| Summary: | The Middle Pleistocene Transition (MPT) is a great significant period of Quaternary environmental evolution, characterized by a change in the dominant glacial-interglacial cycles from ~40 kyrs to ~100 kyrs, a significant increase in global ice volume, a decrease in sea surface temperature, and an intensification of land aridity. While the driving mechanisms of the MPT are still unclear, the most likely hypotheses are related to ice-sheet dynamic feedbacks, such as ice albedo, precipitation at the ice margins, elevation-temperature and the regoliths. Here we focus on the “precipitation at the ice margins†hypothesis. To test this hypothesis, appropriate proxy reconstructions and paleoclimate simulations are needed. The SW Iberian margin is directly under the influence of the westerlies which was an important factor controlling the moisture arrival at the ice margins. In my presentation I will show the pollen-based vegetation and winter rainfall changes in the adjacent landmasses of the southwestern Iberian margin during the MPT. Changes in the reconstructed vegetation from IODP Site U1386 (1.2-0.8 Ma), combined with IODP Site U1385 (0.8-0.67 Ma), and their comparison with changes in the North Atlantic Ocean thermal gradient reveal the variability in the intensity and position of the westerlies and in the position of the moisture source, respectively. Preliminary pollen results reveal a long-term decreasing trend in the Mediterranean forest cover during MIS 31-20 (1.1-0.8 Ma), associated with long-term southward migration of the thermocline water source. This trend abruptly shifted northward at 0.8 Ma, and probably was related to progressive northward shift of the westerlies that bring moisture to the margin of the ice sheets feeding the ice caps, and leading to the shift of the dominant ice sheet cyclicity from 41 kyrs to 100 kyrs.
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