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Improved quantitative reconstruction of temperature during the Younger Dryas cold period in Northern Europe using botanical fossils

Norbert Kühl1, Christian Ohlwein2
1 Steinmann-Institut der Universität Bonn, Bereich Paläontologie
2 Meteorologisches Institut der Univerität Bonn

V 5.3 in Klima und Umwelt der letzten 130.000 Jahre

18.09.2012, 09:10-09:30, H6

Quantitative palaeo climate reconstructions are fundamental for our knowledge  about magnitude and speed of past climate variability. Comparisons of such data with model simulations improve our understanding of the potential causes and processes  of past climate variability. Of particular interest is the reconstruction of cold periods such as the so-called Younger Dryas (c. 12.600-11.500 BP), because climatic conditions and variability during such periods was significantly different from today (Bakke et al. 2009).

Botanical proxies are widely used as proxy for quantitative palaeo climate reconstructions. However, substantial challenges have to be coped with. A major challenge is that only present conditions can be used for determining vegetation-climate relationships. Climate in Europe was very different during the Younger Dryas compared to today, leading to a no analogue-situation. Vegetation-climate relationship as determined for vegetation in Europe today is therefore very problematic to be used for reconstructing climate of the Younger Dryas.

Here a solution to this problem is proposed. Presence of individual plant taxa is used rather than the abundance of their pollen. This enables the use of a wider range of climatic conditions, because geographic areas beyond Europe can be included. In addition, using presence minimizes the problem of non-analogous plant communities. The relation of the taxa to climate is estimated in a probabilistic way (Kühl et al. 2002). The approach allows the use of macrofossils which have the advantage over pollen that they have a higher taxonomic resolution. Good macrofossil data, however, are rare.

In contrast, numerous pollen data are available in Europe. These, however, have the severe problem that cold-adapted plant taxa as they can be determined by pollen analysis generally occur within a wide temperature range. A consequence is a large uncertainty in the reconstructions with insignificant differences between the Allerød and Younger Dryas (Simonis, Hense & Litt 2012). The new approach presented here reduces the uncertainty by including additional background information about the general vegetation type. Reconstructions with data from a Norwegian site were performed from which excellent macrofossil as well as pollen data is available.

The reconstructions deviate from previous reconstructions of Younger Dryas temperature using other methods, particularly for winter temperature. Mean winter temperature was about 25°C lower than it is today in the area. This is in concordance with geological evidence (Isarin 1997). July temperature was only several degrees lower than today (Aarnes, Kühl & Birks 2012). The much more pronounced seasonality compared to today can be understood by the extent of sea ice which reached considerably further south than today and caused a very reduced if not absent North Atlantic current, leading to cold winters. Summer temperatures were less affected, because summer insolation during the Younger Dryas was even higher than today.

Recent developments include the reconstruction of climatic fields (Simonis, Hense & Litt 2012). A worthwhile next step will be to apply the new approach to palaeobotanical investigations from the Younger Dryas of whole Europe and to compare the results with simulation output from climate models.

 

Aarnes, I., Kühl, N., Birks, H.H., (2012): Quantitative climate reconstruction from late-glacial and early Holocene plant macrofossils in western Norway using the probability density function approach. Review of Palaeobotany and Palynology, 170: 27-39.

Bakke, J., Lie, O., Heegaard, E., Dokken, T., Haug, G.H., Birks, H.H., Dulski, P., Nilsen, T., (2009): Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience, 2: 202-205.

Isarin, R.F.B. (1997): Permafrost Distribution and Temperatures in Europe During the Younger Dryas. Permafrost and Periglacial Processes, 8: 313-333.

Kühl, N., Gebhardt, C., Litt, T., Hense, A., (2002): Probability Density Functions as Botanical-Climatological Transfer Functions for Climate Reconstruction. Quaternary Research, 58: 381-392.

Simonis, D., Hense, A., Litt, T., (2012): Reconstruction of Late Glacial and Early Holocene near surface temperature anomalies in Europe and their statistical interpretation. Quaternary International, in press. Doi:10.1016/j.quaint.2012.02.050.

 



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Letzte Änderung 26.07.2012