Extreme Drought around 900ka in Southwestern Iberia: Insights from IODP Pollen Records

The Middle Pleistocene Transition (MPT) is a key period of Quaternary environmental evolution, characterized by a change in the dominant pacing of glacial-interglacial cycles from ~40 kyrs to ~100 kyrs, a significant increase in global ice volume and land aridity, and a decrease in global sea surface temperature. 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, and the decrease of CO2 concentration. Here we focus on the "precipitation at the ice margins" hypothesis. To test this hypothesis, appropriate proxy reconstructions and paleoclimate simulations are needed. The Southwestern Iberian margin is directly under the influence of the westerlies which was an important factor controlling the moisture arrival at the North Hemisphere ice margins. Here we present new pollen records from International Ocean Discovery Program (IODP) Sites U1386 and U1385, which show vegetation variations and winter rainfall changes in Southwestern Iberia during the MPT. Our results show that there was a long-term decreasing trend in the Mediterranean forest (MF) cover during MIS 31-22 (1200-870 ka), that parallels the long-term decrease of the boreal summer insolation at 65°N, with an extreme dryness between 930-870 ka. This period of strongest dryness coincides with the highest North Atlantic Ice Rafted Debris (IRD) concentration, weakest Atlantic Meridional Overturning Circulation (AMOC) and highest dust input from north Africa to the Mediterranean Sea during MIS 24-22, and coeval with the southward migration of the thermocline water source, related to the position of the moisture source, lower than the Iberian margin. After 870 ka, the trend was reversed and the MF thrived again during MIS 21, associated with the increase in insolation and AMOC and the northward shift of the moisture affecting Iberia. During MIS 20, the strong reduction of the MF during this glacial reflects the second extreme regional drought, probably due to the maximum ice volume, supported by the heaviest benthic foraminifer δ18O (δ18Ob) values, and the southernmost position of thermocline water source at around 830 ka. From MIS 19 to MIS 18, the insolation remained low, but the MF increase concomitant with the northward migration of the moisture source that bring precipitation to Iberia through the Westerlies. The increase in insolation, the decrease in ice volume and the optimal position of the moisture source close to Iberia, led the maximum expansion of the MF during MIS 17. A pronounced contraction of the MF after 700 ka suggests the north shift of the westerlies, leading to the strong 100-kyr cycles. Furthermore, the wavelet analysis on the MF, δ13C and δ18Ob records has showed the dominant cyclicity is in the 80-kyr band between 1200 ka to 1050 ka, and faded away from 1050 ka to 840 ka including the mega drought period. After 840 ka, the obliquity and precession cyclicities emerged, and even the 5.5-kyr cyclicity can be found in the MF. Our data suggest the strong IRD-derived freshwater fluxes blurred the predictable North Atlantic orbital climate cyclicity during 930-840 ka.