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1.
A study on two closed salt lake basins, Tal Chapar and Parihara in the eastern margin of the Thar Desert, Rajasthan, was carried out to unravel late Quaternary geomorphic evolution of these saline lakes. Both lakes are elliptical in shape bordered by stabilised dunes, and are oriented in a NE-SW direction, i.e., in the direction of the prevailing summer monsoon wind. Both lakes have been formed in the wind-shadow zones of isolated hills of Precambrian quartzite. Our study indicates that the late Quaternary sediments in the lakes began with the cyclic deposition of laminated fine silt layers (0.5 m thick), rich in organic matter, alternating with ripple cross-bedded sand layers (each ∼1.5–2 m thick). Sand layers that are moderately sorted are separated by laminated silt-clay layers with gypsum/calcite and this unit occurs in the upper most 4 m sequence in deeper sections. The presence of gypsum crystals within the laminated sediments suggests a high concentration of Ca in the inflowing water. At Parihara Lake the organic carbon-rich sediments at 95 cm depth was dated to 7,375 + 155/−150 year BP. At Tal Chapar radiocarbon dates of 7,190 + 155/−150 and 9,903 + 360/−350 was obtained from the sediments rich in organic carbon occurring at a depth of 1.35 m and 1.80 m, respectively. The study reveals strong hydrologic oscillations during the past ∼14,000 year BP (13,090 + 310/−300 year BP). Quaternary geomorphic processes, especially the strong aeolian processes during dry climatic phases, played a major role in the formation of the lake basins, as well as the fringing linear dunes. Geochemical and mineralogical analyses of the lacustrine sediments, supported by radiocarbon dates indicate the existence of an ephemeral lake earlier than ∼13,000 year BP as sediments began to be deposited in a lacustrine environment implying sustained runoff in the catchments. A freshwater lake formed between 9,000 year and 7,000 year BP. The lake dried periodically and this strong fluctuating regime continued until about ∼7,000 year BP. Mid-Holocene was wet and this was possibly due to higher winter rains A saline lake existed between 6,000 year and 1,300 year BP and finally present day semi arid conditions set in since 1,200 year BP. Remnants of a habitation site (hearth and charred bones) on stabilised dune at Devani near Tal Chapar were dated to 240 ± 120 year, while that at Gopalpura was dated to 335 ± 90 year. These historical sites on stabilised dunes were, according to the local accounts, settlements of people who used the lake brine for manufacturing salt.  相似文献   

2.
The recent sediments of two lakes in the NE German lowland became seasonally laminated at different times. Anoxic bottom conditions resulted from a surplus of organic matter (OM), in the early stage indicated by irregularly laminated sediments comprising abundant iron-sulfide framboids. Their diagenetic formation predates the preservation of biochemical calcite varves. In the larger, deeper Lake Tiefer See near Klocksin, anoxia developed stepwise. A first anoxic pulse was contemporary with inflow narrowing by railway-dam construction and accumulation of OM. It was favored by a decrease of the intensity of lake circulation (turnover). Nutrients introduced from artificial fertilizer then increased the primary production (diatoms) to the point of OM surplus and seasonal laminae formation started 40 years later in 1924. In the smaller, shallower Lake Tiefer See in the Uckermark, a massive pulse of iron sulfide was centered around 1960, seven years after installation of piped field drainage into the lake. Anoxia developed rapidly with the nutrients drained from a fertilized groundwater catchment that is 10 times larger than the surface catchment, while surface erosion was reduced. Reducing bottom conditions became regular and the seasonal lamination was preserved after 1967. Morphological criteria to screen lakes for varved sediments should include reductions of natural lake inflow and catchment increase, such as by inflow of field drainage. Similar developments of increased nutrient input or intensity decrease of lake circulation may result from historical human activities but also from natural processes.  相似文献   

