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1.
Based on long-term observations, we examine the seasonal and interannual dynamics of the phytoplankton and chemical composition of the water in Lake Zavernyaikha located in the delta of the Selenga river. The study revealed an intense development of five kinds of phytoplankton under the ice, dominated by dynophytes: Baikal’s endemics G. baicalense and P. baicalense. The main factors for massive algae development under the ice are: the favorable gaseous regime and relationship of nutrients, high content of readily hydrolizable organic matter (RHOM), isolatedness of the water body, and high division rate of dinophytes.  相似文献   

2.
We examine the ecological aspects concerning the status of the surface waters in the delta of the Selenga river for the low-water conditions which have been observed since 1996. The ongoing changes in the most important indicators of the status of the waters in the river delta are assessed on the basis of results of hydrological, hydrochemical and hydrobiological investigations made by these authors in 2013–2015 and with consideration for earlier research efforts. It is established that the low-water conditions resulted in a decrease of the role of the main delta branches, Kharauz and Levoberezhnaya, in streamflow. An increase in the contribution of groundwater in the alimentation of the river led to an increase in concentrations of main ions: for the current low-water period their total content has varied from 119 to 780 mg/L, with a maximum in small delta branches in the wintertime. Changes in concentrations of biogenic elements were recorded: total phosphorus from 10 to 190 μg P/L, mineral phosphorus from 0 to 35 μg P/L, and nitrate nitrogen from 0.01 to 0.81 μg N/L. A minimum of mineral phosphorus compounds and nitrogen was observed at the time of mass development of algae. It is established that the dynamics of organic matter content is determined by changes in hydraulicity, and by the intensity of phytoplankton development. Within the long-term context, a deterioration of water regime and vegetation infill and swamping of shallow delta branches were recorded. The development level of the phytoplankton and changes in its structure point to a progressive eutrophication of the Selenga waters. According to the value of the saprobic index, the quality of the Selenga waters corresponds largely to the category of “moderately polluted” waters (class II and III), and it decreases at low-water periods, especially in shallow delta branches with a high population of cryptophytic algae, to the category of “polluted” waters.  相似文献   

3.
Biogenic Silica (BSi) has been one of the most important proxies for determining the palaeoclimate from Lake Baikal over glacial-interglacial cycles. Concentrations (calculated through a 1% Na2CO3 wet-alkaline digestion) at a site in the north basin, however, reveal consistently low values during MIS 3 compared to greater than tenfold changes in diatom concentrations and biovolumes from c. 53.3–51.5 kyr BP. With similar glacial trends present at other low sedimentation sites, we suggest that significant amounts of BSi are removed from diatoms during glacials due to a relative increase in diatom dissolution at the sediment–water interface. This contrasts with existing results from other, higher sedimentation, sites such as those within the Selenga Delta, which display a strong relationship between diatoms and BSi. Site selection is therefore essential when searching for Heinrich and other glacial millennial-scale events in Lake Baikal, and we recommend that both BSi and diatom concentrations be calculated together in future studies.  相似文献   

