4.3 Environmental reconstruction
The bleached horizon with charcoal that covers the cross-bedded sands of layer 9 (fig. 6), interpreted as the Usselo horizon, is a key horizon in the stratigraphy of the Korhaan sand ridge. The weighted mean of the three individual charcoal pieces that were sampled from this layer is 10,983 ± 37 BP, which is in agreement with the ages of around 10,950 BP on charcoal fragments from pine found in the Usselo horizon at many sites (van der Hammen & Van Geel 2008). The Korhaan data, however, suggest fire events not only at the transition between Allerød and Younger Dryas (YD) but also during the YD (Poz-28515). A disruption of the local vegetation during a very short period with an influx of sands is suggested to explain the split of the Usselo horizon at this particular spot. Its timing remains however unclear: did it occur at 10,983 ± 37 BP or during the YD? The presence of YD charcoal in the upper whitish layer of the Usselo horizon in any case confirms that this soil surfaced during the earlier part of the YD.
The presence in pit B of small but deep frost wedges suggests that the Usselo horizon is buried under a cover of YD aeolian sands, with a maximum thickness of 140 cm and a mean of 80 cm for all observations where the Usselo horizon is attested. In the transect in figure 7 it is on average 75 cm with a maximum of 100 cm. It cannot be excluded that the Usselo horizon has been obliterated elsewhere by the later podzolisation as a result of a thinner YD sand cover. The YD cover thus slightly influenced the height of the sand ridge, but barely its morphology. This is mainly an inherited (pre-)Allerød morphology, which is mostly determined by the unidirectional cross-laminated sands deposited by northwestern winds and prograding towards the southeast.
The top of the peat allows a precise environmental reconstruction for the transect between pit A and the Luifgoor depression at the transition Allerød-Younger Dryas. The Final Palaeolithic artefact scatter (see below) at the top of the Allerød horizon in pit A is situated only 0.5 m above and 35 m from the border of the wet depression (Fig. 8).
A simulation of the wider environment is based on the present-day DEM and the maximum elevation of the peat (fig. 10). It shows an extended marshy area towards the east, possibly with open water in its lower parts. This image is considered to be an underestimation of its real extent because of the later sediment deposition in the area, with the YD cover probably the most important. An interdisciplinary study is in progress to specify the chronology of the ridge building and the paleoenvironment based on investigations of both pollen and macroremains.