New approach to evaluate the intensity of ancient human activities, based on organic matter characteristics using Rock-Eval® thermal analysis.

Archaeological anthrosols constitute a heritage of long-term carbon storage and soil fertility. Their anthropogenic features are affected by the type and intensity of ancient human activities. Human activities can follow a gradation of disturbance intensity, with lower intensity related to a weak human pollution of natural soil, and stronger intensity related to anthropogenic materials inputs (e.g. refuse pits). Soil properties are indeed deeply modified by the addition of objects (e.g. bones, ceramic) and of organic matter with distinct chemical composition and biological stability (e.g. charcoal). The aim of the study was to establish a new analytical approach to distinguish intensities of human activities, based on organic matter characteristics. To this end, we studied intertropical soil profiles (0-120 cm) from Cameroon, Brazil and Bolivia, with spatial or temporal intensity variations of human activities. We used standard compositional parameters (hydrogen index, HI, and oxygen index, OI) and advanced thermal parameters (I-index and R-index) from Rock-Eval^® pyrolysis, as well as magnetic susceptibility, to characterize anthrosols.

Results demonstrated the potential of Rock-Eval^® pyrolysis parameters to identify human activities changes. First, the deviation of I-index (delta-I) between our samples and a reference value from natural sites informed about the intensity of human impacts, allowing for the distinction between artificial infilling of refuse pits and soil profiles with no or few human impacts. Second, positive HI:OI correlation established the importance of charcoal as main organic C source. The magnetic susceptibility informed about the presence of burnt soils in a Brazilian and one of the Bolivian sites. The combination of all these parameters, when represented with soil depth, allowed for the estimation of temporal changes in Brazilian and Bolivian sites. The topsoils were similar for all sites, relative to a low intensity of human activities or to the resumption of natural pedogenesis, thereby alleviating the effects of ancient human activities on organic matter characteristics. In contrast, the subsoils exhibited higher intensities of ancient human activities, with even higher values of intensities in Bolivian sites, thereby evidencing the long-term conservation of their effects on organic matter characteristics.

To conclude, anthropogenic activities may durably affect organic matter characteristics in tropical sites, even after several centuries. Beyond being of interest for archaeological research, this new approach raises questions about the long-term consequences of our current human activities.