4.
Paleoshoreline evidence for postglacial tilting in Southern Manitoba   总被引：1，自引：0，他引：1  
Detailed air photo interpretation and four seasons of field mapping and surveying in southern Manitoba have revealed that the once-level paleoshorelines of Lake Winnipegosis and Dauphin Lake and the Burnside shoreline of former Lake Agassiz have been tilted up to the northeast by postglacial differential rebound. Our investigation has also revealed that Lake Winnipegosis has the best preserved paleoshoreline record of any of the large lakes in southern Manitoba, including lakes Winnipeg and Manitoba. This is because northeasterly uptilting shifts the region's lakes to the southwest. Lakes with southern outlets, like Lake Winnipegosis, undergo general regression as the outlet is lowered relative to the rest of the basin. Lakes with northern outlets, like lakes Winnipeg and Manitoba, undergo general transgression as northeasterly uptilting raises the outlet relative to the rest of the basin. Along the northeastern shore of Lake Winnipegosis a staircase of at least 32 abandoned Winnipegosis shorelines exists that is consistent with northeasterly tilting. The Dawson level represents the major mid-Holocene highstand on Lake Winnipegosis. It persisted for about 500 years, peaking at 5290 14C yr B.P. (early Dawson) and then falling about 3 m by 4740 14C yr B.P. (late Dawson). The early Dawson shoreline is tilted at 13.5 cm km-1 in a direction N24.3°E. Three other shorelines informally named shoreline 4, shoreline 3, and shoreline 2 are also tilted up to the northeast. Their radiocarbon ages (and slopes in cm km-1) are 3330 yr B.P. (2.2), 1510 yr B.P. (1.3), and 1080 yr B.P. (0.7), respectively. On Dauphin Lake shoreline IV is the oldest level mapped for this study. It has a 14C age of 7910 yr B.P. and is tilted at 21.7 cm km-1 in a direction N44.4°E. The Id shoreline marks the major mid-Holocene highstand for Dauphin Lake. It peaked at 4640 14C yr B.P. followed by a rapid decline of about 1 m to the Ib shoreline, which is dated at 4320 14C yr B.P. Id is tilted up at 8.8 cm km-1 in a direction N53.4°E. The next major shoreline is Ia3 which has a 14C age of 3020 yr B.P. and is tilted up at 5.3 cm km-1 in a direction N62.3°E. Tilt directions are significantly more easterly for the Dauphin Lake shorelines than those from Lake Winnipegosis or any of the much older Lake Agassiz shorelines. Taken together, the Winnipegosis and Dauphin isobases indicate that the direction of tilt in southern Manitoba is more complex than a simple uni-directional pattern. The observed pattern of tilting for paleoshorelines in southern Manitoba agrees better with predictions derived from the recently revised loading history model ICE-4G than with those from its predecessor ICE-3G. In general, the calculated tilt based on the ICE-3G load tends to exceed the observed tilt, while ICE-4G tends to underestimate it. Both ice load models appear to disagree most with our observed tilts in this region during the interval before about 9000 cal yr B.P., when deglaciation was proceeding rapidly and the large water load associated with Lake Agassiz covered the region. Because both of these ice load models have been estimated mainly from a global data set of relative sea level curves from marine coast sites, it is not unexpected that model tilts derived from them do not agree well with observations in the North American continental interior. The pattern of postglacial crustal deformation for southern Manitoba described in this paper could be used to further refine ice load models for the North American continental interior.  相似文献   

5.
In order to assess how best to manage impacted lake systems, one needs to understand the trophic functioning of the lake system and the recent states through which the lake may have transitioned. Lakes in the middle and lower reaches of the Yangtze have been heavily impacted over recent decades. In order to understand recent changes in functional status, we examined sediment cores covering the last 120?years from two lakes in the same catchment with differing status: one algal-dominated (Taibai Lake) and the other macrophyte-dominated (Longgan Lake). Chironomid head capsules were identified from both sites and an expanded chironomid-total phosphorus (TP) transfer function (21 sites were added to the 30-lake model previously developed by Zhang et al. 2006) was used to assess the lakes?? response to recent anthropogenic change. Quantitative chironomid-inferred TP (CI-TP) reconstructions showed that Taibai Lake experienced clear changes in trophic status since the 1860s. Before the 1950s, the CI-TP concentration was relatively stable around 50?C80???g?L?1, while it reached to 80?C130???g?L?1 in the latter period. CI-TP for Longgan Lake, however, showed a relative decline from the range of 50?C75???g?L?1 since the 1880s to 30?C40???g?L?1 in recent years, accompanied by strong evidence from the chironomids for increased macrophyte biomass as TP levels declined. Both reconstructions agreed with diatom inferences of TP from the same lakes. The stark difference between these two sites is thought to reflect a function of macrophyte development, with Taibai Lake losing its plants through increased nutrient levels and internal recycling, whereas Longgan Lake, which is much bigger in area and hence potentially more resilient to change, was able to develop macrophyte communities over the same time period. The positive feedbacks associated with abundant macrophytes retained the clear water state of Longgan Lake, but a further increase in nutrients might lead to decrease in resilience of the relatively stable macrophyte state and loss of benthic pathways of primary production, which would push the lake towards eutrophication. Unless nutrient inputs to Longgan are controlled, Longgan Lake might lose macrophyte communities and follow a developmental pathway similar to that observed in Taibai Lake.  相似文献   

