Carbonate deposition and facies distribution in a central Michigan marl lake

Relatively pure lacustrine carbonates referred to as marl are being deposited in Littlefield Lake, central Michigan, a hard-water lake with little terrigenous clastic influx. Thick accumulations of marl form both progradational marl benches along lake margins, and islands or lakemounts in the lake centre. Marl benches develop flat platforms up to 20 m wide in very shallow water and steeply inclined slopes, up to 30°, extending into deep water. The flat landward platform is frequently covered by algal pisoliths while the upper portion of the lakeward-sloping bottom is overgrown by Chara which in the summertime becomes thickly encrusted with low-magnesian calcite. Marl islands are flat-topped features that formed over relict highs on Pleistocene drift which underlies the lake basin. These are fringed by marl benches identical to those found along lake margins. Marl benches are composed of four units: two thin facies deposited on the shallow-water bench platform and two thicker faces deposited on the bench slope developed in moderate water depths. These in turn overlie a fifth facies deposited in deep water. A coarsening-upward sequence is developed in these sediments as a result of both mechanical sorting, and primary production of carbonate sand and gravel in shallow water. In addition to facies sequences and size grading, trends upsection of increasing carbonate content and decreasing insoluble content may serve to identify temperate-region lacustrine carbonate deposits in the rock record.     

Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer

Bacterially produced branched glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous in soils and lake sediments and can potentially be used to reconstruct past temperatures. In lakes, however, it is still unclear if these compounds are derived from eroded soils or if they are produced in situ. To better understand environmental controls on the distributions of these compounds and the sources of branched GDGTs to lake sediments, we compare branched GDGT distributions and concentrations in lake sediments and catchment soils within a 3600 m altitudinal transect in western Uganda. Reconstructed mean annual air temperature (MAAT), determined from the degree of methylation (MBT) and cyclisation (CBT) of branched GDGTs in soils, decreases with increasing altitude, as is expected from the air temperature gradient in our transect. However, we observe significant offsets between observed and reconstructed temperatures in soils from wet, high elevation soils but not in most dry, low elevation soils. Branched GDGT distributions differ significantly between lake sediments and soils at all elevations, with greater differences at low elevations than at high elevations. These data support previous hypotheses that branched GDGTs are produced in situ in lakes and suggest that the abundance of water in soil environments may play a role in controlling the distribution of branched GDGTs. While branched GDGTs in lacustrine sediments can be used to reconstruct temporal temperature variations in lakes, we urge caution in utilizing this proxy in lacustrine systems with high soil loadings, especially if there is evidence of changing clastic fluxes through time.     

Geologic and atmospheric input factors affecting watershed chemistry in upper michigan

The relationships between watershed variables and lakewater chemistry were examined for 53 lakes in the Upper Peninsula of Michigan to identify factors influencing lake sensitivity to atmospheric inputs. The lakes lie in three distinct geologic/geomorphic regions. Acid neutralization capacity (ANC), sulfate, and color were correlated with parameters related to atmospheric loading, watershed area and relief, hydrology, geology, and land use for the entire 53-lake set and for lower alkalinity subsets. Acid-neutralizing capacity was related to atmospheric acidic inputs and, in the southern portion of the Upper Peninsula, to the presence of mineralized groundwater inputs. In the north, ANC is correlated with hydrologic lake type and surficial deposits. Results show the highest density of acidified lakes in the northern region, which is underlain by noncalcareous sedimentary rocks. Color was related to lake size and the presence of organic soils in the watershed, whereas lake sulfate concentration was mainly influenced by atmospheric or groundwater inputs, surficial deposits, and soil type.     

