BOMB C-14 RECORDED IN LAMINATED SPELEOTHEMS - CALCULATION OF DEAD CARBON PROPORTION

We performed radiocarbon measurements using accelerator mass spectrome try (AMS) on 6 stalagmites, 3 stalactites and 7 seepage waters from fo ur different caves in Southwest France and Belgium in order to calcula te the dead carbon proportion (dcp). All the speleothems studied are m odern and annually laminated, which offers the advantage of an accurat e chronology, with better than one-year resolution. Coupled with the f act that very little calcite is necessary for an AMS measurement (betw een 1.5 and 7 yr of calcite deposit), we obtained dead carbon values w ithin an uncertainty limit of +/- 1.5%. Results show that the dead car bon proportion varies from 9.2% to 21.9% for calcite deposits and from 3.6% to 21.9% far water. In each sampling site, the dcp is homogeneou s. Although the inter-site dcp varies by >11%, its average value of 15 .5% +/- 4.4 still lies within the uncertainty range of the accepted va lue of 15% +/- 5 (dilution factor of 0.85 +/- 0.5). We compare the ave rage dcp of each site with the local geology, vegetation and climate. Given similar geology and temperature, the highest dcp values are foun d under forest cover; dcp difference is up to 9%. However, the Belgian site, which is also under a forest, shows a dcp very close to the dcp found under grassland sites of Southwest France, which proves that ot her unknown factors may play an important role in dissolution processe s. Secondary calcite deposition and redissolution in the soil zone or more likely in the fracture system before reaching the cave itself cou ld also explain the inter-site differences. The IAEA isotopic model (P earson model adapted for open systems) is in good agreement with the m easured activities.