Carbon transfer dynamics from bomb-C-14 and delta C-13 time series of a laminated stalagmite from SW France - Modelling and comparison with other stalagmite records.

Twenty-two AMS C-14 measurements have been made on a modern stalagmite from SW France in order to reconstruct the C-14 activity history of the calcite deposit. Annual growth laminae provides a chronology up to 1919 A.D. Resul ts show that the stalagmite 14C activity time series is sensitive to modern atmosphere C-14 activity changes such as those produced by the nuclear wea pon tests. The comparison between the two C-14 time series shows that the s talagmite time series is damped: its amplitude variation between pre-bomb a nd post-bomb values is 75% less and the time delay between the two time ser ies peaks is 16 years +/-3. A model is developed using atmosphere C-14 and C-13 data, fractionation processes and three soil organic matter components whose mean turnover rates are different. The linear correlation coefficien t between modeled and measured activities is 0.99. These results, combined with two other stalagmite C-14 time series already published and compared w ith local vegetation and climate, demonstrate that most of the carbon trans fer dynamics are controlled in the soil by soil organic matter degradation rates. Where vegetation produces debris whose degradation is slow, the frac tion of old carbon injected in the system increases, the observed C-14 time series is much more damped and lag time longer than that observed under gr assland sites. The same mixing model applied on the C-13 shows a good agree ment (R-2 = 0.78) between modeled and measured stalagmite delta(13)C and de monstrates that the Suess Effect due to fossil fuel combustion in the atmos phere is recorded in the stalagmite but with a damped effect due to SOM deg radation rate. The different sources of dead carbon in the seepage water ar e calculated and discussed. Copyright (C) 1999 Elsevier Science Ltd.