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1.
The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.  相似文献   

2.
A 5.52 m long sediment sequence was recovered from Lake Terrasovoje, Amery Oasis, East Antarctica, in order to reconstruct the regional environmental history. The basal sediments, which are dominated by glacial and glaciofluvial clastic sediments, attest to a Late Pleistocene deglaciation of the lake basin. These sediments are overlain by 2.70 m of laminated algal and microbial mats and a few interspersed moss layers. Radiocarbon dating, conducted on bulk organic carbon of 12 samples throughout the organic sequence, provides a reliable chronology since the onset of biogenic accumulation at c. 12,400 cal. year BP. Successful diatom colonization, however, was probably hampered by extensive ice and snow cover on the lake and restricted input of nutrients until 10,200 cal. year BP. A subsequent increase of nutrient supply culminated between 8600 and 8200 cal. year BP and is related to warm summer temperatures and reduced albedo in the catchment. Warm conditions lasted until 6700 cal. year BP, supporting the establishment of a diatom community. Colder temperatures from 6700 cal. year BP culminated in several periods between 6200 and 3700 cal. year BP, when high amounts of sulphur and low abundances of diatoms were deposited due to a perennial ice and snow cover on the lake. During the late Holocene, relatively warm conditions between 3200 and 2300 cal. year BP and between 1500 to 1000 cal. year BP, respectively, indicated by high accumulation of organic matter and reducing bottom water conditions, were interrupted and followed by colder periods.  相似文献   

3.
Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.  相似文献   

4.
Holocene paleoenvironments of Harris Lake, southwestern Saskatchewan, are reconstructed from the ostracode stratigraphy of a 10.4 m sediment core. Twenty three taxa, representing nine genera, were identified and counted from 113 samples. At each depth, a theoretical faunal assemblage was derived from the raw counts. The mean and variance of chemical, climatic and physical variables were inferred from modern analogues of the fossil assemblages, using existing autecological data from 6720 sites, mostly in western Canada. These data suggest four paleoenvironments: an early-Holocene (9240–6400 years BP) variable climate supporting aspen parkland vegetation; the warm dry hypsithermal (6400–4500 years BP); a short transitional period of ameliorating climate and expanding subboreal forest (4500–3600 years BP); and the present environment since 3600 years BP. A change in regional climate with the draining of Glacial Lake Agassiz (ca. 8500 years BP) and landsliding in the watershed (ca. 4000 years BP) caused relatively rapid environmental change. The ostracode record generally corroborates the interpretations of other proxy data previously published for Harris Lake. Most of the discrepancy involves the timing and severity of maximum Holocene warmth and aridity. Peak aridity interpreted from the pollen data is earlier than in the other proxy records. Both the diatoms and ostracodes indicate highest paleosalinity between ca. 6500 and 5000 years BP, but maximum salinity in the diatom record occurs between ca. 6000–5700 years BP, whereas the ostracode-inferred salinity is relatively low at this time and peaks later at ca. 5000 years. Neither of these reconstructions suggests the short episodes of hypersalinity interpreted from the mineralogy.  相似文献   

5.
刘向军 《盐湖研究》2018,26(2):16-26
青海湖是国内最大的内陆湖泊,位于青藏高原东北缘,因其处在东亚夏季风、印度季风和西风带的交替控制区域,对气候变化十分敏感,成为古环境变化研究的热点地区。有关青海湖的形成演化、环境变化和水文变化的研究也存在多种观点。本研究再分析了青海湖已报道的古环境指标和气候模式模拟的夏季、冬季温度和降水变化,力图更加全面地理解青海湖全新世以来的古环境变化。研究发现早全新世11~8 ka夏季降水量和表面蒸发量较大,冬季降水稀少,湖泊水位只有十余米深,使得青海湖周边风沙活动频繁。并且,早全新世的气候不稳定,经历了频繁和较大幅度的波动。全新世气候适宜期出现在8~6 ka,古环境指标指示这一时期为温暖湿润的气候环境,湖盆内植被以森林草原为主,湖泊水位不断上升。青海湖地区的夏季降水自6 ka开始减少,然而冬季降水增加,同时夏季温度和蒸发量减少,使得湖区植被组成由森林草原向高山草甸转变,湖区大范围形成古土壤。湖区古环境条件在晚全新世距今1.5 ka开始恶化,冬季和夏季降水同时减少,湖泊水位下降,风沙活动再次加强。  相似文献   

