TheOldest Dryas[a] is abiostratigraphic subdivision layer corresponding to a relatively abruptclimatic cooling event, orstadial, which occurred during thelast glacial retreat.[1][2] The time period to which the layer corresponds is poorly defined and varies between regions,[1] but it is generally dated as starting at 18.5–17 thousand years (ka)before present (BP) and ending 15–14kaBP.[3][4][5][6][7] As with theYounger andOlderDryas events, the stratigraphic layer is marked by abundance of the pollen and other remains ofDryas octopetala, anindicator species that colonizes arctic-alpine regions. The termination of the Oldest Dryas is marked by an abrupt oxygen isotope excursion, which has been observed at many sites in the Alps that correspond to this interval of time.[8]
In theAlps, the Oldest Dryas corresponds to theGschnitz stadial of theWürm glaciation. The term was originally defined specifically for terrestrial records in the region ofScandinavia, but has come to be used both forice core stratigraphy in areas across the world, and to refer to the time period itself and its associated temporary reversal of the glacial retreat.[1]
In theIberian Peninsula, the glaciers of thePyrenees,Sierra Nevada, Central Range, and Northwestern Mountains, which had almost entirely disappeared by 17,500 BP, began to advance once again. Between 16,800 and 16,500 BP, these glaciers abruptly advanced into montane valleys and deposited moraines near the moraines formed during theLast Glacial Maximum. These glaciers then began to oscillate between advance and retreat until a final glacial advance at 15,500 BP. A thousand years later, following a general glacial retreat, these alpine glaciers were relegated tocirques.[9]
During the Oldest Dryas, Europe was treeless and similar to the Arctictundra, but much drier and grassier than the modern tundra. It contained shrubs and herbaceous plants such as the following:
Species were mainly Arctic but during the Glacial Maximum, the warmer weather species had withdrawn into refugia and began to repopulate Europe in the Oldest Dryas.
The brown bear,Ursus arctos, was among the first to arrive in the north. Genetic studies indicate North European brown bears came from a refugium in theCarpathians ofMoldavia. Other refugia were inItaly,Spain andGreece.
The bears would not have returned north except in pursuit of food. The tundra must already have been well populated. It is likely that the species hunted by humans atLake Neuchâtel inSwitzerland by the end of the period were present during it. Here are other animals present:
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