engleski

A response of 14C activity in recent tufa from the Plitvice Lakes to changes of atmospheric CO2

14C activity of the atmospheric CO2 was almost constant in the past due to established equilibrium between the 14C production by cosmic rays and the radioactive decay of 14C. However, as a consequence of the thermonuclear atmospheric experiments the atmospheric 14C activity increased from 100 pMC in 1950 to almost 200 pMC in 1963. After the nuclear ban treaty the atmospheric 14C activity has declined and returned presently almost to the natural level. The presence of so-called bomb carbon in the earth’ s atmosphere has enabled it to be used as a tracer for the carbon cycle studies. Tufa is a secondary calcium carbonate that precipitates under favourable physico-chemical conditions from fresh water. In the Plitvice Lakes system in Croatia tufa has been growing during Holocene forming numerous lakes and waterfalls. The area has been pronounced as a National Park and is visited by numerous tourists who walk around the lakes along the wooden pathways. Such a pathway was built in the bed of the Korana River, which outflows from the lake system, in 1983. Recent tufa has been formed around the wooden pillar used for the path construction and in 2003 both the pillar and the tufa were extracted from the river bed. During the 20 years about 35 cm thick layer of tufa was formed around the wood. In the first layer, about 6 cm thick, annual tufa layers can be clearly observed. Ten samples were taken along the tufa profile, and 14C activity and stable isotope content (delta13C and delta18O) were measured. Carbonate was dissolved in HCl and the obtained CO2 was used for benzene synthesis. 14C activity was measured by the liquid scintillation counting technique using LSC Quantulus. Mean values delta13C = -8.9‰ and delta18O = -9.6‰ (VPDB) do not differ from stable isotope content of other tufa or lake sediment samples from the same area proving that the tufa has been formed from the dissolved inorganic carbon under conditions close to the isotopic equilibrium. In the first 6-7 samples an increase in 14C activity of tufa from 74.8 + 0.4 pMC (the innermost layer) to 89.9 + 0.5 pMC has been observed, which was then followed by a gradual decrease to 85.3 ± ; ; ; 0.4 pMC in the outermost (surface) layer. Such a distribution of 14C activities in the tufa profile is explained as a response of carbon in tufa to the changes in 14C activity of the atmospheric CO2 in the second half of the 20th century. The peak 14C activity in tufa is damped and it is observed with a remarkable delay of about 35 years in relation to the atmospheric 14C peak. Such a damped and delayed response is caused by different geochemical processes of the exchange of carbon of different origin in water that are going on along the water path from the infiltration to the places where tufa precipitates. Similar effect of delayed 14C peak activity was observed also in speleothems, but the delay was 15 – 20 years due to shorter path of water carrying dissolved inorganic carbon to the speleothem formation site. Work performed under Project 098-0982709-2741 of the Ministry of Science, Education and Sports. The authors thank the Stable Isotope Laboratory SILab of the University of Rijeka and their Project 062-098 2709-0510 for stable isotope measurements.

14C activity; atmospheric CO2; tufa; calcite; delta 13C; delta 18O

nije evidentirano