3.
Glacial landscapes of the Land of Great Masurian Lakes and Suwa?ki Lakelands in northeast Poland are characterized by very high abundance of lakes. These two areas were surveyed for lakes containing laminated sediments. Using bathymetry as a criterion, 60 small, deep lakes, representing preferred conditions for formation and preservation of lacustrine non-glacial varves, were selected for gravity coring. We found laminated sediments in 24 of the lakes, 15 in the Land of Great Masurian Lakes and 9 in the Suwa?ki Lakeland. Seven of these 24 sediment records were laminated in the topmost part only. Analysis of lake morphometric variables showed that the relation between surface area and maximum water depth can be used to identify lakes with laminated sediments. Most of the newly discovered lakes with laminated deposits have surface areas ≤0.3 km2 and maximum depths of 15–35 m. Multivariate statistical analysis (Linear Discriminant Analysis) of the lake dataset identified the morphological features of lake basins and their catchments that largely control preservation of laminated sediments. Microscopic and geochemical analyses revealed a biogenic (carbonaceous) type of lamination typical for lakes in northeast Poland. Such lakes are characterized by a spring-summer lamina that is rich in calcium carbonate and an autumn-winter lamina composed of organic and minerogenic detritus. This pattern may be modified by multiple periods of calcite deposition during a single year or substantial contribution of clastic material. Laminations and high sedimentation rates offer the possibility of high-resolution investigation of past climate and environmental changes through application of myriad biological, isotopic and geochemical proxies.  相似文献   

4.
A study of sedimentary processes and sediment yields in a high arctic meromictic lake (Lake C2, Taconite Inlet, Northern Ellesemere Island, Canada) was undertaken from May 1990 through August 1992 to understand the links between climatic controls, hydrology, and the laminated sediment record preserved in the lake. Understanding the relationships between processes and the sediment record is critical for interpreting the climatic significance of the laminated sediments in a region where high resolution climate proxy records are quite limited.Sediment transport to Lake C2 is dominated by fluvial processes. During the early part of the melt season slushflows transport sediment to the lake surface. Subsequently, suspended sediment is delivered to the lake by the main inlet stream and distributed lakeward by a plume emanating the main inlet channel. Due to the strong density stratification of the water column the plume distributes sediment downlake by overflows and interflows in the epilimnion. In general, overflows are generated by lower discharge events whereas interflows penetrate to the halocline during high discharge with increased suspended sediment concentration.Sediment trap analysis demonstrates that suspended sediment transport and deposition responds to diurnal through annual changes in stream discharge. Seasonal and annual sediment trap yields agree with average accumulation rates determined from varve thickness measurements and cumulative suspended sediment discharge from the main inlet stream indicating a close link between climatological, hydrological, and sedimentological controls and varve deposition.This is the fourth in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.  相似文献   

5.
We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and 137Cs and 14C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans.  相似文献   

6.
Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km2; z max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS 14C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.  相似文献   

7.
We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.  相似文献   

8.
El’gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size, clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic carrier minerals. The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing preserved laminated sequences in the core. A bioturbation index based upon these laminated and non- laminated sequences co-varies with total organic carbon (TOC) and magnetic susceptibility. Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing conditions on the landscape around the lake, chlorite weathers easily and illite/smectite abundance increase, which produces an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes that correlate with shifts in susceptibility. The mean grain-size (6.92 μm) is in the silt-size fraction, with few grains larger than 65 μm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related to grain size. Lake El’gygytgyn’s magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally recognized␣events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment proxies of this age from Chukotka (Fig. 1). This is the tenth in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献   

