E. Rössler

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

Abstract210Pb- and137Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with210Pb/210Po and revealed sediment accumulation rates of (0.055±0.015) g·cm−2·y−1 and (0.09±0.03) g·cm−2·y−1, respectively. A comparable rate of (0.07±0.01) g·cm−2·y−1 has been obtained from137Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm−2·y−1. Constant210Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct137Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of7Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant210Pb activities. Remobilization may also be the reason for an incomplete210Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the210Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.ZusammenfassungSedimente aus dem Zürichsee wurden mit Hilfe von drei unabhängigen Methoden datiert. Für210Pb ergaben sich Sedimentationsraten von 0.055 g·cm−2y−1bzw. 0.09 g·cm−2y−1. Die137Cs-Datierung und die Auszählung von Jahreslagen (Varven) ergab 0.07 g·cm−2y−1. Ähnlich wie in anderen Seen wurden auch im Zürichsee in den obersten 6 cm aller untersuchten Kerne konstante210Pb-Aktivitäten festgestellt. Dies wird in der Literatur in der Regel auf Bioturbation oder auf sekundäre Durchmischung der stark wassergesättigten Sedimente bei der Probennahme zurückgeführt. Im Zürichsee widersprechen die Erhaltung von millimetermächtigen Jahreslagen, das Auftreten eines ausgeprägten137Cs-Maximums und ganzjährig anoxische Bedingungen an der Sediment/Wassergrenzfläche solchen Deutungsversuchen. Auch das Vorhandensein des kurzlebigen Isotopes7Be (HWZ 53 Tage) in den obersten Sedimentschichten beweist eine komplette und ungestörte Probennahme. Für die beobachtete konstante210Pb-Aktivität werden daher Remobilisationsprozesse nach der Ablagerung der Sedimente verantwortlich gemacht. Nachträgliche Remobilisierung könnte auch die Ursache für das beobachtete unvollständige-210Pb-Inventar in den Sedimenten sien, das nur ca. 50% des atmosphärischen Ausfalls beträgt. Die Untersuchungen zeigen, dass die Ergebnisse der210Pb-Datierung in diesem eutrophen See mit Vorsicht interprtiert werden müssen und dass die Auszählung von Jahreslagen—falls vorhanden-die am wenigsten aufwendige und genaueste Methode zur Datierung von Seesedimenten darstellt.

Sedimentation rates in the central lake constance determined with210Pb and137Cs

Sediment cores from central Lake Constance were dated with210Pb and137Cs. A sedimentation rate of (0.11±0.02) g·cm−2·y−1 was determined with the210Pb method.137Cs measurements revealed sedimentation rates of (0.11±0.01) g·cm−2·y−1 and (0.08±0.01) g·cm−2·y−1 respectively for two different cores sampled at the same location. The lower Cs-dated value indicates incomplete core recovery and demonstrates the sensitivity of this simple dating method to small losses of material at the water/sediment interface. An unambiguous application of the137Cs method is, therefore, only possible if complete core recovery is ensured. Sedimentation rates based on particulate matter, collected in sediment traps at various water depths, agree with the results of the radioisotope methods. Estimates of 30–125 days residence times for suspended particulate matter were calculated from7Be measurements.

Mass distribution in the reactor-neutron-induced fission of thorium-232

This investigation was undertaken to improve the data on product yields in the reactor-neutron-induced fission of /sup 232/Th. The mass distribution in the reactor-neutron-induced fission of /sup 232/Th was studied using radiochemical techniques and direct gamma-ray measurements. Absolute fission yields of the standard nuclides /sup 99/Mo and /sup 132/Te were obtained from /sup 232/Th sources irradiated in a fission counter. The yields of 36 mass chains, including three that have been measured for the first time, were determined both from measurements relative to /sup 235/U(n/sub th/, f) nth,f) yields and from /sup 232/Th data alone. 39 refs.

Radiochemical investigation of the mass distribution and probability in stopped μ− induced fission of 238U

Abstract Radiochemical measurements of 24 fission products show that the mass-split is asymmetric in stopped μ − induced fission of 238 U. The mass distribution is similar to 14 MeV neutron-induced fission. The main difference is a smaller peak-to-valley ratio no point indicating a mean excitation energy of about 20 MeV. The fission probability (prompt and delayed) is 0.15 ± 0.03 per stopped muon.

Sorption of Nickel and Cobalt on a Size-Fraction of Unconsolidated Glaciofluvial Deposits and on Clay Minerals

In earlier studies we investigated the sorption behaviour of cesium, barium and strontium on size fractions of glaciofluvial materials and on the clay minerals montmorillonite, illite and chlorite. Here, we report on sorption and desoiption experiments with nickel and cobalt in synthetic groundwater on the <32 μπι size-fraction of a glaciofluvial deposit, on the clay minerals montmorillonite and illite, and on synthetic Si0 2 and CaC03. In general, sorption and desorption equilibrium is not reached for nickel and cobalt, which is in striking contrast to the behaviour observed for strontium and barium. A significant increase in the distribution ratios was observed for nickel and cobalt up to sorption times of 182 and 56 days, respectively. In most cases, the sorption process was largely irreversible. The sorptive power of the < 32 μπι materials with artificially changed mineralogies is considerably reduced as compared to that of the <32 μπι material with the original mineralogical composition, i. e., during the removal of the carbonates a significant decrease in the sorptive power occurred. A comparison of the sorption behaviour of nickel and cobalt with that of cesium, strontium and barium is also shown.

Sorption of Barium on Unconsolidated Glaciofluvial Deposits and Clay Minerals

Drinking water is recovered in the alpine and peri-alpine regions to a large extent from shallow aquifers in unconsolidated deposits of quaternary glaciofluvial origin. Due to the importance of drinking water in the exposure pathway for many radionuclides we are studying sorption processes on samples from this aquifer and, in addition, on clay minerals. Here, we report on batch measurements of sorption, desorption and exchange with barium in the concentration range of 1.0E-08 to 1.0E-04 Μ on glaciofluvial materials from Glattfelden, Switzerland, and on the clay minerals montmorillonite, illite and chlorite. For the aquifer material, the influence of grain size, mineralogy of the < 32-μηι size fraction and composition of the synthetic groundwater was investigated. The results suggest that reversible cation-exchange mechanisms on clay minerals are responsible for the sorption behaviour of barium on the glaciofluvial aquifer material. Its dependence on the composition of the groundwater was reproduced with a 3-site model based on ion-exchange. The sorption isotherm for the aquifer material resembles that of the clay minerals illite and chlorite, but deviates considerably from that of montmorillonite.

Apparatus for semi‐automatic grain‐size separation of small samples

A semi-automatic sieving apparatus for separation of small samples (50-500 mg) of granular material into several uniform grain-size fractions is described. The achieved separations are reproducible, yielding more than 95% of the initial sample. Such separations are needed for investigations of surface phenomena in very fine powders. It is demonstrated that the apparatus is very suitable to handle highly radioactive materials, keeping the radiation doses very low for the operator. The separations into several fractions are faster, cleaner, and more reproducible than those obtained in ordinary hand-sieving techniques.