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1.
Anton VaksMiryam Bar-Matthews Avner AyalonBettina Schilman Mabs GilmourChris J Hawkesworth Amos FrumkinAaron Kaufman Alan Matthews 《Quaternary Research》2003,59(2):182-193
High-resolution 230Th/234U ages and δ18O and δ13C compositions of speleothems in Ma’ale Efrayim Cave located to the east of the central mountain ridge of Israel enable us to examine the nature of the rain shadow aridity during glacial and interglacial intervals. Speleothem growth occurred during marine glacial isotopic periods, with no growth during the two last marine isotope interglacial intervals and during the peak of the Last Glacial Maximum. This contrasts with speleothem growth in caves located on the western flank of the central mountain ridge, in the Eastern Mediterranean semiarid climatic zone, which continued throughout the last 240,000 yr. Thus, during glacial periods water reached both sides of the central mountain ridge. A comparison of the present-day rain and cave water isotopic compositions and amounts at the Ma’ale Efrayim Cave site with those on the western flank shows that evaporation and higher temperatures on the eastern flank are major influences on isotopic composition and the lack of rainfall. The δ18O and δ13C profiles of the speleothems deposited between 67,000 and 25,000 yr B.P. match the general trends of the isotopic profiles of Soreq Cave speleothems, suggesting a similar source (eastern Mediterranean Sea) and similar climatic conditions. Thus, during glacial periods the desert boundary effectively migrated further south or east from its present-day location on the eastern flank, whereas interglacial periods appear to have been similar to the present, with the desert boundary at the same position. The decrease in overall temperature and a consequent reduction in the evaporation to precipitation ratios on the eastern flank are viewed as the major factors controlling the decay of the rain shadow effect during glacial periods. 相似文献
2.
北京石花洞的区域地质意义 总被引:1,自引:0,他引:1
在全国范围内,北京石花洞的层数最多,次生化学沉积物的叠置关系明显。该洞的大形态反映了北京西山新构造运动的历史,可以同华北地文斯和永定河阶地对比。洞内次生化学沉积物记录了中更新世以来,西山地区古环境变化的信息,可以和周口店洞穴中碎屑堆积物进行对比。 相似文献
3.
Wet climatic episodes are known to have prevailed in the Egyptian Sahara several times during the late Quaternary, most recently during the Holocene 8000 yr ago. Earlier wet episodes have been recognized as having occurred during the past 300,000 yr and have been dated by U-series methods in speleothems and in lake travertines. We show here that the times of enhanced groundwater movement can also be determined by 230Th/234U dating of secondary U in ores of uranium, iron, and phosphate. We also present evidence that such acceleration of groundwater movements is indicated by relatively low 234U/238U activity ratios in the secondary uranium. Our new data show that pluvial periods in Egypt occurred during marine oxygen isotope stages 4, 5, 6, and 7 and therefore are consistent with the view that the wet episodes are the results of migration of the tropical monsoonal belt driven primarily by the 23,000-yr precession cycle of the Milankovich curve, modulated by the 100,000-yr eccentricity cycle. 相似文献
4.
Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the 230Th/234U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from 234U/238U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature. 相似文献
5.
The correlation of geological records is commonly based on subjective and arbitrary interpretation by a researcher. Geological data are usually collected as a series on a depth scale, and then chronologies of the profiles are constructed. The correlation of geological records based on matching quality indicators requires an efficient method of searching for the best position. GenCorr is a software based on a genetic algorithm, for correlation of records with or without accompanying chronological control. Where their age information is available, points on both records may be moved in the range limited by the age uncertainties. GenCorr has been developed by the authors in Borland RAD Studio 2007 environment. The authors tested the impact of matched record length, noise level, sampling resolution, or additional linear components and hiatuses on the correlation results using artificial test data. Finally, the genetic algorithm was applied to stable isotope records obtained from two stalagmites collected from the Demianova Cave System (Low Tatras, Slovakia). The results show that the genetic algorithm correlation method is similar to that used in pattern recognition by the human eye without prior knowledge of the ages. The genetic algorithm is able to correlate in multidimensional space and is free from human biases, but we accept that there are also occasions where additional expert opinion may be also important. GenCorr can be run on every PC with MS Windows operating system, the package was not tested in Mac environment. GenCorr is available as a freeware from the authors. For downloading, please contact: dzeq@twarda.pan.pl. 相似文献
6.
