Bomb 14C time history recorded in two modern stalagmites — importance for soil organic matter dynamics and bomb 14C distribution over continents

Carbon 14 activity measurements made by Accelerator Mass Spectrometry on two modern stalagmites from the Han-sur-Lesse cave (Belgium) and from the Postojna Cave (Slovenia) permit the construction of 14C activity (a14C) time series over the last 50 years. A high precision chronology is given by annual laminae in the first stalagmite and by a specific mark (explosion in the Postojna Cave in 1944) in the second one. In both stalagmites, 14C activity increase due to nuclear tests in the atmosphere is remarkable. However, instead of a sharp peak like the one observed in the atmosphere around 1963–1964, the 14C activities of the stalagmite CaCO3 show an abrupt increase, with an offset of 1–10 years, followed by a high activity plateau for the Han-sur-Lesse sample and a slight decrease for the Postojna sample. For both stalagmites, the variation of the a14C amplitude between pre- and post-bomb period is much lower than the atmospheric record, which demonstrates the damping effect of the soil carbon reservoir. We have modeled the CaCO3 activities using fractionation processes between atmosphere CO2, soil CO2 and organic matter (OM), dissolved inorganic carbon and stalagmite CaCO3. In both cases studied, the model and former soil studies suggest that CO2 from soil organic matter (SOM) decomposition, which has a slow turnover (i.e. >1 y), is of major importance in winter, when the development of speleothem is the most important. Combined with the fact that 80–90% of the stalagmite carbon comes from soil CO2, this produces a damping effect on the speleothem a14C. Consequently, the `geochemical time resolutionʹ, at least for speleothem carbon, is much lower than the structural resolution given by annual laminae alternations and is mainly controlled by soil carbon dynamics: a14C and δ13C are smoothed over several years. Differences between the 14C time series of the Han-sur-Lesse and Postojna stalagmites are likely to be due to the double amount of precipitation in Postojna, which produces a faster soil OM turnover and thus a `systemʹ which is more sensitive to atmospheric changes.