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

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

Aug 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.160..795g&link_type=abstract

Earth and Planetary Science Letters, vol. 160, Issue 3-4, pp.795-809

Computer Science

6

C-14, Absolute Age, Stalagmites

Scientific paper

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 14 C activity ( a 14 C) 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, 14 C 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 14 C activities of the stalagmite CaCO 3 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 a 14 C 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 CaCO 3 activities using fractionation processes between atmosphere CO 2 , soil CO 2 and organic matter (OM), dissolved inorganic carbon and stalagmite CaCO 3 . In both cases studied, the model and former soil studies suggest that CO 2 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 CO 2 , this produces a damping effect on the speleothem a 14 C. Consequently, the `geochemical time resolution', 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: a 14 C and 13 C are smoothed over several years. Differences between the 14 C 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 `system' which is more sensitive to atmospheric changes.

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