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151.
152.
深海钻探揭示的古新统—始新统界线上许多全球性变化引起了地质学家的普遍关注。这些变化包括:岩性上从海绿石石英砂岩变化到粘土岩、粘土矿物组合从伊利石/蒙脱石为主变化为高岭石为主、大陆植物群的绝灭和迁移、钙质超微化石物种更新速度加快、底栖有孔虫绝灭和分异度降低、碳氧同位素强烈负异常、大陆哺乳动物演化发生变化、大气环流强度减弱、海洋环流模式改变、海平面上升、海底热液活动呈1~2个数量级加强。通过对古新世—始新世的地层沉积学、古生物学、古气候学和古海洋学研究,确认古新统—始新统地层界线是全球构造事件的结果,表现为全球板块边界重组、扩张中心和转换断层模式改变、海底热液活动呈1~2个数量级加强。 相似文献
153.
Application of bivariate extreme value distribution to flood frequency analysis: a case study of Northwestern Mexico 总被引:1,自引:0,他引:1
Carlos Escalante-Sandoval 《Natural Hazards》2007,42(1):37-46
In Mexico, poverty has forced people to live almost on the water of rivers. This situation along with the occurrence of floods
is a serious problem for the local governments. In order to protect their lives and goods, it is very important to account
with a mathematical tool that may reduce the uncertainties in computing the design events for different return periods.
In this paper, the Logistic model for bivariate extreme value distribution with Weibull-2 and Mixed Weibull marginals is proposed
for the case of flood frequency analysis. A procedure to estimate their parameters based on the maximum likelihood method
is developed. A region in Northwestern Mexico with 16 gauging stations has been selected to apply the model and regional at-site
quantiles were estimated. A significant improvement occurs, measured through the use of a goodness-of-fit test, when parameters
are estimated using the bivariate distribution instead of its univariate counterpart. Results suggest that it is very important
to consider the Mixed Weibull distribution and its bivariate option when analyzing floods generated by a␣mixture of two populations. 相似文献
154.
High-resolution climate variability of southwest China during 57-70 ka reflected in a stalagmite δ18O record from Xinya Cave 总被引:1,自引:0,他引:1
Li TingYong Yuan DaoXian Li HongChun Yang Yan Wang JianLi Wang XinYa Li JunYun Qin JiaMing Zhang MeiLiang Lin YuShi 《中国科学D辑(英文版)》2007,50(8):1202-1208
A 26-cm-long stalagmite (XY2) from Xinya Cave in northeastern Chongqing of China has been ICP-MS 230Th/U dated, showing a depositional hiatus at 2.3 cm depth from the top. The growth of the 2.3–26 cm interval determined by
four dates was between 57 ka and 70 ka, with a linear growth rate of 0.023 mm/a. We have analyzed 190 samples for δ
18O and δ
13C, mostly in the 2.3–26 cm part. The δ
18O and δ
13C values between 57 ka and 70 ka reveal decadal-to-centennial climatic variability during the glacial interval of Marine Isotope
Stage 4 (MIS4), exhibiting much higher resolution than that of the published Hulu and Dongge records during this interval.
Speleothem δ
18O in eastern China, including our study area can be used as a proxy of summer monsoon strength, with lighter values pointing
to stronger summer monsoon and higher precipitation, and vice versa. Two decreases in the δ
18O signature of XY2 record around 59.5 and 64.5 Ka are argued to correspond to the Dansgaard-Oeschger (D-O) events 17 and 18
respectively. The Heinrich event 6 (H6) can be identified in the record as a heavy δ
18O peak around 60 ka, indicating significant weakening of the monsoon in Chongqing during the cold period. The XY2 δ
18O record shows very rapid change toward to the interstadial condition of the D-O event, but more gradual change toward to
the cold stadial condition. This phenomenon found in the Greenland ice core records is rarely observed so clearly in previously
published speleothem records. According to SPECMAP δ
18O record, the glacial maximum of MIS 4 was around 64.5 ka with the boundary of MIS 3/4 around 60 ka. Unlike the marine record,
the speleothem record of XY2, China, exhibits much high frequency variations without an apparent glacial maximum during MIS
4. However, the timing of MIS 3/4 boundary seems to be around 60 ka when the H6 terminated, in agreement with the marine chronology.
The growth period of sample XY2 during glacial times probably reflects a local karstic routing of water, rather than having
climatic significance.
Supported by the National Natural Science Foundation of China (Grant Nos. 40672165, 90511004, 40672202) and the Academician
Special Project of Chongqing Science Committee (Grant No. 2003-7835) 相似文献
155.
