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1.
The Sierra de Los Tuxtlas is a recently active volcanic field, with eruptions in 1664 and 1792. It holds one of the reserves of tropical evergreen forest in Mexico, as well as several maar lakes. One of them, Lago Verde, was chosen for a three-fold study (1) on its present limnological conditions, (2) on the algal community living in the water column and preserved in the surface sediments; and (3) on its recent history (ca. 340 yr). The palaeolimnological study was based on multiproxy analyses on core material dated by 210Pb, 137Cs and 14C. Lago Verde is a small, shallow lake with dilute, slightly alkaline water (CO32− + HCO3− > Cl− > SO42−, Na+ + K+ > Ca2+ > Mg2+). It is turbid, eutrophic, with high phosphorus levels. It is a warm polymictic lake, with thermal and oxygen stratification establishing by midday during the warm months. The lake does not stratify in winter. Diatoms dominate the phytoplankton community in the cold ‘nortes’ season, Cyanobacteria in summer, and Chlorophyta in autumn. Cyanobacteria (Chroococcales) are not well preserved in the surface sediments while Chlorophyta are better preserved. Sedimentary diatoms are well preserved, dominated by the three most abundant species in the water column: Achnanthidium minutissimum, Fragilaria capucina and Aulacoseira granulata. The base of the studied sequences is constrained by the historic eruption of 1664. The period from 1664 to 1963 is characterised by a meso-eutrophic lake. Tropical Forest vegetation reaches maximum values between ca. 1800 and 1963. Between ca. 1785 and 1885 the lake was slightly shallower than in the rest of the 1664–1963 period, probably recording climatic variability. An early pulse of anthropogenic disturbance was recorded by ca. 1921 and after ca. 1963 intense forest clearance and high erosion rates led to a more turbid, more productive, nutrient-rich lake. The highest anthropogenic impact was reached by ca. 1988; afterwards the lake and its basin reached a new balance, with the establishment of the present modern conditions.  相似文献   

2.
Chrysophyte cysts and scales and sponge spicules were enumerated, along with diatoms, from the surface sediments of 102 western Canadian lakes. The salinities of these lakes ranged from 0.042 to 369 g L−1 in late summer. Sponge spicules and chrysophyte cysts and scales were more common in fresher waters, although chrysophyte cysts were also present in subsaline and hyposaline waters at lower relative percentages. These siliceous microfossils can easily be distinguished and counted along with diatom valves, with little extra effort. It is likely that using these additional indicators will strengthen paleolimnological inferences of past lakewater salinity.  相似文献   

3.
Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Zmax 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to 137Cs and 210Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year−1 during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water  相似文献   

4.
A textural analysis of the siliceous sediments component of a small saline lake in south-eastern South Australia (Lake Cantara South) was undertaken. The data indicated periods of marine incursion, lagoonal dominance and elevated freshwater inputs. Lake Cantara originated as a marine embayment; then followed periods of intermittent marine connection; finally, the lake became athalassic (mostly saline but with occasional freshwater inundations). This history accords well with that derived from an examination of mollusc fossils (marine, estuarine, athalassic saline, and freshwater forms) and with previously published mineralogical, geochemical and geomorphological evidence.  相似文献   

5.
Seasonal changes of soluble reactive phosphorus concentrations in sediments of Fuente de Piedra, a saline lake, have been observed and linked to salinity changes. Phosphate availability in sediments was studied in the laboratory under different salinities, from 10 to 70 g L−1. Changes in adsorption of dissolved phosphate appear to be controlled by salinity variations. The adsorption coefficient increases with salinity and the amount of phosphorus adsorbed ranged between 10 and 30 per cent of the total. Salinity-modulated adsorption is proposed as the primary mechanism explaining the seasonal dynamics of phosphorus in this saline lake and its effects on progressive eutrophication.  相似文献   