6.
Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-1 throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.  相似文献   

7.
Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal. yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities.  相似文献   

8.
Lake Nicaragua, the largest lake in Central America, is a promising site for paleolimnological study of past climate change, tectonic and volcanic activity, and pre-Columbian agriculture in the region. It is near the northern limit of the Intertropical Convergence Zone (ITCZ), which brings the rainy season to the tropics, so effects of decreasing precipitation due to southern migration of the ITCZ through the Holocene should be observable. Because fault zones and an active volcano lie within the lake, the long-term impact of tectonic and volcanic activity can also be examined. Finally, the fertile volcanic soils near the lake may have encouraged early agriculture. We analyzed diatoms, biogenic silica (BSi), total organic carbon (TOC), water content, volcanic glass, and magnetic susceptibility in a sediment core from Lake Nicaragua with eleven accelerator mass spectroscopy radiocarbon dates, spanning ~5,700 years. Sediment accumulation rates decreased from the bottom to the top of the core, indicating a general drying trend through the Holocene. An increase in eutrophic diatom abundance suggests that pre-Columbian agriculture impacted the lake as early as ~5,400 cal yr BP. Above a horizon of coarser grains deposited sometime between ~5,200 and 1,600 cal yr BP, planktonic diatoms increased and remained dominant to the top of the core, indicating that water depth permanently increased. Although magnetic susceptibility peaked and water content dipped at the coarse horizon, volcanic glass fragments did not increase, suggesting that the coarse horizon and subsequent increase in water depth were caused by tectonic rather than by volcanic activity. Decreased accumulation rates of BSi and TOC indicate that water became clearer when depth increased.  相似文献   

9.
A continuous, 1,420-cm sediment record from Lake Pupuke, Auckland, New Zealand (37°S) was analysed for diatom taxonomy, concentration and flux. A New Zealand freshwater diatom transfer function was applied to infer past pH, electrical conductivity, dissolved reactive phosphorus and chlorophyll a. A precise, mixed-effect regression model of age versus depth was constructed from 11 tephra and 13 radiocarbon dates, with a basal age of 48.2?cal kyr BP. Diatom-inferred changes in paleolimnology and climate corroborate earlier inferences from geochemical analyses (Stephens et al. 2012), with respect to the timing of marked climate changes in the Last Glacial Coldest Phase (LGCP; 28.8?C18.0?cal kyr BP), the Last Glacial Interglacial Transition (LGIT; 18.0 to ca. 12?C10?cal kyr BP) and the Holocene, the onset of which is difficult to discern from LGIT amelioration, but which includes an early climatic optimum (10.2?C8.0?cal kyr BP). The LGCP is readily defined by a reduction in lake level and effective precipitation, whereas the LGIT represents a period of rising lake level, with greater biomass during the Holocene. There was limited change in diatom assemblage structure, influx or inferred water quality during a Late Glacial Reversal (LGR; 14.5?C13.8?cal kyr BP), associated with heightened erosional influx. In contrast, an LGIT peak in paleoproductivity is recorded by increased diatom influx from 13.8 to 12.8?cal kyr BP. Changes in sediment influx and biomass record complex millennial-scale events attuned to the Antarctic Cold Reversal (ACR; 14.5?C12.8?cal kyr BP). Additional millennial-scale environmental change is apparent in the Holocene, with marked changes in lake circulation beginning at 7.6?cal kyr BP, including the onset of seasonal thermal stratification and rapid species turnover at 5.7?cal kyr BP. The most rapid diatom community turnover accompanied widely varying nutrient availability and greater seasonality during the last 3.3?cal kyr. Rising seasonality appears to have been linked to strengthened Southern Westerlies at their northern margins during the middle and late Holocene.  相似文献   

10.
The character and impact of climate change since the last glacial maximum (LGM) in the eastern Mediterranean region remain poorly understood. Here, two new diatom records from the Ioannina basin in northwest Greece are presented alongside a pre-existing record and used to infer past changes in lake level, a proxy for the balance between precipitation and evaporation. Comparison of the three records indicates that lake-level fluctuations were the dominant driver of diatom assemblage composition change, whereas productivity variations had a secondary role. The reconstruction indicates low lake levels during the LGM. Late glacial lake deepening was underway by 15.0 cal kyr BP, implying that the climate was becoming wetter. During the Younger Dryas stadial, a lake-level decline is recorded, indicating arid climatic conditions. Lake Ioannina deepened rapidly in the early Holocene, but long-term lake-level decline commenced around 7.0 cal kyr BP. The pattern of lake-level change is broadly consistent with an existing lake-level reconstruction at Lake Xinias, central Greece. The timing of the apparent change, however, is different, with delayed early Holocene deepening at Xinias. This offset is attributed to uncertainties in the age models, and the position of Xinias in the rain shadow of the Pindus Mountains.  相似文献   

