Variation among fossil chironomid assemblages in surficial sediments of Bodensee-Untersee (SW-Germany): implications for paleolimnological interpretation     

Alexandra Schmäh 《Journal of Paleolimnology》1993,9(2):99-108

In paleolimnology, subfossil head capsules of chironomids play an important role as ecological indicators of lake history. It is important to determine, therefore, whether fossil assemblages are representative of former biocoenoses. There is evidence that headcapsules washed in from other places can make up a significant percentage of the total. As interpretations are usually drawn from the examination of a single core, it is of special interest to know whether a fossil assemblage of a single site properly reflects limnological conditions of the whole lake. This study examined the taxonomic distribution of subfossil chironomids in the surficial sediments of the Bodensee-Untersee, with the aim of assessing the variability in chironomid assemblages. Apparently, most of the head capsules of the profundal fossil assemblages in the Untersee had been washed in from the littoral zone or from the slope. Although the Bodensee-Untersee is a rather large lake, variability is surprisingly low among all samples. Therefore a correct interpretation from a single core may be possible.  相似文献   

40Ar/39Ar laser probe evidence concerning the age and associated hazards of the Lake Nyos Maar,Cameroon     

G. Brent Dalrymple  John P. Lockwood 《Natural Hazards》1990,3(4):373-378

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent ⁴⁰Ar/³⁹Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic ⁴⁰Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a ¹⁴C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria.  相似文献   

Assessment of episodic acidification in Sierra Nevada,California     

Nikolaos P. Nikolaidis  Vicki S. Nikolaidis  Jerald L. Schnoor 《Aquatic Sciences - Research Across Boundaries》1991,53(4):330-345

Monte-Carlo simulations were used to assess the extent of shortterm alkalinity depressions occuring in Sierra Nevada lakes due to acidic deposition events. The Episodic Event Model (EEM) was used to simulate spring snowmelt events. Snow course data, precipitation data and lake acidification surveys were used to derive values for the EEM parameters. Spring snowmelt events were shown to have great impacts on the water quality of Sierran lakes. Lakes are likely to be most affected by the early-spring snowmelt event because the epilimnion depth is at a minimum, which indicates minimum dilution. Under annual average loading conditions, no Sierran lake has been reported as acidic although 29% of the lakes have alkalinities less than 40 µeq/L indicating a sensitivity to acidification. In simulations of early-spring snowmelt events, using present-day acidic loading conditions, it was estimated 79% ± 9% of the lakes would experience shortterm lake alkalinity depressions to levels less than 40 µeq/L. The results provided by the model simulations are valuable in establishing upper and lower limits on the extent of possible episodic acidification to lake-resources-at-risk. The most critical parameters controlling the magnitude of lake alkalinity depressions during snowmelt episodic events are a) the lake area to watershed area ratio — a measure of input loading, and b) the epilimnion volume — a measure of dilution and mixing.  相似文献   

洱海湖盆晚第四纪地层的14C年龄测定及洱海的演变   总被引：2，自引：0，他引：2       下载免费PDF全文

彭贵  焦文强 《地震地质》1991,13(2):179-183

从大理洱海湖盆西部平原湖沼相地层的分布和相关的14C年龄测定以及文物考古资料,阐述了晚更新世中期以来洱海的演变。研究表明,距今5万年以来洱海西部古湖岸线是从西向东迁移的。期间曾有过几次摆动。引起洱海古湖岸线变迁的主导因素是点苍山断块的间歇性抬升和洱海湖盆的下沉,地质历史时期的气候干湿演变也起了重要作用  相似文献   

第四纪盐湖型钾盐矿床的航空γ能谱异常特征及找矿模式     

徐东宸 《铀矿地质》1994,(4)

本文以航空γ能谱测量资料为依据，简述了与找钾有关的部分科研成果，建立了第四纪盐湖型钾盐矿床的找矿模式，进而通过对别勒滩、马海等钾盐矿例剖析，说明航空γ能潜测量在寻找第四纪盐湖型钾盐矿床方面确有明显效果。  相似文献   

Carbohydrates in recent marine sediments—II. Occurrence and fate of carbohydrates in a recent stromatolitic deposit: Solar Lake,Sinai     