Lake Shala: Water chemistry,mineralogy and geochemistry of sediments in an Ethiopian Rift lake

Lake Shala, the deepest lake in the internal Galla lakes basin of the Ethiopian Rift, fills a depression in Pleistocene volcanic rocks. Its sodium-bicarbonate (-chloride) water (salinity 16 g/l) is remarkably low in earth alkalines and sulphate. Stratification is indicated by different ion concentrations in the surface and bottom waters and by a thermocline in a water depth of 50–70 m. Hot soda springs emerging on the shores of Lakes Shala and Langano are believed to be derived from a hot saline underground reservoir recharged by meteoric waters. The ion composition of the hot spring waters is uniform and matches that in the Galla Lakes except for total salinities. Anomalous heavy metal concentrations are lacking in lake and hot spring waters. Sediments of Lake Shala belong to an extremely fine-grained group of deposits. They are poorly sorted and the lateral distribution of the grain sizes does not follow the normal scheme for aquatic depositional environments. A belt, 50–100 m below lake level containing the finest-grained sediments, separates the shallow periphery of the lake bottom from the deep center, both characterized by coarser-grained deposits. The sediments consist of a large portion of glassy components. A poorly cristallized smectite is most abundant in the clay mineral group. The components of the sand fraction are quartz, feldspar, glass particles and occasionally calcite. Nickel, cobalt and lead are depleted in the Lake Shala sediments compared with the averages of shales. Iron, manganese and zinc are relatively high. Silver, cadmium and some of the rare earth elements are enriched by factors of > 5.     

Post‐deposition diffusion of 137Cs in lake sediment: Implications for radiocaesium dating

Peak activities of radiocaesium (¹³⁷Cs) in lake sediments have frequently been used to infer the ages of sediments deposited in the 1960s (¹³⁷Cs derived from nuclear bomb testing) or in 1986 (Chernobyl derived ¹³⁷Cs). Records of the vertical distribution of ¹³⁷Cs in sediments can thus be used to provide accurate dates for a critical period in which palaeoecological reconstructions often overlap contemporary monitoring data. However, knowledge regarding how the distribution of ¹³⁷Cs in sediments is affected by post‐depositional processes is limited to interpretations based on the ¹³⁷Cs distribution in sediments sampled at a single given date. This study assesses the extent to which the ¹³⁷Cs record in annually laminated (varved) lake sediments is affected by post‐depositional diffusion, using 11 archived sediment cores sampled between 1986 and 2007. The sediment record reveals how Chernobyl ¹³⁷Cs incorporated into the 1986 varve diffused downwards in the core at a decreasing rate over time, whereas the surface sediments continued to receive inputs of ¹³⁷Cs mobilized from the catchment soils or lake margin. In spite of these processes, all cores post‐dating the Chernobyl accident had a clear and well‐resolved peak in the 1986 varve, justifying the use of this feature as a fixed chronostratigraphic feature. Because of the very high levels of Chernobyl fallout at this site, downwards migration of Chernobyl ¹³⁷Cs has, however, completely masked the nuclear weapons ¹³⁷Cs fallout peak that had been clearly preserved in the 1964 varve of a pre‐Chernobyl core sampled just three weeks before the Chernobyl accident. In consequence, the weapons fallout marker is likely to be of little use for determining ¹³⁷Cs dates in areas strongly affected by high levels of Chernobyl fallout.     

Distributions of branched GDGTs in a tropical lake system: Implications for lacustrine application of the MBT/CBT paleoproxy

Branched glycerol dialkyl glycerol tetraether (GDGT) lipids are abundant and ubiquitous in lake sediments, potentially allowing for a paleolimnological application of the so-called MBT/CBT proxy (methylation index of branched tetraethers/cyclization ratio of branched tetraethers). To investigate the origin and characteristics of these compounds in lacustrine environments, we examined the distributions of GDGTs in soils, river sediments and lake sediments from Lake Towuti on the island of Sulawesi, Indonesia. We found significant differences in the degree of methylation and cyclization (expressed by way of the MBT and CBT indices) between the soil samples and the aquatic samples, suggesting that there may be in situ production of GDGTs in the aquatic environment. Based on these findings, we urge caution in the application of the MBT/CBT paleoproxy to lake sediments and encourage more rigorous study of these compounds in freshwater environments.     

