Dating the Younger Dryas in lake sediments - comparison of different records by I. Hajdas', S.D. Ivy', G. Bonani' ,A. Fischer 2 , S.A.G. Leroy3,', A. Lotter 4 and B. Zolitschka 5 Teilchenphysik ETH Hönggerberg, CH-8093 Zürich, Switzer/and Ge%gy ETHZ, CH-8092 Zürich, Switzerland 3 Paléontologie et Paléogéographie, VCL, Pl. L. Pasteur, B-1348 Louvain-la-Neuve, Belgium 4 EAWAG, CH-8600 Dübendorf, Switzerland 5 FB VI/Geologie, Vniversity Trier, D-54286 Trier, Germany I 2 Recently, the radiocarbon and absolute time sc ales ofthe last deglaciation we re improved significantly (Bard et al., 1993; Hajdas et al., 1993; Hajdas et al., in prep.; Kromer and Becker, 1993; Taylor et al., 1993). Proxy-data archives, such as those from ice cores or laminated lake sediments, provide a high resolution signalof climate change. Various methods have been applied to date these continental records and associated climatic signals derived from stabie isotopes, dust, pollen and sediments. We propose to evaluate the present knowledge concerning the dating of the Younger Dryas. We compare the dating ofthe Younger Dryas biozone (i.e. the boundaries Aller0d (AL) / Younger Dryas (YD) and YD/Pre-Boreal (PB) obtained from varved sediment sequences in Soppensee (Switzerland) and Holzmaar (Germany) with evidence derived from Greenland ice cores, tree rings, sediments from lake Gosciaz (Po land) and dating of corals. AMS dating ofterrestrial macrofossils selected from sediments ofSoppensee and Holzmaar provided a radiocarbon time scale for these two records. The absolute time scale is based on corrected varve chrono logies (Hajdas et al., 1993; Hajdas et al., in prep.). Figure 1 shows the radiocarbon-varve chronologies plotted together with the tree ring curve (Kromer and Becker, 1993) and results from U-Th dating on corals (Bard et al., 1993; Edwards et al., 1993). A very good agreement between both varve chronologies was obtained. In the combined varve chronology, the oldest radiocarbon age of 12.5 ka 14C BP corresponds to ca. 14 ka cal BP. However, the trend ofthe radiocarbon calibration curve based on varve chronologies is different from th at derived by U/Th dating of corals. The offset between both chronologies is ca. 1000 years at 11 ka 14C • Present address: PAGES, Bärenplatz 2, CH-3011 Bern, Switzerland 179 13 - - tree ring. ~N" <> 12 e Sopp,"" verve. Holzmeer UlTh deted corel. a:ce ~ Ol Cl 1\:1 Ü - 11 10 v 9 B 9 10 11 12 14 13 15 Dendro / varve / U-Th age (kyr BP) Fig. I. Varve chronologies from Soppensee and Holzmaar - comparison with dendrocalibration curve and results from U/Th dating of corals. BP (12.2 kyr cal BP varves, ca 13 ka cal BP corals). In consequence, the absolute dating of the YD (boundaries, duration) proposed by varve chronologies and U/Th dating of corals is also different. Therefore, the solution to the problem varve versus coral data is connected to the dating of the late glacial events, in particulary, to the dating of the YD period. Figure 2 summarizes the dating of the YD in various records. A duration of Soppensee Holzmaar Gosciaz kyr tree rings eorals iee cores cal BP 11.(} - - ~~...., 12.0 _ 10 kyr 14C BP -- W-.~-~"" 13.(} - - - --- pollen& S180 ~.JI1-~t'~~ 11 pollen& sedim. pollen& S18 0 SO& S1 3C 14C ages LJ kyr 14C BP dust& S18 0 Fig. 2. Comparison of dating of the YD in sediments of Soppensee and Holzmaar with the results from lake Gosciaz, ice core, tree rings and dating of corals. Radicarbon ages of \0 and 11 kyr are shown as hatched 'curves' connecting all records. 180 ca. 1150 to 1300 years was found in ice cores (GRIP and GISP2) and sediments from Gosciaz (1140) and Soppensee (1140). The shorter, ca. 540 years long, YD in Holzmaar sediments cannot be explained by missing varves. In such a case, the same number of varves would also be missing in Soppensee. Additional correction in Soppensee would resul t in a YD of ca.1700 years. The age of the upper boundary of the YD in ice cores (8 180, dust) (Alley et al., 1993; Johnsen et al., 1992) is ca. 500 years older than in tree rings (8 l3 C, dD) and Soppensee sediment (pollen, 818 0). The varve chronology of lake Gosciaz is still fioating so that the YD/PB boundary (pollen, 818 0) is not dated precisly (option land 11). A calibration of radiocarbon ages of 10 kyr and 11 kyr BP (age of bounadries of the YD) corresponding to the 14C_U/Th dating of corals shows that the YD was ca. 1200 to 1700 years long (option land 11). The duration of the YD found in lakes and ice cores as weIl as the character of changes observed in stabIe isotopes (Fischer and McKenzie, submitted; Goslar et al., 1993) suggest the same timing ofthe event. 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