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1.
Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high-seismicity settings where both earthquake-generated and flood-generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south-central Alaska, using standard core logging and grain-size data. A novel statistical approach is presented, in which varve-thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain-size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake-wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain-size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide.  相似文献   

2.
《Sedimentology》2018,65(5):1777-1799
Sequences of lake sediments often form long and continuous records that may be sensitive recorders of seismic shaking. A multi‐proxy analysis of Lake Bohinj sediments associated with a well‐constrained chronology was conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia). A seismic reflection survey and sedimentological analyses identified 29 homogenite‐type deposits related to mass‐wasting deposits. The most recent homogenites can be linked to historical regional earthquakes (i.e. 1348 ad , 1511 ad and 1690 ad ) with strong epicentral intensity [greater than ‘damaging’ (VIII ) on the Medvedev–Sponheuer–Karnik scale]. The correlation between the historical earthquake data set and the homogenites identified in a core isolated from local stream inputs, allows interpretation of all similar deposits as earthquake related. This work extends the earthquake chronicle of the last 6600 years in this area with a total of 29 events recorded. The early Holocene sedimentary record is disturbed by a seismic event (6617 ± 94 cal yr bp ) that reworked previously deposited sediment and led to a thick sediment deposit identified in the seismic survey. The period between 3500 cal yr bp and 2000 cal yr bp is characterized by a major destabilization in the watershed by human activities that led to increases in erosion and sedimentation rates. This change increased the lake's sensitivity to recording an earthquake (earthquake‐sensitivity threshold index) with the occurrence of 72 turbidite‐type deposits over this period. The high turbidite frequency identified could be the consequence of this change in lake earthquake sensitivity and thus these turbidites could be triggered by earthquake shaking, as other origins are discarded. This study illustrates why it is not acceptable to propose a return period for seismic activity recorded in lake sediment if the sedimentation rate varies significantly.  相似文献   

3.
A high‐resolution sedimentological and geochemical study of a high‐altitude proglacial lake (Lake Blanc, Aiguilles Rouges, 2352 m a.s.l.) revealed 195 turbidites, 190 of which are related to flood events over the last 1400 years. We used the coarsest sediment fraction of each turbidite as a proxy for the intensity of each flood event. Because most flood events at this locality are triggered by localized summer convective precipitation events, the reconstructed sedimentary record reveals changes in the frequency and intensity of such events over the last millennium. Comparisons with other temperature, palaeohydrological and glacier reconstructions in the region suggest that the most intense events occurred during the warmest periods, i.e. during the Medieval Climate Anomaly (AD 800–1300) and the current period of global warming. On a multi‐decadal time scale, almost all the flood frequency peaks seem to correspond to warmer periods, whereas multi‐centennial variations in flood frequency appear to follow the regional precipitation pattern. Consequently, this new Alpine flood record provides further evidence of a link between climate warming and an increase in the frequency and intensity of flooding on a multi‐decadal time scale, whereas the centennial variability in flood frequencies is related to regional precipitation patterns. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

4.
Lake El′gygytgyn is situated in a 3·6 Myr old impact crater in North‐eastern Siberia. Its sedimentary record probably represents the most complete archive of Pliocene and Quaternary climate change in the terrestrial Arctic. In order to investigate the influence of gravitational sediment transport on the pelagic sediment record in the lake centre, two sediment cores were recovered from the lower western lake slope. The cores penetrate a sub‐recent mass movement deposit that was identified by 3·5 kHz echo sounding. In the proximal part of this deposit, deformed sediments reflect an initial debris flow characterized by limited sediment mixture. Above and in front of the debrite, a wide massive densite indicates a second stage with a liquefied dense flow. The mass movement event led to basal erosion of ca 1 m thick unconsolidated sediments along parts of its flow path. The event produced a suspension cloud, whose deposition led to the formation of a turbidite. The occurrence of the turbidite throughout the lake and the limited erosion at its base mainly suggest deposition by ‘pelagic rain’ following Stokes’ Law. Very similar radiocarbon dates obtained in the sediments directly beneath and above the turbidite in the central lake confirm this interpretation. When applying the depositional model for the Late Quaternary sediment record of Lake El′gygytgyn, the recovered turbidites allow reconstruction of the frequency and temporal distribution of large mass movement events at the lake slopes. In total, 28 turbidites and related deposits were identified in two, 12·9 and 16·6 m long, sediment cores from the central lake area covering approximately 300 kyr.  相似文献   

