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1.
《Applied Geochemistry》2003,18(9):1497-1506
Sedimentation and benthic release of As was determined in Baldeggersee, a eutrophic lake in central Switzerland. Sediment traps recorded As sedimentation during 1994, including a flood event in spring. Diagenetic processes were studied using porewater profiles at the sediment–water interface and in deeper sediment strata deposited in the mesotrophic lake (before 1885). Sediment cores were used to calculate the accumulation and to construct the balance of sedimentation and remobilisation. The results showed that the lake sediment acts as an efficient sink for As. Only 22% of the particulate As flux reaching the sediment surface was remobilised at the sediment–water interface. The As accumulation in the recent varved section of the eutrophic lake was 40 mg As m−2 a−1. Iron reduction in older sediment caused a remobilisation of 1.2 mg As m−2 a−1. This upward flux from the deeper sediment was quantitatively immobilised in the recent sulfidic sediments. The flood event in spring contributed about 34% of the yearly sediment load and led to distinct peak profiles of dissolved As in the porewater. This evidence for rapid remobilisation disappeared within months.  相似文献   

2.
The 1979 Nice turbidity current is modelled using a visco-plastic analysis of flow velocity because the initial flow concentrations are expected to have been very high. The complete history of the failed sediment from debris flow to turbidity current plume is therefore addressed. The turbidity current portion is considered as a steady state flow divided into a dense bottom flow and an upper plume. Model results show that a dense flow can be generated from the debris flow by the disaggregation of the initial slide. The dense flow would be strongly erosive and able to create and maintain a low-density plume at its surface. The depth of erosion of the channel floor by the dense flow is predicted to reach 6–11 m in overconsolidated sediments, with the main erosion taking place in Var Canyon and the Upper Fan Valley. The eroded volume (108 m3) provides additional material to the sediment mass of the initial failure. The dense flow appears able to inject fine sand and silt into the overlying plume during 90 km, and would disintegrate before being able to deposit sediment. The extensive sand layer along the travel path of the turbidity current may have been deposited from the tail of the trailing plume: a result of the velocity difference between the plume and the dense flow. Observations on sedimentary structures, erosion features and distribution of the sand deposit are quite in agreement with our modelling approach. For example, gravel waves can be generated when loose deposits are reworked by the supercritical dense flow. The methodology and equations presented here provide a good estimate of the geological consequences of a high-velocity gravity flow undergoing rheological transition.  相似文献   

3.
ABSTRACT Surface sediments, cores and seismic reflection profiling delineate sedimentary environments and processes of sedimentation in Lake Tekapo. Sedimentation is dominated by the Godley River which forms an extensive delta in the northern third of the lake. Delta growth accounts for 55% of annual sediment deposition. In winter sandy muds are deposited at the top of the delta slope, where they may move under gravity as a surficial slide. Oversteepening of the upper slope also generates deep seated failures. The entire 20 km2 of delta slope is subjected to rotational slumping which episodically reworks large volumes of sediment. Down the delta slope sedimentation rates decrease, surface sediments get finer and varves become better developed.
In the lake basin sediments are parallel bedded varves, which contain typical winter-summer annual cycles as well as minor, non-annual flood varves. Annual varve thickness and semi-annual varve frequency are determined by variations in the discharge of the Godley River. Sedimentation in the basin accounts for 40% of the budget and sedimentation rates decrease with distance from the delta, except at the distal end of the basin, where turbid underflows are stopped by the rising lake floor. Beyond the basin, sedimentation rates decrease abruptly. Coriolis deflection of inflowing river water increases sedimentation rates down the eastern shore. The remaining 5% of the sediment is deposited on the lateral slopes and shoulders where sediments form a thin muddy veneer over basement, which occasionally slumps to the basin floor.  相似文献   

