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1.
This study is focused on the endorheic Uyni-Coipasa Basin located in the southern Bolivian Altiplano. Stratigraphical and fossil diatom studies based on a detailed radiocarbon chrnology revealed six phases in water-level changes and paleosalinity variations. At 15,430±80 yr B.P., lacustrine conditions settled in the southern Bolivian Altiplano. A saline lake, characterized by benthic meso-metasaline species, reached +4 m altitude above the present bottom of the basin. After 15,430±80 yr B.P., the level rapidly rose to +27 m, as suggested by a tychoplanktonic mesosaline flora. Between 14,500 years and 13,000 years, finely lanminated sediments at +32 m contained successively a dominance of epiphytic mesosaline to hypersaline species and tychoplanktonic oligosaline diatoms, indicating weak fluctuations in water-level and salinity. At 13,000 years, strong changes in the diatom flora occurred; epiphytic oligo-hypersaline diatoms were replaced by planktonic meso-polysaline species. They indicate a deep salt lake (the lake reached +100 m). After 12,000 years, the lake level abruptly dropped, as suggested by fluviatile sediments with a benthic mesopolysaline diatom flora. The main lake was replaced by shallow saline ponds. A wet pulse occurred at 11,400 years, characterized by low water level (+7 m) and high salinity. This lacustrine phase remained until 10,400 yr B.P. These data indicate changes in Precipitation minus Evaporation (P-E). Our regional interpretations are based on a comparison with teh available data on the northern (Lake Titicaca) and southern (Lipez are) Bolivian Altiplano and on the northern Chilean Altiplano (Atacama Desert).  相似文献   

2.
Criteria for removing training set lakes and taxa in chironomidbased inference models, due to low abundances, have largely been ad hoc. We used an anoxia inference model and a hypolimnetic oxygen model from southcentral Ontario to determine what effect subfossil head capsule abundance and taxa deletion criteria have on fossil inference statistics. Results from six training set lakes suggest that a minimum abundance of 40–50 head capsules is sufficient for use in inference models, however more diverse samples likely require more than 50 head capsules. Taxa deletion criteria substantially improved the predictive ability of inference models (lowered the root mean squared error of prediction (RMSEP)). The common practice of including taxa with only 2% abundance in at least two lakes was one of the deletion criteria that much improved inference models. Similar deletion criteria, such as 2% in at least 3 lakes and 3% in at least 1 lake, produced comparable improvements (up to 18% reduction in RMSEP).  相似文献   

3.
With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.  相似文献   

4.
We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (18Op) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (18Oc). Comparison of the calcite oxygen isotope proxy for 18Op in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This 18Oc/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite 18O values since the 1960s. Possible explanations for this recent 18Oc shift, as it is not related to mean annual air temperature, include changes in 18Op due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in 18Oc data.  相似文献   

5.
Canonical correspondence analysis (CCA was used to explore and identify statistically significant relationships between the distributions of planktonic diatoms and the physical and chemical properties of 50 Connecticut lakes. Six variables (pH, total nitrogen, calcium, sulfate, potassium and chlorophyll- a concentrations) were found to be significantly correlated with either or both of the first two extracted axes. The pH and calcium concentration, and to a lesser extent total nitrogen concentrations, were the most important variables controlling the distributions of planktonic diatoms in this suite of lakes. Paleolimnological inference models were developed for pH, total nitrogen (TN) and specific conductivity. Weighted averaging with (WAtol) and without (WA) tolerance downweighting, with and without bootstrap resampling techniques, and using either classical or inverse deshrinking methods were used to develop inference models for each variable. The pH and TN yielded sufficiently high 1/2 ratios and a highly significant first (constrained) axis when entered as single variables in both constrained and partially constrained CCA analyses, supporting the idea that reliable inference models could be developed for these variables. The r2 and RMSE of prediction values ranged from 0.73 to 0.86 and 0.37 to 0.6, respectively for pH, and from 0.4 to 0.64 and 59 g/l to 95 g/l, respectively for TN. Inference models for specific conductivity also yielded significant goodness-of-fit statistics. However, because specific conductivity was removed from the CCA analysis due to its high variance inflation factor and did not yield a significant relationship when entered as the sole variable in a partial constrained CCA, inference models for this variable will probably not yield any additional environmental information. The use of only planktonic diatoms in construction of inference models is discussed.  相似文献   