5.
湖泊相浊积岩的主要特征及其地质意义   总被引：11，自引：3，他引：11  
引言1855年,Forel首先在瑞士康士坦司湖和日内瓦湖中发现由悬浮物引起的高密度流。1939年,Johnson引入浊流的概念,随后,在许多天然湖泊和人工水库中,都观察到了与高密度流有关的沉积现象。不仅混浊河流可以引起高密度流,滑坡作用也是重要的触发机制(Lawson 1919;Daly 1936;Grover and Howard 1938;Johnson1939;Bell 1947;Gould 1953,1960).但是，自从五十年代经过Kuenen等人(1950)的研究，将巨厚复理石的成因与深海浊流联系起来以来，湖泊浊流沉积作用反而被忽视了。  相似文献   

6.
The evolution of flood activity with global warming remains uncertain. To better assess flood–climate relationships, lake sediments are increasingly being investigated because they could provide regional flood histories long enough to cover past climate changes. However, site‐specific sedimentary processes may bias flood reconstructions. The aim of this article is to investigate these effects through the reconstruction of two distinct flood records from independent, neighbouring sedimentary basins of the same lake (Lake Allos in the Mediterranean French Alps), i.e. under the same climate conditions. Understanding of sedimentary processes is crucial in order to adapt the sampling strategy and the flood‐intensity proxy to each sedimentary system and, thereby, reconstruct a complete and reliable flood history. Thanks to this detailed approach, the main trends of the regional flood variability can be reproduced; i.e. periods of high flood‐frequency, ranges of flood‐frequency values and timing of the most intense events. In detail, some differences appeared associated to the various stream capacities to erode and transport flood sediments to the lake system, implying variable sensitivity of sedimentary systems in recording floods. Comparing regional flood records based on independent sedimentary systems from similar environments could thus be a complementary approach to assessing past flood intensity. Such an approach could open particularly interesting perspectives because reconstructing the long‐term evolution of flood intensity is a key challenge in the geosciences.  相似文献   

7.
鄂尔多斯盆地西南部延长组长7段浊积岩沉积特征   总被引：2，自引：0，他引：2  
郝松立  李兆雨  李文厚 《地质通报》2016,35(203):424-432
根据野外露头、岩心观察及粒度分析,对鄂尔多斯盆地西南部延长组长7段浊积岩的沉积特征进行了研究。结果表明,浊积岩主要为岩屑长石砂岩,以次棱角状为主,成分成熟度及结构成熟度较低;粒度分布上具有典型的浊流沉积的特征,发育粒序层理、包卷层理、沟模、槽模、重荷模、滑塌变形等沉积构造;常见的鲍马序列层序组合有AB型、ABC型、ADE型、AE型。研究区浊积扇可分为上扇、中扇、下扇亚相及主沟道、辫状沟道等相应的沉积微相,上扇、中扇分布范围较广,是浊积扇的主体。在古地理演化过程中,长73浊积岩体规模较小,长72—长71湖侵作用减弱,深湖线收缩,浊积岩逐渐发育,在华池—庆城一带连片展布;湖盆地形、物源供给及构造运动是影响浊积岩发育及分布的重要原因。  相似文献   

8.
Seismically‐induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well‐documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically‐induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 Mw 9·5 Valdivia and the 2010 Mw 8·8 Maule earthquakes in 17 lakes in South‐Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain‐size analysis allows identification of five types of seismically‐induced event deposits: (i) mass‐transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (lacustrine turbidites type 2) and (v) megaturbidites. These seismically‐induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post‐earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near‐shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically‐induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically‐induced event deposits allows the reconstruction of the complete impact of an earthquake.  相似文献   

9.
Earthquakes are widely recognized as triggers for turbidites, submarine debris flows and slumps. In tectonically active areas, surprisingly small changes in stress can trigger seismic events, implying that past sea level changes may be important in controlling the timing of seismicity and the occurrence of turbidites. We apply this idea in an analysis of turbidites from the upper Cretaceous–Paleocene Scaglia Rossa Formation of the Umbria‐Marches region of Italy. These turbidites are composed of resedimented foraminiferal tests derived from fluidizing deep‐water (∼1500 m), pelagic sediments; seismic triggering is the most likely triggering mechanism given this setting and composition. The timing of these turbidites (and associated synsedimentary slumps), constrained by biostratigraphy and magnetostratigraphy, reveals an unusual, non‐random temporal pattern that appears to correlate well with proposed eustatic fluctuations. This correlation between turbidites and eustatic fluctuations leads us to suggest that stress and pore fluid pressure changes associated with changing sea level may trigger periods of increased seismicity in the geological past.  相似文献   