6.
Diatom assemblages in surficial sediments, sediment cores, sediment traps, and inflowing streams of perennially ice-covered Lake Hoare, South Victorialand, Antarctica were examined to determine the distribution of diatom taxa, and to ascertain if diatom species composition has changed over time. Lake Hoare is a closed-basin lake with an area of 1.8 km2, maximum depth of 34 m, and mean depth of 14 m, although lake level has been rising at a rate of 0.09 m yr-1 in recent decades. The lake has an unusual regime of sediment deposition: coarse grained sediments accumulate on the ice surface and are deposited episodically on the lake bottom. Benthic microbial mats are covered in situ by the coarse episodic deposits, and the new surfaces are recolonized. Ice cover prevents wind-induced mixing, creating the unique depositional environment in which sediment cores record the history of a particular site, rather than a lake-wide integration. Shallow-water (<1 m) diatom assemblages (Stauroneis anceps, Navicula molesta, Diadesmis contenta var. parallela, Navicula peraustralis) were distinct from mid-depth (4–16 m) assemblages (Diadesmis contenta, Luticola muticopsis fo. reducta, Stauroneis anceps, Diadesmis contenta var. parallela, Luticola murrayi) and deep-water (26–31 m) assemblages (Luticola murrayi, Luticola muticopsis fo. reducta, Navicula molesta). Analysis of a sediment core (30 cm long, from 11 m water depth) from Lake Hoare revealed two abrupt changes in diatom assemblages. The upper section of the sediment core contained the greatest biomass of benthic microbial mat, as well as the greatest total abundance and diversity of diatoms. Relative abundances of diatoms in this section are similar to the surficial samples from mid-depths. An intermediate zone contained less organic material and lower densities of diatoms. The bottom section of core contained the least amount of microbial mat and organic material, and the lowest density of diatoms. The dominant process influencing species composition and abundance of diatom assemblages in the benthic microbial mats is episodic deposition of coarse sediment from the ice surface.  相似文献   

7.
The primary producer community of Lake Apopka, a large (125 km2), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.  相似文献   

8.
The main limnological features of Lake Issyk-kul are described. The lake is a large (6,236 km2), deep (zm, 668 m), closed lake in eastern Kirgizia. It lies at ∼1,607 m above sea level, but water-levels have been dropping since the last century. It is slightly saline (salinity, ∼6g L−1), with Na+, Mg2+, Cl− and SO 4 2− the dominant ions. Nutrient levels are low and the lake is considered ultra-oligotrophic. Characeae dominate the macrophytes. About 300 and 117 taxa of, respectively, phytoplankton and zooplankton have been identified, withArctodiaptomus salinus the most numerous in the zooplankton. Chironomids dominate the benthos. Several endemic taxa of fish occur, of whichLeuciscus bergi was dominant until the 1970s. The fish fauna has been supplemented by many introduced species. Three mysids were introduced in 1965–8 and are now a significant part of the ecosystem. The present annual fish catch permitted is 320 t. The most important value of the lake is as a recreational resource. To promote and sustain this value requires careful, ongoing management. The most significant threats to the lake are local pollution, visitor pressure, and declining water-levels.  相似文献   

9.
Tephras are often used in paleolimnology and other stratigraphicapplications as a chronostratigraphic marker. Where analytical errors inradiocarbon or other dating methods make precise comparison between sitesdifficult, tephras provide an absolute stratigraphic reference that can be usedto assess the relative ages of events across a region. Applications oftephrochronology typically make the assumption that a tephra is deposited atwhat was the top of the stratigraphic sequence at the time of deposition, andthat the contact between the tephra and underlying sediments is anisochron. This paper presents evidence from two lakes in western Canadawhich suggest that tephras may be very mobile within the sedimentary column,particularly in low-density organic lake sediments. Analysis of sedimentcores from Copper Lake, Alberta, suggest that the 6730 BP Mazama tephra moveddown-core by the equivalent of more than 3000 years. A vertical exposureof the Holocene sediments from Doal Lake, Yukon Territory, reveals that the1200 BP White River (WR) tephra, found throughout the southern Yukon, moveddown-sequence through organic lake sediments, to a stratum dating toapproximately 10,000 BP, creating conformable tephra beds at that level. Thistype of stratigraphic displacement has important consequences for the use oftephrochronology in lakes and other soft-sediment environments andunderscores the importance of properly identifying tephras and criticallyassessing their stratigraphic context within a lake core.  相似文献   