9.
Climate signal in varve thickness: Lake La Cruz (Spain), a case study   总被引：1，自引：0，他引：1  
Lake La Cruz is a meromictic, karstic lake with annually laminated sediment formed by summer pulses of calcite deposition. The aim of this study was to explore the potential use of the laminated sediment from Lake La Cruz as a quantitative climate proxy, by calibrating lamina thickness against instrumental climate data. Statistical analysis of the relation between lamina thickness and the meteorological dataset indicated a high correlation between calcium carbonate lamina thickness and rainfall from December to March (r  = 0.725, P < 0.01, n = 35). Winter rainfall anomalies in the area are, in turn, highly negatively correlated with the North Atlantic Oscillation index (NAO, r = −0.832; P < 0.01; n = 53). We propose a regression model to infer past winter rainfall from calcium carbonate laminae thickness. These results highlight new possibilities for paleoenvironmental research using calcite laminated sediment records as climate proxies, especially to study past rainfall variability.  相似文献   

10.
High-resolution (annual to sub-decadal) quantitative reconstructions of climate variables are needed from a variety of paleoclimate archives across the world to place current climate change in the context of long-term natural climate variability. Rapid, high-resolution, non-destructive scanning techniques are required to produce such high-resolution records from lake sediments. In this study we explored the potential of scanning reflectance spectroscopy (VIS-RS; 380–730 nm) to produce quantitative summer temperature reconstructions from minerogenic sediments of proglacial, annually laminated Lake Silvaplana, in the eastern Swiss Alps. The scanning resolution was 2 mm, which corresponded to sediment deposition over 1–2 years. We found correlations up to r = 0.84 (p < 0.05) for the calibration period 1864–1950, between six reflectance-dependent variables and summer (JJAS) temperature. These reflectance-dependent variables (e.g. slope of the reflectance 570/630 nm, indicative of illite, biotite and chlorite; minimum reflectance at 690 nm indicative of chlorite) indicate the mineralogical composition of the clastic sediments, which is, in turn, related to climate in the catchment of this particular proglacial lake. We used multiple linear regression (MLR) to establish a calibration model that explains 84% of the variance of summer (JJAS) temperature during the calibration period 1864–1950. We then applied the calibration model downcore to develop a quantitative summer temperature reconstruction extending back to AD 1177. This temperature reconstruction is in good agreement with two independent temperature reconstructions based on documentary data that extend back to AD 1500 and tree ring data that extend back to AD 1177. This study confirms the great potential of in situ scanning reflectance spectroscopy as a novel non-destructive technique to rapidly acquire high-resolution quantitative paleoclimate information from minerogenic lake sediments.  相似文献   

11.
A sediment sequence from the Gotland Basin, representing the Ancylus Lake/Litorina Sea transition, was analysed for pigments, stable isotopes (δ13C and δ15N), diatoms and carbon and nitrogen content. The negative correlation between the cyanobacterial specific pigment zeaxanthin and δ15N supports the hypothesis that cyanobacteria in the early Litorina Sea indeed were nitrogen fixers. Their incorporation of nitrogen could have acted as a trigger for eutrophication during the Litorina Sea stage of the Baltic Sea. As cyanobacteria normally flourish in eutrophicated waters, the increasing concentrations of zeaxanthin during early Litorina Sea also corroborate that high primary production was an important part of the formation of organic rich (sapropel) sediments in the Baltic Sea. The first occurrence pre-dates the formation of laminated sediments, but the peaks of both zeaxanthin and organic carbon are within laminated sequences. This implies that the oxygen conditions of the bottom water also play a major role in the formation of sapropel sediments.  相似文献   