4.
The temporal and spatial variations in the rate of sedimentation and sediment resuspension in Lake Rehtijärvi (southern Finland) were studied by sediment traps deployed in the stratifying and non‐stratifying regions of the lake. Both the gross sedimentation rate and the resuspension rate recorded by near‐bottom traps were significantly higher in the stratifying region (>20 m depth) than in the shallow part of the lake (2 m depth). These rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m?2 day?1 in July 2005, whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m?2 day?1 throughout the study. The elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish, but was due to the occurrence of internal seiches. The seiche amplitude was on average the highest in July, although the wind velocities were low; this was because winds temporarily were blowing along the longitudinal axis of the lake. Due to the steep slopes, the sediments of Lake Rehtijärvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations.  相似文献   

5.
Proglacial Quaternary lacustrine sediments deposited along the Caribou River Valley, Yukon, Canada, formed in a lake impounded by glacial ice that was retreating downslope. Sedimentation in the lake was dominated by turbid sediment underflows generated from the upslope, previously deglaciated region. The base of the sedimentary succession indicates a gradual transition from sporadic low-density distal flows to higher density proximal flows. Continued sediment accumulation resulted in the construction of a subaqueous clay and silt bank. Sedimentation was dominated by deposition of suspension load clay carried by subseasonal bottom countercurrents induced by katabatic winds. This sedimentation pattern prevailed until the subaqueous bank was disturbed by mass movement. Removal of the sediment bank increased the depth of the nearshore area sufficiently to allow turbid underflows to dominate sedimentation once more. The changing sedimentation patterns reflect events in the areas away from the ice front, rather than changes in the activity of the impounding glacier. Similar successions could be developed in other glacial lakes impounded by glaciers which moved up topographical slopes, either pre-existing or generated by glacioisostatic depression.  相似文献   

6.
Lake Houston is a man-made reservoir located northeast of Houston, Texas. The purpose of this investigation was to document suspended sediment transport, sedimentation, and resuspension in the lake with a view towards estimating the influence of sedimentation on water quality. Sediment traps were placed in strategic locations in the lake to collect suspended sediments. Samples were analyzed for bulk density, grain size, organic carbon, and a number of trace elements. These data were analyzed along with meteorological data to examine those factors which regulate suspended sediment input and dispersal, and the role of suspended sediments in controlling water quality within the lake. Sediment input to the lake depends primarily on the intensity of rainfall in the watershed. Sediment movement within the lake is strongly influenced by wave activity, which resuspends sediments from shallow areas, and by wind-driven circulation. The increased residence time of suspended sediments due to resuspension allows greater decomposition of organic matter and the release of several trace elements from sediments to the water column. Virtually all samples from sediment traps suspended between 1 and 5 m above the lake bottom contain medium to coarse silt, and even some very fine sand-sized material. This implies that circulation in Lake Houston is periodically intense enough to transport this size material in suspension. During winter, northerly winds with sustained velocities of greater than 5 m/sec provide the most suitable condition for rapid (<1 d) transport of suspended sediment down the length of the lake. Fluctuations in current velocities and the subsequent suspension/deposition of particles may explain variations in the abundance of coliform bacteria in Lake Houston.  相似文献   

7.
Resuspension estimates given by two different trap methods in a shallow lake were compared. The sensitivity of the methods to errors in estimates of gross sedimentation and organic fraction of trapped material was explored. The methods were label method, in which resuspension is estimated by determining the organic fraction of surface sediment, suspended seston and trapped material, and SPIM/SPM method, where the relationship between settling particulate inorganic matter (SPIM) and total settling particulate matter (SPM) is used. During the whole 111 day study period, according to the label method, at a sheltered station 1949 g m−2 dry weight of sediment was resuspended, whereas SPIM/SPM gave an estimate of 1815 g m−2. The difference in the estimates was probably due to mineralization loss of organic material in the traps during the two week exposure periods. Sensitivity analysis showed that of the two methods, the label method was more sensitive to variations in the organic content of trapped material. At a wind-exposed station, the total amounts of resuspended matter given by the label method and by the SPIM/SPM method were 4966 g m−2 and 4971 g m−2, respectively. Due to wind effects, escape of trapped material took place, which caused underestimation of gross sedimentation and compensated the effects of mineralization loss to diminish the difference between the methods. Of the two methods, the SPIM/SPM method seems thus more suitable for lakes, where bacterial activity is high. If cyanobacterial blooms take place, the label method is probably more reliable, providing that the exposure time of sediment traps is kept adequately short.  相似文献   