10.
During the Middle Pleistocene late Saalian glaciation of northern central Europe numerous pro‐glacial lakes formed along the southwestern margin of the Scandinavian Ice Sheet. Little is known about the drainage history of these lakes, the pathways of glacial lake outburst floods and their impacts on erosion, sedimentation and landscape evolution. This study investigated the impact of the late Saalian Weser and Münsterland Lake (Germany) outburst floods. In particular, we reconstructed the routing and flow dynamics of the lake outburst flood and analysed the flood related sediments. We employed one‐dimensional hydraulic modelling to calculate glacial lake outburst flood hydrographs. We modelled the flow pathway and local flow conditions along the pathway based on the boundary conditions of two different hydrographs and two different ice‐margin positions. The modelling results were compared with geomorphological and sedimentological field data in order to estimate the magnitude and impact of the flood on erosion and sedimentation. Two major lake drainage events are reconstructed for the study area, during which approximately 90–50 km3 of water was released. Modelling results indicate that the lake outburst floods created a high‐energy flood wave with a height of 35–50 m in confined valley areas that rapidly spread out into the Lower Rhine Embayment eventually flowing into the North Sea basin. The sedimentary record of the outburst floods comprises poorly sorted coarse‐grained gravel bars, long‐wavelength bedforms and sandy bedforms deposited by supercritical and subcritical flows. Some parts of the sandy flood deposits are rich in reworked mammoth bones or mammoth and horse teeth, pointing to reworking of older fluvial sediments, hydraulic concentration and subsequent re‐sedimentation of vertebrate remains. These deposits are preserved in sheltered areas or at high elevations, well above the influence of postglacial fluvial erosion. The flood‐related erosional features include up to 80‐m‐deep scour pools, alluvial channels and streamlined hills.  相似文献   

11.
洪泽湖流域洪涝灾害的成灾机理分析与探讨   总被引：3，自引：0，他引：3  
叶正伟 《水文》2006,26(4):85-87,42
本文从洪泽湖历史洪涝灾情入手,分析了洪涝灾害的孕灾机理,认为自然因素叠加了人类不合理的活动共同导致了洪泽湖洪涝发生。典型洪涝孕灾区的过渡性地理位置是导致洪涝发生的大环境背景原因；流域暴雨降水高度集中的特征是洪涝的触发器；黄河夺淮历史和泥沙淤积而成的“悬湖”和“倒比降”的脆弱性地形地貌是洪涝发生的历史根本原因；下游排洪通道标准低、行洪能力不足以及人类过度围垦造成库容和湖面减小的影响都加重加大了洪涝灾害的程度和频次。  相似文献   

12.
李光明  雍永源 《地球学报》2000,21(4):373-378
藏北那曲地区中上侏罗统拉贡塘组发育典型的深水浊流沉积,根据浊积岩鲍马序列和粒度特征,可以进一步将其区分为近源砂质浊积岩和远源细屑浊积岩。微量及稀土元素地球化学研究表明,拉贡塘组浊积岩富集Cr、Co、Sc、Th等元素,无Ce负异常或负异常不明显,沉积物物源主要来自于变质岩区或再循环的沉积岩分布区。那曲盆地是冈底斯中生代多岛弧盆系统的组成部分之一,中上侏罗世深水浊流沉积形成于活动大陆边缘中的弧后盆地环境,该弧后盆地的形成与班公湖怒江中特提斯洋盆的洋壳向南的俯冲作用有关。  相似文献   

13.
The Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget is documented by sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. Six high-amplitude reflectors within the lacustrine drape can be correlated to periods of enhanced inter- and underflow deposition in sediment cores. Based on the synthesis of major environmental changes in the NW Alps and on the age-depth model covering the past 7500 years in Lake Le Bourget, periods of enhanced Rhone River flood events in the lake can be related to abrupt climate changes and/or to increasing land use since c. 2700 cal. yr BP. For example, significant land use under rather stable climate conditions during the Roman Empire may be responsible for large flood deposits in the northern part of Lake Le Bourget between AD 966 and 1093. However, during the Little Ice Age (LIA), well-documented major environmental changes in the catchment area essentially resulted from climate change and formed basin-wide major flood deposits in Lake Le Bourget. Up to five 'LIA-like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget are documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP. These abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels. Correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation.  相似文献   

