Abstract
The Weichselian Lateglacial (~14.7-11.7 ka BP) is marked by major climatic and environmental changes in the North Atlantic region. The long-term warming trend arising during the Lateglacial was interrupted by a number of rapid climate oscillations, including the cold Younger Dryas Stadial (~12.8-11.7 ka BP) and at least two centennial-scale
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cold oscillations (Greenland Interstadial events 1b and 1d) during the relatively warm Interstadial (~14.7-12.8 ka BP). These cold phases are thought to originate from meltwater pulses into the Atlantic Ocean, leading to a decreased North Atlantic thermohaline circulation. Thus, the most pronounced cooling during these cold phases is expected in regions close to the Atlantic Ocean. Understanding how these changes propagated inland will provide insights into the influence of the Atlantic Ocean on climatic and environmental changes in northwest Europe. The main research objective of this study was to establish how and to what extent Lateglacial climatic changes, and more specifically changes in summer temperature, were associated with vegetation changes in northwest Europe. To achieve the objective, calcareous lake sediment records from western Ireland, the Netherlands and northeast Germany were analysed for multiple proxies to reconstruct temperature and vegetation changes along a west-east transect through northwest Europe. Mean July air temperatures were inferred from the fossil chironomid assemblages, while pollen analyses on the same records enabled a direct comparison with Lateglacial vegetation development. Further, results from these records were integrated with additional Lateglacial northwest European records. Reconstructed average Interstadial mean July chironomid-inferred temperatures (C-IT) varied from ~11.5°C in the British Isles to ~16.5°C in continental northwest Europe, while the minimum mean July C-IT for the Younger Dryas varied from ~7.5°C in the British Isles to ~11.0°C in continental northwest Europe. Comparison with northwest European vegetation development suggests a strong link between summer temperatures and vegetation, even though other climatic factors presumably also affected vegetational development to a considerable degree. Reconstructed mean July temperatures for the Interstadial that exceeded 10-12°C facilitated expansion of birch and pine forests in large parts of northwest Europe, while a summer temperature decline to well below 10-12°C during the Younger Dryas in large parts of the British Isles probably strongly limited forests. The chironomid records also provided evidence for the occurrence of centennial-scale cold oscillations during the Interstadial throughout northwest Europe, correlated to Greenland Interstadial 1b and 1d. It appears that mainly tree and shrub communities were affected by these climatic oscillations, probably due to a combination of climatic factors. Finally, chironomid-based inferences indicate that July temperature gradients were comparable to the present-day summer temperature gradient during the Interstadial, while results provide evidence for an increased west to east gradient during the coldest phase of the Younger Dryas compared to the present-day gradient. This may indicate that oceanic influences were more important during the Younger Dryas than during the warm Interstadial.
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