6.
We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 14C yr BP), pollen data suggest moist conditions, but high 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in 18O values and associated change in ostracod species at 6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 14C yr BP may indicate early human disturbance. By 2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.  相似文献   

7.
Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.  相似文献   

8.
A multi-proxy investigation of two sediment cores from the large closed-basin Lake Qinghai provides evidence of abrupt changes in paleolimnological conditions across the late-glacial/Holocene transition. The chronology of the lacustrine sediment sequence is framed by four AMS 14C ages for aquatic-plant macrofossil seeds. Four distinct stratigraphic units are identified on the basis of abrupt shifts in lithology, carbonate composition, 18O of authigenic carbonates, magnetic susceptibility characteristics, and total nitrogen content. These units represent four environmental stages that were each initiated by three abrupt changes in hydro-climatic regime at 11,600, 10,700, and 10,000 14C yrs B.P. Each of the four environmental stages thus represents a characteristic precipitation-to-evaporation balance for the lake catchment. The paleoenvironmental evidence indicates that the lake before 11,600 14C yrs B.P. was very shallow with carbonate production and organic productivity much lower than in the Holocene, suggesting a much colder and drier climate than in the Holocene. From 11,600 to 10,700 14C yrs B.P., the presence of clastic laminations and Ruppia fossil seeds suggests an increased inflow of sediment-laden waters into the lake. Between 10,700 and 10,000 14C yrs B.P., the development of a carbonate playa lake indicates that a negative water balance persisted. From 10,000 14C yrs B.P. an abrupt increase in rainfall is suggested by a sudden termination of the playa lake environment and the diluted lake waters, as evidenced by negative shift in both total carbonate content and 18O values of mineral carbonate. However, the lake level during the early Holocene was about 20 m shallower than today, indicating that the effective moisture then was much lower than it is today. The multi-proxy record suggests a step-wise pattern of climatic change across the late-glacial/Holocene transition along with abrupt shifts in P-E balance on the N. E. Tibet-Qinghai Plateau. This pattern is characterized by reorganization of Asian monsoon circulation, which probably was determined by increasing summer insolation and changes in surface boundary conditions accompanying regional deglaciation. The arid event at 10,700–10,000 14C yrs B.P. is interpreted as a Younger Dryas equivalent, although climatic cooling is not indicated by the evidence at hand.  相似文献   

9.
The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have 13C and 18O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high 13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in 13C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The 13C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to 13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of 18O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the 18O variability.  相似文献   

10.
A diatom transfer function to infer epilimnetic total phosphorus (TP) concentration was derived using surface sediment diatom data from 68 medium-sized (10–1000 ha) lakes in Southern Finland. Publicly available monitoring records were used in lake selection to avoid gradients caused by pH and humic substances. Constrained and partially constrained ordinations indicated that TP was an important variable influencing diatom assemblages. A long floristic gradient in relation to TP was also apparent and therefore an inference model was developed for TP using unimodal-based regression and calibration methods. The final model included 61 lakes with epilimnetic TP concentrations between 3 and 89 g P l–1, measured during the autumnal circulation period. It has a jackknifed-estimated root mean squared error of prediction of 0.16 log g P l–1, a maximum bias of 0.28 log g P l–1, and an r2 jack of 0.76.The model was tested in the presently eutrophic Lake Valkjärvi (epilimnetic [TP] 60–85 g P l–1), located in Southern Finland. It successfully predicted the measured autumnal epilimnetic TP concentration for the past twenty years and the changes in inferred [TP] reflected disturbances known to have occurred before that time. The diatom-based inferences show that Lake Valkjärvi was oligo-mesotrophic as late as the 1930's and has become eutrophic because of nutrient inputs from agriculture and, especially, municipalities. However, epilimnetic TP concentration has not increased further.  相似文献   