8.
A series of laboratory experiments was conducted in order to determine how settling-driven convection influences the length-scale over which the majority of particles settle beneath a buoyant sediment-laden plume spreading over a denser saline layer. This system is analogous to sediment-laden river water spreading into a lake or the coastal ocean. The key dimensionless parameter that controls the settling dynamics of such flows is the density ratio, defined as the ratio of density differences due to the added salt and sediment. For a buoyant plume, this ratio has to be greater than unity, so that the experiments in the current study were performed for density ratios between one and five. When density ratio was close to one, settling-driven convection was vigorous and the length-scale over which sedimentation occurred was very small. A strong secondary turbidity current was generated in this case. On the other hand, for larger values of density ratio, the predicted length-scale over which a secondary plume was generated increased in proportion to the density ratio. A complete mathematical expression for this length-scale was developed using recent theory that described the timescale over which settling-driven convection evolved. The theoretically predicted propagation length-scale showed very good quantitative agreement with laboratory experiments. The use of the dimensionless density ratio allows the expression to predict which sediment-laden river plumes in lakes and the coastal ocean could quickly form secondary turbidity currents.  相似文献   

9.
Sedimentation from gravity currents generated by turbulent plumes   总被引：4，自引：0，他引：4  
Sedimentation from radially spreading gravity currents generated at the top of ascending sediment-laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant, B, which depends on flow rate, Q, and particle settling velocity, v (B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle-laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant, B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = -Br2, where r is the radial distance, S is the sediment mass flux per unit area and S0 is the value of S at r=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical model.  相似文献   

10.
The Grande Prairie region of northwestern Alberta was partially covered by glacial Lake Peace, which was dammed against the retreating Laurentide ice sheet. Two levels of glacial Lake Peace are identified in the study are by closely spaced groups of strandlines and minor deltas lying at 805 to 840 m a. s. l., and 655 to 710 m a.s.l. Sedimentation associated with the upper of the two lake levels is marked by rhythmites of silt and clay deposited by turbid underflow, interbedded with diamicton deposited by debris flow. Dropstones and dump structures indicate common ice-rafting. Thick sequences are only found on the axes of major valleys, where sediment gravity flows were concertrated. Thin sequences of ice proximal glaciolacustrine sediments reflect topographic setting and do not indicate a short-lived lake. Retreat of the ice front resulted in a decrease in ice-rafted material and diamicton in sediments. The fall in lake level to the second stage resulted in deposition of sequences of vaguely laminated silt and clay close to the modern Beaverlodge River. These sediments were deposited by suspension settling from interflow or overflow of the Beaverlodge River as it entered the lake. Lake sedimentation was dominated by inflow from unglaciated areas, rather than the ice front.  相似文献   

11.
Sediments deposited in two small ice-contact lakes with low rates of sediment input have been studied in subaerial exposures. Sediment characteristics are a function of the water source (glacial meltwater versus non-meltwater), proximity to the glacier margin and lake shore, amount of supraglacial debris, and lake duration. Calving Lake expanded (and later partially drained) as a calving ice margin retreated. Nearshore deltas contain 1 × 105 m3 stratified sand and gravel deposited at rates up to 1 m/yr during a 9-yr interval. Deltaic sediment contains types A and B ripple-drift cross-lamination, draped lamination, and scour surfaces caused by variations in water-flow velocity and the amount of sediment settling from suspension. Most water inflow came from non-subglacial meltwater sources and was sediment-poor, so overflow and interflow sedimentation processes dominated the offshore environment. Offshore sediment generally contains massive silt or silt interbedded with fine-grained sand deposited at rates of 1.3-1.5 cm/yr. Iceberg gravity craters observed on the lake plain were formed when icebergs impacted the lake floor during calving events. In Bruce Hills Lake, proximity to glacier ice and the presence of supraglacial sediment formed coarsening-upward successions when debris fell directly from an ice ledge onto silty lacustrine sediment.  相似文献   

