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1.
Columnar stalagmites in caves of the Guadalupe Mountains during the late Holocene record a 4000-year annually resolved climate history for the southwestern United States. Annual banding, hiatuses, and high-precision uranium-series dating show a present day-like climate from 4000 to 3000 years ago, following a drier middle Holocene. A distinctly wetter and cooler period from 3000 to 800 years ago was followed by a period of present day-like conditions, with the exception of a slightly wetter interval from 440 to 290 years before the present. The stalagmite record correlates well with the archaeological record of changes in cultural activities of indigenous people. Such climate change may help to explain evidence of dwelling abandonment and population redistribution. 相似文献
2.
Titanium and iron concentration data from the anoxic Cariaco Basin, off the Venezuelan coast, can be used to infer variations in the hydrological cycle over northern South America during the past 14,000 years with subdecadal resolution. Following a dry Younger Dryas, a period of increased precipitation and riverine discharge occurred during the Holocene "thermal maximum." Since approximately 5400 years ago, a trend toward drier conditions is evident from the data, with high-amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the "Little Ice Age." These regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone (ITCZ), potentially driven by Pacific-based climate variability. The Cariaco Basin record exhibits strong correlations with climate records from distant regions, including the high-latitude Northern Hemisphere, providing evidence for global teleconnections among regional climates. 相似文献
3.
On occasion the decametric radio bursts from Jupiter contain pulses of millisecond duration. Study of data for 2 years shows that the distribution in Jovian longitude of these fast pulses is different from that of the more common pulses of longer duration. The two classes of pulses also appear to be differently affected by the position of the innermost Galilean satellite. 相似文献
4.
Umina PA Weeks AR Kearney MR McKechnie SW Hoffmann AA 《Science (New York, N.Y.)》2005,308(5722):691-693
Geographical clines in genetic polymorphisms are widely used as evidence of climatic selection and are expected to shift with climate change. We show that the classic latitudinal cline in the alcohol dehydrogenase polymorphism of Drosophila melanogaster has shifted over 20 years in eastern coastal Australia. Southern high-latitude populations now have the genetic constitution of more northerly populations, equivalent to a shift of 4 degrees in latitude. A similar shift was detected for a genetically independent inversion polymorphism, whereas two other linked polymorphisms exhibiting weaker clinal patterns have remained relatively stable. These genetic changes are likely to reflect increasingly warmer and drier conditions and may serve as sensitive biomarkers for climate change. 相似文献
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6.
We describe a method for storing sequences of optical data pulses by converting them into long-lived acoustic excitations in an optical fiber through the process of stimulated Brillouin scattering. These stored pulses can be retrieved later, after a time interval limited by the lifetime of the acoustic excitation. In the experiment reported here, smooth 2-nanosecond-long pulses are stored for up to 12 nanoseconds with good readout efficiency: 29% at 4-nanosecond storage time and 2% at 12 nanoseconds. This method thus can potentially store data packets that are many bits long. It can be implemented at any wavelength where the fiber is transparent and can be incorporated into existing telecommunication networks because it operates using only commercially available components at room temperature. 相似文献
7.
利用方波脉冲电场增强乳酸菌细胞通透性,通过将乳酸菌细胞离心,制成浓度为108cfu.mL-1菌悬液,采用不同方波电脉冲条件对其进行处理。结果表明,在0.5 mol.L-1蔗糖的等渗脉冲介质中,电场强度2.25kV.cm-1,脉冲宽度15μs,脉冲数5,间隔时间2 s,此时,乳酸菌细胞的存活率平均为78.24%,细胞内平均荧光强度为5 480。表明脉冲电场可有效增强乳酸菌细胞通透性。 相似文献
8.
