Farid El-Daoushy

A late Holocene record of land-use history, soil erosion, lake trophy and lake-level fluctuations at Bjäresjösjön (south Sweden)

Land-use history, soil erosion, lake trophy and lake-level fluctuations during the last 3000 years were reconstructed through a multidisciplinary palaeolimnological study (pollen, plant macrofossils, diatoms, physical and chemical analysis, magnetic measurements and radiometric methods) of a small eutrophic lake in southern Sweden (Bjäresjösjön, Scania). There are striking responses in diatom, chemical, sediment yield and magnetic records to land-use changes documented by pollen analysis or historical sources, and to lake-level changes identified from sedimentary changes. Our multidisciplinary approach assists interpretation of the processes controlling long-term changes and separation of the effects of different factors (land-use changes, lake-level fluctuations) on individual biostratigraphical records. Climate has controlled processes in the lake indirectly, through lake-level fluctuations, from the Late Bronze Age to the Viking Age (700 BC-AD 800). Since the Viking Age, land-use controlled most of the changes observed in the lakes development and soil erosion processes. Major changes in lake development occurred during the last 200 years, due to a drastic increase in soil erosion and water eutrophication during a period of agricultural modernization.

Dating of marine sediments by an incomplete mixing model

Abstract A marine sediment core from the Kattegat area (Sweden) has been studied for dating purposes. The 137 Cs and 210 Pb profiles have been measured and described by a model of incomplete activity mixing. A time-dependent 137 Cs flux function for the sediment has been constructed from atmospheric data and the 137 Cs contents in water from the Sellafield nuclear reprocessing plant. It has been used as the 137 Cs input function for our model. The results show that the model clearly reproduces the measured profile.

Accounting for incomplete charge collection in Monte Carlo simulations of the efficiency of well-type Ge-detectors

The influence of the process of incomplete charge collection on the performance properties of HPGe well-type detectors is studied. A simplified sub-routine, that takes into account the total collection time of charges within the Ge-crystal, has been developed and coupled to the Monte Carlo photopeak efficiency. Only the manufacturers data of the detectors geometry and the detectors operating voltage was used in the sub-routine. The corrected photopeak efficiency simulations, as based on this new sub-routine, showed much better agreement with the experimental data, especially for energies above 40 keV. The ratio between the experimental and simulated efficiency at 1460 keV were, for example, improved from 0.73 to 0.96 after performing these corrections on our largest well-type Ge-detector.

Determination of alpha-emitting Pu isotopes in environmental samples

This paper presents an improved radiochemical procedure for the determination of alpha-emitting Pu isotopes in environmental samples (soils, sediments, vegetation) by alpha-particle spectrometry. Quantitative Pu recovery yields were obtained (average 60%), 0.1 mBq being the average minimum detectable activity by the complete technique. Special efforts were made to ensure the removal of traces of different natural alpha-emitting radionuclides, which can interfere with the correct determination of 239+240Pu and 238Pu concentrations. The radiochemical procedure was validated by application to reference material and by participation in intercomparison exercises. This radiochemical procedure was applied to the different layers of a high-resolution sediment core taken from a lake in Sweden. The 239+240Pu and 238Pu/239+240Pu profiles obtained in the high-resolution sediment core correctly reproduced the expected evolution of these quantities as observed historically in the atmosphere, validating the procedure for this purpose and showing the power of these radionuclides for dating purposes.

Säynäjälampi and the difficulties inherent in the dating of sediments in a hard-water lake

Previous measurements of the 14C activity of lake sediments and Potamogeton plants from Säynäjälampi, collected in 1970, indicated a severe hard-water effect (Donner et al. 1971). Samples from new cores, collected in 1977, were sliced into thin sections down to 18 cm. The 14C concentration in samples down to 7 cm was similar to that obtained by the Helsinki Laboratory. The corresponding apparent 14C age is almost 2 000 years. Light-coloured layers, apparently due to catchment drainage, were used together with density diagrams to correlate the cores. The drainage history gives a time-scale showing that the lowest samples dated to correlate the cores. The drainage history gives a time-scale showing that the lowest samples dated were just old enough to be pre-bomb sediments. Three samples of submerged plants were also examined, indicating some exchange with the air and an appreciable reservoir age. The activity of the plants was significantly lower in 1978 than in 1970. One sample, well below the lowest drainage stratum, indicates a reservoir age of ca. 1 100 years. This sample yielded an apparent age of the INS fraction which was significantly different from that of the SOL fraction. The difference was determined to ca. 600 years.Total 210Pb was determined via 210Po using isotope dilution while 226Ra was measured with the help of radon emanation. The 226Ra was used to estimate the supported 210Pb. The unsupported 210Pb showed a higher sediment accumulation rate for the top nine cm of one core (the length investigated was 16.5 cm). The varying accumulation rate was explained by human activities, as could be seen from the heterogenous sediment composition of the cores.Recent vegetational change is described from direct observations since 1957. Pollen analysis does not give any reliable indication of agriculture.