12.
Dams and reservoirs are often efficient sediment traps, and conventional wisdom holds that fluvial sediment supplies are reduced well downstream. However, there are reasons to question the extent to which fluvial and alluvial sediment supplies are reduced more than a few kilometers downstream of dams. Sedimentation in bottomlands of Loco Bayou, east Texas, was investigated at a site less than 16 km downstream of Loco Dam and Lake Nacogdoches, which controls 86% of the 265-km2 drainage area. Turbidity levels are generally as high or higher than those on Loco Bayou upstream of the lake. Sedimentation rates on the lower floodplain since the dam was completed are 11 mm year-1 or more. This rate is high enough to suggest that the dam has no effect on sediment supplies 16 km downstream. The spatial pattern of sedimentation and the vegetation distribution suggest that the elevation and frequency of flooding, not fluvial sediment availability, are the critical factors in determining sediment supplies to these floodplains.  相似文献   

13.
Submarine lobes have been identified within various deep-water settings, including the basin-floor, the base of slope and the continental slope. Their dimensions and geometries are postulated to be controlled by the topographic configuration of the seabed, sediment supply system and slope gradient. Ten experiments were conducted in a three-dimensional-flume to study the depositional characteristics of submarine lobes associated with: (i) different basin floor gradients (0 to 4°); (ii) different sediment concentrations of the parent turbidity current (11 to 19% vol); and (iii) varying discharge (25 to 40 m3 h−1). Most runs produced lobate deposits that onlapped onto the lower slope. Deposit length was proportional to basin-floor angle and sediment volume concentration. A higher amount of bypass is observed in the proximal area as the basin-floor angles get steeper and sediment concentrations higher. Deposits of runs with lower discharge could be traced higher upslope while runs with higher discharge produced an area of low deposition behind the channel mouth, i.e. discharge controlled whether lobe deposits were attached or detached from their channel-levée systems. A particle-advection-length scale analysis suggests that this approach can be used as a first order estimation of lobe element length. However, the estimations strongly depend on the average grain size used for calculations (for example, silt is still actively transported after all sand has been deposited) and the method cannot be used to locate the main depocentre. Furthermore, attempted reconstructions of turbidity current velocities from natural systems suggest that the method is not appropriate for use in inversions from more complex composite bodies such as lobes.  相似文献   

14.
Sediments deposited in a lake at the front of a glacier in the Svartisen area, Norway, have been studied between 1957 and 1974. Until 1959, they were almost completely covered by an outwash plain (sandur), but subsequent erosion has exposed glacial lake sediments more than 70 m deep within a rock basin about 2·5 km long and 1 km wide. The basin was filled by sand and silt carried from beneath the glacier Austerdalsisen by two rivers, each of which deposited a delta in the lake. As the deltas advanced, laminated pro-delta silt was covered by crossbeds of fine sand and silt, and by near-horizontal sheets of fine sediments laid down between the delta-fronts and the distal end of the rock basin. Although both slumping and loading caused minor disturbance of sediments at the lake floor, deformation was of local significance only. Movement of a mass of sediment across the floor, probably triggered by a ‘seismic event’ related to movement of the glacier or to calving at the floating tongue, created a recumbent fold in laminated sand and silt, but transfer of sediment over the lake bed was rare once it had been deposited. Varves are not common at Austerdalsisen, indicating that water temperature, lake chemistry or variations of water and sediment discharge from the glacier were unfavourable for their formation; rhythmic deposition from density flows of sediments carried from beneath the glacier rarely occurred within the Austerdalsisen basin.  相似文献   