Flood-triggered versus earthquake-triggered turbidites: A sedimentological study in clastic lake sediments (Eklutna Lake,Alaska)

Lake sediments in Eklutna Lake, Alaska, reveal the presence of turbidites within varved sequences. These turbidites, which result from flood events and earthquakes, show a similar macroscopic appearance. In order to use turbidites to reconstruct flood variability and/or seismic history in the lake basin, it is crucial to determine the trigger of the turbidity currents. This study examined the turbidite caused by the ad 1964 Great Alaska earthquake as well as turbidites linked to historical flood events in order to differentiate between these earthquake-triggered and flood-triggered turbidites. In a suite of samples from throughout the lake, distinctive proxies are identified that can be associated with event-specific flow characteristics. The study presents straightforward discrimination methods related to the sedimentology and geochemical components of the turbidites. These methods are also applicable to other lakes, particularly proglacial lakes where the sediment composition of onshore and offshore sources is similar. Finally, the discrimination of the turbidite trigger can be used to reconstruct the palaeoflood and seismic history.     

陆相湖盆深水重力流混合事件层沉积及沉积学意义

重力流混合事件层在陆相湖盆广泛发育,其形成和分布对理解重力流沉积演化过程及重力流沉积常规与非常规油气勘探开发意义重大。以涠西南凹陷流沙港组一段和鄂尔多斯盆地延长组7段重力流沉积为研究对象,分析湖盆重力流混合事件层的沉积特征、类型、成因及沉积模式,并进一步探讨其地质意义。湖盆主要发育滑动、滑塌重力驱动块体搬运沉积和砂质碎屑流、泥质碎屑流、高密度浊流和低密度浊流等重力流流体沉积,同时广泛发育重力流混合事件层沉积。湖盆重力流混合事件层包含多层结构、双层结构和频繁互层三种大的类型;其中,双层结构的重力流混合事件层进一步根据上下两个沉积单元厚度的差异可细分为两个亚类。多层结构的混合事件层主要为流体侵蚀或砂体液化成因,多发育于混合事件层沉积近端;双层结构与频繁互层结构的混合事件层主要为流体减速膨胀、泥质碎屑流中碎屑颗粒的差异沉降成因,多发育于混合事件层沉积远端。相同沉积单元组成的沉积层在垂向上的规律叠置是岩芯中识别重力流混合事件层沉积的可靠依据;在未明确其沉积过程的情况下可能会导致沉积信息的错误解读。同时,重力流混合事件层的发育会导致重力流沉积非均质性增强,不利于常规油气的储集;但是,重力流混合事...     

Authigenic calcium carbonate flux in groundwater-controlled lakes: Implications for lacustrine paleoclimate records

Groundwater dominated lakes are an important feature of many landscapes. Their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce. Where groundwater and lake composition are favorable, carbonate mineral precipitation, evaporative concentration of lake water, and microbial processes can combine to strongly deplete dissolved Ca relative to influent groundwaters. The authigenic carbonate flux (ACF) can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge. Here we analyze sedimentary records from two marl-producing, groundwater-controlled lakes and demonstrate a link between one-dimensional ACF and the Palmer Drought Severity Index (PDSI), a measure of land surface wetness. In a restricted outflow lake with high-carbonate alkalinity, ACF is enhanced during historically wet climatic periods in response to increased aquifer recharge rates. ACF in this lake declines during droughts. A neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between ACF and PDSI history. Ionic chemistry, carbonate mineral equilibria, and δ¹³C patterns of dissolved inorganic carbon show that the sensitivity of the ACF signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover.     

Discovery of lacustrine shale deposits in the Yanshan Orogenic Belt,China:Implications for hydrocarbon exploration

