晚第四纪柴达木盆地东部古湖泊高湖面光释光年代学

高湖面是湖泊演化的鼎盛期,指示区域的温暖湿润气候。关于青藏高原湖泊高湖面的年代有不同的观点。一种观点(主要是基于14C测年)认为在氧同位素三阶段晚期青藏高原普遍存在大湖期或泛湖期,并且其温度和降水可能比全新世还高。另一种观点(主要基于释光、铀系测年等)认为青藏高原湖泊的最高古湖面出现在氧同位素五阶段,之后湖面逐渐下降。柴达木盆地位于青藏高原的东北部,其高湖面年代的研究可为认识青藏高原环境演化提供基础资料。本文选择柴达木盆地东部的托素湖和尕海湖高出现代湖面的湖相沉积和湖岸砂堤为研究对象,利用石英光释光测年方法建立其年代序列。根据沉积物沉积特征和光释光年代结果认为,尕海湖和托素湖古高湖面出现在82～73ka、63～55ka、34.4ka和全新世早期。通过与青藏高原及周边湖泊高湖面年代记录对比,认为尕海湖和托素湖的最高湖面主体出现在氧同位素五阶段,之后湖面逐渐下降。     

Preliminary reconstruction of sediment-source linkages for the past 6000 yr at the Petit Lac d'Annecy, France, based on mineral magnetic data

An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.     

Lakes on fractal surfaces: A null hypothesis for lake-rich landscapes

A class of stochastic processes known as fractional Brownian motion (fBm) provides strikingly realistic simulations of certain types of terrain, particularly those which appear to be unmodified by geomorphological and geological processes. In addition to their less serious applications in video games and science fiction movies, fractal terrain simulations have proven useful in a number of areas of spatial analysis. For example, they can provide sample data sets for testing the efficiency of data structures and algorithms designed for topographic applications. Previous work has shown that stream networks simulated on fBm surfaces show the same deviations from accepted theories of channel network topology as do real stream networks, implying that such deviations originate in the geometrical constraints of packing channels onto surfaces, rather than from geological or other environmental controls. In effect, this work demonstrates the usefulness of fBm as a null hypothesis for terrain. One difficulty, however, stems from the abundant pits which occur in the simulations, because peaks and pits are equally likely. Flooding of pits on fBm surfaces was simulated to obtain lakes. Lake-rich stream networks were extracted and represented with a suitable integer code. The relative frequencies of various network topologies and groups of topologies were compared to known characteristics of channel networks on real lake-rich landscapes. Lake-string topologies are significantly less abundant than in glaciated landscapes. Lake areas show good fits to hyperbolic distributions, but lake in-degrees do not fit the negative binomial model. fBm surfaces are appropriate null hypotheses of scale-free, lake-rich landscapes.This paper was presented at Emerging Concepts, MGUS-87 Conference, Redwood City, California, 13–15 April 1987.     

鄱阳湖影响过湖对流强度的数值模拟及机理分析

湖陆下垫面的非均匀性对强对流天气的发展演变有很大的影响。鄱阳湖是我国最大的淡水湖,湖面积具有明显的月变化和季节变化,而模式中的下边界一般默认湖面积不变,这与实际情况的差异较大,必然带来模式预报误差。利用WRF模式对夏季夜间发生在鄱阳湖地区的一次强对流天气过程进行数值模拟,并通过湖面积变化的敏感性试验,深入研究鄱阳湖对强对流天气发展演变的影响及其机理,结果表明：夏季夜间湖面上空2 m温度明显高于陆面,向湖陆风在湖面上空辐合上升,岸边则存在下沉辐散气流。这导致降水在湖西岸减弱、湖上空增强。随后用去湖敏感性试验印证了鄱阳湖的暖湖效应,湖泊的存在能够通过激发陆风次级环流对湖西岸（湖面）上空降水起抑制（促进）作用。去湖试验的降水在湖西岸增强20%,在湖面上空减弱16%,体现出湖泊对降水强度的重要影响。此外,还发现湖面积扩大1.5、2.5、3.5、4.0倍的扩湖敏感性试验的降水在湖面上空分别增幅7%、16%、30%、43%,进一步证实了强对流强度对鄱阳湖面积变化较为敏感。这指示我们在预报夏季夜间穿湖而过的强对流天气时,应重点关注其可能存在的入湖前减弱、入湖后增强的变化趋势。同时,在利用数值模式模拟湖区强对流天气过程时,如果湖面积与模式中默认的湖面积相差较大,则应考虑将实际湖面积引入模式下边界,以期提升模式对于湖区对流的预报能力。     