6.
We investigated the influence of deep forced temperature convection on aeration of the bottom water layer in the pelagic zone of Baikal. Temperature T (± 0.002°C) and oxygen O2 concentration (± 0.01 mg/L) were measured by using the SBE-25 probe with the SBE-43 oxygen sensor in May-July 2006?2007 and 2009?2013. Oxygen input into the bottom layer with cold intrusions was determined for 79 cases. The study revealed a quantitative correlation between the heat deficit and the rise in oxygen content in the bottom layer during cold intrusions. This correlation was used to reconstruct the values of intrusion aeration of the bottom layer during 1993?2005. It is ascertained that the intrusion-caused 2006?2013-average oxygen input into Baikal’s bottom layer is virtually close to the annual oxygen demand for oxidation processes in the water column and bottom sediments. In the southern and middle parts of Baikal, intrusion-caused input of oxygen corresponds to a higher rate of its demand (4.5?4.6x10-4 mg/L per day). A slight increase in oxygen concentration was detected in the bottom layer of the South and Middle Baikal and a slight decrease in the bottom layer of North Baikal during 1993?2013.  相似文献   

7.
It is found that pollution of the territory from local sources of the zone of atmospheric influence spreads over several tens of kilometers along the prevailing wind direction, and in the central zone it is replaced by regional pollution. Snow pollution in the water area of Lake Baikal was recorded near coastal settlements and in the mouth of the Selenga river. It is established that the southern hollow of Baikal holds the lead in the input of NO2, NO3, NH4, PO4, F, Al, Na, Ba, Mo, Mn, Pb, Cu, Zn, Sr, Hg and oil products to the lake’s water area in spite of the fact that it is twice as small as the northern hollow. The northern hollow receives much larger amounts of SO4, H2CO3, Cl, Ca, Mg and K than the northern hollow, and much larger amounts of Be, V, Cr and Co than the middle hollow, which is associated with the operation of coastal enterprises, boilers, thermoelectric plants, stove heating, automobile exhaust emissions, and with the natural transport of dust in snow-deficient areas of the middle hollow. It is determined that that last five years saw a slight increase in regional pollution for separate components. It is shown that in the late 1990s and in the early 2000s, pollution decreased twice, which was due to the setback in industrial production.  相似文献   

8.
A comprehensive characterization of the flood hazard on the rivers of the Baikal region is presented, which was obtained by investigating the streams within the basins of the Angara, Upper Lena (with the Vitim and Olekma), Lower Tunguska (headwaters), Upper Amur and Lake Baikal (including the entire Selenga basin). The main flood indicators for the period 1985–2017 are estimated: the genesis, recurrence, duration, flooded area and the force of impact. The influence of changes in the river runoff characteristics on the flood risk is shown by results of correlation analysis and analysis of integro-differential curves. An assessment is made of the changes in the flood frequency at gauging stations during the period of pronounced climatic changes (from 1981 to 2014), compared with the earlier period. The flood hazard within the spatial context for municipalities in the rank of administrative districts is determined on the basis of the dual (socio-economic and natural) nature of floods. It has been confirmed that the most dangerous in the Baikal region are rainfall floods in the southern areas of Irkutsk oblast, the Republic of Buryatia and Zabaikalskii krai. They have the highest frequency, and the largest flooding areas and force of impact and are characterized by the greatest damage and by the largest number of victims and evacuated people. At the same time, the frequency of floods at gauging stations in recent years has decreased compared to the earlier period against the background of the observed low-water period, which is most pronounced on the rivers of South Baikal and in the Selenga river basin.  相似文献   

