RECORDS OF LATE-GLACIAL PIONEER FORESTS ON THE SWISS PLATEAU PROVIDE A HIGH RESOLUTION ARCHIVE

Lateral melt water channels were formed along the margins of the Alpine glacier lobes on the Swiss plateau after the last glacial maximum (c. 16 ka BP). As soon as these outlets became inactive, landslide, solifluction, and surface water processes started to fill the channels with loamy alluvia. Pioneer forests (birch, pine) established themselves in response to the abrupt warming at the beginning of the Late-glacial. While the pioneer vegetation developed, continuous sedimentation processes led to the tree stumps being buried in sediments some 10 m thick, with subfossil pines ranging in age between 12,350 and 9,400 14C BP. This process ended when the Boreal vegetation began to migrate to higher elevations, suppressing the Late-glacial and Preboreal pioneer forests.

The Bølling/Allerød forest is represented in the floating chronologies obtained from Daettnau, while the ages of the subfossil forests in the Zurich area range in age between 11,800 and 9350 14C BP, spanning the Allerød, Younger Dryas and Preboreal. The chronologies derived for the later half of Younger Dryas and the Preboreal have been matched with the absolute pine sequences from Hohenheim, extending these back to 12,454 cal BP. Two construction sites, Gaenziloo and Landikon, excavated for the new highway tunnel through the Uetliberg in Zurich, provided a total of 144 fossil pine stumps. The resulting floating chronologies cover the main part of the Allerød from c. 11,800 to 10,850 14C BP. A 14C plateau of >200 years at 11,100 14C BP complicates attempts to develop a continuous Allerød chronology. Tree finds are less frequent but continue into the Younger Dryas. From the early part of the YD until the beginning of the absolute pine chronology, only 9 tree fossils have been recovered. Six of these form a floating chronology of some 189 years, while 3 single trees with ring counts of 245, 144, and 145 dendro-yr, and which do not cross-match, span an interval of at least 430 dendro-years.