Two paleoecological histories spanning the period of human settlement in southeastern Brazil     

Marco F. Raczka  Paulo E. De Oliveira  Mark Bush  Crystal H. McMichael 《第四纪科学杂志》2013,28(2):144-151

The absence of human occupation sites in southeastern Brazil during the mid Holocene has been referred to as the 'Archaic Gap' (8970–1940 cal. a BP) and is predicted to have resulted from increased aridity. A ca. 14 000 cal. a pollen history from two well‐dated lake sediment cores located within the archeological district of Lagoa Santa, in the State of Minas Gerais, southeastern Brazil, was used to test this hypothesis. Our analyses indicated that the present cerrado and tropical semi‐deciduous forest mosaic persisted throughout the mid Holocene, until ca. 5500 cal. a BP, and the Lagoa Santa region did not experience especially dry conditions during the Holocene period. The early Holocene pollen spectra contained an assemblage of cold‐adapted taxa such as Podocarpus, Myrsine and Araucaria, co‐occurring with taxa from cerrado, e.g. Caryocar. A replacement of cold taxa by the modern cerrado–semi‐deciduous forest vegetation took place progressively, but appears to have been completed by the mid Holocene. No evidence of sustained drought was found in sedimentation or forest composition, nor any prolonged dry event in the study region. Holocene dryness as an explanation for the abandonment of Lagoa Santa region is not supported by the palynological analyses conducted in this study. Rather it is suggested that unpredictable climate may have underlain that abandonment. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Characterizing changes in the sedimentary environment of a varved lake sediment record in southern central Finland around 8000 cal. yr BP     

Antti E. K. Ojala  Atko Heinsalu  Tommi Kauppila  Teija Alenius  Matti Saarnisto 《第四纪科学杂志》2008,23(8):765-775

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初夏至盛夏东亚副热带西风急流变化与江淮出梅的关系          下载免费PDF全文

董丽娜  郭品文  张福颖   《大气科学学报》2010,33(1):74-81

利用1961—2005年逐候资料对东亚副热带西风急流初夏至盛夏变化与江淮出梅的关系进行了分析。结果表明,多年平均7月初夏至盛夏急流中心由西太平洋地区西跳至青藏高原的同时我国东部地区急流北跳至37.5°N以北,比梅雨结束早1候;急流北跳使得我国东部高空强辐散中心北移至华北地区,江淮地区上空辐散显著减弱,上升运动减弱,从而使得江淮梅雨结束,雨带北移;而急流中心的西跳仅使得我国东部地区高空辐散中心减弱,降水减弱,有利于雨带北移。我国东部急流北跳与江淮地区梅雨结束时间显著正相关,在北跳偏早(晚)年份梅雨结束早(晚),长江中下游地区降水偏少(多),而急流中心西跳早晚对我国华北北部地区和淮河附近地区降水有较大影响。可见,我国东部急流北跳与梅雨结束关系密切,可作为梅雨结束的先期信号。  相似文献   

河西地区重力动态变化的二维离散小波多尺度分解   总被引：7，自引：5，他引：2  

朱桂芝  祝意青  唐文  程宏宾 《大地测量与地球动力学》2004,24(1):46-49

采用二维离散小波的多尺度分解技术，对1994～1995年和2002～2003年的河西地区重力动态变化进行分解，提取该地区不同层次的重力动态变化异常，结果反映了不同深度在一定尺度意义上的介质密度的动态变化分布。  相似文献   

THE RELATIONSHIP BETWEEN INLAND LAKES EVOLUTION AND CLIMATIC FLUCTUATION IN ARID ZONE   总被引：2，自引：0，他引：2  

秦伯强  施雅风  王苏民 《中国地理科学(英文版)》1991,1(4):316-323

The evolution of the inland lakes in arid and semi-arid zones is accorded with the climatic fluctuation. The humid climate is in harmony with the higher water level and greater lake water quantity budget while arid climate is in correspondence with the lower water level and little water budget. Based on the analysis of the lake fluctuation and lake budget change, with the aid of the data of geom'orphology, palynology, sedimentology and chronology, It is found that the climate experienced a warm and humid period during 7000-3500 yr. B. P. and showed a drying and warming trend in the last century in the Central Asia.  相似文献   