Lead-210 and heavy metal contents in dated ombrotrophic peat hummocks from Finland

Abstract Two Sphagnum fuscum hummock cores, core 1, Karpansuo bog and core F9, Kunonniemensuo bog, from Finland were used in this study. The peats are ombrotrophic and were dated using the moss-increment method. The mosses in both cores were carefully examined for their botanical composition, degree of humification, ash percentage and bulk density. The total accumulated dry peat-matter in the Kunonniemensuo core was almost double that in the Karpansuo core. The total 210 Pb and the supported 210 Pb were measured by isotope dilution and the radon emanation technique. Materials in the same peat samples were analysed for their 210 Pb content at the Institute of Physics, Uppsala, Sweden and the Technical Research Centre of Finland, Espoo, Finland. The annual content of unsupported 210 Pb in the dated peat-layers showed that peat materials are effective traps which could yield information on atmospheric-fluxes both chronologically and regionally. Lead, copper, zinc, iron and manganese were also measured by flame atomic absorption spectrometry (AAS). The total accumulated amounts over the past 150 y of the heavy metals investigated are almost identical for both cores apart from manganese which is considerably higher in the Kunonniemensuo core. However, the metal profiles studied exhibit discontinuity zones more pronounced in the Kunonniemensuo core. The 210 Pb data indicate that growth rate and bulk density variations in ombrotrophic peat bogs affect the accumulation of 210 Pb and similar trace metals.

Speciation of Pb-210/Po-210 in aquatic systems and their deposits

Abstract Lead-210 speciation was carried out on previously examined aquatic deposits of known ages and geochemical histories. The main separated fractions were fulvic, humic, humin and mineral-bonded humics. these organic components were wet-oxidized and their 210Po was extracted by coprecipitation with Fe3+, distillation at 450°C and self-deposition on silver-discs. The 210Po activities were determined by isotope dilution and alpha spectrometry techniques. Nineteen samples from three lakes and one ombrotrophic peat-hummock in southern Sweden; and three fjords of Skagerrak, North Sea, were investigated. Humic materials proved to trap and preserve aquatic unsupported 210Pb, especially in lakes and peats. Fulvic compounds, on the other hand, seem to enhance the mobility of unsupported 210Pb particularly under acidic conditions. However, the fulvic fraction of marine sediments showed higher amounts of unsupported 210Pb. Our results show that the supported 210Pb is incorporated with the organo-mineral complexes, the NaOH-insoluble fraction. The 210Pb activity of this fraction was found to converge to the expected supported 210Pb of sediment samples older than 100 years or so. Some younger sediments may also show the last mentioned observation. This might serve as an alternative for evaluating the supported 210Pb in 210Pb dating.

Plutonium, americium and cesium records in sediment cores from Blelham Tarn, Cumbria (UK)

Abstract238Pu, 239-240Pu, and 241Am were measured, by α-counting after radiochemical separation, in two lake sediment cores. The profiles of these transuranics were compared to those of the fission product 137Cs. Datation of nuclear events and data on behavior of the studied elements were deduced.

Plutonium and americium inventories in atmospheric fallout and sediment cores from Blelham Tarn, Cumbria (UK)

The objective of this paper is to report on the results of a study of 238Pu, 239 + 240Pu and 241Am inventories onto Blelham Tarn in Cumbria (UK). The atmospheric fallout inventory was obtained by analysing soil cores and the results are in good agreement with the literature: 101 Bq m(-2) for 239 + 240Pu; 4.5 Bq m(-2) for 238Pu and 37 Bq m(-2) for 241Am. The sediment core inventory for the whole lake is compared to the atmospheric fallout inventory. The sediment activity is 60-80% higher than the estimated fallout activity, showing a catchment area contribution and in particular the stream input.

A multidisciplinary study of the lake Bjäresjösjön (S Sweden) : land-use history, soil erosion, lake trophy and lake-level fluctuations during the last 3000 years

The lake Bjäresjösjön, Southern Scania, Southern Sweden, was studied in the context of the project ‘The cultural landscape of the past 6000 years in Southern Sweden’. Pollen, plant macrofossils, diatoms, physical and chemical analysis, magnetic measurements and radiometric methods (210Pb, 14C) have been used to study palaeoecological changes, i.e. climate, land use, lake trophy and soil erosion during the past 3000 years. This multidisciplinary study shows striking responses of diatom communities, physical and chemical characteristics, sediment yields and magnetic parameters to land-use changes and lake-level fluctuations. Moreover, the latter are closely related to the settlement history at the site, inferred from archaeological records and historical sources.Before 650 AD, the limnological development was affected mainly by lake-level fluctuations, but partly also by human impact (extensive forest clearings and dominant pastoral farming). With the expansion of arable farming (around 650 AD), human impact on the landscape was the major factor influencing soil erosion processes in the catchment and limnological changes in the lake.