洞穴钟乳石类的分类方案 总被引:11,自引:7,他引:4
为填补我国洞穴钟乳石类尚无一较系统的分类方案的空白,根据我国洞穴调查与探测的现有研究程度与发现,和我国主要属“雨水型喀斯特”的基本特点,提出了一个以钟乳石类形成的水动力机制为基础,以形态特征为主导,较适宜于中国洞穴应用的分类方案,共有重力水沉积、非重力水沉积、协同沉积、叠置沉积和异因同形沉积五大类,列出和讨论了在国内洞穴中已被发现分属于各类的60多种个体形态名称及其部分特征。 相似文献
7.
8.
Dominik Fleitmann Stephen J BurnsUlrich Neff Augusto ManginiAlbert Matter 《Quaternary Research》2003,60(2):223-232
Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δDFI) are between −60 and −20‰ (VSMOW) and δ18O values of speleothem calcite (δ18OC) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods. 相似文献
9.
This article highlights the relationship between speleothems growing inside gypsum caves and the particular climate that existed
during their development. Speleothems in gypsum caves normally consist of calcium carbonate (calcite) or calcium sulphate
(gypsum) and the abundance of such deposits greatly differs from zone to zone. Observations carried out over the last 20 years
in gypsum caves subjected to very different climates (Italy, Spain, New Mexico, northern Russia, Cuba, Argentina) highlight
wide variation in their cave deposits. In arid or semi-arid climates, the speleothems are mainly composed of gypsum, whilst
in temperate, humid or tropical regions, carbonate formations are largely predominant. In polar zones no speleothems develop.
These mineralogical details could be useful paleoclimatic indicators of climate change. The interpretation proposed is based
on the fact that in gypsum karst the kind of speleothems deposited is determined by competition between the two principal
mechanisms that cause precipitation of calcite and gypsum. These mechanisms are completely different: calcite speleothem evolution
is mainly controlled by CO2 diffusion, while gypsum deposits develop mostly due to evaporation. Therefore, the prevalence of one kind of speleothem over
the other, and the relationship between the solution–precipitation processes of calcite and gypsum, may provide evidence of
a specific paleoclimate. Additionally, other non-common deposits in gypsum caves like moonmilk, cave rafts and dolomite speleothems
can be used as markers for the prevalence of long, dry periods in humid areas, seasonal changes in climate, or rainfall trends
in some gypsum areas. Moreover, the dating of gypsum speleothems could contribute paleoclimatic data relating to dry periods
when calcite speleothems are not deposited. In contrast, the dating of calcite speleothems in gypsum caves could identify
former wet periods in arid zones. 相似文献
10.
Győző Szeidovitz Gergely Surányi Katalin Gribovszki Zoltán Bus Szabolcs Leél-Őssy Zsolt Varga 《Journal of Seismology》2008,12(1):21-33
The examination of speleothems in the Hajnóczy and Baradla caves (northeastern Hungary) allows estimating an upper limit for
horizontal peak ground acceleration generated by paleoearthquakes. The density, the Young’s modulus and the tensile failure
stress of the samples originating from a broken speleothem have been measured in a laboratory, whereas the natural frequency
of intact speleothems was determined by in situ observations. The value of horizontal ground acceleration resulting in failure,
the natural frequency and the inner friction coefficient of speleothems were assessed by theoretical calculations. The ages
of the samples taken from a stalagmite 5.1 m in height (Baradla cave) have been determined by inductively coupled plasma mass
spectrometry analysis and alpha spectrometry. The measured ages fall between 140,000 and 70,000 years; therefore, we assume
the speleothem has not been changed since the end of this time interval. According to our modeling results, this speleothem
has not been excited by a horizontal acceleration higher than 0.05 g during the last 70,000 years. 相似文献