Abstract/Summary

This thesis presents reconstructions of the vegetation and climate history of the Eastern Mediterranean region during the Holocene, based on a comprehensive pollen data set, the development and application of new reconstruction techniques, and rigorous statistical analyses. The Eastern Mediterranean is of interest because it experienced one of the earliest transitions to permanent agriculture, facilitating the expansion of human populations in the region. Nevertheless, there were significant changes in population across the region during the Holocene, as witnessed by rise and often abrupt decline of specific ancient civilizations. There is an ongoing debate about whether climate changes influenced vegetation, which in turn affected the availability of resources for ancient societies, or if human activities altered vegetation and subsequently those changes impacted the climate. The current uncertainty arises from a focus on records from individual sites and the lack of a comprehensive regional understanding of vegetation and climate history. Understanding historical events in the Eastern Mediterranean region necessitates robust reconstructions of the vegetation and climate in the region during the last 12000 years in order to disentangle the causal factors underpinning observed changes in vegetation and societal events in the region. The thesis leverages the Eastern Mediterranean-Black Sea-Caspian Corridor (EMBSeCBIO) project, which collected modern and fossil pollen data for the Eastern Mediterranean region, to develop or apply statistically based predictive models for vegetation and climate reconstruction. Standardized data syntheses, when analysed using statistical techniques, are an effective tool for generating objective regional environmental reconstructions, addressing uncertainties and reducing analytical biases. In this thesis, a new technique for reconstructing vegetation is developed and calibrated by using a modern pollen training dataset to characterize the composition and variability in the abundance of pollen taxa within different biomes. The new technique performs better than existing vegetation reconstruction methods, is more robust when applied to pollen time series, and allows the explicit recognition of assemblagesthat have no analogue in the modern vegetation. This new method was then applied to fossil pollen data from the Eastern Mediterranean region to predict changes in vegetation types through the Holocene. These reconstructions were used to address several controversies about the regional vegetation history. These analyses showed that the greatest extent of non-analogue vegetation occurred during the early Holocene, the timing of re-afforestation was broadly synchronous across the region, that the maximum expansion of temperate deciduous forest occurred between 5.5 and 5 ka, and that the increase in drought-adapted vegetation in the late Holocene was regionally heterogeneous. The thesis addresses climate changes during the Holocene by applying a recently developed method, frequency-weighted (fx) Tolerance-weighted Weighted Averaging Partial Least Squares (fxTWA-PLS) to the EMBSeCBIO database in order to reconstruct four climate variables crucial to vegetation growth and relevant to human well-being. Reconstructions were obtained for the mean temperature of the coldest month (MTCO), mean temperature of thewarmest month (MTWA), growing degree days above a threshold of 0° C (GDD0 ) and plant-available moisture, represented by the ratio of actual to equilibrium evapotranspiration (α), for 71 individual pollen records. The reconstructions were compared with independent evidence for climate changes and with climate model simulations in order to identify the drivers of observed changes. The glacial-Holocene transition and the early part of the Holocene were characterised by conditions colder and drier than present. Rapid increases in temperature and moisture occurred between ca 10.3 and 9.3 ka,which is considerably after the end of the Younger Dryas. MTCO showed a gradual increase from 9 ka to the present, indicating that winter temperatures were forced by orbitally induced increases in insolation during the Holocene. MTWA also showed an increasing trend from 9 ka to 5 ka, followed by a gradual decline towards present-day conditions. The initial increase in summer temperature is not consistent with a response to orbital forcing, but comparison with climate model simulations suggests that a delayed response to summer insolation changes is likely a reflection of the persistence of the Laurentide and Fennoscandian ice sheets; summercooling post-5 ka is consistent with the expected response to insolation changes. After the initial rapid increase in plant-available moisture between 11 and 9.3 ka, subsequent changes were small. This thesis represents the first effort to obtain a comprehensive and quantitative reconstruction of regional vegetation and climate changes in the Eastern Mediterranean. In addition to providing a secure foundation for documenting the environmental history of the region through the Holocene, it also provides the necessary basis for understanding historical events and their potential impact on the regional vegetation. It has resolved a number of persistent controversies about the nature of regional vegetation changes and provides a resource to explore the remaining controversies about the interaction between climate, vegetation and human activities in a region that is of great interest to multiple scientific communities.