Calculation of past dead carbon proportion and variability by the comparison of AMS14C and TIMS U/Th ages on two holocene stalagmites

Twenty-two radiocarbon activity measurements were made by accelerator mass spectrometry (AMS) on 2 Holocene stalagmites from Belgium (Han-stm1b) and from southwest France (Vil-stm1b). Sixteen thermal ionization mass spectrometric (TIMS) U/Th measurements were performed parallel to AMS analyses. The past dead carbon proportion (dcp) due to limestone dissolution and old soil organic matter (SOM) degradation is calculated with U/Th ages, measured calcite C-14 activity and atmospheric C-14 activity from the dendrochronological calibration curves. Results show that the dcp is different for the 2 stalagmites: between 10,800 and 4780 yr from present dcp=17.5% (sigma=2.4; n=10) for Han-stm1b and dcp=9.4% (sigma=1.6; n=6) between 3070 and 520 yr for Vil-stm1b. Despite a broad stability of the dcp during the time ranges covered by each sample, a slight dcp increase of about 5.0% is observed in the Han-stm1b sample between 8500 and 5200 yr. This change is synchronous with a calcite delta(13)C increase, which could be due to variation in limestone dissolution processes possibly linked with a vegetation change. The dcp and delta(13)C of the 2 studied samples are compared with 5 other modem stalagmites from Europe. Results show that several factors intervene, among them: the vegetation type. and the soil saturation leading to variable dissolution process systems (open/closed). The good correlation (R-2=0.98) between the U/Th ages and the calibrated C-14 ages corrected with a constant dcp validates the C-14 method. However, the dcp error leads to large C-14 age errors (i.e. 250-500 yr for the period studied), which is an obstacle for both a high-resolution chronology and the improvement of the C-14 calibration curves, at least for the Holocene.