LATE GLACIAL TREE-RING CHRONOLOGIES – STATUS AND PERSPECTIVE

Tree-ring chronologies provide a versatile archive for late Quaternary dating and climate research. Over the past 30 years we have been engaged in the construction of Holocene and Late-glacial tree-ring series. The absolute chronology developed by the Hohenheim lab is based on German oak for most of the Holocene and dates back to 10,340 yr BP. The older part, based on pine samples also from Switzerland, extends the absolute record back to 12,454 yr BP. In the pine series we observe a drastic growth reduction at c. 11,590 yr BP, which we associate with the YD/PB transition. Hence the absolute chronology extends for c. 860 years back into the Younger Dryas.

For the Bølling/Allerød, a number of floating chronologies have been developed based on samples from the Swiss Plateau (Daettnau, Gaenziloh, Ollon), Germany (Reichwalde, Pfuhl, Eching, Danube Valley, Lausitz, Guenz valley), and Northern Italy (Avigliana). The oldest sections are 14C dated to 12,350 yr BP (c. 14,300 cal BP) with the Bølling/Allerød well covered by >300 individual tree records, most of them dated by 14C. We observe regional dendro-matches within the series from Germany and within those from Switzerland, as well as between several of the Swiss and German chronologies. However, the sequences cannot yet be amalgamated into one continuous record.

The earliest two centuries of the Younger Dryas, between 10,850 and 10,600 BP, are represented by only one tree sample which has 245 annual rings, while there are several specimens that fall within the interval between 10,600 BP and the start of the absolute chronology. The collective data already enable us to calculate the minimum duration of the Bølling/Allerød and of the Younger Dryas.

Presently the pre-12 ka INTCAL98 14C calibration data-set is based on marine 14C data which assume a constant marine reservoir error of 400 years for conversion to atmospheric 14C levels. Because our chronologies for the Late-glacial are floating, they are not yet suitable for 14C calibration. Due to their annual nature, however, we can at least test the assumption of a time-invariant marine reservoir age. Using a 400-yr marine reservoir correction, our data are compatible with the high-resolution Cariaco 14C data-set during the Bølling and the first half of the Allerød, but suggest a much higher reservoir age for the later half of the Allerød.