9.
The sedimentary record from Lake Baikal (Siberia, Russia) has been an important source of information about paleoclimatic variability in the northern hemisphere and dynamics of continental rift development. A lack of reliable chronology has, however, been a major obstacle to fully utilizing the Baikal archive for time scales beyond about 4-5 Myr. In this paper we use the distribution of 10Be to establish a new chronology for the longest core drilled in Lake Baikal so far. The 10Be-based chronology spans the last 8 Myr and provides better constraints on sedimentation rates and consequently on an east-west tectonic extension in the lake, which has been apparently coeval with other rifts in Asia that are related to the Tibetan plateau uplift. Our data also show higher 10Be flux in the sediment section older then 5 Myr compared with the younger period. This can be explained partly by warm and humid climatic conditions of the Miocene and partly by a high cosmic ray flux to the Earth resulting from possible low geomagnetic field intensity during that time.  相似文献   

10.
Climate records during the last millennium are essential in placing recent anthropogenic-induced climate change into the context of natural climatic variability. However, detailed records are still sparse in Alaska, and these records would help elucidate climate patterns and possible forcing mechanisms. Here we present a multiple-proxy sedimentary record from Kepler Lake in south-central Alaska to reconstruct climatic and environmental changes over the last 800?years. Two short cores (85 and 101?cm long) from this groundwater-fed marl lake provide a detailed stable isotope and sediment lithological record with chronology based on four AMS 14C dates on terrestrial macrofossils and 210Pb analysis. The ??18O values of inorganic calcite (CaCO3) range from ?17.0 to ?15.7???, with the highest values during the period of 1450?C1850 AD, coeval with the well-documented Little Ice Age (LIA) cold interval in Alaska. The high ??18O values during the cold LIA are interpreted as reflecting shifts in atmospheric circulation. A weakening of the wintertime Aleutian low pressure system residing over the Gulf of Alaska during the LIA would have resulted in 18O-enriched winter precipitation as well as a colder and possibly drier winter climate in south-central Alaska. Also, elevated calcite contents of >80?% during the LIA reflect a lowering of lake level and/or enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature and water depth. This interpretation is also supported by high ??13C values, likely reflecting high aquatic productivity or increased residence times of the lake water during lower lake levels. The lower lake levels and warmer summers would have increased evaporative enrichment in 18O, also contributing to the high ??18O values during the LIA. Our results indicate that changes in atmospheric circulation were an important component of climate change during the last millennium, exerting strong influence on regional climate in Alaska and the Arctic.  相似文献   

11.
The computer simulation of a Lake Baikal seismic profile located in the Selenga River Delta area resulted in a lake level record of the last 600 kyr. This curve demonstrates several low-magnitude episodes and both a dramatic 300 m fall and a more than 150 m increase of the lake level relative to present situation. The greatest change in paleo-lake depth at 300 ka corresponds in time with the major glaciation in the Eastern Siberia and is probably the response of the lake to this climatic phenomenon. The results of this study conform with existing hypotheses on the regional tectonic history and climatic events.  相似文献   

12.
Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS 14C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??13C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??18O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??18OPisidiidae and ??13CPisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??18O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers.  相似文献   

13.
This paper provides an overview of the documents on the preservation and sustainable use of natural resources within the Lake Baikal drainage basin. An analysis is made of the results of the environmental activity during the period of planned economy and post-perestroika. In the reform years, Baikal was included on the List of the World Natural Heritage Sites, the Federal Law “On the protection of Lake Baikal” was adopted as well as three federal target programs of the protection of the lake and socioeconomic development of the Baikal natural territory. Due to funding shortages, however, they are hardly implemented. The latest legal acts substantially restrict the rights of local residents to land privatization, the development of the social infrastructure, and to use natural resources. Furthermore, in spite of a large number of federal nature conservation directives, closure of many enterprises and reductions in emissions, the ecological situation on Lake Baikal is not improving. Among the problems is the negative attitude of local residents toward the restriction of their constitutional rights to property and economic activity. In addition, all ecological legislative acts are adopted without any public discussion and funding of environmental protection expenditures is not a priority. An improvement of the ecological situation on Baikal is possible only through a further development of the region’s environmentally safe economy and by raising the living standards of the local population on the principles of sustainable development with due regard for the interests of future generations.  相似文献   

14.
Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in 210Pb dated and pre-210Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal.  相似文献   

