of three downslope‐directed debris flows dated between 8th and 2nd century B.C. Some of these deposits were channelized via a stream bed that once extended from high reliefs to Pompeii. It is proposed that one of these events may have been partially responsible for urban decline during the 4th century B.C. These mass flows are interpreted as having been triggered primarily by intense rains in a manner similar to those that have occurred and destroyed towns in this region during the past 50 years. Our investigation shows that Pompeii and adjacent areas in the past, much as in recent time, have likely been most frequently susceptible to episodic damage by avalanches and mass flows of volcaniclastic material resulting from hydrological effects rather than from volcanic events, earthquake tremors, and societal disruptions such as wars. 相似文献