J. Klok  H.C. Cox  M. Baas  J.W. de Leeuw  P.A. Schenck 《Organic Geochemistry》1984,7(2):101-109

In a study on the diagenesis of carbohydrates in a recent stromatolitic deposit (Solar Lake, Sinai) monosaccharides are quantitated after mild acid hydrolysis and gas chromatographic analysis as alditol acetates. From the depth profiles of the individual monosaccharides relative to the total organic carbon three categories of carbohydrates can be distinguished. The behaviour of the various categories upon burial is discussed in terms of their potential to become a part of the organic matter that survives geological periods. The distribution patterns of especially the mono-O-methyl monosaccharides at various depths are characteristics for the bacterial communities present in the corresponding parts of the sediment column.  相似文献   

西藏扎布耶湖ZK91-2钻孔沉积特征与气候环境演化   总被引：12，自引：1，他引：12  

齐文  郑绵平 《湖泊科学》1995,7(2):33-140

扎布耶盐湖位于西藏高原腹地,1991年在其南部干盐滩中钻取了20m深湖相沉积,以5～10cm/个的密度进行高分辨率取样,详细探讨了沉积标志、矿物组合（尤其Mg、Ca、Li碳酸盐）、干旱与潮湿两种气候类型的元素组合等气候环境指标,推导了晚更新世以来该湖区湖面涨缩、水质咸谈及气温高低变化。  相似文献   

滆湖沉积物理化特征及磷释放腄   总被引：4，自引：0，他引：4  

范成新 《湖泊科学》1995,(4)

对滆湖表层底泥、柱状层祥和间隙水进行理化分析,模拟不同条件下的沉积物磷释放。结果表明,0～20cm深度范围内,各项理化指标变化较大;大于2cm后差异较小。当提高沉积物-水体系温度、降低氧含量(或Eh),提高pH及施以水动力作用时,可促进沉积物磷释放进程。无菌条件对磷释放有抑制作用。采用实验室模拟和间隙水浓度扩散模型计算得到的磷总释放量分别为10.65t/a和9.40t/a,其中湖面网围区释放量占全湖总量的28.2～35.4％。建议适度控制网围养殖规模,以减少内源磷污染。  相似文献   

High resolution time series of lead and zinc in sediments of Lake Constance   总被引：3，自引：0，他引：3  

Martin Wessels  Angela Lenhard  Federico Giovanoli  Andreas Bollhöfer 《Aquatic Sciences - Research Across Boundaries》1995,57(4):291-304

Sediments in Lake Constance have been dated by radiometric methods and lamination counting. Investigations into the concentrations of lead and zinc in the sediments were carried out to get detailed information about environmental history. Maximum concentrations were found at the beginning of the 1960's. The lead maximum contamination occurred 3 years before the zinc maximum and has an unidentified source. The use of coal, leaded gasoline or the remobilization within the sediment could be excluded to result in the lead maximum. The fluctuations in zinc contamination could be explained with increasing and decreasing pollution (industry, coal burning, building of sewage plants). The greatest fluctuations of both metals occur during periods of great change in the economic history of Germany.  相似文献   

青藏高原北缘哈拉湖近800年来 湖泊沉积及其环境意义^*   总被引：5，自引：5，他引：5  

曹洁  张家武  张成君  陈发虎 《第四纪研究》2007,27(1):100-107

文章以青藏高原北缘高山祁连山湖泊——哈拉湖作为研究对象,利用放射性核素²¹⁰ Pb和¹³⁷ Cs测年资料,通过对此高海拔湖泊沉积碳酸盐氧碳同位素、碳酸盐含量、总有机碳含量、磁化率等环境代用指标的分析,揭示了这一地区近800年来的气候环境变化过程。分析表明哈拉湖记录的气候环境变化经历了3个阶段:1206~1700A.D.环境较为寒冷、湿润,并有持续变湿的趋势;1700~1920A.D.环境由冷湿的状况趋向温暖干旱,1771年之后,环境保持相对稳定;1920~2002A.D.是自1206年以来最为温暖干旱的阶段。  相似文献