10.
Zooplankton collections were made during 1985, 1986, 1988 and 1989 from 17 lakes in Saskatchewan and 3 in Alberta. Salinity ranged from 2.8 to 269 g L−1 (total filtrable residue). A total of 35 species was present in four taxa: Anostraca (3 species), Cladocera (11), Copepoda (7) and Rotifera (14). Species richness was greatest at salinities <7 g L−1 (15–16 species). Lakes with salinities between 7 and 100 g L−1 generally had 6–8 zooplankton species, while the most saline lakes (>100 g L−1) had 2–5 species. The largest concentrations of zooplankton occurred at <30 g L−1, but some species (Brachionus plicatilis, Hexarthra polyodonta, Artemia franciscana, Diaptomus connexus) were abundant at salinities >50 g L−1. Eurysaline species included the rotifersAsplanchna girodi (3–111 g L−1),Brachionus plicatilis (13–146) andKeratella quadrata (2.8–103).Artemia franciscana (33–269—but absent from Big Quill Lake, 49–82),Daphnia similis (3–104).D. connexus (9–82),Diacyclops thomasi (3–72), andCletocamptus albuquerquensis (17 to 126—but never abundant in the plankton). About half the species were restricted to hyposaline waters (3–20 g L−1), but some (Hexarthra fennica, Moina hutchinsoni, Hexarthra polyodonta) occurred only at intermediate salinities. The latter two species were also only present at high pH values (>9.2). There was a trend of decreasing species richness with increasing salinity. TWINSPAN classification of 94 lake samples (six parameters) based on zooplankton species abundances yielded a dendrogram with 14 ‘indicator’ species characteristic of seven lake groups related partly to a salinity gradient, but with other environmental factors such as water column depth, pH, Secchi disk transparency, water temperature and month sampled also influenced lake separation. *** DIRECT SUPPORT *** A02GG004 00002  相似文献   

11.
During the past twelve years, Lake Poopo, located on the Bolivian Altiplano, has had two main types of morphometry. Before 1985, its level was low and the depth shallow (maximum 3 m); there was no outlet and a strong salinity gradient existed from north to south. After 1985, the depth doubled, an outlet developed and the salinity became uniform throughout the lake (∼10 g L−1). Before 1985, the phytoplankton was distinguished by a high number of diatom taxa and by the dominance of diatoms in the algal biomass. After 1985, while diatoms were still numerous in terms of species composition, Peridiniales or Chlorophyceae dominated the algal biomass. *** DIRECT SUPPORT *** A02GG003 00002  相似文献   

12.
A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC23) and δ13C values of TOC and nC23 indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C20 highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C25 HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC17-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and δ13C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The δ13C values of nC23 range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ13C value of the inorganic carbon source may vary, the offset between the δ13C values of nC23 and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.  相似文献   

13.
Chrysophyte scales have been used in several paleolimnological studies to track long-term environmental change, however little data exist for the many lakes in the Maritime provinces of eastern Canada. As part of a multi-disciplinary investigation of acidification and other environmental stressors in the Maritimes, chrysophytes scales were identified and enumerated from the sediments of 52 lakes from Nova Scotia and two lakes from New Brunswick. A total of 25 chrysophyte taxa were identified from the surface sediments, reflecting the modern-day chrysophyte assemblages. The dominant species included Mallamonas duerrschmidtiae and Mallomonas acaroides. Taxa of the genus Synura were present in some lakes, but mainly in the more southern sites. In general, the floras were less diverse than those recorded from similar studies in other temperate regions. This may be related to the fact that the calibration lake set contained only a relatively short limnological gradient, and the assemblages reflect the acidic to circumneutral conditions of these lakes. Synura petersenii, a taxon that has been linked to imparting taste and odor problems to lakes, and had been shown to increase in the recent sediments of many other Canadian lakes, was only rarely present. In contrast to other studies, scaled chrysophytes were very rare in the pre-industrial sediments, with substantial nineteenth century populations only present in four relatively deep (>19 m) lakes. Detailed stratigraphic analyses of eight sediment cores revealed that scaled-chrysophyte assemblages increased dramatically during the latter part of the twentieth century. Limnological changes associated with climate (e.g. increased thermal stratification due to a 1.5°C temperature increase since ~1850) may have influenced chrysophyte distributions in these lakes.  相似文献   

14.
Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.  相似文献   

15.
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.  相似文献   

16.
This paper introduces the results of an integrated project designed to compare high resolution analysis of proxy records of climate change in the sediments of seven mountain lakes across Europe with reconstructed instrumental records of climate change over the last 200 years. Palaeolimnological methods used include radiometric dating (210Pb, 137Cs), mineral magnetics, dry weight, loss-on-ignition, carbon, nitrogen, sulphur, pigments, diatoms, chrysophyte cysts, cladocera and chironomids. Changes in fossil assemblages were summarised using principal components analysis. The stratigraphic data were compared with the instrumental record using linear regression techniques. The dated sediment records for each proxy from each site were treated as the response variables and the various attributes of the instrumental climate record as the predictor variables. The predictor variables were generated for each site for the period 1781 to 1997 using temperature reconstructions based on meteorological records. To harmonise the climatic predictors and the response variables, the climatic variables were smoothed along time with a LOESS regression. The results of the various analyses at the seven sites are presented in the following papers. A synthesis of the project and the relative performance of the different proxy methods are discussed in the final paper.  相似文献   