Bilateral stimulation of the medial forebrain bundle with pulses of varying interpulse intervals elicited a pattern of self-stimulation behavior in the rat indicative of temporal synaptic summation: the shorter the interval the greater the response. In contrast, the effectiveness of unilateral stimulation at very short intervals was limited by neuronal refractory periods. The results support the notion that there is convergence of the medial forebrain bundle self-stimulation system from the two sides of the brain and suggest ways of studying the degree of convergence. They also suggest a technique for behaviorally comparing heterosynaptic and homosynaptic mechanisms of summation. 相似文献
9.
基于双区间两阶段随机规划的黑河中游主要农作物种植结构优化 总被引：3，自引：2，他引：1
为优化有限农业水资源量下的种植结构以实现最大经济效益,考虑种植结构优化过程中存在的水文要素与社会经济要素中存在的不确定性,构建以经济效益最大为目标的双区间两阶段随机规划模型。在农业水资源量测算时出现了双区间特点,故使用双区间理论对农业水资源量进行表征计算。将构建的模型用于黑河中游甘肃省张掖市甘州区、临泽县、高台县的5种水文年(枯水年、较枯水年、平水年、较丰水年以及丰水年)下的种植结构优化,比较了优化结果与现状水平年种植结构与经济收益。结果表明:通过双区间两阶段随机规划模型得到的3种种植结构优化方案即保守决策方案、趋于平均决策方案和乐观决策方案产生的经济效益比现状水平年种植结构下的经济效益分别提高[1.97,8.53]、[4.20,11.04]、[6.43,13.56]亿元。 相似文献
10.
A new technique in which elicited behavior of the freely moving rat is used to measure the poststimulation excitability cycle of the central neurons mediating that behavior has been adapted from accepted methods of neurophysiology. A continuous train of pairs of brief pulses was delivered to pain systems in the midbrain. Rate of lever pressing to achieve 3-second rests from this stimulation was measured as a function of the interval separating the pulses within pairs. Evidence for latent addition, absolute refractory period, temporal summation, and adaptation was demonstrated. Obtained relationships suggested that three sets of fibers may carry the aversive signal and that synaptic integration of pain in the brain may be related to Stevens' power law functions. 相似文献
11.
刘长海 《仲恺农业技术学院学报》2002,15(2):36-40
对现有几种新型气流干燥器的特点进行了介绍 ,并就气流干燥系统的优化设计问题进行了探讨 鉴于传热系数随高度增加而下降是直管气流干燥器的主要缺点 故采用新型气流干燥器 ,如脉冲气流干燥器等可以改变这种情况 两种以上设备构成的组合式气流干燥器是今后发展的方向 另外 ,干燥系统中增加粉碎装置可以扩大设备的使用范围 提高空气的入口温度 ,有利于提高热效率 . 相似文献
12.
Carbon istotope ratios of modern, 10,000-year-old, and more than 40,000-year-old Atriplex confertifolia (C(4)) material from Nevada caves indicate that the C(4) photosynthetic pathway was operating in these plants over that period. Samples of a plant with crassulacean acid metabolism, Opuntia polyacantha, were also measured, and a shift in the 8(13)C value from -21.9 per mil (more than 40,000 years ago) to -13.9 per mil (10,000 years ago) was observed. This provides unique physiological evidence to support the hypothesis that the late Pleistocene pluvial climate in the region already had become drier about 10,000 years ago. 相似文献
13.
滚筒式牧草干燥机性能研究 总被引：4，自引：2，他引：2
为优化苜蓿干燥机结构参数,提高干燥机性能,在自行设计的滚筒式牧草干燥机上进行试验研究,得出喂入率、干燥温度和滚筒转速对其性能的影响,在喂入率为25～30kg·min-1,干燥温度为400℃,滚筒转速10r·min-1时,苜蓿最终含水率可达安全贮藏水分。研究还定量分析出各参数对干燥机性能指标的影响规律,得出不同温度下喂入率与最终含水率的数学模型,为确定合理干燥机结构和作业参数提供参考。 相似文献
14.