15.
Sea‐floor topography of deep‐water folds is widely considered to have a major impact on turbidity currents and their depositional systems, but understanding the flow response to such features was limited mainly to conceptual notions inspired by small‐scale laboratory experiments. High‐resolution three‐dimensional numerical experiments can compensate for the lack of natural‐scale flow observations. The present study combines numerical modelling of thrusts with fault‐propagation folds by Trishear3D software with computational fluid dynamics simulations of a natural‐scale unconfined turbidity current by MassFlow‐3D? software. The study reveals the hydraulic and depositional responses of a turbidity current (ca 50 m thick) to typical topographic features that it might encounter in an orthogonal incidence on a sea‐floor deep‐water fold and thrust belt. The supercritical current (ca 10 m sec?1) decelerated and thickened due to the hydraulic jump on the fold backlimb counter‐slope, where a reverse overflow formed through current self‐reflection and a reverse underflow was issued by backward squeezing of a dense near‐bed sediment load. The reverse flows were re‐feeding sediment to the parental current, reducing its waning rate and extending its runout. The low‐efficiency current, carrying sand and silt, outran a downslope distance of >17 km with only modest deposition (<0·2 m) beyond the fold. Most of the flow volume diverted sideways along the backlimb to surround the fold and spread further downslope, with some overspill across the fold and another hydraulic jump at the forelimb toe. In the case of a segmented fold, a large part of the flow went downslope through the segment boundary. Preferential deposition (0·2 to 1·8 m) occurred on the fold backlimb and directly upslope, and on the forelimb slope in the case of a smaller fold. The spatial patterns of sand entrapment revealed by the study may serve as guidelines for assessing the influence of substrate folds on turbiditic sedimentation in a basin.  相似文献   

16.
The chronology of near-surface sediments in Lake Illawarra has been investigated using radiocarbon dating and anthropogenically derived substances including trace metals, ash, and137Cs. Sediments at depths about 1 m below the water-sediment interface ranged in age from Modern to 786 calendar years bp on the basis of radiocarbon dating ofNotospisula trigonella valves. Multiple marker (for example ash-trace metals) depth-concentration sediment profiles yielded estimates of sedimentation rate ranging from 3 to 5 mm yr–1 at Griffins Bay to more than 16 mm yr–1 at Macquarie Rivulet. Sedimentation rates of approximately 10 mm yr–1 appeared to be typical of the western and southwestern portions of the lagoon. Rates of sediment accretion, prior to catchment clearing, urbanization, and industrialization have been estimated at less than 1 mm yr–1, thus indicating a general tenfold increase in sediment accumulation adjacent to the western foreshore caused by catchment development. Accelerated sedimentation in shallow coastal lagoons constitutes significant environmental impacts including shoaling, degradation of seagrass beds, and increased turbidity with consequent loss of aesthetic appeal. Management policy should be directed at attempts to reduce the amount of sediment input by the construction of strategically placed sediment retention ponds. Siltation within the shallow embayments of Lake Illawarra could be ameliorated by a carefully planned program of dredging.  相似文献   

17.
In Fayetteville Green Lake, past sedimentation rates can be accurately and precisely estimated by separating annual couplets or varves in dried sediment samples. Two measures were used, which serve as upper and lower limits on estimated sedimentation rate. They agree within 5 % with average annual sedimentation rate in couplets for recent years. Between 3 and 5 replicate samples are needed to reduce the half-width of 95 % confidence intervals on individual couplet sedimentation rates to 30 g m?2 yr?1 about 5 % of average recent rates. In the late 1800s sedimentation rate averaged 392 g m?2 yr?1 and ranged between 324 and 466 g m?2 yr?1, while in the 1970s the rate averaged 581 g m?2 yr?1 by the same measure, and ranged between 384 and 646 g m?2 yr?1. Sedimentation rate averaged for 13 years does not vary over short distances in the profundal zone, but lateral variation in sedimentation rate can be detected for individual years over the same distance. Not all this variation was associated with the non-uniform distribution of dark sublaminae and thin turbidites which cannot be separated from the annual layers. This indicates that although precise estimates of sedimentation rates can be made at different points in the lake, estimates will have to be made at numerous points before annual sedimentation rates for the lake as a whole can be accurately assessed.  相似文献   

