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1.
Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment
profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating
methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the 210Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, 137Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by 210Pb for three cores from the central mud zone of the lake. Unsupported 210Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely
mixed for at least the last 75 years. Onset and maximum activity of the radioisotope 137Cs in the cores coincides with the 210Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of 137Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically
deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation
in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted 210Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to
be about 1950 by 210Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured
in one core, and the observed peak corresponds to a 210Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines
in native types correspond generally with the expected dates assigned by 210Pb dating. Conformity between the 210Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and
reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions. 相似文献
2.
ABSTRACTThe sinking of the MV Rena on Astrolabe Reef (Otaiti) in the Bay of Plenty, New Zealand, resulted in the release of oil and ship debris, including dangerous goods carried as cargo. Two key questions of concern to the public and environmental managers were posed immediately: what was the impact of the Rena oil spill and how long would it take for the marine environment to recover? The research that began immediately after the ship grounded provided answers, as documented in this special issue. 相似文献
3.
DR. M. ERDÉLYI 《水文科学杂志》2013,58(1):44-52
SUMMARY Many hydrological systems exhibit long periods of quiescent behaviour, broken only occasionally by short periods of comparatively intense activity. Clear examples arise in stream run-off measurements and in ground water table observations. The number of measurements recorded can then be considerably reduced by reducing the frequency of recording over the quiescent periods. The paper describes an algorithm whereby this reduction is attained in an optimum manner. When recordings can be accepted directly by a computer, the algorithm simply provides a translation programme Whereby the quality of data may be very considerably reduced. Other applications are, however, discussed. The problems of Optimum representations is first presented, and choices of norm indicated. Using a particular norm, it is shown that the optimum representation is characterised, under certain limiting conditions, by the property that the second moments of the second derivatives are constant over all optimum intervals. This characterisation is used to construct an iterative difference scheme, whereby a certain truncation error is accepted. Sufficient conditions for stability of the scheme are determined and these incorporated into a computer programme. The programme has been tested on a variety of data with satisfactory results. In conclusion, a further development of the method is indicated. 相似文献
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Significant population growth over the last three decades, as well as efforts to improve surface-water quality mandated by the Clean Water Act, potentially have had opposing influences on aquatic ecosystems in the U.S. Because historical data on water-quality trends are limited over this time period, we developed a diatom-based transfer function to reconstruct chloride, color, acid neutralizing capacity (ANC), total phosphorus (TP), and pH in 55 Minnesota lakes. The lakes span three different ecoregions, as well as the Twin Cities metropolitan area, and differ in their history of settlement and land use, and in surficial geology, climate, and vegetation. Lakes in the Northern Lakes and Forest ecoregion are nearly pristine, whereas those in the other regions likely are strongly affected by urban or agricultural pollutants. Reconstructions of water-chemistry trends since 1970 suggest that recent human activities have had substantial impacts in both urban and rural areas. Chloride concentrations have increased in many Metro lakes, which may be due to road salts, and phosphorus levels have been steady or rising in agricultural regions. The majority of Metro lakes show some decline in TP, although many of the changes are not statistically significant based on our reconstruction techniques. There is no evidence that atmospheric deposition of sulfate or nitrate has caused acidification or changes in trophic state for remote lakes in the northeastern part of the state. 相似文献
6.
Water isotopic and hydrochemical evolution of a lake chain in the northern Great Plains and its paleoclimatic implications 总被引:2,自引:2,他引:0
Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The 18O and 2H of water from the chain of lakes, when plotted in 2H - 18O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (18O = –6 ± 1 VSMOW) and show no correlation with salinity (which ranges from 1 to 65). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the 18O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower 18O value (–16.7 in 1998) than that of mean annual precipitation (–11) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that 18O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached. 相似文献
7.
D. R. Engstrom C. Whitlock S. C. Fritz H. E. Wright Jr. 《Journal of Paleolimnology》1991,5(2):139-174
Recent sediments of eight small lakes in the northern winter range of Yellowstone National Park were cored to examine stratigraphic records of past changes in limnology and local environment that might be attributed to grazing and other activities of elk, bison, and other large ungulates. Cores of undisturbed sediment were analyzed at close intervals to depths covering the last 100–150 years according to chronologies established by lead-210 dating. Pollen analyses were made to show change in regional vegetation, and diatom and geochemical analyses were made to reveal possible limnological changes resulting from soil erosion and nutrient input from the lake catchments.Variations in sedimentary components prior to establishment of the Park in 1872 indicate some natural variability in environmental factors e.g., erosional inputs in landslide areas west of Gardiner. All lakes had abundant nutrient inputs.After the Park was founded, fire suppression may have been responsible for small increases in pollen percentages of various conifers and Artemisia tridentata (big sagebrush) at different times in different lakes. Perceptible decreases in pollen of willow, aspen, alder, and birch at different times may reflect local ungulate browsing, although drier climatic conditions may have been a factor as well.The most striking manifestation of accelerated erosion in a catchment was found at a lake located beside a road constructed in the 1930s. In contrast to changes at this site, the record of erosion at other lakes is hardly perceptible. Changes in sediment-accumulation rates seen at most sites result from redistribution of sediment within the lake after initial deposition.In the century following Park establishment, the abundance of planktonic diatoms relative to benthic taxa varies among lakes and may reflect differential nutrient inputs or changes in lake level. Four of the five lakes analyzed for diatoms show in the last few decades an increase in planktonic relative to benthic species, implying elevated nutrient inputs. The recent flora, however, is similar to that in pre-Park levels which suggests that these lakes have not been perturbed outside their normal range. Increased nutrient supply in recent decades for at least two of the lakes is supported by the geochemical data, which show an increase in biogenic silica and in organic matter.As a whole, our investigation of the sedimentary record does not support the hypothesis that ungulate grazing has had a strong direct or indirect effect on the vegetation and soil stability in the lake catchments or on the water quality of the lakes. 相似文献
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10.
The geochemistry of ostracode shells and bulk carbonates in a 19-meter sediment core documents at century-scale resolution the evolution of water chemistry in Coldwater Lake, North Dakota, providing a continuous paleohydrologic record of Holocene climate change in the northern Great Plains. A combination of 18O, 13C, Mg/Ca and Sr/Ca in ostracode calcite aided by Sr/Ca in bulk carbonates are used to constrain the paleoclimatic reconstructions. A fresh-water phase in the early Holocene, indicated by the absence of Candona rawsoni and low concentrations of Sr/Ca in bulk carbonate, was followed by a sharp increase in salinity between 10800 and 8900 yr B.P. The climate was predominately dry during the late part of the early Holocene and most of the middle Holocene (8900–5000 yr B.P.), when the lake was very sensitive and recorded a series of dry and wet oscillations. Maximum salinity occurred around 5500 yr B.P. and was followed by a gradual decrease between 5000 and 2400 yr B.P. From 2400 yr B.P. the 18O, Mg/Ca, and Sr/Ca in the ostracodes indicate generally wet conditions interrupted by a series of lesser salinity and temperature oscillations lasting until 600 yr B.P. Ostracode geochemistry indicates that a warm and dry climate returned at about the time of the Little Ice Age (600–150 yr B.P.). Ostracode 13C shows a ong-term increasing trend during the Holocene, which suggests that lake productivity and atmospheric CO2 exchange made greater contributions to the hypolimnetic carbon pool as the lake became shallower with time. 相似文献