大黄鱼的发声及信号特性研究 总被引：3，自引：0，他引：3
对大黄鱼Pseudosciaena crocea觅食及产卵时的发声及信号特性进行了研究。结果表明:不同行为下大黄鱼发声信号的频谱特性一致,即约在800 Hz频率处有一个明显的谱峰;但不同行为下发声的时域信号差距较大,即觅食时大黄鱼的发声信号都是简单的单脉冲,脉冲间隔大多集中在1~30 ms,脉冲间隔比较小,而产卵时大黄鱼的发声信号则大部分是连续的双脉冲或三脉冲,只有极个别的单脉冲或多脉冲,脉冲间隔则集中在100~130 ms,脉冲间隔要大得多。 相似文献
15.
Cvancara AM Clayton L Bickley WB Jacob AF Ashworth AC Brophy JA Shay CT Delorme LD Lammers GE 《Science (New York, N.Y.)》1971,171(3967):172-174
A unique late Quaternary lacustrine deposit has been discovered recently on the Missouri Coteau of North Dakota. A diverse, extremely well-preserved biota of more than 160 species has been recovered primarily from an organic mud deposited about 9500 years before the present. The lacustrine body shallowed gradually as the climate became drier. 相似文献
16.
The cause of decreased pan evaporation over the past 50 years 总被引：17，自引：0，他引：17
Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration. 相似文献
17.
Abrupt first-order shifts in strontium and phosphorus concentrations in stalagmite calcite deposited in western Ireland during the 8200-year event (the major cooling episode 8200 years before the present) are interpreted as responses to a drier climate lasting about 37 years. Both shifts are centered on 8330 +/- 80 years before the present, coinciding with a large oxygen isotope anomaly and a change in the calcite petrography. In this very high resolution (monthly) record, antipathetic second-order oscillations in phosphorus and strontium reveal decreased growth rates and increased rainfall seasonality. Growth rate variations within the event reveal a two-pronged structure consistent with recent model simulations. 相似文献
18.
Weyhenmeyer CE Burns SJ Waber HN Aeschbach-Hertig W Kipfer R Loosli HH Matter A 《Science (New York, N.Y.)》2000,287(5454):842-845
Concentrations of atmospheric noble gases (neon, argon, krypton, and xenon) dissolved in groundwaters from northern Oman indicate that the average ground temperature during the Late Pleistocene (15,000 to 24,000 years before present) was 6.5 degrees +/- 0.6 degrees C lower than that of today. Stable oxygen and hydrogen isotopic groundwater data show that the origin of atmospheric water vapor changed from a primarily southern, Indian Ocean source during the Late Pleistocene to a dominantly northern, Mediterranean source today. The reduced northern water vapor source is consistent with a drier Last Glacial Maximum through much of northern Africa and Arabia. 相似文献
19.
冬小麦保护性耕作法与传统耕作法的田间对比试验 总被引：28，自引：0，他引：28
对６种保护性耕作法和传统耕作法进行了连续６ａ的田间对比试验研究,结果表明,采用保护性耕作法的冬小麦地夏休闲期蓄水量明显高于传统耕作,平均多蓄水９％,水分利用效率比传统耕作平均高１３．２％;产量比传统耕作平均增加约１４％,其中旱年增产幅度较大,平、丰水年增产幅度较小。 相似文献
20.
利用1971～2007年抚顺地区章党站、清原站、新宾站3个气象站的气候资料,采用线性气候倾向率方法,对年和春、夏、秋、冬四季的光、水等主要农业气候资源要素进行了诊断分析。结果表明,近37年来,抚顺地区降水量出现减少趋势,趋势并不剧烈,但季节性不同,秋季降水减少最显著,与其相关的蒸发潜力变化趋势也并不明显;太阳辐射与日照时数变化趋势一致,均呈下降趋势,夏季更少。抚顺地区总体的气候变化存在以降水、日照逐渐减少为主要特点的暖干化趋势,秋季更为显著。 相似文献