Signatures of slope failures and river‐delta collapses in a perialpine lake (Lake Lucerne,Switzerland)

Historical reports from the 17th Century document two destructive tsunamis with run‐ups exceeding 5 m, affecting proximal basins of Lake Lucerne (Switzerland). One event in ad 1601 is coeval with a strong nearby earthquake (M_W ca 5·9) which caused extensive slope failures in many parts of the lake. The second event in ad 1687 is associated with an apparently spontaneous partial collapse of the Muota river delta. This study combines high‐resolution bathymetry, reflection seismic and lithological data to document the sedimentary and morphological signatures of the two subaqueous mass movements that probably generated the observed tsunamis. Such mass movements are significant as a common sedimentation process and as a natural hazard in fjord‐type lakes and similar environments. The deposits, covering large parts of the basins with thicknesses reaching >10 m, consist of two subunits: A lower ‘massflow deposit’ contains variably deformed sediments from the source areas. Its emplacement affected pre‐existing sediments, incorporating thin sediment slices into the deposit and increasing its volume. Deep‐reaching deformation near basin margins is expressed as bulges on the lake floor. An overlying ‘megaturbidite’, featuring a graded, sandy base and a thick homogeneous muddy part, was deposited from suspended particles. The source area for the ad 1601 event, gently dipping lateral slopes with an unconsolidated hemipelagic sediment cover, hosts a pronounced slide scar with sharp escarpments and sliding surfaces. The source area for the ad 1687 event on an active delta slope has been overprinted by continued sedimentation and does not show an unambiguous scar. The case studies are exemplary for end‐member types of source areas (lateral versus delta slopes) and trigger mechanisms (seismic versus aseismic); they show that morphological mapping and reconstructions of past events are key components of a hazard assessment for mass movement‐generated tsunamis.     

Influence of sediment grain size and mineralogy on testate amoebae test construction

Testate amoebae are increasingly used for environmental monitoring as well as paleoenvironmental reconstructions. Paleoecological interpretations of testate amoebae assemblages depend on the understanding of the ecological processes operating today. We then ask the question of the link between testate structure and its environment. This study analyses both the grain size and mineralogical assemblage of tests of common species belonging to the genus Centropyxis and Difflugia. It is concluded that grain size is a limiting factor for test construction, whereas mineral composition is not. Hence, when analyzing agglutinated testate amoebae for paleoenvironmental reconstructions, it should be taken into account the mean grain size of the sediment. A non-appropriate grain-size probably inhibits the development of a testate amoebae specific assemblage.     

湖泊生态系统对过去1000年气候变化的响应研究——以鄱阳湖生态系统动力学模拟为例

与陆地生态系统相比,水生生态系统对全球气候变暖的响应敏感而脆弱。鄱阳湖是我国最大面积的淡水湖泊,近年来湖泊水位下降、湖床干涸频发,发生了一系列水生态灾害事件以及生态系统的退化,但气候变化对湖泊生态灾害发生和生态系统退化的过程和机制目前还不清楚。本文基于Lotka-Volterra 动力模型的原理和结构,首先构建湖泊初级生产者浮游植物(藻)与高等水生植物(草)的竞争模式,其次构建初级生产者草、藻与顶级猎食者鱼的捕食模式,最后利用气候-湖泊水量与生态因子的物质传递原理,构成气候与生态的动力方程组,设置现代和历史不同时期的试验方案进行数值模拟研究。模拟结果表明,鄱阳湖藻、草、鱼生物群落在各自的环境容纳量、增长率和相互依存/抑制关系下,年内和年际生物量变化构成了稳定和准循环系统(试验Ⅰ)。在湖泊水量与生态因子相互作用中,水量生态效应可保持生态系统的动态平衡(试验Ⅱ)。应用到过去1000年湖泊生态系统中,模拟发现在气候-水量变化的边界条件下,湖泊生物量增加和减退的速度不对称(试验Ⅲ)。水量的反馈可使增加阶段的生物量速度减缓、使衰减阶段的速度加速,从而量化了气候水量变化在生态系统中的负反馈效应。设置小冰期气候的试验Ⅳ模拟表明,在流域降水减少19% 、湖泊面积减少33% 和湖泊水量减少7% 的状况下,湖泊生态系统在600~700个月内振荡并衰退。当这个极端气候持续600个月以上,水生群落系统全面崩溃。小冰期中鄱阳湖的沙生和旱生植被替代水生和湿生植被的沉积孢粉证据验证了这个模拟,表明在极端干旱条件下,边界条件变化显著超过了生态系统的应对能力,湖泊生态系统在长期震荡后崩溃。