Landscape changes, cave site formation and human occupation during the Late Pleistocene: a geoarchaeological study from the Ach and Lone valleys (Swabian Jura, SW Germany)

Abstract

In the Swabian Jura (Southwest Germany) the Ach and Lone valleys host a number of cave sites, which have been repeatedly occupied by groups of hominins since at least the last interglacial. From the excavation of these Prehistoric cave deposits generations of archaeologists have discovered deep sequences and spectacular finds that help to illuminate the evolution of our species and the extinction of Neanderthals. The current understanding of the relationship between caves, cave deposits, human occupation and landscape evolution in the Ach and Lone valleys relies on data coming almost exclusively from cave sites and rockshelters. Conversely, the few geological studies that focused on the geomorphological evolution of the Ach and Lone valleys were mostly designed to answer questions unrelated to the study of the human occupation of this region. This dissertation intends to bridge the gap existing (not only in the Ach and Lone valleys) between the micromorphological analysis of cave deposits and the study of the paleolandscape. To this end, we investigated the deposits accumulated inside the cave site of Hohlenstein-Stadel (in the Lone Valley) with micromorphological analysis and FTIR methods. Additionally we studied the sediment archives preserved in the Ach and Lone valleys by means of auggering, deeper coring, geophysics, micromorphology, FTIR and 14C dating. By integrating our results with previous studies we have been able to evaluate the impact of landscape and environmental changes on the sedimentary and diagenetical processes that occurred inside the cave sites of this region. Our findings carry significant implications for models used to explain the arrival of early modern humans in the Swabian Jura and the human depopulation that this region seem to have experienced around the Last Glacial Maximum (LGM). Previous studies have argued that the early Aurignacian deposits at the sites of Hohle Fels and Geißenklösterle are characterized by deposition of fresh loess, less intensive phosphatization, and the formation of ice lenses. These data support the hypothesis that modern humans entered the Ach Valley in the course of a period characterized by cold climate. Our micromorphological analysis showed that inside Hohlenstein-Stadel (in the Lone Valley) the transition from the Middle Paleolithic to early Aurignacian deposits displays decreasing phosphatization. However, at Hohlenstein-Stadel we did not observe features diagnostic for ground freezing (such as ice lenses). Therefore inside this cave there is no direct evidence supporting the occurrence of a colder climatic period at the time of the arrival of modern humans. The data we collected from Ach and Lone valleys suggest that this region has been variably shaped by phases of soil formation, soil denudation, loess accumulation and river valley incision. Our study revealed that these changes in the landscape might have shaped significantly the archaeological record originally accumulated inside the cave sites of this region. Accordingly to a widely accepted model, Southwest Germany was abandoned by Gravettian groups around 26.000 14C BP. After a major chronostratigraphic gap Magdalenians recolonized the region starting 13.500 14C BP. Such chronostratigraphic gap (12.500 14C years) have been regarded as indicative of the depopulation that the region experienced during the LGM. Our data, however, show that the beginning of this long-lasting phase of apparent depopulation coincides with the occurrence of erosional phases at some cave sites of Ach and Lone valleys. Although the relation between cave erosion and Gravettian emigration needs to be further verified, we hypothesize that humans might have been present in this region also during the period 26.000-13.500 14C BP and erosional processes might have cancelled the archaeological evidence for such occupations.