11.
A paleolimnological study of eutrophied Lake Arendsee (Germany)   总被引：1，自引：0，他引：1  
To study the algal microfossil assemblages of eutrophic Lake Arendsee (Germany) prior to the beginning of a restoration project, a 47-cm long freeze core, dating back to ca 1800, was taken from the deepest area of the lake. Based on the CRS modeled 210Pb and 137Cs profiles from the core, 1948 is around 15 cm and the sedimentation rate has increased from 21.2 mg cm-2 yr-1 in 1900 to 56.6 mg cm-2 yr-1 in 1986. The sediments were dominated by three centric diatoms. Stephanodiscus binatus, a species associated with eutrophic environments, dominated the upper 19 cm of the core. Cyclotella rossii, a species commonly found in less productive freshwater systems, was found to dominate the lower portion of the core and was absent above 16 cm. S. agassizensis was found throughout the core. In addition to the centric diatoms, three penate diatoms were found to be abundant. Fragilaria crotonensis was found throughout the core, but was most abundant from 19 cm to 16 cm. Asterionella formosa was prevalent below 15 cm, while Diatoma elongatum was found to be common from 17 cm to the surface. The abundances of algal remains of cyanobacteria, chlorophytes, cryptophytes and dinoflagellates decrease dramatically below 25 cm. Zooplankton remains were most abundant around 20 cm, with copepod spermatophores, fecal pellets and protozoa remains most common in the lower portion of the core. The major species shifts observed in the core from Lake Arendsee occur in a transition zone between 20 cm and 15 cm (1920–1940), a time when agricultural production was being increase with the use of inorganic fertilizer.  相似文献   

12.
Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km2 of catchment area and 40 km2 of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. 210Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr–1, which also was confirmed by 137Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (LF), susceptibility of anhysteretic remanent magnetism (ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/LF) had much more information to reveal environmental changes. The ARM/LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.  相似文献   

13.
Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High 18O and low 13C values are associated with abundant Mn-carbonates. Low 18O and high 13C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.  相似文献   

14.
Multivariate numerical analyses (DCA, CCA) were used to study the distribution of chironomids from surface sediments of 100 lakes spanning broad ecoclimatic conditions in northern Swedish Lapland. The study sites range from boreal forest to alpine tundra and are located in a region of relatively low human impact. Of the 19 environmental variables measured, ordination by CCA identified mean July air temperature as one of the most significant variables explaining the distribution and the abundance of chironomids. Lossonignition (LOI), maximum lake depth and mean January air temperature also accounted for significant variation in chironomid assemblages. A quantitative transfer function was created to estimate mean July air temperature from sedimentary chironomid assemblages using weightedaveraging partial least squares regression (WAPLS). The coefficient of determination was relatively high (r2 = 0.65) with root mean squared error of prediction (RMSEP, based on jack-knifing) of 1.13 °C and maximum bias of 2.1 °C, indicating that chironomids can provide useful quantitative estimates of past changes in mean July air temperature. The paper focuses mainly on the relationship between chironomid composition and July air temperature, but the relationship to LOI and depth are also discussed.  相似文献   

15.
Diatom-based transfer functions for inferring epilimnetic total phosphorus (TP) have been developed from a data set of 33 southeastern Australian water storages. Regular institutional monitoring of these sites has allowed comparison of models developed from TP data covering different time periods. A model based on mean annual TP performs better than models derived from winter maximum TP, spring minimum TP or TP nearest the time of diatom sampling. A mean annual TP model (WA-PLS 2 component) has a jack-knifed diatom-inferred versus measured TP correlation coefficient (r 2 jack) of 0.69 and a root-mean-square-error of prediction (RMSEP) of 0.246 log10g TP l–1, while alternative models have RMSEP > 0.27. Deletion of two samples with uncharacteristic species composition and environmental conditions improved performance of the mean annual TP model (r 2 jack= 0.74; RMSEP = 0.233 log10g TP l–1). Comparison with other published diatom-TP calibration models indicates that this model performs relatively well, with possible contributing factors including the extensive characterisation of TP (with an average 15 determinations making up the annual mean) and the dominance of planktonic diatoms in most sites. Downcore application of the model will allow the reconstruction of reservoir nutrient histories since commissioning, and thus provide a basis for understanding and management of reservoirs.  相似文献   