The mechanism of formation of lacustrine deposits within stable orogenic belts and their potential for shale oil and gas exploration are frontier themes of challenge in the fields of sedimentology and petro-leum exploration.Orogenic belts witness strong tectonic activities and normally cannot host stable lacus-trine basins and deep shale formations.Therefore,basins in orogenic belts are considered to have no potential to form shale hydrocarbon reservoirs.Here we investigate the Luanping Basin located in the Yanshan orogenic belt where previous studies regarded rivers and fan deltas as the major main Mesozoic deposits.Based on detailed field exploration and scientific drilling,we report the finding of a large number of lacustrine shale continental deep-water deposits in the Mesozoic strata.Our finding of the occurrence of active shale oil and gas also in this basin also subvert the previous perceptions.We report SHRIMP zircon U-Pb age that define the bottom boundary of the target interval as 127.6±1.7 Ma belonging to the early Cretaceous strata.Tectonics and climate are considered to be the main factors that controlled the deep-water sedimentation during this period.The drill cores revealed evidence of shale gas and the TOC of shale is 0.33％-3.60％,with an average value of 1.39％and Ro is 0.84％-1.21％,with an average value of 1.002％.The brittleness index of shale is between 52.7％and 100％.After vertical well fracturing,the daily gas production is more than 1000 m3.Our findings show that the basin has consid-erable potential for shale oil and gas.The geological resources of the shale gas in the Xiguayuan Fm.are estimated as 1110.12 × 108 m3,with shale oil geological resources of 3340.152×104t.Our findings indi-cate that the Yanshan orogenic belt has potential exploration prospect.This work not only redefines the Luanping Basin as a rift deep-water Mesozoic Lake Basin,but also rules out the previous notion that the basin is dominated by shallow water sediments.The discovery of shale oil and gas also provides an important reference for subsequent petroleum exploration and development in this basin.Our study shows that shale oil and gas reservoirs can be found in the lacustrine basins of orogenic belts which were strongly influenced by volcanism.These results have significant implications for the sedimentology and oil exploration in the Qinling and Xingmeng Orogenic Belts of China,as well as those in other terranes of the world including the New England Orogenic Belt in Australia.     

A sedimentological and stable isotopic study of travertines and associated sediments within Upper Triassic lacustrine limestones, South Wales, UK

The Upper Triassic in South Wales is composed of up to 100 m of red, dolomitic mudstones of the Mercia Mudstone Group which overlie and are laterally equivalent to basin margin coarse clastic deposits. In the Sully Island and Dinas Powys areas, a series of carbonate deposits was laid down within small basins adjacent to the main Bristol Channel Basin. The rocks consist of dolomites containing replaced evaporites, overlain by perilittoral freshwater limestones. The limestones are fenestral intrasparites and contain abundant pedogenic and stromatolitic horizons, as well as locally developed travertines. Evidence for vadose diagenesis within the limestones is common. The travertines consist of sheets of fibrous calcite (flowstone) associated with pisoids and flöe calcite. Most of the travertines consist of single sheets several millimetres in thickness although ‘mounds’up to 1 m in height and 5 m in diameter are also present. The topmost metre of the limestones, which has been dolomitized, is deformed into tepee and megapolygonal structures. The carbon and oxygen stable isotopic composition of the limestones suggests that they were precipitated in low salinity waters. Successive samples from individual bands of flowstone show a covariance of δ¹⁸C and δ¹³C which is consistent with the mixing of resurgent groundwaters with pools of more evolved waters at the surface. The sedimentological and geochemical evidence suggests that the limestones were deposited in a small, enclosed basin fed by upwelling meteoric groundwaters in an environment distinct from that in which the laterally equivalent gypsiferous red mudstones were formed.     

The chemistry of suspended matter in Esthwaite Water,a biologically productive lake with seasonally anoxic hypolimnion

Ten detailed vertical water column profiles were taken between April and November, 1979, in Esthwaite Water (English Lake district), a lake with high biological productivity and a seasonally anoxic hypolimnion. Measurements of the major-element particle composition (organic C, P, S, Si, Al, Ti, K, Mg, Ca, Fe, Mn, and Ba) and hydrochemical constituents (temperature, pH, dissolved oxygen, total suspended load, dissolved Fe, Mn, P, and Ba) were carried out. These have revealed new information about the mechanisms and kinetics of biogeochemical cycles in a lake.Pronounced seasonal cycles exist in which large excess concentrations (those unsupported by detrital components) of particulate organic C, Fe, Mn, P, S, Mg, K, Ba, and Ca are being generated and lost in situ in the water column (15m deep). In the epilimnion these elements (excepting Fe and Mn) are incorporated into the organic components of growing phytoplankton during the spring and summer. Simultaneously, in the hypolimnion there is a build-up and then a decrease in the excess concentrations of particulate C, P, S, Mg, K, Ba and Ca; this cycle is due to the indirect involvement of these elements with the iron redox cycle. As the hypolimnion becomes anoxic, dissolved ferrous Fe is released from the sediments and large concentrations of excess particulate iron (III) oxides accumulate; these oxides act as adsorbing substrates for the above mentioned elements. As conditions become more reducing, these same elements are solubilized as the iron (III) oxide particles are reduced to dissolved ferrous iron.Adsorption equations are derived from the field data which relate the concentration of excess particulate Fe to those of POC, P, S, Ca, Mg, Ba, and K. At the last stages of anoxia (before the lake overturns) large populations of bacteria and the formation of iron sulfide particles control the concentrations of excess particulate C, S, P, Mg, K, and Ca.     

