14C in Atmosphere

The knowledge of the concentration of natural radioactive isotope 14C in the atmosphere and its global and local distribution in of great importance for ecology, climatology and hydrogeology, as well as for the monitoring of the carbon cycle in the environment. A significant increase of the concentration of 14C in the atmospheric CO2 in the whole atmosphere and biosphere during the sixties was caused by intensive thermonuclear bomb tests in the atmosphere. Various nuclear facilities may also contribute to the increase of 14C in the atmosphere, while the use of great amounts of fossil fuels causes a dillution of 14C, i.e., a decrease of its concentration. In the Radiocarbon and Tritium Laboratory of the Rudjer Boskovic Institute we have monitored the 14C activity of the atmospheric CO2 since 1978. Our results show a continuous decrease of the atmospheric 14C activity in this period (Fig. 1), in accordance with the global trend of 14C decrease. Between 1995 and 1997 we measured 14C concentration in monthly samples collected at station Puntijarka, Mt. Medvednica. 14C concentration at this station is mostly higher than in the Zagreb city area (Fig. 2), and the difference is significant during the winter months due to fossil fuel combustion in the Zagreb area. The average 14C activity for period 1995 - 1997 in the Zagreb area is Δ14C = (70±30)promil, while at the Puntijarka station Δ14C = (86±20)promil. Monthly samples of atmospheric CO2 were also collected outside and inside the Postojna Cave, Slovenia (Fig. 3). D14C values measured inside the cave range from -3promil to 55promil, with the mean value (26±16)promil. Outside the cave the mean value was (95±11)promil, and the monthly 14C activities ranged from 76promil to 113promil. (author)