K. M. Tomkins G. S. Humphreys M. T. Wilkinson D. Fink P. P. Hesse S. H. Doerr R. A. Shakesby P. J. Wallbrink W. H. Blake 《地球表面变化过程与地形》2007,32(7):1013-1031
Short‐term (contemporary) and long‐term denudation rates were determined for the Blue Mountains Plateau in the western Sydney Basin, Australia, to explore the role of extreme events (wildfires and catastrophic floods) in landscape denudation along a passive plate margin. Contemporary denudation rates were reconstructed using 40 years of river sediment load data from the Nattai catchment in the south‐west of the basin, combined with an analysis of hillslope erosion following recent wildfires. Long‐term denudation rates (10 kyr–10 Myr) were determined from terrestrial cosmogenic nuclides, apatite fission track thermochronology and post‐basalt flow valley incision. Contemporary denudation rates average several times lower than the long‐term average (5·5 ± 4 mm kyr?1 versus 21·5 ± 7 mm kyr?1). Erosion of sediment following wildfires accounts for only a small proportion (5%) of the contemporary rate. Most post‐fire sediment is stored on the lower slopes and valley floor, with the amount transported to the river network dependent on rainfall–run‐off conditions within the first few years following the fire. Historical catastrophic floods account for a much larger proportion (35%) of the contemporary erosion rate, and highlight the importance of these events in reworking stored material. Evidence for palaeofloods much larger than those experienced over the past 200 years suggests even greater sediment export potential. Mass movement on hillslopes along valleys incised into softer lithology appears to be a dominant erosion process that supplies substantial volumes of material to the valley floor. It is possible that a combination of infrequent mass movement events and high fluvial discharge could account for a significant proportion of the discrepancy between the contemporary and long‐term denudation rates. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
156.
157.
1999年 5期《第四纪研究》刊登的“青藏高原冰期环境与冰期全球降温”论文 ,内容丰富 ,提出了许多重大问题和新鲜讯息 ,发人深思。本文就部分内容进行讨论 ,认为根据新近若干冰期降温值较大的信息 ,就得出冰期降温幅度为过去认识的两倍的普遍性推断 ,还为时过早。青藏高原新近研究表明夏季降温值是较小的 ;冰川平衡线高度取决于以夏季温度为标志所提供的消融热量与全年降雪积累量的平衡 ,单纯就降温值 ,不能决定平衡线下降值 ;全球各地气候、地形差别很大 ,冰期变化也很悬殊 ,不存在全球均一的 1 0 0 0m左右平衡线下降值 ,干旱区的下降值多低于此数。青藏高原末次冰盛期 (LGM)冰川堆积 (终碛、侧碛 )和侵蚀形态(冰斗、槽谷 )一般保存良好、形态鲜明 ,较易识别 ,近年已获取测年资料 ,证明过去地貌法判别的LGM冰川规模 ,并以此决定的LGM平衡线位置基本恰当 ,当然也有误判者。从 2 0世纪 2 0年代起 ,研究者逐渐认识高原内部主要是中西部LGM平衡线下降值较小 ,经过多次改进的LGM平衡线高度分布图显示约有 1 /3左右地区 ,平衡线下降值不足 30 0m ,实为世所罕见。其原因可能主要在高原西北部是全球惟一高海拔寒旱核心区域 ,为极大陆型或极地型冰川分布区 ,以现代西昆仑山崇测冰川、古里雅冰帽为例 ,平衡线高度 相似文献
158.
159.
南海北部1987年9月~1988年10月沉积物捕获器中颗粒物质和硅藻通量的季节性变化受到季风气候的控制.颗粒物质与硅藻在东北和西南季风盛行期增加,在季风转变期减少.浅层和深层的颗粒总通量、蛋白石通量、碳酸钙通量、蛋白石/颗粒总通量比值、碳酸钙/颗粒总通量比值、有机碳/磷的比值以及浅层硅藻Thalassionema nitzschioides、Coscinodiscus excentricus、Coscinodiscus nodulifer、Nitzschia marina和Rhizosolenia bergonii的通量在东北季风期间明显地增加了,这些变化可能与1987年~1988年发生的EINino事件相关. 相似文献
160.
影响事件层保存潜力的主要因素有 :纯粹的沉积速率、生物成因的混合速率以及机械改造的规模,可以从时间规模 (过渡时间和消除时间 )的角度来考虑事件层保存潜力。基底的机械改造同样对于事件层和正常天气下的沉积物的保存具有重要影响。综合分析影响事件层保存潜力的各种因素表明,在近基的临滨带中、上部及滨外带下部,事件层具有较高的保存潜力,而在近基及远基两端之间的临滨带下部及滨外带上部,事件层保存潜力较低. 相似文献