A lacustrine record from Lop Nur,Xinjiang, China: Implications for paleoclimate change during Late Pleistocene

Climate variability during the Late Pleistocene is studied from the proxies in core CK-2 drilled from the Luobei Depression (91°03′E, 40°47′N), Lop Nur in the eastern Tarim Basin, Xinjiang, China. Geophysical and geochemical properties, including magnetic susceptibility, granularity, chroma, carbonate content, loss on ignition and trace elements, have been determined to reconstruct the environmental evolution of the area during 32–9 ka BP. The chronology is established by uranium–thorium disequilibrium dating techniques.Our data suggest four paleoclimate stages, indicating glacial variations between cold–humid and warm–arid environments. A period of extreme humidity occurred during 31,900–19,200 yr BP is attributed the last glacial maximum (LGM). The period was followed by a warm–arid episode during 19,200–13,500 yr BP. Then a cold–humid interval during 13,500–12,700 yr BP may correspond to another cooling phases at high latitudes of the Northern Hemisphere. The last stage from 12,700 to 9000 yr BP has a trend that the climate turned warm and arid. The Lop Nur region is characterized by particularly humid stadials and arid interstadials. The climate variability in Lop Nur was constrained by global climate change because it is correlated with Dansgaard–Oeschger and Heinrich events, which were observed at the northern high latitudes. The synchroneity of the palaeoclimatic events suggested that cold air activity at the northern high latitudes was the most important factor that influenced the climate evolution in the Lop Nur region. A probable mechanism that involves the migration of westerly winds is proposed to interpret this synchroneity.     

Holocene Saharan dust deposition on the Cape Verde Islands: sedimentological and Nd-Sr isotopic evidence

Holocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north-eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin-suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.     

镭同位素示踪盐湖地下水排放通量 ——以大柴旦盐湖为例

为了解大柴旦盐湖地下水排放通量,测试了深部热水、浅层地下水、河流水、盐湖表层水中镭同位素(223 Ra、224 Ra、226 Ra和228 Ra)的活度值.研究结果表明湖水中223 Ra、224 Ra、226 Ra和228 Ra的活度值,在盐度较低时随着盐度的增加而升高,当盐度大于168.99‰时,则随着盐度的增加而降...     

Subglacial and subaqueous processes near a glacier grounding line: sedimentological evidence from a former ice-dammed lake, Achnasheen Scotland

Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.     

内蒙古黄旗海全新世湖泊沉积物粒度分析及其沉积学意义

对内蒙古黄旗海湖心、湖滨2个现代沉积剖面在前期磁组构研究的基础上,进行了沉积物粒度分析。结果显示,2个剖面从下到上都表现出粗颗粒含量逐渐增加的趋势,反映水动力逐渐增强。对比之前的磁组构结果发现,靠近湖心的HQW剖面,粒度与磁组构具有较好的对比性。HQW剖面的磁各向异性度等磁组构参数从下到上表现出逐渐增大的趋势,显示矿物定向和水流流速逐渐增强。另外,在20cm、40cm、65cm深度附近,粒度与磁组构参数都发生突然变化,表明存在水动力增大的异常沉积事件。HQE剖面的磁组构未能有效指示异常沉积事件,但其粒度分析中的细砂百分比含量和中值粒径在这些位置附近明显减小。若这是由同一异常沉积事件造成的,则表明异常沉积事件的沉积过程比较复杂,在研究异常沉积事件时不同的沉积部位需使用不同的研究方法。     

