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1.
Analyses of marine carbonates through the interval 63.9 to 65.4 million years ago indicate a near-constant flux of extraterrestrial helium-3, a tracer of the accretion rate of interplanetary dust to Earth. This observation indicates that the bolide associated with the Cretaceous-Tertiary (K-T) extinction event was not accompanied by enhanced solar system dustiness and so could not have been a member of a comet shower. The use of helium-3 as a constant-flux proxy of sedimentation rate implies deposition of the K-T boundary clay in (10 +/- 2) x 10(3) years, precluding the possibility of a long hiatus at the boundary and requiring extremely rapid faunal turnover. 相似文献
2.
Inaccurate stratigraphic correlations in the Hell Creek area, Montana, have led to the assumption that transitional vertebrate faunas (Bug Creek Anthills) exist in the latest Cretaceous, refuting a catastrophic turnover at the Cretaceous-Tertiary boundary. Establishment of the transitional faunas in Paleocene channels that cut down through the Cretaceous-Tertiary boundary renders the terrestrial faunal record compatible with the marine record and with catastrophic extinction. 相似文献
3.
Renne PR Ernesto M Pacca IG Coe RS Glen JM Prévot M Perrin M 《Science (New York, N.Y.)》1992,258(5084):975-979
The Paraná-Etendeka flood volcanic event produced approximately 1.5 x 10(6) cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New (40)Ar/(39)Ar data combined with earlier paleomagnetic results indicate that Paraná flood volcanism in southern Brazil began at 133 +/- 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paraná flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary. 相似文献
4.
Lithologic, faunal, seismic, and isotopic evidence from the Blake Nose (subtropical western North Atlantic) links a massive release of biogenic methane approximately 55.5 million years ago to a warming of deep-ocean and high-latitude surface waters, a large perturbation in the combined ocean-atmosphere carbon cycle (the largest of the past 90 million years), a mass extinction event in benthic faunas, and a radiation of mammalian orders. The deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming, massive carbon input to the global exogenic carbon cycle, pelagic carbonate dissolution, a decrease in dissolved oxygen, and a benthic foraminiferal extinction event. These events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the Blake Nose as a site of methane release. 相似文献
5.
Wang K 《Science (New York, N.Y.)》1992,256(5063):1547-1550
The properties of microspherules recovered from an Upper Devonian marine limestone immediately overlain by a geochemical anomaly of siderophile and chalcophile elements are similar to those of impact-derived microtektites. These microspherules are glass, have splash-form shapes, contain spherical vesicles and lechatelierite inclusions, and show oxide compositional variations similar to those in known microtektites. These characteristics suggest that these Upper Devonian microspherules have an impact origin. A bolide impact may have occurred about 365 million years ago on the South China Plate and caused a faunal extinction on eastern Gondwana. 相似文献
6.
Martin PS 《Science (New York, N.Y.)》1973,179(4077):969-974
I propose a new scenario for the discovery of America. By analogy with other successful animal invasions, one may assume that the discovery of the New World triggered a human population explosion. The invading hunters attained their highest population density along a front that swept from Canada to the Gulf of Mexico in 350 years, and on to the tip of South America in roughly 1000 years. A sharp drop in human population soon followed as major prey animals declined to extinction. Possible values for the model include an average frontal depth of 160 kilometers, an average population density of 0.4 person per square kilometer on the front and of 0.04 person per square kilometer behind the front, and an average rate of frontal advance of 16 kilometers per year. For the first two centuries the maximum rate of growth may have equaled the historic maximum of 3.4 percent annually. During the episode of faunal extinctions, the population of North America need not have exceeded 600,000 people at any one time. The model generates a population sufficiently large to overkill a biomass of Pleistocene large animals averaging 9 metric tons per square kilometer (50 animal units per section) or 2.3 x 10(8) metric tons in the hemisphere. It requires that on the front one person in four destroy one animal unit (450 kilograms) per week, or 26 percent of the biomass of an average section in 1 year in any one region. Extinction would occur within a decade. There was insufficient time for the fauna to learn defensive behaviors, or for more than a few kill sites to be buried and preserved for the archeologist. Should the model survive future findings, it will mean that the extinction chronology of the Pleistocene megafauna can be used to map the spread of Homo sapiens throughout the New World. 相似文献
7.