15.
We applied the 32Si dating technique to a sediment core from Lake Baikal to obtain the sediment chronology for the last millennium. The core was recovered about 4 km offshore from the north slope of the South Basin in 1,366 m water depth. The sediment material consisted of continuously accumulated diatom-rich geogenic-terrigenous mud, intercalated with a number of dark olive-grey turbidite layers. The sediment layers containing the turbidites were excluded from 32Si sampling to obtain the chronology that is representative of the continuous sedimentation. The initial 32Si activity of 31.3 dpm kg?1 SiO2, measured in sediment trap samples, confirms the trend of 32Si specific activities of biogenic silica found in other Northern Hemisphere lakes. The four sediment core samples from depth 0–48 cm have 32Si specific activities between 23.5 and 0.5 dpm kg?1 SiO2, with corresponding ages between 60 and 860 years and constant sedimentation rate of 0.036 ± 0.004 cm year?1 over the most recent 800 years. 32Si allowed us for the first time to date the uppermost turbidites in the South Basin of Lake Baikal, to 1030, 1310 and 1670 ad. Given these dates, the last long-distance turbidity current triggered by slope instabilities had occurred 330 years before 2000 ad, and the intervals between the 1310 and 1670 ad event and between the 1030 and 1310 ad event were 360 and 280 years, respectively. The 32Si ages allow unprecedented time resolution for reconstruction of the former environmental and climatic conditions during the past millennium.  相似文献   

16.
Detailed depth profiles of photosynthetic pigments in a sediment core (G-12) collected at the BDP93 site, the Buguldeika saddle, of south Lake Baikal, along with depth profiles of total organic carbon (TOC) and biogenic silica, were studied to elucidate the temporal changes of phytoplankton assemblages in the lake during the past 28 kyr. In addition to the quantification of carotenoids by high-performance liquid chromatography with photodiode-array detection (HPLC-PDA), steryl chlorin esters (SCEs) were analyzed by HPLC-PDA, HPLC-mass spectrometry (LC-MS) and sterols in SCEs by gas chromatography–mass spectrometry (GC–MS) to enrich the taxonomical information on the phytoplankton composition. Allochthonous input of organic matter from the Selenga River resulted in the higher TOC contents in core G-12 than in a previously reported core (G-6) collected at another site from the southern basin. The poorer correlation in core G-12 than in G-6 between TOC and chlorophyll-a-originating pigments, which are indicative of autochthonous production, also indicated a significant allochthonous input at the site. The abundance of lutein among the carotenoids detected, and the good correlation of total chlorophyll a and b shows that green algae represented a significant portion of the phytoplankton, accompanying the diatoms at the G-12 site, after the last glacial period. The presence of cryptomonads and cyanobacteria were confirmed from marker carotenoids in the sediment core. GC–MS analysis of sterols in SCEs detected marker sterols of diatoms, green algae, chrysophytes and dinoflagellates. The depth profiles of the measured indicators gave consistent features for temporal changes in phytoplankton assemblage at the G-12 site of Lake Baikal after the last glacial maximum. Notably, the profile of a chrysophyte-specific sterol in SCEs was consistent with the reported distribution of chrysophyte cysts during the Holocene. The presence of phytoplankton, such as green algae, diatoms and chrysophytes, in Lake Baikal during the late last glacial period was indicated by the analysis of sterols in SCEs. Sedimentary carotenoids and sterols in SCEs were found to give complementary information about phytoplankton composition. These molecular indicators allow us to reconstruct past lake phytoplankton assemblages responding to environmental changes with a time resolution as high as age–depth relationship in sediments attainable at present.  相似文献   

17.
“Heavier-than-air flying machines are impossible” Lord Kelvin ca. 1895, British Mathematician and Physicist2“But what ... is it good for?” Engineer at the Advanced Computing Systems Division of IBM, 1968, commenting on the microchip.3  相似文献   