湘南宝山矿床花岗闪长斑岩中黑云母的矿物学特征及其指示意义

湖南宝山矿床处于坪宝矿带的北端,是湘南地区最大的铜多金属矿床。为了进一步探讨矿区内花岗质岩石的形成条件及成矿潜力,文章在详细的野外地质和岩相学观察的基础上,对与成矿密切相关的花岗闪长斑岩中的黑云母进行了详细的矿物化学分析。电子探针分析结果表明:宝山花岗闪长斑岩中的黑云母为铁质黑云母和镁质黑云母,其中,Ti介于0.18~0.30,且Mg/(Mg+Fe2+)值介于0.42~0.58,属于典型的岩浆成因黑云母;黑云母的氧化系数为0.16~0.26,w(Mg O)为8.17%~11.72%,平均9.3%,MF值范围为0.38~0.50,指示其岩体属于壳幔混源型的I型花岗岩;岩体中以黑云母的全铝含量计算的结晶压力为97~174 MPa,相应的结晶深度为3.67~6.57 km,平均深度为5.12 km。其log f(O2)变化范围为-14.5~-12.8,表明黑云母是在较高氧逸度条件下结晶形成的,有利于铜矿的形成。     

Lake Kivu: structure,chemistry and biology of an East African rift lake

Geophysical, geochemical and biological data are integrated to unravel the origin and evolution of an unusual rift lake. The northern basin of Lake Kivu contains about 0.5 km of sediments which overlie a basement believed to be crystalline rocks of Precambrian age. Volcanic rocks at the northern end of the lake have created large magnetic anomalies of up to 300. Heat flow varies from 0.4 to 4 hfu. The extreme variability may be due in part to sedimentation or recent changes in the temperature of the bottom water. Sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers. Such convecting cells are the result of the increase in both temperature and salinity with depth.Concentrations of the major dissolved gases in the deep water, CO₂ and CH₄, approach saturation but do not exceed it at any depth. The salts are supplied mainly by hydrothermal discharges at the bottom of the lake which we calculate to have a salinity of 4 which is about 60% higher than the salinity of the bottom water. The annual discharge at the present time is about 0.5 km³. Zinc anomalies in the water are explained by the accumulation of sphalerite-containing globules at certain depths.Stratigraphic correlation of sediments is possible across the entire lake, based on physical, geochemical and paleontological criteria. Sedimentation rates are of the order of 30 cm/1000 years implying a Pliocene age for the deepest part of the lake. Periods of hydrothermal activities and heightened volcanism, as recorded in the sediments, appear to have coincided with pluvial times.Enrichment of the surface waters of Lake Kivu by nutrients has led to explosive speciation in the diatom genusNitzschia. Several new types of methane oxidizing and-producing bacteria were isolated. Bacterial degradation of recent plankton appears insufficient to explain the amount of methane in the lake, and some of it is derived diagenetically.

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Zusammenfassung Geophysikalische, geochemische und biologische Daten werden vorgelegt und miteinander in Beziehung gebracht, um einen besseren Einblick in Entstehung und Evolution eines ungewöhnlichen Sees zu gewinnen. Das nördliche Becken des Kivu-Sees enthält Sedimente von etwa 0,5 km Mächtigkeit, die dem präkambrischen kristallinen Grundgebirge überlagert sind. Die im Norden des Sees vorliegenden vulkanischen Gesteine erklären die hohen magnetischen Anomalien, die bis zu 300 betragen. Der Wärmefluß schwankt zwischen 0,4 und 4 cal/cm²/sec. Diese Schwankungsbreite erklärt sich zum Teil aus den Sedimentationsverhältnissen oder den lokalen Temperaturveränderungen im Tiefenwasser. Scharfe Grenzflächen in der vertikalen Temperatur- und Salinitätsstruktur des Wassers über den Gesamtsee sind das Ergebnis von Konvektion, die zu übereinanderliegenden Konvektionszellen führt, in denen jeweils Temperatur und Salinität konstant sind. Die Bildung, Anzahl und Stabilität solcher Zellen hängt von dem Verhältnis der durch Temperatur und Salzgehalt hervorgerufenen Dichteveränderungen ab.Die Konzentrationen der im Tiefenwasser gelösten Gase, d. h. von Kohlendioxyd und Methan, liegen für alle Tiefen unterhalb der Löslichkeit. Die vorliegenden Salze entstammen weitgehend hydrothermalen Lösungen, die dem Seeboden entweichen und deren Salinität etwa 4 beträgt; der Vergleichswert für das Tiefenwasser beträgt 2,5 Diese hydrothermalen Ausschüttungen haben eine Größe von etwa 0,5 km³ pro Jahr, was etwa ein Tausendstel des Gesamtseevolumens ausmacht. Zinkanomalien im Wasser sind ebenfalls hydrothermal bedingt.Physikalische, geochemische und paläontologische Indikatoren erlauben eine stratigraphische Korrelation aller Sedimentkerne. Die Sedimentationsraten liegen bei 30 cm/ 1000 Jahren, und ein pliozänes Alter errechnet sich daraus für das tiefe nördliche Becken. Perioden hydrothermaler Aktivitäten und verstärkter vulkanischer Tätigkeit, die sich in den Sedimenten nachweisen lassen, scheinen mit Pluvialzeiten zu koinzidieren.Die Anreicherung der Oberflächenwässer vom Kivusee durch Mineralstoffe führte zu einer explosionsartigen Spezisierung in der GattungNitzschia. Verschiedene neue Arten von Methan-oxidierenden und -produzierenden Bakterien wurden isoliert. Das Auftreten von Methan ist zum Teil bakteriell und zum Teil diagenetisch bedingt.