12.
The recent (1950–1996) varve record from the proximal sediments in Nicolay Lake, Cornwall Island, Nunavut, Canada (77°46′ N, 94°40′ W) contains distinct subannual rhythmites. Deposition of these subannual rhythmites is due primarily to nival snow melt, with additional sedimentary units resulting from major summer precipitation and subaqueous mass wasting events. In order to evaluate the potential hydroclimatic signal contained in the varves from the unglacierized catchment, the nival deposition record was estimated by delineating the initial subannual rhythmite within each varve. When the record is split into temporal segments based on two phases that exhibit different sediment deposition patterns in the lake, the nival rhythmites are significantly correlated to annual cumulative melting degree days (MDD) from the nearest weather station Isachsen (78°47′ N, 103°32′ W) (1950–1962 AD and 1963–1977 AD with r = 0.55 and r = 0.82, respectively). A similar analysis with data from Resolute (74°43′ N, 94°59′ W) yields slightly weaker correlations (1950–1962 AD, r = 0.60; 1963–1994, r = 0.59). The strong positive correlation with both the Isachsen and Resolute thermal records suggests that the paleoclimatic signal in the sediments reflects regional climate conditions. Notably, the signal is strongest when the entire melt season is considered; weaker correlations with instrumental weather records are associated with comparisons limited to the peak melt or early season melt periods. We attribute this to the ongoing supply of snowmelt through the season in this polar region and the availability of sediment for transport throughout the melt season. These results indicate that a high resolution hydroclimatic signal is present in the sediments from Nicolay Lake and can be used for paleoclimate reconstruction provided sedimentary depositional controls are taken into account.  相似文献   

13.
Western Poland is located in the central European climatic transition zone, which separates the mild and humid Atlantic climate of Western Europe and the East European continental climate. This region is sensitive to lateral shifts of the European climate zones and is particularly suitable for reconstructing Holocene climate variability. This paper presents detailed analyses of the sedimentary record from Lake Strzeszyńskie since the Late Pleistocene. These include smear-slide and thin-section observations, X-ray fluorescence core scanning, magnetic susceptibility measurements, pollen analyses, and radiocarbon dating. The sediment record reveals three distinct sedimentary units consisting of: (1) an alternation of sand layers and laminated silt and clay deposits accumulated prior to 14,600 cal yr BP; (2) faintly laminated calcareous sediments intercalated with organic matter-rich layers deposited between 14,600 and 10,200 cal yr BP; and (3) massive calcareous mud deposited after 10,200 cal yr BP. The Holocene period is marked by nine phases of organic-rich sedimentation and enhanced Fe deposition, which occurred at ca. 10.1, 9.3, 6.4–6.1, 5.5–5.1, 4.7–4.5, 2.7–2.4, 1.3–1.2, 0.8–0.6, 0.4–0.2 kyr cal BP. These phases are associated with high lake levels and correspond with wet periods recognized in several other records from Poland and central Europe. These phases partly coincide with North Atlantic cold periods, which may suggest that high lake levels are triggered by an ocean-continent linking mechanism.  相似文献   

14.
Diatom and geochemical data from Crawford Lake, Ontario, have been used to document limnological responses to periods of cultural disturbance resulting from native Iroquoian occupation of the watershed (1268–1486 AD) and Euro-Canadian agriculture and deforestation (1867 AD–present). Here, we further develop the high-resolution nature of the Crawford Lake sediment record to examine the physical, chemical and biological aspects of limnological response to human disturbances in the lake catchment area with exceptional detail. We report detailed diatom abundance and flux data for individual taxa from Crawford Lake, and further describe the relationship between assemblage composition and environmental conditions using canonical correspondence analysis (CCA). Diatom assemblage data are used to calculate diatom inferred-total phosphorus (DI-TP) concentrations for the past ∼1,000 years. We also examine the diatom community response during and after periods of disturbance by Iroquoian and Euro-Canadian populations, and compare this response to existing geochemical proxies of lake production and new elemental geochemical indicators of catchment area erosion. In particular, we explore the differing limnological response to the two distinct periods of cultural eutrophication and examine the limnological processes that occurred during the period of␣low (or no) human activity (1487–1866 AD), when geochemical indicators of lake production recovered to pre-disturbance conditions, but diatom assemblages notably did not. Our results illustrate the highly susceptible nature of diatom communities to periods of anthropogenic disturbance, and emphasize that ecological indicators (such as diatom assemblages) should be included with other proxies (such as nutrient concentrations and physical characteristics) when assessing disturbance and recovery in lake systems.  相似文献   

