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Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ13Cmean = −2.49 ± 0.99‰; δ18Omean = −9.43 ± 1.40‰). The δ13C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ18O values are typical of fresh water carbonates. Covariation between δ13C and δ18O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ13Cmean = −2.06 ± 0.84‰; δ18Omean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)molar, δ13C and δ18O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records. 相似文献
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Aquatic dance flies (Empididae; Clinocerinae and Hemerodromiinae) are important components of freshwater assemblages, especially in running waters. They are predators as larvae and adults and thus essential for understanding aquatic food webs. This study was conducted in Plitvice lakes National Park (Croatia) representing a wide variety of freshwater habitats (springs, streams, lakes and tufa barriers). Adults were collected monthly from March 2007 until March 2009 using pyramid-type emergence traps at 13 locations. A total of 3865 specimens comprising 18 species were collected. The dominant genus was Chelifera, while the most abundant species was Hemerodromia unilineata. All species were univoltine except Chelifera precabunda, Chelifera pyrenaica and Chelifera stigmatica that were bivoltine. Considerable differences in composition and structure of aquatic dance flies assemblages were recorded along a longitudinal gradient of studied sites, primarily related to differences in physical and chemical parameters of water. Water temperature was the main factor influencing the timing of emergence. Hemerodromia species preferred variable water temperature throughout the year while the majority of the Chelifera species preferred stable water temperature characteristic of spring sites. Furthermore, discharge affected assemblage composition of aquatic dance flies. The highest abundance of aquatic dance flies was recorded in lotic habitats with fast water current over substrates of moss, gravel and particulate tufa with detritus. These results give a new insight on microhabitat preference and their distribution on unique karstic habitats. 相似文献
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英国钙华苔藓植物区系特征及其主要钙华沉积类型 总被引：9，自引：5，他引：4
根据1998— 2000年作者采自英国39个钙华沉积区1245份的野外苔藓植物标本研究, 英国钙华苔藓植物区系具有下列特征:(1)区系种类由19科34属50种(含变种亚种)组成, 其中含英国钙华苔藓新记录26种;(2)区系生活型含高丛集型(10 %)、矮丛集型(32%)、交织型(44 %)、扇型(2 %)和平埔型(12 %)5 种类型;(3)区系地理成分含北温带分布(46 %)、温带欧洲分布(6 %)、欧洲-非洲分布(2 %)、欧洲-北美分布(10 %)、旧世界温带分布(4%)和世界广泛分布(34%)等6种成。根据钙华生长的环境特征, 英国苔藓植物钙华可划分为泉华、瀑华、溪流钙华和洞穴弱光带钙华4 种基本类型及12 种小类型。 相似文献
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Janet S. Wright 《The Australian geographer》2000,31(3):333-347
Four types of tufa occur along creeks draining the Oscar Range in the Kimberley region of Western Australia; stream-bed tufa, tufa dams, stream-bed tufa waterfalls, and rimstone pools. Well-developed tufa dam and waterfall formations occur at significant breaks of slope within the channel long profile. This suggests that disruption of the hydraulic flow regime and increased turbulence at these points has an important role to play in determining the location of tufa deposits. However, the best-developed stream-bed tufa deposits are located immediately upstream of dam and waterfall formations, which indicates that the evaporation of water ponded behind these flow obstructions may lead to calcium carbonate precipitation during the dry season. The presence of plant and cyanobacterial communities on, and the incorporation of organic material into, the best-developed tufa formations indicates that biological activity may have an important role to play in determining both the rate of tufa deposition and the internal structure of the resulting accumulations. 相似文献
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Gail M. Ashley Doris Barboni Manuel Dominguez-Rodrigo Henry T. Bunn Audax Z.P. Mabulla Fernando Diez-Martin Rebeca Barba Enrique Baquedano 《Quaternary Research》2010,74(3):304-314
The 1959 discovery of the hominin fossil Zinjanthropus boisei brought the world's attention to the rich records at Olduvai Gorge, Tanzania. Subsequent excavations of archaeological level 22 (FLK Zinj) Bed I uncovered remains of Homo habilis and a high-density collection of fossils and Oldowan stone tools. The occurrence of this unusual collection of bones and tools at this specific location has been controversial for decades. We present paleoecological data that provide new insights into the origin of FLK Zinj. Our recent excavations 200 m north of the site uncovered a 0.5-m-thick tufa mound draped by Tuff IC, in the same stratigraphic horizon as level 22. Stable isotope analyses indicate that the carbonates were deposited by a freshwater spring. Phytolith analysis of the waxy clay under Tuff IC revealed abundant woody dicotyledon and palm phytoliths, indicating that the site was wooded to densely wooded. The time equivalency and close physical proximity of the two environments indicate the two are related. This study has provided the first documented evidence of springs in Bed I and these data have important implications for the interpretation of hominin behavior in meat acquisition and the ongoing debate on scavenging versus hunting. 相似文献
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This research assesses the collapse mechanisms of tufa deposits. The city of Antalya, located on the Mediterranean coast of Turkey, has been settled on tufa deposits. By the end of 1980s, the area behind the tufa cliffs became the site of high-rise residential buildings. Some of these buildings have suffered from foundation instabilities, which have given rise to cracking and fissuring of the walls, and overall tilting. The collapsible behaviour upon loading and/or wetting of some tufa deposits has caused foundation settlement.
Voids, bonding of grains and grain properties of the collapsible tufa were investigated using optical and scanning electron microscopes. Our studies indicate that the collapse behaviour of tufa is affected mainly by depositional environment and microfabric. Laboratory tests have been performed in order to determine the index properties and the collapse potential of tufa samples. Young tufa deposits near the surface are weakly bonded with thin films of carbonate cement, giving rise to high void ratios. Analysis of geotechnical parameters has shown that the collapse potential of tufa is closely related to the natural void ratio and the amount of cement binding the grains. 相似文献
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方解石沉积速率控制的物理化学机制及其古环境重建意义 总被引：14，自引：1，他引：13
本文总结了作者有关方解石沉积动力学研究的结果,发现方解石沉积速率受环境温度、CO2分压、水动力条件、水层厚度和溶液中钙浓度等多因素的控制。总的规律是温度愈高、CO2分压愈低、流速愈快、水层厚度愈大、溶液中钙浓度愈高,方解石沉积速率愈大。而且,方解石沉积速率随水中钙浓度的增加呈近似线性增加。文中对自然界类似环境条件下的速率系数和我国南北气候环境条件下的方解石沉积速率进行了总结和分析。文章最后对利用钙华和石笋的沉积速率进行古环境重建的复杂性进行了简要的讨论。 相似文献
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The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras
Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre,
New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P.
and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated
for the most widespread members are all less than 0.1 km3. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little
tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every
100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which
mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free
crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised
in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field
characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members
(Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric
water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000
years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras
are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part
of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the
Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and
Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events.
Received: 14 September 1996 / Accepted: 6 June 1997 相似文献
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