14.
龚一鸣 《沉积学报》1993,11(2):75-83
新疆北部泥盆系火山碎屑浊积岩包括粗粒、中粒和细粒火山碎屑浊积岩三大类,后者尚可进一步划分为粉砂质、泥质和生物成因浊积岩。岩性上以含火山物质有别于陆屑、钙屑浊积岩。在火山碎屑浊积岩中识别出20种层序结构,它们可归并为完整鲍马序列、连续不完整鲍马序列和间断不完整鲍马序列三类。文章认为:间断不完整鲍马序列是多个浊流事件或浊流事件与其他事件相互叠加、干扰的产物,为活动构造环境,特别是火山岛弧海中非单源、单方向浊流沉积或火山碎屑浊积岩或细粒浊积岩的重要特征。  相似文献   

15.
Human activities such as river corrections and deviations, lake‐level regulations and installations of hydropower plants affect and often strongly modify natural processes in lacustrine systems. In 1714, the previously bypassing Kander River was deviated into peri‐alpine Lake Thun. This pioneering geo‐engineering project, the first river correction of such dimensions in Switzerland, doubled the water and sediment input to the lake. In order to evaluate the sedimentary consequences of the Kander River deviation, the lacustrine sediments were investigated using a combined approach of high‐resolution (3·5 kHz) reflection seismic data and sediment cores (maximum length 2·5 m). The significance of this study is increased by the possible hazard represented by ammunition dumped into the lake (from 1920 to 1960) and by the recent installation of a gas pipeline on the lake floor in 2007/2008. The first 130 years after the river deviation were dominated by an extremely high sediment input, which led to the frequent occurrence of subaquatic mass movements. Slope failures primarily occur due to rapid sediment accumulation, but were occasionally triggered in combination with earthquake‐induced shocks and lake‐level fluctuations. After 1840, mass‐movement activity and sedimentation rates decreased due to a reduced sediment input as the Kander River adjusted to its new base level and, to a smaller degree, by further engineering of the Kander River bed and gravel withdrawal at the Kander Delta. A further consequence of the Kander River deviation is that the shores around Lake Thun have been more frequently affected by flooding due to the increased water input. In the time span from 1850 to 2006, six historically and/or instrumentally documented flood events could be correlated to flood turbidites in the sediment cores. This study demonstrates the significant usefulness of lacustrine sediments, not only in archiving natural hazards and human impact but also in assessing the consequences of future anthropogenic interventions on lacustrine systems.  相似文献   

16.
东濮凹陷濮卫环洼带层序划分与沉积体系   总被引：6，自引：2，他引：4       下载免费PDF全文
根据地层基准面原理,通过对岩心、测井、录井资料的综合分析,将研究区Es3.3-Es3.2层段的地层划分出4个中期地层旋回（层序）：MSC1,MSC2,MSC3,MSC4。其中大致发育2种类型的层序,即陆源碎屑岩层序和膏盐岩层序。陆源碎屑岩层序多形成于基准面上升期,以发育泥质岩夹浊积砂体、三角洲前缘砂质沉积为主;膏盐岩层序多形成于基准面下降期,以发育厚层的盐岩、膏盐岩、膏岩、泥膏岩、膏泥岩夹浊积席状砂为特征。识别出3种类型的沉积体系：较深水湖－浊积扇、较深水盐湖、浅湖－三角洲体系,并在层序格架内分析了各旋回的沉积体系构成和储层砂体的发育情况。综合分析生、储、盖条件后认为,在垂向上,MSC2上升半旋回为本区最有利的储集层段;在平面上,本区的油气勘探应主要寻找洼陷东、西两侧断层下降盘的浊积砂体,主要储层砂体类型为浊积水道及浊积席状砂。  相似文献   

17.
In recent decades, there have been discussions and predictions regarding the impact of climate on floods, due to its socioeconomic and environmental consequences. For accurate prediction of future flood events and their impacts, it is crucial to have an improved understanding on past flood events. Lacustrine sediments have been used as a natural archive to study the past flood events. Here, we study the impact of 1954 flood event on the lacustrine environment of Bengas and Rupa Lake in central Nepal based on X-ray fluorescence spectrometry (XRF) element analysis, magnetic susceptibility (MS), total organic carbon (TOC) and the biomarker molecular compositions. Results showed that 1954 flood event had significant impacts on the two lakes in terms of detrital input, organic matter deposition and aquatic production. Before the flood event, both two studied lakes had relatively lower catchment erosion rate, lower organic matter deposition and aquatic production. During the flood event, catchment erosion and aquatic production increased in both lakes due to mass transport deposits and the increased nutrition loading attributed to flood event. Following the flood event, Begnas Lake showed the sharp increase in organic matter deposition, whereas in the Rupa Lake organic matter deposition showed minor changes. The difference in organic matter deposition in lakes during flooding event is likely due to detrital material brought and deposited by the flood activity. Overall our results suggest that lacustrine sediments are sensitive to the extreme event and would be an ideal archive for the reconstruction of flood events.  相似文献   