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Résumé Des données géophysiques, géochimiques et biologiques sont présentées et collationnées pour donner une vue meilleure sur l'origine et l'évolution d'un lac particulier de la «Rift valley». La baie septentrionale du lac Kivu contient environ 500 mètres de sédiments qui recouvrent le socle cristallin d'âge précambrien. Des épanchements volcaniques au Nord du lac expliquent les fortes anomalies magnétiques qui atteignent 300. Les valeurs du flux thermique varient entre 0.4 et 4 cal/cm²/sec. Cette importante variation s'explique en partie par la sédimentation ou par des changements locaux de la température de l'eau de fond. Des surfaces-limites brusques dans la structure verticale de la répartition de la température et de la salinité de l'eau dans l'étendue du lac sont dûs à la convection; celle-ci conduit à la superposition de cellules de convection dans lesquelles la température et la salinité sont constantes. La formation, le nombre et la stabilité de telles cellules dépendent du rapport des variations de densité dues à la température et à la teneur en sels.Les concentrations des gaz dissouts dans l'eau profonde, en l'occurrence CO₂ et CH₄, sont, à toute profondeur, inférieures à la saturation. Les sels minéraux proviennent surtout de solutions hydrothermales qui émanent du fond du lac, et dont la salinité est voisine de 4; la valeur comparative pour l'eau profonde est de 2,5. L'apport annuel de ces sources est de l'ordre de 0,5 km³, soit 1/1000 du volume total du lac. Les teneurs anormales en Zn sont dues également à ce caractère hydrothermal.Des données physiques, géochemiques et paléontologiques permettent la corrélation stratigraphique des sédiments. Les vitesses de sédimentation sont de l'ordre de 30 cm/1000 ans, donnant ainsi un âge pliocène pour la partie profonde de la baie septentrionale. Les périodes d'activité hydrothermale et de renforcement de la vulcanicité qui se manifestent dans les sédiments, semblent coïncider avec les périodes pluviales.L'enrichissement des eaux de surface du lac Kivu en substances minérales a entraîné un développement explosif des diatomées, en particulier du genreNitzchia. Différentes espèces nouvelles de bactéries oxydant et produisant CH₄ ont été isolées. La présence de méthane est due en partie à la destruction du plancton par les bactéries et en partie à une transformation diagénétique.

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Aeolian component records in lacustrine sediments and its atmospheric circulation significance for evolution of Qarhan salt lake,NE Qinghai-Tibetan Plateau

正1 Introduction The Qarhan Playa of Qaidam Basin is located at the northeastern QinghaiTibetan Plateau(QTP),in which thick lacustrine and evaporate sediments deposited since the late Pleistocene(Chen et al.,1985,1986).As surrounded by capacious Gobi and yardang fields and suffered strong westerly-northwesterly wind,abundant aeolian materials were trapped in these sediment sequences in the