11.
Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.  相似文献   

12.
The vegetation history and development of three different types of lakes, lakes Valday, Kubenskoye and Vishnevskoye (northwest of the East European Plain) were reconstructed using paleolimnological techniques. Watershed vegetation demonstrates a close connection with climate fluctuations: gradual expansion of the southern broad-leaved trees to the North during the Holocene with the maximum extent during the climate optimum (8000–5000 BP); and their subsequent retreat afterwards; followed by the extension of spruce during the cold and dry Subboreal time; and dominance of pine-spruce-birch forests in the Subatlantic time. The Late Pleistocene and Holocene climate changes resulted in lake-level fluctuations and other ecosystem changes. Valday Lake was formed ca. 12,500 BP as an oligotrophic, deep water basin. The lake level decreased during the dry Boreal, then increased again during the humid Atlantic period. The large shallow Kubenskoye Lake was formerly a part of an ice margin lake, which was then separated (ca. 13,000 BP) and developed into the Sukhona Basin with an outflow to the northwest. During the Atlantic, the outflow direction changed to the east. As a result, the ancient Sukhona Lake disappeared and Kubenskoye Lake formed in its modern size and shape. Vishnevskoye Lake, on the Karelian Isthmus, was formed at the beginning of the Preboreal after the disappearance of the Baltic Ice Lake. It was flooded by waters of the Boreal Ancylus transgression of the Baltic Basin and had become a small eutrophic lake by the time.  相似文献   

13.
Diatom dissolution in saline lakes represents an important obstacle to the quantitative reconstruction of water chemistry and climate from lake sediment archives. This problem is here approached experimentally by artificially dissolving diatom-bearing core sediment from Lake Manyara, Tanzania. Manyara holds one of the longest continuous palaeolimnological records from tropical Africa although its interpretation is based on a fragmentary diatom record due to frustule dissolution. These experiments have revealed clear changes in assemblage composition as dissolution operated differentially with respect to diatom taxa. Differential dissolution has considerable impact on the water chemistry estimates derived from transfer functions. Taphonomy, rather than environmental change, may have been responsible for minor fluctuations in the diatom assemblages from Manyara, although major palaeohydrological changes during the Late Pleistocene and Early Holocene can be identified. Particularly well represented by MANE-87 is a period of intermediate lake level between 27 500 and 23 000 14C yr BP which has regional palaeohydrological significance.  相似文献   

14.
Lake Ohrid is considered to be of Pliocene origin and is the oldest extant lake in Europe. A 1,075-cm-long sediment core was recovered from the southeastern part of the lake, from a water depth of 105 m. The core was investigated using geophysical, granulometric, biogeochemical, diatom, ostracod, and pollen analyses. Tephrochronology and AMS radiocarbon dating of plant macrofossils reveals that the sediment sequence spans the past ca. 39,500 years and features a hiatus between ca. 14,600 and 9,400 cal. year BP. The Pleistocene sequence indicates relatively stable and cold conditions, with steppe vegetation in the catchment, at least partial winter ice-cover of the lake, and oxygenated bottom waters at the coring site. The Holocene sequence indicates that the catchment vegetation had changed to forest dominated by pine and summer-green oak. Several of the proxies suggest the impact of abrupt climate oscillations such as the 8.2 or 4.0 ka event. The observed changes, however, cannot be related clearly to a change in temperature or humidity. Human impact started about 5,000 cal. year BP and increased significantly during the past 2,400 years. Water column mixing conditions, inflow from subaquatic springs, and human impact are the most important parameters influencing internal lake processes, notably affecting the composition and characteristics of the sediments.  相似文献   