On the acceleration of solar-flare charged particles in a collapsing magnetic trap with an electric potential     

V. A. Kovalev  B. V. Somov 《Astronomy Letters》2002,28(7):488-493

We consider the kinetic problem of charged-particle acceleration in a magnetic trap with converging magnetic mirrors. We show that for a positive electrostatic potential of the trap plasma relative to the mirrors, the efficiency of confinement and acceleration increases for electrons and decreases for ions.  相似文献   

Geochemical evidence (C,N and Pb isotopes) of recent anthropogenic impact in south‐central Chile from two environmentally distinct lake sediment records     

Nathalie Fagel  Sebastien Bertrand  Nadine Mattielli  Delphine Gilson  Luis Chirinos  Gilles Lepoint  Roberto Urrutia 《第四纪科学杂志》2010,25(7):1100-1112

In this paper, we compare the elemental and isotopic (C, N, Pb) geochemistry of lake sediments from two contrasted environments in south‐central Chile. The first lake, Laguna Chica de San Pedro (LCSP), is situated in the urbanised area of the Biobio Region (36°S). The second lake, Lago Puyehue (40° S), is located 400 km to the southeast of LCSP and within an Andean national park. Our aim is to identify environmental impacts associated with increasing industrial activities and land degradation during the last 150 a. In LCSP, shifts in C/N atomic ratios, δ¹³C and δ¹⁵N from 1915–1937 to the late 1980s are attributed to successive land degradation episodes in the lake watershed. Based on a Pb isotopic mixing model, we estimate that up to 20% of lead in LCSP sediments is supplied from urban atmospheric pollution. By contrast, human impact in the watershed of Lago Puyehue is very limited. We observe no change in organic geochemistry during the last 150 a and lead contamination remains lower than 5%, even during the last decades. Although contamination levels are much higher in LCSP than in Lago Puyehue, a peak in anthropogenic Pb is recorded during the same period (1974–1976) at both sites. This maximum contamination level is consistent with increased industrial activity in the vicinity of Concepción. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

小秦岭金矿田507脉成矿期内构造活动及矿体定位关系     

郭晓东  王广兰 《河南国土资源》1997,(3)

根据构造控矿的观点，通过对成矿期内容矿断裂活动分析，阐述了构造活动与矿体定位的关系。  相似文献   

秦巴山区环境污染及治理     

谢娟 《地球科学与环境学报》2001,23(2):65-67

对秦巴山区的开发利用，各种工业、厂矿在生产过程产生的“三废”(废气、废水、废渣)未经处理即任意排放，造成河流水质恶化，影响着秦岭的环境质量，提醒人们及早关注环境生态变化，加强环保意识，保护秦巴山区的生态平衡。  相似文献   

强质子耀斑活动区的形态和旋转特征     

周树荣 《中国天文和天体物理学报》1996,(3)

本文研究结果表明：同一黑子群在日面期间的顺或反时针方向的旋转运动会先后并存．质子耀斑前1～2无，黑子群的旋转角速度达到极大．耀斑后，磁绳的松弛，黑子群可能会反向旋转，强的剪切过程和质子耀斑可能会再度出现．强质子耀斑活动区的共同特征是：（1）形态为单个团状结构δ型黑子，即众多异极性本影核紧锁在同一黑子半影中；（2）黑子面积＞1000×10－6半球面积，日面跨度＞10°；（3）黑子群都有快速的旋转运动．这类活动区，如果在日面西部活动性明显地增强，那么这个活动区在未来转到日面边缘及其背后、或再次从日面东边缘转出时，定能再次爆发耀斑和伴随较强质子事件。  相似文献