18.
Distribution of uranium and thorium isotopes in a short sediment core obtained offshore of the Selenga Delta in Lake Baikal, Siberia, was investigated to establish their sedimentary behaviors and to look for a linkage to paleoenvironmental changes. The sediments were composed of dominantly fine detrital materials (70–85%) and a relatively high sedimentation rate (ca. 0.03 cm y−1). The depth profile of 238U content in bulk sediment samples showed a large variation of 70–123 Bq kg−1, while 232Th profile showed a relatively narrow range from 36 to 56 Bq kg−1. The observed 234U/238U activity ratios revealed a marked disequilibrium ranging from 1.53 to 1.84 with a mean value of 1.71 ± 0.07, demonstrating the presence of 50–80% authigenic 238U in the bulk sediments. The distribution of this authigenic 238U did not display any clear correlation with variations in sediment composition (organic, carbonate, Bio-SiO2 and mineral contents) including grain size median. The profile of terrigenous 238U showed a relatively similar pattern to that of 232Th. Results of sequential leaching indicate that 238U in Fe–Mn oxyhydroxides fractions were responsible for the distribution of authigenic 238U rather than in Bio-SiO2 fraction. The distribution of authigenic 238U in the bottom sediments may be explained by the fluctuation of U adsorption capacity on particles including organic matter and Fe–Mn oxyhydroxides before they entered the lake. This study highlights the potential use of authigenic and terrigenous U (Th) signatures in sediments to trace the behavior of U (Th) and to reconstruct environmental (e.g., hydrological) changes in the lake catchment area.  相似文献   

19.
The Bunger Hills in East Antarctica occupy a land area of approximately 400 km2. They have been exposed by Holocene retreat of the Antarctic ice sheet and its outlet glaciers. The accompanying sea level rise flooded the marine inlets that now separate the northern islands and peninsulas from the major part of the hills. During deglaciation the continental ice sheet margin retreated south‐eastwards with several temporary halts, during which ice‐dammed lakes were formed in some valleys. These lakes were maintained long enough to permit formation of beaches of sand and gravel, and for the erosion of shore platforms and low cliffs in bedrock. Around the western end of Fish Tail Bay impressive shoreline features 20 m above sea level define a former ice‐dammed lake that was 5.5 km long. A similar 7 km long former ice‐dammed lake was formed at Lake Dolgoe. The more extensive and deeper glacial lake is revealed by well‐developed and preserved shoreline features cut at 29 m which is 16 m above present lake level. In addition, several small ice‐dammed lakes existed temporarily near Lake Shchel and in the valley to the west. Lake Fish Tail existed more than 6,900 14C years ago and Lake Shchel probably more than 6,680 14C years ago. It is inferred that the shore platforms and beaches were formed by lake ice and wave action over considerable periods when the lakes were impounded by steep cold ice margins. There appears to have been a balance between meltwater input and evaporative loss from the lakes in the cold dry continental climate. There is no evidence for rapid lake level fluctuations, and there was very little input of clastic sediment. This resulted in poor development of deltaic and rhythmically laminated lake floor deposits. It is suggested that such deposits are more characteristic of ice‐dammed lakes formed in association with wet‐based temperate ice than those associated with dry‐based polar ice.  相似文献   

20.
The analysis of sediment chemistry and biota in drill cores from Lake Khubsugul in Mongolia (KDP-01) and Lake Baikal in Siberia (BDP-96/1), two great Eurasian freshwater lakes, detected prominent climate and biological events at 460–420 and 670 kyrs BP in addition to the orbital cycles of precession, tilt and eccentricity. The revealed long-term events were associated with notable changes in biodiversity and geography/landscapes, mainly in water budgets and weathering patterns. The span between 460–420 and 670 kyrs BP was the time when the climate and geographic conditions differed from those before and after these events. The corresponding 33–24 m (670–460 kyr) interval of the Khubsugul core lacked the usual signature of the Milankovitch glacial/interglacial cycles. Events of approximately these ages were found in some other continental ecosystems and in oceanic δ13C records. The two events may mark the phases of a 300–500-kyr long supercycle (or megastadial) in the evolution of continental ecosystems. Among other causes (e.g., regional tectonic events), this periodicity, being globally correlated, may be associated with the 400-kyr cycle of the Earth’s orbital eccentricity.  相似文献   

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