15.
Upper Klamath Lake, in south-central Oregon, contains long sediment records with well-preserved diatoms and lithological variations that reflect climate-induced limnological changes. These sediment archives complement and extend high resolution terrestrial records along a north–south transect that includes areas influenced by the Aleutian Low and Subtropical High, which control both marine and continental climates in the western United States. The longest and oldest core collected in this study came from the southwest margin of the lake at Caledonia Marsh, and was dated by radiocarbon and tephrochronology to an age of about 45 ka. Paleolimnological interpretations of this core, based upon geochemical and diatom analyses, have been augmented by data from a short core collected from open water environments at nearby Howards Bay and from a 9-m core extending to 15 ka raised from the center of the northwestern part of Upper Klamath Lake. Pre- and full-glacial intervals of the Caledonia Marsh core are characterized and dominated by lithic detrital material. Planktic diatom taxa characteristic of cold-water habitats (Aulacoseira subarctica and A. islandica) alternate with warm-water planktic diatoms (A. ambigua) between 45 and 23 ka, documenting climate changes at millennial scales during oxygen isotope stage (OIS) 3. The full-glacial interval contains mostly cold-water planktic, benthic, and reworked Pliocene lacustrine diatoms (from the surrounding Yonna Formation) that document shallow water conditions in a cold, windy environment. After 15 ka, diatom productivity increased. Organic carbon and biogenic silica became significant sediment components and diatoms that live in the lake today, indicative of warm, eutrophic water, became prominent. Lake levels fell during the mid-Holocene and marsh environments extended over the core site. This interval is characterized by high levels of organic carbon from emergent aquatic vegetation (Scirpus) and by the Mazama ash (7.55 ka), generated by the eruption that created nearby Crater Lake. For a brief time the ash increased the salinity of Upper Klamath Lake. High concentrations of molybdenum, arsenic, and vanadium indicate that Caledonia Marsh was anoxic from about 7 to 5 ka. After the mid-Holocene, shallow, but open-water environments returned to the core site. The sediments became dominated (>80%) by biogenic silica. The open-water cores show analogous but less extreme limnological and climatic changes more typical of mid-lake environments. Millennial-scale lake and climate changes during OIS 3 at Upper Klamath Lake contrast with a similar record of variation at Owens Lake, about 750 km south. When Upper Klamath Lake experienced cold-climate episodes during OIS 3, Owens Lake had warm but wet episodes; the reverse occurred during warmer intervals at Upper Klamath Lake. Such climatic alternations apparently reflect the variable position and strength of the Aleutian Low during the mid-Wisconsin.  相似文献   

16.
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   

17.
Holocene development of aquatic plant communities in subarctic Lake Njargajavri, Finnish Lapland, was studied using plant macrofossil analysis. Sediment lithology, grain size, and C/N ratios showed distinct lithological phases, indicating past water-level fluctuations. The colonization of limnophytes took place right after the formation of the lake (after ca. 11,500 cal. BP). The earliest plant macrofossil assemblages indicate nutrient-rich conditions and a warmer climate than at present. After this primary succession phase, aquatic vascular plants were replaced by aquatic bryophytes (before ca. 10,200 cal. BP). Together with lithological evidence, we interpret this as being related to the lowering water table. According to palynological, chronological, and sedimentological evidence, Njargajavri underwent a very shallow phase between ca. 10,000 and 9500 cal. BP and dried out for an unknown period of time between ca. 8000 and 5000 cal. BP. After the dry phase, the water level started to rise and sedimentation at the coring point began again. Despite re-establishment of the lacustrine habitat, late-Holocene plant macrofossil data show no marked recolonization of either vascular limnophytes or bryophytes. The reason for all limnophytes being presently absent from the lake remains speculative. The lack of nutrients and/or the cooling climate (especially shortening of the open-water season) during the latter part of the Holocene may explain why limnophytes failed to recolonize the lake.  相似文献   