17.
The northern Great Plains of Canada stretch from the Precambrian Shield near Winnipeg, Manitoba, westward for ∼1,700 km to the Rocky Mountains foothills. This vast region of flat to gently rolling terrain contains a very large number of salt lakes. Major ion chemical data on ∼500 of them are available. Although the average brine (salinity, 37 ppt) is a Na+−SO4 2− type of water, the lakes exhibit a wide range of salinities and ionic compositions. This diversity is confirmed by Q-mode cluster analysis; it identified thirteen major water chemistry types. Most ions display distinct trends, both spatially and with increasing salinity. All dissolved components increase with increasing salinity, but at different rates. The relative proportions of Ca2+ and HCO3 −+CO3 2− ions show a strong decrease with increasing brine salinity, whereas SO4 2− ions increase with increasing salinity. The ionic proportions of Na+, Mg2+, K+ and Cl− exhibit no significant relationship with salinity. R-mode factor analysis of the lake water chemistry, combined with selected environmental parameters, identifies groundwater composition, climate, and the elevation of the lake within the drainage system as most important in controlling brine chemistry and salinity on a regional basis. Variability in source of ions, reaction processes and products are undoubtedly key factors in helping to explain brine chemistry of an individual basin or variation from a local perspective, but these factors are generally poorly understood and not quantified on a regional basis. Palliser Triangle Global Change Project Contribution Number 3.  相似文献   

18.
We reported a special type of lamination formed in the sediments of Lake Xiaolongwan, northeastern China. The lamination consists of light- and brown-colored laminate couplets in the thin sections. The brown-colored layer is composed mainly of dinoflagellate cysts. The grey-colored layer consists of other organic and siliceous matter (plant detritus, diatoms, chrysophyte cysts) and clastics. Preliminary sediment trap results show that a distinct peak of dinocyst flux occurred in November. The dinocyst flux maximum also corresponds to the peaks of diatom flux and chrysophyte stomatocyst flux. These suggest that "red tide blooms" occur in this freshwater lake. We speculate that the dinocyst flux maximum could be related to autumn overturn due to increased nutrients, and the availability of cysts for germination from the lake bottom. Additionally, it may also reflect increasing dissolved organic matter after leaf fall. An independent chronology derived from 137Cs and 210Pb shows a good agreement with counted laminations. From the sediment trap data and the independent chronology data, the dinocyst microlaminae appear to be annually laminated, and probably could be called dinocyst varves. Although vegetative (thecate stage) cells of Peridinium volzii and Ceratium furcoides are found in the water samples, it is not possible to relate the dinocysts to these two dinoflagellate species. Based on morphological and ecological analyses, we suggested that they have affinities with species of Peridinium (sensu lato), most probably to P. inconspicuum. Detailed investigations should be carried out to understand the red tide history in this freshwater lake. Annually laminated dinocyst microlayers in freshwater and marine sediments not only provide an uncommon archive for understanding the history of red tides and harmful algal blooms, and why and how certain species periodically bloom over several thousands years, but also provide important records of paleoenvironmental and paleoclimatic changes at seasonal to annual resolution.  相似文献   

19.
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   

20.
A 40-cm sediment core from Big Moose Lake (Adirondack Mountains, New York, U.S.A.) was analyzed for recent changes in chrysophyte composition of chrysophyte species to assess if legislated reductions of sulphur emissions have resulted in changes in biological composition and inferred lakewater pH in this lake. This core, taken in September 1994, had a record of stratigraphic changes in chrysophyte assemblages remarkably similar to those in 210Pb-dated sediment cores taken in 1983 and 1988. Abrupt changes in chrysophyte stratigraphy were used to date the newest sediment core by stratigraphic correlation. We observed relatively constant composition of chrysophyte species and inferred-pH values in post-1970 sediments, which suggests that there has been little change in lakewater pH values despite the reductions in sulfate deposition since 1970 in the Adirondacks. These results are consistent with recent monitoring measurements in the Adirondacks. Further research is necessary to ascertain the applicability of these findings to other lakes in the Adirondacks and to determine the reasons for the lack of recovery.  相似文献   

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