18.
MIKE DICKMAN 《Sedimentology》1985,32(1):109-118
In a small meromictic lake near Toronto, Canada, a mass mortality of photosynthetic bacteria followed the ventilation of the chemocline during fall (autumn) and resulted in 3–8 g m-2 day-1 of organic matter being deposited as a dark layer in sediment traps which were suspended in the permanently anaerobic zone. This mass mortality of photosynthetic bacteria occurred in late autumn following the annual thermal destratification of the lake's mixolimnion. Wind mixing during this period of homeothermy resulted in the introduction of low levels of dissolved oxygen into the lake's chemocline. The ensuing mass mortality of photosynthetic bacteria resulted in the release of elemental sulphur as the sulphur-rich bacteria decomposed and sank to the bottom of the lake. The ferrous ions in the water below a depth of 15 m in Crawford Lake reacted with this sulphur to form black ferrous sulphides and pyrite which formed a dark microlamina on the lake floor. Each dark microlamina was overlain by a light coloured (calcite-rich) layer which was deposited each spring and summer during the 3 yr period of this study. The mechanism of microlamina formation elucidated here has been based on the examination of bi-weekly sediment trap information. This approach has permitted an explanation of the mechanisms by which specific events such as calcite precipitation and phytoplankton seasonal succession are transcribed into the sediment record.  相似文献   

19.
The deeply dissected Southwest Grand Banks Slope offshore the Grand Banks of Newfoundland was investigated using multiple data sets in order to determine how canyons and intercanyon ridges developed and what sedimentary processes acted on glacially influenced slopes. The canyons are a product of Quaternary ice‐related processes that operated along the margin, such as ice stream outwash and proglacial plume fallout. Three types of canyon are defined based on their dimensions, axial sedimentary processes and the location of the canyon head. There are canyons formed by glacial outwash with aggradational and erosional floors, and canyons formed on the slope by retrogressive failure. The steep, narrow intercanyon ridges that separate the canyons are composite morphological features formed by a complex history of sediment aggradation and degradation. Ridge aggradation occurred as a result of mid to late Quaternary background sedimentation (proglacial plume fallout and hemipelagic settling) and turbidite deposition. Intercanyon ridge degradation was caused mainly by sediment removal due to local slump failures and erosive sediment gravity flows. Levée‐like deposits are present as little as 15 km from the shelf break. At 30 km from the shelf, turbidity currents spilled over a 400 m high ridge and reconfined in a canyon formed by retrogressive failure, where a thalweg channel was developed. These observations imply that turbidity currents evolved rapidly in this slope‐proximal environment and attained flow depths of hundreds of metres over distances of a few tens of kilometres, suggesting turbulent subglacial outwash from tunnel valleys as the principal turbidity current‐generating mechanism.  相似文献   

20.
Studies of the upper 447 m of the DEEP site sediment succession from central Lake Ohrid, Balkan Peninsula, North Macedonia and Albania provided important insights into the regional climate history and evolutionary dynamics since permanent lacustrine conditions established at 1.36 million years ago (Ma). This paper focuses on the entire 584-m-long DEEP sediment succession and a comparison to a 197-m-long sediment succession from the Pestani site ~5 km to the east in the lake, where drilling ended close to the bedrock, to unravel the earliest history of Lake Ohrid and its basin development. 26Al/10Be dating of clasts from the base of the DEEP sediment succession implies that the sedimentation in the modern basin started at c. 2 Ma. Geophysical, sedimentological and micropalaeontological data allow for chronological information to be transposed from the DEEP to the Pestani succession. Fluvial conditions, slack water conditions, peat formation and/or complete desiccation prevailed at the DEEP and Pestani sites until 1.36 and 1.21 Ma, respectively, before a larger lake extended over both sites. Activation of karst aquifers to the east probably by tectonic activity and a potential existence of neighbouring Lake Prespa supported filling of Lake Ohrid. The lake deepened gradually, with a relatively constant vertical displacement rate of ~0.2 mm a−1 between the central and the eastern lateral basin and with greater water depth presumably during interglacial periods. Although the dynamic environment characterized by local processes and the fragmentary chronology of the basal sediment successions from both sites hamper palaeoclimatic significance prior to the existence of a larger lake, the new data provide an unprecedented and detailed picture of the geodynamic evolution of the basin and lake that is Europe’s presumed oldest extant freshwater lake.  相似文献   

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