Tree-ring δ13C of archeological charcoals as indicator of past climatic seasonality. A case study from the Neolithic settlements of Lake Chalain (Jura, France)

Charcoal fragments from the Neolithic settlements of Lake Chalain (Jura Mountains, France) were characterized by dendro-anthracology (charcoal-pith distance, tree-ring width, earlywood/latewood proportion) and ring-scale isotope geochemistry (13C) to assess the relevance of this combined approach for paleoclimate reconstructions. Two differing climatic periods were investigated: (i) a climatic deterioration period characterized by cool and moist conditions and (ii) a climatic improvement period characterized by slightly less precipitation and warmer temperature. Latewood proportion in charcoal tree-rings was similar for the two studied climatic periods. However, the charcoal tree-rings exhibited width and 13C-content significantly different between the two studied periods, in agreement with previously inferred climatic difference. Monitoring ring-to-ring 13C variation within each charcoal fragment revealed no noticeable climatic trend, for none of the studied periods. However, calculation of the difference in 13C-content between earlywood and latewood of a given tree-ring suggested that the cool and moist climatic period also corresponded to higher seasonal contrast than the dryer climatic period. Although this exploratory study needs further confirmation, it opens promising developments for paleoclimatic reconstructions based on the stable carbon isotope composition of archeological charcoals: the potential for recording subtle paleoclimatic variations and seasonal contrasts.