Rapid glacial-to-interglacial transitions in Europe and southwestern Asia during the Quaternary

Background/Question/Methods Southern and central Europe contain a number of long Quaternary pollen records covering multiple climatic cycles. These show that nearly all glacial-to-interglacial transitions are characterized by the rapid expansion of deciduous Quercus, often accompanied with an expansion of Corylus, and a rapid decrease of herbaceous plants characteristic of dry steppes. Forest succession during the later part of the interglacial intervals, however, substantially differ from one interglacial to another. Until recently, the lack of long pollen records from circum-European regions has prevented a comparison of European vegetation dynamics with adjacent territories.  

Results/Conclusions A new 200 000-year pollen record from Lake Urmia in NW Iran now allow such a comparison to be made. This record indicates that, unlike Europe, the vegetation dynamics during the beginning of the Last Interglacial was different from that recorded during the early Holocene. Whereas the Last Interglacial is marked by a rapid replacement of dry steppes with oak/juniper woodlands, the expansion of these vegetation types during the Holocene occurred several thousand years later ca 6.2 ka BP. Instead, the early Holocene is marked by a prominent expansion of grass steppes and a considerable amount of micro-charcoals indicating more frequent fire events. The modern atmospheric circulation pattern over SW Eurasia suggests that this marked difference in early interglacial vegetation dynamics most probably derives from variations in the complex interaction between the Indian Summer Monsoon (ISM) and the North Atlantic Oscillation (NAO). While NAO is the main control on the distribution of humidity for the European continent, its role diminishes eastwards e.g. in the continental Middle East where the long-distance effect of the ISM modulates the seasonality of precipitation, the latter being the main driving factor in the growth and expansion of the dominant oak species in the Zagros–Anti-Taurus Mountains. We propose the hypothesis that the ISM intensification during the early Holocene in continental Middle East reduced late spring precipitation and prolonged the duration of summer drought, thereby impeding the development of oak woodland. In contrast, during the early part of the Last Interglacial, a weaker ISM increased the influence of the NAO, with higher late spring rainfall favoring the early expansion of oak woodland and the decline of steppe vegetation. Further investigations are needed in the field of pollen-vegetation calibrations, bioclimatic requirements of the major biomes and pollen analysis of long sedimentary records from other parts of the Middle East to consolidate this hypothesis.