Environmental context of the Magdalenian settlement in the Jura Mountains using stable isotope tracking (13C, 15N, 34S) of bone collagen from reindeer (Rangifer tarandus)

The Jura Mountains are considered to be a region where phases of ice cap extension and retreat in response to climatic variation during the Upper Pleniglacial and Lateglacial (ca. 24,000–12,800 cal BP) are well reflected in the vegetation and animal spectrum composition. A new set of direct AMS radiocarbon dates of collagen from reindeer (Rangifer tarandus) bones found at archaeological sites indicated an almost continuous occupation of the Jura region since the end of Last Glacial Maximum, at ca. 24,000 cal BP, until its local disappearance around 14,000 cal BP. To investigate a possible change in reindeer ecology, isotopic analysis of carbon, nitrogen, sulfur in collagen (δ13Ccoll, δ15Ncoll, δ34Scoll) were performed on the dated specimens. A decrease in the δ13Ccoll and δ15Ncoll values of Jura reindeer was found at the beginning of the Lateglacial period around 16,300–15,600 cal BP. While the change in δ13Ccoll values was better explained by a change in diet composition with a decreasing input of lichens, the relative low δ15Ncoll values of the reindeer during the Lateglacial was consistent with a geographical pattern of soil maturity inherited from the Last Glacial Maximum. The same pattern was also seen in the δ15Ncoll values of the Lateglacial horse (Equus sp.) and red deer (Cervus elaphus) until ca. 14,000 cal BP. The decrease in reindeer δ15Ncoll around 16,300–15,600 cal BP and around 21,000–20,000 cal BP in the Jura region may be linked to the occupation of territories recently released by glaciers that formed during the Heinrich event 1 and the Last Glacial Maximum, respectively. The associated high δ15Ncoll and δ34Scoll values found in two specimens indicate the occurrence of areas of high soil activity in a globally cold context. This might correspond to the occupation of refugia in the close surroundings of the Jura region. Such local refugia could explain the capacity of the reindeer to occupy rapidly the newly available territories during phase of glacier retreat. The intensification of the Magdalenian human settlement could have been favored by these local ecosystem expansions.