Abstract of talk by Bas van Geel(1) and Hans Renssen(2)
- The Netherlands Centre for Geo-ecological Research, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands. E-mail: vanGeel[at]bio.uva.nl
- The Netherlands Centre for Geo-ecological Research, University of Utrecht, P.O.B. 80115, 3508 TC Utrecht, The Netherlands.
A sudden and sharp rise in the 14C-content of the atmosphere, which occurred between ca 850 and 760 calendar years BC (ca 2750-2450 BP on the radiocarbon time scale), was contemporaneous with an abrupt climate change. In NW-Europe (as indicated by palaeoecological and geological evidence) climate changed from relatively warm and continental to oceanic (cooler and wetter). Archaeological and palaeoecological evidence for the abandonment of low-lying areas at the Bronze Age/Iron Age transition in The Netherlands is interpreted as the effect of a rise of the water table and the extension of fens and bogs. Palaeoecological, geological/ geomorphological and archaeological evidence for a synchronous climatic change elsewhere in Europe and on other continents is presented. The discussed oscillation of 14C at ca 2650 BP was probably caused by a reduced solar activity which could also have been the forcing mechanism behind the recorded cool event. An important effect of a reduced solar activity is an increase in the cosmic-ray flux leading to a higher production of 14C in the stratosphere and an increased cloudiness and precipitation. The reduction in solar (ultraviolet) radiation may also have lead to a decline in ozone production in the lower stratosphere and the latter process may have been the trigger mechanism responsible for the inferred climate changes, because a decrease in the stratospheric ozone content produced cooling of the lower stratosphere by the absorption of less sunlight. This could have resulted in a decrease of the latitudinal extension of the Hadley Cell circulation and a possible associated weakening of monsoons, which would be consistent with inferred drier conditions in the tropics. Furthermore an expansion of the Polar Cells and a repositioning of the main depression tracks at mid-latitudes towards the equator may have occurred. This probably has resulted in the reconstructed change to cooler and wetter conditions at middle latitudes in both hemispheres around 2650 BP.
Kilian, M.R., van der Plicht, J. and van Geel, B. (1995) Dating raised bogs: new aspects of AMS 14C wiggle matching, a reservoir effect and climatic change, Quaternary Science Reviews 14, 959-966. * Magny, M. (1993) Un cadre climatique pour les habitats lacustres prehistoriques?, Compte Rendu de l'Academie des Sciences Paris 316, Serrie II, 1619-1625. * Svensmark, H. and Friis-Christensen, E. (1997) Variation of cosmic ray flux and global cloud coverage - a missing link in solar-climate relationships. Journal of Atmospheric and Solar-Terrestrial Physics 59, 1225-1232. * van Geel, B. (1978) A palaeoecological study of Holocene peat bog sections in Germany and the Netherlands, Review of Palaeobotany and Palynology 25, 1-120. * van Geel, B. and Mook, W.G. (1989) High-resolution 14C dating of organic deposits using natural atmospheric 14C variations, Radiocarbon 31, 151-156. * van Geel, B., Buurman, J. and Waterbolk, H.T. (1996) Archaeological and palaeoecological indications of an abrupt climate change in The Netherlands, and evidence for climatological teleconnections around 2650 BP, Journal of Quaternary Science 11, 451-460.
BAS van GEEL is a paleo-ecologist in the research group Palynology and Paleo/Actuo-ecology at the University of Amsterdam, The Netherlands. He studies pollen and other fossil objects from peat deposits, lake sediments and archaeological sites and reconstructs changes in vegetation and climate in the past. Climate change during the Bronze Age and especially the abrupt climate change at the Bronze Age/Iron Age transition was already one of the main topics in his Ph.D.- thesis (1976). Recently he applied the new strategy of 14C wiggle-match dating of peat deposits, which appeared to be of crucial importance for understanding the world-wide dramatic changes in climate around 800 BC. Changes in solar activity and related changes in cosmic rays, 14C-production, cloudiness and precipitation were very important factors for climate change in the past, and such changes in solar activity probably will also dominate climate change in the future. Bas van Geel is (co)author of more than 40 papers in international scientific journals and 6 chapters in books.