Karl Otto Münnich

Tritium/3He dating of shallow groundwater

Combined tritium/3He data from three multi-level sampling wells (DFG 1, DFG 4, DFG 7) located at Liedern/ Bocholt, West Germany, are presented and principles of the tritium/3He method in shallow groundwater studies are discussed. The3He excess produced by radioactive decay of bomb tritium (released mainly between 1952 and 1963) is clearly reflected in the data. The tritiogenic3He signal can be detected with a good resolution (signal/1σ error: ≈ 350). The confinement of the tritiogenic3He is estimated to approximately 77–85% at site DFG 4. For the bomb tritium peak the deviation of the tritium/3He age from the age determined by identifying the groundwater layer recharged between 1962 and 1965 is about 3 years (15%). The deviation can be explained by diffusive3He loss across the groundwater table and by flow dispersion.

Tritiogenic 3He in shallow groundwater

Tritium, helium isotope and neon data from a multi-level sampling well (DFG 7) at Liedern/Bocholt (West Germany) are presented and discussed. The presence of a radiogenic helium component leads to 3He/4He ratios below that of atmospheric helium (minimumδ3He values≈ −60%) below about 20 m depth. The3He profile can be corrected for the nucleogenic3He component using the neon measurements. Based on the “Vogel” model of a shallow aquifer the tritium/3He distributions are simulated for the years 1987, 2000 and 2025. The model results show that under favourable conditions the tritiogenic3He peak will be detectable in shallow aquifers for at least the next 4 decades. The influence of the vertical flow velocity and the transversal dispersion coefficient on simulated distributions are estimated.3He confinement is calculated as a function of the vertical flow velocity and the transversal dispersion coefficient. There is a critical value of the vertical flow velocity (about 0.25–0.5 m/year) below which the3He loss increases rapidly to high values.

Groundwater age dating with chlorofluorocarbons

CFCs are useful tracers for age dating young water. Two critical assumptions are typically invoked: (1) the initial concentration needs to be known, and (2) the tracer must be stable. A series of 8000 CFC air data from four sites on the west European continent (from high-alpine clean air down to urban sites) show site-specific CFC excesses relative to the global background ranging from 125% at an urban site (Heidelberg) to only 30% at a rural site (Wachenheim). In both cases we find regular diurnal, weekly, and annual variations but also episodic variations due to changing air mass and sporadic local emission as well as a decrease of the CFC excess due to decreasing emissions in Europe. However, in soil air above the groundwater table the CFC excess variation is low-pass filtered, allowing reconstruction of the site-specific CFC input anomaly in relation to the global background. Thus local CFC soil air measurements can provide a valuable local correction factor for CFC dating and extend the applicability of CFC studies in specific environments. To investigate the chemical stability assumption, five field sites and two laboratory experiments show CFC 11 and CFC 12 degradation under anoxic conditions with degradation rates ranging from 0.05 up to 3×105 pmol L−1 yr−1. However, the CFC 12/CFC 11 degradation ratio is found to be constant over six orders of magnitude, with CFC 12 always being less reactive by a factor of about 10.

Carbon-14 and carbon-13 in soil CO2.

Carbon isotope measurements in soil CO 2 are presented and discussed. Soil CO 2 concentration and 13 C profiles were measured using a new technique. A simple model describing the CO 2 transport from the soil to the atmosphere is derived. The finding that CO 2 in the soil is richer in 13 C than the CO 2 leaving the soil is attributed to isotopic fractionation in molecular diffusion.

Radiocarbon in Atmospheric Carbon Dioxide and Methane: Global Distribution and Trends

For many years, there has been a growing concern in the field of atmospheric chemistry about anthropogenic and natural perturbations of the major atmospheric cycles of carbon, nitrogen and sulfur, and recently, oxygen. The concern is mainly due to the implications of these trace gases on global climate. In view of the atmospheric carbon cycle, the most abundant trace gases, carbon dioxide and methane, just recently became the subject of detailed 14C investigations. These may play an important role in providing the supplementary and independent information needed to better evaluate the current observations.

Lead and cesium transport in european forest soils

The downward velocity of Pb and Cs in undisturbed European forest soils is determined from the depth distribution of atmospheric 210Pb , and from the penetration depth of the bomb Cs peak in the soil. The downward velocity of Pb and Cs shows no correlation with soil type and pH. The downward migration of Pb and Cs is found to be due to an apparent downward movement of organic material, caused by the sedimentation of soil organic matter and subsequent turnover into CO2 . Cs migration, however, shows an additional velocity component which is found to depend on the turnover rate of soil organic matter.

Fractionation, precision and accuracy in 14C and 13C measurements

Abstract During the past ten years, AMS has become a powerful tool in radiocarbon dating. In some applications, an overall accuracy comparable to that of conventional high precision low level counting is required. To achieve this accuracy, fractionation during sample preparation and measurement has to be constant. Comparison of series of 13 C 12 C ratios measured with AMS and a conventional mass spectrometer indicates that systematic errors in the carbon isotope ratio measurements are in the order of 0.1 to 0.2%. With mg size samples prepared by catalytic reduction on iron, delivering beam currents of about 12 μA for at least 1 h, statistical uncertainties for 14C of 0.3% and a total error of 0.4 to 0.5% for the 14 C 12 C ratio (including the calibration error and uncertainty in the background subtraction) for modern carbon has been obtained.

Co2 Gas Proportional Counting in Radiocarbon Dating — Review and Perspective

Gas counting is a mature and powerful technique central to radiocarbon dating. The method was taken from the detection techniques used in nuclear physics and adapted to the special requirements of low-level counting of the carbon gases. A compilation made by W G Mook in 1983 lists 174 gas counters used in 14C dating, the counting gases being CO2 (115 counters), CH4 (38), C2H2 (20) and C2H6 (1). In the present contribution, the current status of CO2 gas counting is reviewed. The emphasis on CO2 is justified by several observations: 1) CO2 is the primary gas to be produced in all methods; 2) routine techniques are able to achieve high purity CO2 gas, so further conversion to hydrocarbons appears unwarranted; 3) the cryogenic properties of CO2 facilitates handling and thus minimizes contamination; 4) all gas counting laboratories involved in high-precision work, eg, for calibration, use CO2 as counting gas. Most of the techniques mentioned in this review were already available around 1975 (see the Proceedings of the Ninth International Radiocarbon Conference (Berger & Suess 1979)); since that time, gas proportional counting has become a routine operating technique in several laboratories. Wherever possible, reference is made to individual laboratories; however, as the more technical aspects of the technique that are central to this chapter are rarely fully documented in the literature, we take most examples from the Heidelberg laboratory, and are fully aware that this description gives heavy weight to a single installation.

C14-Altersbestimmung von Swasser-Kalkablagerungen

It has been shown that the C:/sup 14/ content of recent hard water is approximately 85% of that found in new wood. For the age determination of lime deposits, it must be assumed that inital C/sup 14/ content of deposited lime must not vary from the 85% value. The possible causes of deviations from this value are discussed, and the corrections to be made are described. (J.S.R.)

Turnover of Eastern Caribbean deep water from 14C measurements

Abstract A 14 C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 10 5 m 3 /sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on 14 C concentrations at a station in the Venezuelan Basin which average Δ 14 C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a 14 C concentration of the Atlantic inflow of Δ 14 C= −71% . as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm 2 /sec. The oxygen consumption, and silica and CO 2 regeneration, rates below 2500 m depth are obtained as −0.18, + 0.08, and + 0.2 μmole kg −1 yr −1 , respectively. The CO 2 regeneration has but a negligible effect on the 14 C balance.