15.
The history of hardwater lake Sacrower See (Brandenburg, northeastern Germany) was reconstructed back to the 17th century based on a multi-proxy study of five short sediment cores dated by varve chronology, 210Pb and 137Cs isotopes. We were able to distinguish three main phases: The lake was mesotrophic prior to the 1830s with an oxic hypolimnion. From the early 19th century on, δ13C of organic matter indicates that primary productivity starts to increase slowly. Between the 1830s and 1872, the lake went through a transition towards eutrophy. Low calcite contents in the homogeneous sediment are caused by dissolution connected to increasing primary productivity and growing importance of decomposition processes. After 1873, and accelerated since 1963, Sacrower See is characterised by growing nutrient input, and thus further increasing primary productivity. The lake is eutrophic, and decomposition of organic matter causes high oxygen consumption in the hypolimnion, which becomes regularly anoxic during thermal summer stratification. Biogenic varves are preserved in the sediment, characterised by layers of autochthonous, biochemically precipitated calcite crystals. In this study, we were able to demonstrate that Sacrower See is an example of exceptional slow increase of anthropogenically enhanced nutrient input, and of the imprint which these processes have on sediments of a hardwater lake.  相似文献   

16.
The varved sediments of the dimictic Lake Sihailongwan (Long Gang mountain area, Jilin Province, Northeast China) represent a palaeoclimatic archive which documents the local precipitation frequency during the summer monsoon, and variations in the aeolian flux of dust with their remote sources in the arid and semi-arid regions of inner Asia. Based on a detailed discussion of sediment genesis in Lake SHL, dust flux rates and palaeohydrological conditions were reconstructed on a decadal scale over the past 220 years. The aeolian influx by dry and wet deposition was quantified and characterised in its chemical composition. Photosynthetic production in the lake is positively correlated with the inflow of nutrient-rich groundwater. The groundwater discharge largely reflects the strength of the summer monsoon. Net accumulation rates for biogenic silica were determined for annually laminated sediments from the centre of the U-shaped lake basin based on sediment data. In a Si-balance model of the modern lake, the depositional flux of biogenic silica could be independently quantified on the base of hydrochemical monitoring data. Comparison of the both estimates allowed to asses the focussing of the particle flux in the lake. Though water retention in Lake SHL is rather high (ca. 30 years), changes in the hydrological conditions are sensitively recorded in the sediments because (i) nutrient-rich groundwater discharges into the productive zone of the lake, (ii) a substantial proportion of the total dissolved Si-inventory of the mixed lake (ca. 30%) is annually consumed by diatom growth, and (3) sediment accumulation is substantially focussed towards the flat bottom of the lake basin. The bulk siliciclastic sediment fraction (ca. 75 wt.%) largely originates from influx of dust of remote provenance. In sediment thin-sections, the dry-deposited dust fraction is microscopically identifiable as seasonal silt layer. Aeolian input by wet-deposition shows a distinctly higher variability than the influx of dust by dry-deposition. As diatom production, wet-deposition of dust is positively correlated with the rainfall during the summer monsoon. The inferred positive correlation between rainfall and dust flux during the summer monsoon implies that dust deposition is determined by the out-wash efficiency of mineral particles for a permanent high atmospheric dust concentration over Northeast China in the last 220 years.  相似文献   

17.
Freshwater Lake Ulubat (z mean = 1.5–2.0 m and Area = ~138 km2), NW Anatolia, Turkey was filled in by fine-to-medium-grain silts during the late Holocene. Deposition in Lake Ulubat has been 1.6 cm year−1 for the last 50 years, but the sedimentation rate over the last ~1,600 years was lower (0.37 mm year−1). The organic matter and carbonate contents of the infill show cyclic changes that reflect environmental fluctuations. The silt-dominated lithology and the vertically uniform heavy metal distributions are probably due to wind-controlled sedimentation in the lake. Heterogeneous mud, derived from a large, mountainous drainage basin, is deposited in the lake mostly during summer, June to October, when conditions are hot and calm. Winter months are stormier and sediments are re-suspended due to the shallow water depth and the effect of waves on the lake bottom. It is likely that re-suspended sediments, particularly fine-grained particles, together with the heavy metals, are transported out of the lake via the outlet, especially during periods of high lake level. This resuspension and removal process probably caused the lake sediments to become silt-dominated and depleted in heavy metals. The role of broad shallow lakes in sequestering sediments and heavy metals can be described more accurately when wind data are considered. Such information may also be helpful for land-use planning in downstream areas.  相似文献   