Martinez RN Sereno PC Alcober OA Colombi CE Renne PR Montañez IP Currie BS 《Science (New York, N.Y.)》2011,331(6014):206-210
Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement. 相似文献
8.
The Siberian Traps represent one of the most voluminous flood basalt provinces on Earth. Laser-heating (40)Ar/(39)Ar data indicate that the bulk of these basalts was erupted over an extremely short time interval (900,000 +/- 800,000 years) beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year. Such rates are consistent with a mantle plume origin. Magmatism was not associated with significant lithospheric rifting; thus, mantle decompression resulting from rifting was probably not the primary cause of widespread melting. Inception of Siberian Traps volcanism coincided (within uncertainty) with a profound faunal mass extinction at the Permo-Triassic boundary 249 +/- 4 million years ago; these data thus leave open the question of a genetic relation between the two events. 相似文献
9.
Paleomagnetic study of antarctic deep-sea cores 总被引：1，自引：0，他引：1
The magnetic inclinations and inten sities of about 650 samples from seven deepsea cores taken in the Antarctic were measured on a spinner magnetometer. This series of measurements provided a magnetic stratigraphy, based on zones of normally or reversally polar ized specimens for each core, which was then correlated with the magnetic stra tigraphy of Cox et al. (1). One core (V16-134) gave a continuous record of the paleomagnetic field back to about 3.5 million years. When selected samples were subject ed to alternatingfield demagnetization, most were found to have an unstable component that was removed by fields of 150 oersteds; all samples from two cores were partially demagnetized in a field of 150 oersteds. The average inclination in these two cores was then in good agreement with the average inclination of the ambient field for the latitude of the core site. It was also found that the intensities of the samples decreased at the points of reversal; this finding is to be expected if, as has been postulated by the dynamo theory, the intensity of the dipole field decreases to zero and builds again with opposite polarity. We believe that the magnetiza tion of the cores results from the pres ence of detrital magnetite, although other magnetic minerals also may be present. Four faunal zones (, X, , and ) have been recognized in these Antarctic cores on the basis of upward sequential disappearance of Radiolaria. The faunal boundaries and reversals consistently have the same relations to one another, indicating that they are both timedependent phenomena. Using previously determined times of reversal, one may date the following events in the cores: 1) Radiolarian faunal boundaries:-X, 2 million years; X-, 0.7 million years; -, 0.4 to 0.5 million years. These dates are in good agreement with ages previously extrapolated from radio metric dates. 2) Initiation of Antarctic diatom ooze deposition, approximately 2.0 mil-lion years ago. 3) First occurrence of ice- rafted detritus, approximately 2.5 million years ago. One can also calculate rates of sedi mentation, which vary in the cores studied from 1.1 to about 8.0 millimeters per 1000 years. Sedimentation rates for the Indian Ocean cores are higher than for the Bellingshausen Sea cores. The near coincidence of faunal changes and reversals in the cores suggests but does not prove a causal relation. We conclude from this study that paleomagnetic stratigraphy is a unique method for correlating and dating deep sea cores, and that future work with such cores may provide a complete or nearly complete record of the history of the earth's magnetic field beyond 4 million years. 相似文献
10.
2006～2009年,作者对内蒙古地区的跳蝽科(Saldidae)昆虫进行了整理分类,共整理出内蒙古地区跳蝽23种,隶属于2亚科2族7属,并对其进行了区系分析。结果表明,在世界动物地理区系分布中,内蒙古跳蝽科昆虫具有显著的古北界特性,在中国动物地理区系中,显示出蒙新区特性。 相似文献
11.