18.
希夏邦马峰东坡冰川与冰川湖泊变化遥感监测   总被引：21，自引：9，他引：12  
车涛  李新  P K Mool  许建初 《冰川冻土》2005,27(6):801-805
1977-2003年的遥感影像显示,希夏邦马峰东坡的冰川在迅速退缩,而其相应的冰川湖泊在迅速增大.南部的吉葱普冰川每年的退缩速度57099 m2,冰舌退缩48 m·a-1,相应的卢姆池米冰湖面积增加速度大约为79048 m2·a-1;北面的热强冰川退缩速度在63224 m2·a-1,冰舌退缩71 m·a-1,相应的扛西错冰湖面积增加约73 425 m2·a-1.从这两个冰湖的类型和变化分析,认为其具有发生冰川湖泊溃决洪水的潜在危险.  相似文献   

19.
济阳坳陷牛庄洼陷沙三段三角洲前缘浊积岩特征   总被引：13，自引：0，他引：13  
根据地质、测井、地震资料的综合分析,对济阳坳陷牛庄洼陷沙河街组三段三角洲前缘的浊积岩特征进行了研究。结果表明,该区存在砂质浊积岩体和细粒浊积岩体两种浊流沉积物。其中砂质浊积岩体粒度较细、结构成熟度和成分成熟度较低,结构和构造均反映了砂体具有滑塌再沉积的特点,可用Bouma序列来描述,常发育CDE,BCD,ABCD型浊流组合。砂质浊积岩体可进一步划为有根式和无根式两类。有根式砂体常呈扇形,可分为内扇槽道、中扇辫状水道、水道间、水道前缘和外扇无水道五种微相;无根式砂体常呈片状、舌状,可分为中心微相和边缘微相两个相带。细粒浊积岩属于低密度流的产物,不能用Bouma序列来解释,主要发育递变纹层泥岩和不均匀的块状泥岩两种细粒浊积岩。根据两类沉积物的沉积特征,建立了该区三角洲一浊积岩体综合沉积模式。论述了三角洲前缘浊积岩的成因及石油地质意义。  相似文献   

20.
《Quaternary Science Reviews》2007,26(3-4):287-299
High-resolution seismic data from Lake Tana, the source of the Blue Nile in northern Ethiopia, reveal a deep sedimentary sequence divided by four strong reflectors. Data from nearshore cores show that the uppermost strong reflector represents a stiff silt unit, interpreted as a desiccation surface. Channel cuts in this surface, bordered by levee-like structures, are apparent in the seismic data from near the lake margin, suggesting fluvial downcutting and over-bank deposition during seasonal flood events. Periphytic diatoms and peat at the base of a core from the deepest part of the lake overlie compacted sediments, indicating that desiccation was followed by development of shallow-water environments and papyrus swamp in the central basin between 16,700 and 15,100 cal BP. As the lake level rose, open-water evaporation from the closed lake caused it to become slightly saline, as indicated by halophytic diatoms. An abrupt return to freshwater conditions occurred at 14,750 cal BP, when the lake overflowed into the Blue Nile. Further reflection surfaces with downcut structures are identifiable in seismic images of the overlying sediments, suggesting at least two lesser lake-level falls, tentatively dated to about 12,000 and 8000 cal BP. Since Lake Victoria, the source of the White Nile, was also dry until 15,000 cal BP, and did not reach overflow until 14,500 cal BP, the entire Nile system must have been reduced to intermittent seasonal flow until about 14,500 cal BP, when baseflow was re-established with almost simultaneous overflow of the headwater lakes of both the White and Blue Nile rivers. Desiccation of the Nile sources coincides with Heinrich event 1, when cessation of northward heat transport from the tropical Atlantic disrupted the Atlantic monsoon, causing drought in north tropical Africa. The strong reflectors at deeper levels in the seismic sequence of Lake Tana may represent earlier desiccation events, possibly contemporaneous with previous Late Pleistocene Heinrich events.  相似文献   

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