Vasily Alfimov

Ultra-trace analysis of 36 Cl by accelerator mass spectrometry: an interlaboratory study

A first international 36Cl interlaboratory comparison has been initiated. Evaluation of the final results of the eight participating accelerator mass spectrometry (AMS) laboratories on three synthetic AgCl samples with 36Cl/Cl ratios at the 10−11, 10−12, and 10−13 level shows no difference in the sense of simple statistical significance. However, more detailed statistical analyses demonstrate certain interlaboratory bias and underestimation of uncertainties by some laboratories. Following subsequent remeasurement and reanalysis of the data from some AMS facilities, the round-robin data indicate that 36Cl/Cl data from two individual AMS laboratories can differ by up to 17%. Thus, the demand for further work on harmonising the 36Cl-system on a worldwide scale and enlarging the improvement of measurements is obvious.

36Cl exposure dating of paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey

Based on historical and instrumental data, societies in the Eastern Mediterranean and Middle East have survived at least 150 large earthquakes (generally M > 6) during the past 2500 yr. Beyond this time span, an earthquake chronology is mostly unknown, which hampers the production of reliable long-term earthquake models. Since the only remaining evidence of seismic activity is a bedrock scarp, cosmogenic 36 Cl is the only suitable nuclide to be applied in the determination of the seismic history and slip rate of an active limestone fault plane. In this study,

Dating Fan Deposits with Cosmogenic Nuclides

Cosmogenic nuclides (3He, 10Be, 14C, 21Ne, 26Al, 36Cl) are produced in minerals due to nuclear reactions induced by cosmic rays. Measured cosmogenic nuclide concentrations are used to determine how long rocks and sediment have been exposed at or near the surface of the earth. The timing of abandonment of alluvial fan lobes can be determined directly with cosmogenic nuclide methods. For fans with abundant boulders, the top surfaces of the largest boulders are sampled for exposure dating which is based on the build-up of cosmogenic nuclides. 10Be, 26Al, and 36Cl have been most commonly applied. On fans with surfaces that were abandoned more than about 100-200,000 years ago, the boulders are often weathered, collapsed and crumbled. Such fans, as well as those that never had boulders at the surface, are dated with individual or amalgamated clast samples. Both post-depositional fan surface modification (cryo- or bioturbation), as well as the presence of inherited nuclide concentrations, may hinder obtainment of accurate ages. The former leads to exposure ages that are younger and the latter to ages that are older than the actual age of fan abandonment. Cosmogenic nuclide depth-profile dating is called upon. Samples are taken every 10-20 cm up to a depth of about 3 m. Alluvial fan sediment at greater depths can be dated with burial dating or burial isochron dating, which are based on the differential decay of two nuclides (for example 10Be and 26Al). Optimally samples are taken from depths of more than 30 m. Although in the majority of published studies cosmogenic nuclide dating of fans has been used to determine tectonic slip rates, much has been learned about fan construction and, in turn, catchment processes.

Retrospective dosimetry of Iodine-131 exposures using Iodine-129 and Caesium-137 inventories in soils – A critical evaluation of the consequences of the Chernobyl accident in parts of Northern Ukraine

The radiation exposure of thyroid glands due to (131)I as a consequence of the Chernobyl accident was investigated retrospectively based on (129)I and (137)Cs inventories in soils in Northern Ukraine. To this end, soil samples from 60 settlements were investigated for (129)I, (127)I, and (137)Cs by AMS, ICP-MS and gamma-spectrometry, respectively. Sampling was performed between 2004 und 2007. In those parts of Northern Ukraine investigated here the (129)I and (137)Cs inventories are well correlated, the variability of the individual (129)I/(137)Cs ratios being, however, high. Both the (129)I and (137)Cs inventories in the individual 5 samples for each settlement allowed estimating the uncertainties of the inventories due to the variability of the radionuclide deposition and consequently of the retrospective dosimetry. Thyroid equivalent doses were calculated from the (129)I and the (137)Cs inventories using aggregated dose coefficients for 5-year old and 10-year-old children as well as for adults. The highest thyroid equivalent doses (calculated from (129)I inventories) were calculated for Wladimirowka with 30 Gy for 5-years-old children and 7 Gy for adults. In 35 settlements of contamination zone II the geometric mean of the thyroid equivalent doses was 2.0 Gy for 5-years-old children with a geometric standard deviation (GSD) of 3.0. For adults the geometric mean was 0.47 Gy also with a GSD of 3.0. In more than 25 settlements of contamination zone III the geometric means were 0.82 Gy for 5-years old children with a GSD of 1.8 and 0.21 Gy for adults (GSD 1.8). For 45 settlements, the results of the retrospective dosimetry could be compared with thyroid equivalent doses calculated using time-integrated (131)I activities of thyroids which were measured in 1986. Thus, a critical evaluation of the results was possible which demonstrated the general feasibility of the method, but also the associated uncertainties and limitations.

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.

41Ca, 14C and 10Be concentrations in coral sand from the Bikini atoll.

Activation measurements of materials exposed to nuclear bomb explosions are widely used to reconstruct the neutron flux for retrospective dosimetry. In this study the applicability of coral CaCO3 as a biogenic neutron fluence dosimeter is tested. The long-lived radioisotopes (41)Ca, (14)C and (10)Be, which had been produced in nuclear bomb explosions, are measured in several coral sand samples from the Bikini atoll at the 600 kV and 200 kV AMS facilities of ETH Zurich. Elevated concentrations of all studied isotopes are found in a sample from the crater that was initially formed by the high-yield nuclear explosion Castle Bravo in 1954 and that had been used as site for several tests afterward. The observed (14)C concentration is considered too large to originate from neutron irradiation of CaCO3 alone. The relatively low concentration of (10)Be found in the crater sample indicates that production of (10)Be during nuclear bomb testing is generally minor. A simple neutron fluence reconstruction is performed on basis of the (41)Ca/(40)Ca ratio.

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps: ISOCHRON-BURIAL DATING OF GLACIOFLUVIAL DEPOSITS

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.

Advance in the Mapping of the 1717 AD Triolet Rock Avalanche Deposit (Mont Blanc Massif, Italy) Using Cosmogenic Exposure Dating

The granitic deposit of the upper Val Ferret was shown in 1980 to result from a large September 1717 rock avalanche. A huge volume of rock and ice travelled onto the Triolet Glacier>7 km downvalley. Boulder accumulations and irregular ridges spread out over a distance of 2 km on the valley floor terminate with an arcuate front. A 2009 reconstruction with radiocarbon dating proposed a smaller lateral extension of the landslide on the valley floor preserving Lateglacial moraines, but cosmogenic exposure dating questions it. Consistency of 10Be dates suggests that the deposit results from the 1717 rock avalanche, with a rock volume>10×106 m3, and a likely similar ice volume. The importance of rock avalanches has to be emphasised to help to prevent risk in inhabitated valleys. Even for the recent period, the contribution of cosmogenic exposure dating to geomorphological analysis of complex landform assemblages is valuable.