J H Hollis 《Science (New York, N.Y.)》1968,159(3822):1487-1489
A retarded child with a high stereotyped rocking rate was conditioned to pull a ball on a reinforcement schedule in which the fixed ratio aof rewarded to nonrewarded responses was 100. Results show no rocking movements during ball-pulling; but when ball-pulling was on extinction, rocking returned to its original rate. Chlorpromazine blocked rocking movements during extinction, but had no effect on ball-pulling. Delivery of one free reinforcer was sufficient to reinstate ball-pulling after extinction, but the stimulus properties of the free reinforcer were not affected by the drug. 相似文献
12.
LAI-2000冠层分析仪在不同植物群体光分布特征研究中的应用 总被引：32，自引：1，他引：32
【目的】寻求快速准确确定各种农林植物冠层透光率及其随时间变化规律的方法，以及通过冠层分析确定农林植物冠层消光系数的方法。【方法】应用LAI-2000冠层分析仪和LI-1400数据采集器连接LI-190、LI-191传感器，测定了高秆作物（玉米）、矮秆作物（大豆）和林木群体（楸树、白蜡、栾树苗圃幼林冠层）的LAI、叶倾角、冠层开度及群体透光率。【结果】（1）LAI-2000测得的冠层开度，与全天群体透光率相关非常密切。相关系数达0.9308，回归直线的截距为0.015，接近于0，斜率为1.0668，接近于1。所以，完全可以使用冠层开度估计透光率。阴天比晴天估计效果更好。（2）用实测的LAI和群体透光率，准确计算出了农林植物群体的消光系数。消光系数有明显的日变化，玉米冠层一日中出现两个极大值，中午最低。【结论】LAI-2000冠层分析仪能给出群体内不同天顶角方向的天空开度，能更详细地表达植物群体结构特征；群体内某一点的冠层开度与该点的冠层透光率在数值上几乎相等，所以冠层内透光率测定不再需要冠层上下同时测光计算，而直接由测冠层开度得到；LAI-2000无损测定冠层LAI，可用于计算群体消光系数，文中所计算的消光系数，处在文献报道的正常范围。 相似文献
13.
Hastings A 《Science (New York, N.Y.)》2003,301(5639):1525-1526
Simple metapopulation models with random disturbances are used in species conservation or management, relying on the condition for persistence that the per-patch colonization rate be greater than the per-patch extinction rate. For a general model incorporating the role of succession or patch age-dependent extinction, I show that persistence requires the per-patch colonization rate to be greater than the inverse of mean patch age, where age is the time since the patch became available for colonization. Because mean patch age can be arbitrarily large relative to the extinction rate, currently persisting metapopulations may be persisting despite small colonization rates, and management approaches that do not consider the role of mean patch age may unnecessarily doom populations to extinction. 相似文献
14.
土壤动物在陆地生态系统物质循环和能量流动中起着重要作用,直接或间接的参与土壤有机质的分解与矿化;长期施肥对土壤理化性质产生影响的同时,改变了土壤动物群落组成.为查明紫色土长期施肥对土壤动物群落的影响及其响应关系,于2008年的5、7、9和11月分别对紫色土农田无肥对照(CK)、单施氮肥(N)、常规化肥氮磷钾(NPK)、有机肥(OM)、有机肥与化肥氮磷钾混施(OMNPK)、秸秆还田(RSD)和秸秆还田与化肥氮磷钾混施(RSDNPK)等7种长期施肥定位试验地的土壤动物群落进行调查,采用改良的干漏斗和湿漏斗两种方法,共获得土壤动物9454只,隶属7门17纲24目.分析表明,OM和RSDNPK两种施肥方式下土壤动物群落的多样性显著高于CK、N和NPK等3种施肥方式,说明有机物料的长期投入有利于提高土壤动物群落丰富度和多样性.方差分析表明施肥方式对土壤动物主要类群密度的影响差异性极显著(F=42.412,P=0.0001),对土壤动物群落类群影响存在不均衡性.施肥方式主要影响农田土壤动物类群的种群个体数量、线虫动物门个体数量、大蚓类个体数量、甲螨亚目个体数量、密度-类群指数DG及土壤动物群落类群数等六个指标,初步认为这些主要类群因素能够预测长期施肥引起的土壤肥力变化,可能对指示土壤质量的变化具有一定潜力. 相似文献
15.