18.
Remarkably high rates of sedimentation (up to nearly 60 kg m−2 yr−1) are documented for offshore areas of Freefight Lake, a deep, hypersaline, meromictic lake in the northern Great Plains of Canada. Most material in the offshore sediment traps consists of soluble and suparingly soluble endogenic salts. Deposition of allogenic sediment is minor. The observed high sedimentation rates are in response to several mechanism: (i) freeseout precipitation of salts from the mixolimnion; (ii) redistribution of shallow water endogenic precipitates by turbidity flows and interflow processes; and (iii) precipitation of salts from within the monimolimnion Because the bottom water mass of the lake is essentially isothermal and isochemical, large scale temperature induced remobilization of these salts at or near the sediment-water interface is unlikely. Similarly, the morphology of the basin suggests that resuspension of bottom sediment by waves in the offshore areas of the basin is negligible. Although the sedimentation rates observed in Freefight Lake are extraordinarily high and considerably greater than other modern lacustrine sedimentation rates documented in the literature to date, the observed rates are entirely compatible with suggested accumulation rates of ancient evaporite sequences. Palliser Tringle Global Change Contribution Number 6.  相似文献   

19.
The sediment trap technique was used to study the sedimentation processes in two parts of Lake Goci, central Poland, the deep of the main basin and the shallow bay area. From field and lab data was noted that seasonal and annual deposition of pollen grains on the lake bottom was disturbed in the main basin by an increase in water mass dynamics and by resuspension and focusing. Greater concordance between pollen sedimentation and the flowering periods of plants was found in the bay area, where sediment resuspension took place sporadically and with less intensity, compared to the main basin. Futhermore, it is suggested that resuspension of pollen can occur even if laminated sediments are formed, a process which therby diminishes the chronostratigraphic value of sediments and pollen analyses.  相似文献   

20.
Geomorphic, lithologhic, and stratigraphic field studies as well as pollen data and mineralogical study have been used to propose Pliocene and Pleistocene paleogeographic reconstructions of the El’gygytgyn meteorite crater area. The moment of impact is recorded above the early Pliocene hill denudation plain as a “chaotic horizon” consisting of fragments of impactite rocks. This chaotic horizon lies between layers of late Pliocene alluvial sediments. During the second half of the late Pliocene, the region was tectonically active, when the Anadyr lowland was uplifted causing alluvial sediments to accumulate in the basins to the south of the crater. Regional climatic cooling, which supported the spread of tundra and the formation of permafrost is characteristically to late Pliocene. The 35–40 m high terrace that roughly follows the 530 m contour interval along the Enmyvaam River formed during the middle Pleistocene. This terrace represents the maximum lake level. Erosion and incision of the upper Enmyvaam River increased due to another wave of uplift. Additionally, El’gygytgyn Lake discharge increased causing lake level to begin to drop in the Middle Pleistocene. Cooling continued, which led to the development of herb-dominated arctic tundra. middle and late Pleistocene glaciations did not reach the El’gygytgyn lake region. The 9–11 m high lacustrine terrace was formed around the lake during the late Pleistocene and the 2–3 m high lacustrine terrace formed later during the Holocene. During the last 5000 years, the lake level has continued to drop as the modern coastline developed. This is the third in a series of eleven papers published in this special issue dedicated to initial studies of El’gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献   

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