16.
In this article we describe the basic framework of the computerized geologic mapping system cigma. The system, whic is based on a mathematical formulation of geologic concepts, consists of the following six subsystems: (1) input of geologic data set; (2) inference of stratigraphic sequence; (3) construction of logical models of geologic structures; (4) determination of three-dimensional geologic boundary surfaces; (5) construction of three-dimensional solid model of geologic structures; and (6) graphical presentation. Geologic structures are summarized in several tables called logical models of geologic structures. Each model is constructed automatically from input data on structural relations between geologic bodies. The model interprets the data automatically to create data files necessary to determine the shapes of geologic boundaries; it also provides a threedimensional solid model of geologic structures referring to the shapes of boundaries. As a prototype, we introduce two types of contacts corresponding to conformity and unconformity into the logical model and show that it is possible to draw a geologic map automatically. More complex geologic structures can be introduced into the geologic mapping system through further formulation of geologic structures.  相似文献   

17.
An environmental magnetic and magnetic fabric study of sedimentscollected from Lake Waynewood, a post-glacial lake in the PoconoMountains of Pennsylvania, USA, provided a history of the lake's watersheddynamics over the past 3500 years. Two 5 m long, Mackereth coresof lake sediments and three watershed soil profiles were analyzed magnetically.Paleosecular variation and 14C measurements allowed timing ofchanges in the lake's watershed which are documented by changes in ARM,SIRM,, S-ratio, SIRM/, ARM/ and ARM/SIRMdowncore. Prior to 2900 years BP, there is little evidence for allogenicinflux. Dramatic changes in mineral magnetic properties and a strong AARmagnetic fabric appear approximately 2900 years BP, suggesting major changes inwatershed conditions, either in the hydrologic regime or in vegetative cover.Between 2900 and 1800 yrs BP, large variations in magnetic mineralogy areapparent, whereas about 1800 years BP, a single sediment source began todominate the magnetic mineralogy. About 100–200 years ago, conditionsagain changed, probably due to clearcutting and settlement of the watershed.Topsoil erosion appears to have dominated the magnetic signal. S and Mnconcentration downcore indicate that there is little evidence for reductiondiagenesis having caused the changes observed in magnetic mineral type andconcentration, except in the top 10 centimeters of the sediment column.  相似文献   

18.
Over the last 12600 years, lake levels in the eastern Lake Erie basin have fluctuated dramatically, causing major changes in drainage patterns, flooding and draining ephemeral Lake Wainfleet several times and widening and narrowing the Niagara Gorge as the erosive effects of Niagara Falls waxed and waned. The control sill for Lake Erie levels was at first the Fort Erie/Buffalo sill, before the Lyell/Johnson sill in Niagara Falls took over due to isostatic rebound. This sill, in time, was eventually eroded by the recession of Niagara Falls and the Fort Erie/Buffalo sill regained control. The environmental picture is complicated by catastrophic outbursts from glacial Lake Agassiz and Lake Barlow-Ojibway, changes in outlet routes, isostatic rebound and climatic changes over the Great Lakes basins. Today, the flow of water into Lake Erie from the streams and rivers surrounding it only accounts for about 13% of the flow out of it, therefore, the importance of flow from the Upper Great Lakes, specifically the flow from Lake Huron, has a great effect on Lake Erie levels. While the changing control sills, Lyell/Johnson and Buffalo/Fort Erie would affect Lake Erie levels, overall they are mostly input driven by the amount of waters received from the Upper Great Lakes. Since Lake Erie's water level changes are so closely tied to Lake Huron's water level changes we have decided to use names assigned to Lake Huron such as the two Mattawa highstands and three Stanley lowstands rather than inflict a whole new set of names on the public. While the duration of each high and lowstand in Erie and Huron may not always be the same, they always happen within the same time frame. The datum elevations used for Lake Huron (175.8 m) and Lake Erie (173.3 m) are historically recorded averages. The Lake Erie levels proposed in this paper reflect Lake Hurons effects on Lake Erie and the levels occuring at the eastern end of the Erie Basin throughout the last 12600 years. All dates in this paper are uncorrected 14 C dates unless the date was obtained from shells, then the date has been corrected for hard-water effects. Also, all heights are given as modern day elevations and are not adjusted for isostatic rebound.  相似文献   