Kerr RA 《Science (New York, N.Y.)》2000,289(5485):1666b-1667b
Two hundred and fifty million years ago, at the end of the Permian period and the opening of the Triassic, 85% of the species in the sea vanished in a geologic moment of less than half a million years. Now from South Africa comes evidence that the Permian-Triassic extinction of land plants was equally brutal and swift. In a paper on page 1740 of this issue of Science, researchers report that rocks that started as sediments laid down in South Africa's Karoo Basin 250 million years ago tell of an abrupt switch in style of sedimentation, as if the land had been permanently stripped of the rooted plants that held it in place. But in the absence of any trace of an impact, researchers are groping for an equally far-reaching explanation. 相似文献
16.
【目的】以绿豆为原料,添加不同的稳定剂优化绿豆饮料的稳定性。【方法】以绿豆饮料离心沉淀率为指标,在调查羧甲基纤维素钠(CMC-Na)、三聚磷酸钠(STPP)、六偏磷酸钠(SHMP)3个单体稳定剂影响绿豆饮料稳定性单因素试验结果的基础上,采用响应面分析确定最佳稳定剂配比。【结果】结果表明,当复合稳定剂CMC-Na、STPP和SHMP的添加量分别为0.02%、0.10%、0.06%时,绿豆饮料的离心沉淀率最小值为9.19%。【结论】降低沉淀率,部分解决绿豆饮料沉淀问题,为绿豆饮料生产提供参考。 相似文献
17.
根据剑角蝗科（Acrididae）在我国各动物地理区的种类组成和区系特点,发现华北区和华中区的区系特点不明显,前者为南北成分的混交区,后者为特有种可跨区分布,这与蝗虫有明显的区域和地带分布的特点不相符。 相似文献
18.
氮肥用量对面包小麦品种产量和品质的影响 总被引：7，自引：1，他引：7
通过在小麦拔节期追施不同的氮肥用量来研究各处理下面包小麦的产量和品质,结果表明:小麦产量受基因型、供氮背景及其互作的影响,表现为拔节期适当的氮肥用量能提高产量;而品质表现主要受基因型和供氮背景的影响.其中蛋白质含量、湿面筋含量受氮肥用量影响较大,供氮效应超过了基因型效应;SDS沉降值和角质率受到基因型效应和供氮效应两方面影响,但基因型效应是主要的;比沉降值、伯尔辛克值和面筋-乳酸溶液透光率只受基因型效应的影响,供氮效应可以忽略. 相似文献
19.
应用氚化胸苷示踪法研究了滆湖浮游物与沉降物中细菌生长速率的动态变化，同时测定了滆湖有机碎屑沉降量、水体中有机物含量以及C／N的动态变化。结果表明：滆潮浮游物与沉降物中细菌生长速率的变化具有很明显的季节性，其与水温、水环境中的有机物含量、碳氮比、有机碎屑沉降量等存在密切的联系。我们认为，细菌生长速率是阐明湖泊生态系结构与功能的主要参数，也是水体渔业开发的重要依据之一。 相似文献
20.
滆湖水域中细菌生长速率的测定及其动态研究 总被引：2，自引：0，他引：2
应用氚化胸苷示踪法研究了滆湖浮游物与沉降物中细菌生长速率的动态变化，同时测定了滆湖有机碎屑沉降量、水体中有机物含量以及C／N的动态变化。结果表明：滆潮浮游物与沉降物中细菌生长速率的变化具有很明显的季节性，其与水温、水环境中的有机物含量、碳氮比、有机碎屑沉降量等存在密切的联系。我们认为，细菌生长速率是阐明湖泊生态系结构与功能的主要参数，也是水体渔业开发的重要依据之一。 相似文献