18.
The late Quaternary diatom records from alpine Opabin Lake (altitude 2285 m a.s.l.) and sub-alpine Mary Lake (altitude 2054 m a.s.l.), located in Yoho National Park, British Columbia (lat. 51 ° 21N; long. 116 ° 20), have been analyzed, and changes in these records have been used to reconstruct lake histories. The results have also been related to independently inferred vegetation and climate changes. Following deglaciation, when both lakes were receiving high inputs of clastic materials, benthic diatom taxa dominate the records of these two shallow lakes with small species ofFragilaria being particularly prominent. During the early to mid-Holocene period, when treeline was at a higher elevation than today, the diatom flora of both lakes became more diverse with previously minor species becoming more prominent.Cyclotella radiosa occurs in cores from both Mary Lake, and much deeper, neighbouring Lake O'Hara during the warm early Holocene, and may reflect this warmer climate, a longer ice-free season than presently, and perhaps less turbid water, or its presence may reflect a subtly higher nutrient status of the lake water during this period. The Neoglacial is marked by increased amounts of sediments originating from glacial sources in Opabin Lake, which undoubtedly led to very turbid water, and by the presence ofEllerbeckia arenaria f.teres andCampylodiscus noricus v.hibernica in Opabin Lake; however, these species are absent from Mary Lake which has not been influenced by either glacial activity since the recession of the glaciers prior toc. 10 000 years BP or water originating from Opabin Lake. The impact of the two tephras during the Holocene was dramatic in terms of increased diatom production, as exemplified by the increases in diatom numbers, but there was little effect upon species composition. The diatom records and changes in the diatom:cyst ratio suggest that the chemical status of these two small, shallow lakes has changed little during the Holocene, other than after deposition of the two tephras. These results provide evidence that shallow alpine and high sub-alpine lakes are sensitive recorders of past environmental changes.  相似文献   

19.
A 12 m sediment core recovered from the south basin of Lake Turkana, northwestern Kenya, reveals four major diatom assemblages that span approximately 5450 to 1070 years BP based on AMS radiocarbon analyses. The oldest assemblage, Zone D (5450 to 4850 yr BP), is dominated by Melosira nyassensis and Stephanodiscus spp. and is interpreted to reflect higher lake levels, fresher water and more variable seasonal mixing of the water column than the modern lake. Melosira dominates the assemblage in Zone C (4850 to 3900 yr BP) with some Surirella engleri and Stephanodiscus. This assemblage indicates a continuation of relatively high lake levels and seasonal mixing of a stratified lake. The brief peak of Surirella, interpreted as benthic, suggests an episode of slightly lower lake level. Thalassiosira rudolfi and Surirella predominate since the beginning of Zone B (3900 to 1900 yr BP), reflecting a decrease in lake level and increase in water column salinity. Increasing dominance of Surirella in Zone A (1900 to 1070 yr BP) may suggest that the lake continued to decrease in depth. Salinity probably rose to levels comparable with the modern lake. These results are consistent with paleoclimatic interpretations based on carbonate abundance, lamination thickness, oxygen isotope and bulk geochemistry profiles from this core and cores recovered from the north basin. It extends the known paleolimnology beyond 4000 yr BP of the earlier research to 5450 yr BP and into the early to mid Holocene pluvial phase in northern intertropical east Africa.  相似文献   

20.
The late Quaternary diatom record from subalpine Crowfoot Lake, Banff National Park, Alberta (lat. 51° 61N; long. 116° 31W) has been analyzed. Results are related to independently inferred vegetation and climate changes. No diatoms were found in the basal diamict that predates 11330 14C yr BP. Very few occur until ca. 10 10014 C yr BP probably due to the short time between de-glaciation and an advance of the Crowfoot Glacier during the Younger Dryas Chron. Initial pioneering species were characteristic of alkaline water and calcareous organic sediments. They appeared as sediments became organic and laminated suggesting increasing water clarity, and as the Pinus-dominated forest expanded and the climate warmed. After ca. 9060 14C yr BP diatom numbers increased rapidly, reaching a maximum prior to the Mazama tephra; they remained high until ca. 3500 14C yr BP. The period between ca. 9060 and 3500 14C yr saw timberline elevation increase and the dominance of xerophytic taxa. These are consistent with early to mid-Holocene warmth and aridity. Diatom productivity reflects the warm climate and presumably longer ice-free season, a stable catchment and transparent water. Decreases in diatom productivity coincide with a vegetation change with reduction of xerophytic taxa and the appearance of a closed Picea-Abies forest, hence a cooler, wetter climate at ca. 4100 to 3500 14C yr BP. The diatom numbers during the Neoglacial were of the same magnitude as prior to ca. 9060 14C yr BP. Small species of Fragilaria (overwhelmingly Fragilaria construens v. venter) became extremely dominant during the period of high diatom productivity, and remained so thereafter. Recovery of the lake appears to have been rapid after deposition of the Mazama tephra. Maximum occurrence of Cyclotella radiosa occurred ca. 8000 14C yr BP during the warm early Holocene and may reflect this warmer climate, a longer ice-free season than presently, perhaps less turbid water, or it may reflect a subtly higher nutrient status of the lake water. The diatom record of Crowfoot Lake has responded with sensitivity, particularly in terms of productivity, to the Holocene vegetation and climate changes.  相似文献   

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