19.
The return of hundreds to millions of adult sockeye salmon (Oncorhynchus nerka), which have returned from the ocean to their natal nursery lake environment to spawn, can result in significant nutrient loading. By analyzing sedimentary diatom assemblages from nursery lakes, we demonstrated that a salmon-derived nutrient signal could be traced over time and be used to infer past sockeye salmon population dynamics. We conducted a 2,200 year paleolimnological study of two Alaskan sockeye salmon nursery lakes, Karluk and Frazer lakes. The two lakes are very similar, except that sockeye salmon were only introduced into Frazer Lake in 1951 (first spawners returned in 1956). In both lakes we found a strong correspondence between diatom assemblages and the number of adult salmon spawners recorded in the historical data (40 and 70 years for Frazer and Karluk lakes, respectively). Given this robust relationship, we then used our analyses of diatoms from Karluk Lake over the past 2,200 years to gain insight into salmon-derived nutrient loading changes (which are directly related to the number of sockeye salmon spawners). The diatom record from Karluk Lake recorded dramatic species changes on both decadal and century timescales, and was strongly correlated with an independent indicator of sockeye salmon abundances, 15N. Together, these data suggest pronounced variability in sockeye salmon abundances at Karluk Lake over the past 2,200 years. The direct impacts of regional environmental variability were not likely responsible for the patterns apparent in Karluk Lake, as the diatom and 15N profiles from Frazer Lake were relatively stable prior to the introduction of sockeye salmon. Application of total phosphorus transfer functions to the Karluk and Frazer lakes' diatom records revealed that sockeye salmon carcasses substantially increased the trophic status in these lakes, which has important implications for the health of juvenile salmon that rear in nursery lakes. Overall, this paper illustrates the potential use of diatoms in reconstructing past sockeye salmon population dynamics, which in turn can lead to a greater understanding of the mechanisms influencing abundances of sockeye salmon.  相似文献   

20.
A sedimentary blue-green algal record has been investigated through measurement of myxoxanthophyll and oscillaxanthin in two cores taken from deep and shallow sites in Lake Wabamun, Alberta, Canada (Longitudes 114° 26 and 114° 44 W; Latitudes 50° 30 and 50° 35 N). Blue-green algae have been a component of the algal flora of this lake throughout the Holocene period. Myxoxanthophyll and oscillaxanthin maxima occur in early Holocene sediments (ca. 9000 years BP), whereas oscillaxanthin concentrations are high between 7000 and 3800 years BP. High oscillaxanthin levels suggest that a phytoplankton assemblage, which included Oscillatoria spp., existed during this latter period and the lake was more eutrophic than at present. Decreases in the number of planktonic diatoms in the core from the deep site (Seba core) appear to be related to increased eutrophy, increased salinity, and sediment redistribution as well as possible competition with Oscillatoria. That the lake has been less productive during the last 2500 years in supported by the diatom record, the diatom: chrysophyte statospore (stomatocyst) ratio and concentrations of the blue-green algal pigments. In the core from the shallow site (Moonlight Bay) concentrations of blue-green algal pigments are initially high, which along with the diatom assemblage, indicates a younger basal age of the sediments. It is possible that benthic blue-green algae contributed significantly to sedimentary pigment concentrations in the Moonlight Bay core. Major fluctuations in the Osc: Myx ratio, particularly in the Seba core, casts some doubt upon the usefulness of this ratio, and suggests that it is not degradation-independent.  相似文献   

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