Polycyclic Aromatic Hydrocarbons in stalagmites: Occurrence and use for analyzing past environments

This article presents a preliminary investigation into the value of Polycyclic Aromatic Hydrocarbons (PAHs) as markers for interpreting past environments in karst areas. We analyzed a core sample representing the last 400 years' growth from a massive stalagmite in the Chevaline Cave (Vercors, France), together with the soil overlying the karst directly above the stalagmite. During the last millennium, this area has been profoundly affected by human activities, such as slash and burn agriculture, charcoal burning and forestry. These activities may have induced changes in PAH production (in the soil) and thus in the PAH record (in the stalagmite). PAHs were extracted using both the Soxhlet method and the ultrasonic method in order to compare extraction yields. The ultrasonic method, in conjunction with classic HPLC/fluo analysis, was adopted for the analyses as it gave the greater yields. PAH distributions were obtained for the stalagmite and for the overlying soils. The sum of the PAH measured in the stalagmite ranges from 18 to 30 ng/g. Furthermore, the more soluble and labile PAHs (lighter than anthracene) dominated the PAH distribution in the stalagmite (about 90% of low molecular weight PAH). However, we did not find any obvious source fingerprints in the soil or the stalagmite, and we observed an unexpected discrepancy between the PAH distributions in the stalagmite and its stratigraphy, which is controlled by mineral and organic inclusions. Although PAH analyses can provide valuable data about the transfer of organic compounds in the karst, it is difficult to extract meaningful paleoenvironmental information from PAHs in stalagmites over the time scale of this study.