14C in Urban Secondary Carbonate Deposits: a New Tool for Environmental Study

Abstract : Secondary carbonate deposits (similar to speleothems) in urban undergrounds, have been recently highlighted as powerful archives for reconstruction of the historical anthropogenic imprint on the environment. The precise chronology of these secondary carbonate deposits is a key issue for the accurate time reconstruction of environmental conditions. We present three 14C data sets for urban speleothem-like deposits that developed in contrasted man made environments. The first one was sampled in an underground technical gallery of the Palace of Versailles (France), and the other two in a manhole (Saint-Martin spring) of a historical underground aqueduct in Paris (France). The comparison of these records with the bomb peak and relative chronology (laminae counting) allowed us to identify: i) fast carbon transfer from the atmosphere to the urban underground; ii) a high proportion of dead carbon and a high damping effect in relation to possible old carbon stored within urban soils and/or the influence of local fossil carbon burning. This study also shows that the lamination of these deposits is biannual in these highly urbanized sites.