Millennial-Scale Climate Variability Potentially Shaped the Early Interglacial Optimum in Southern Europe

The seasonal and latitudinal distribution of insolation is considered the main factor controlling the magnitude and timing of interglacial periods. However, despite small differences in insolation forcing, vegetation and hydrology in southern Europe during past interglacials are variable and the gradual change in insolation cannot explain the observed short-lived forest optimum. Here we focus on vegetation and hydroclimatic changes at orbital- and suborbital-scales in southwestern Europe during two past warm interglacial periods with reduced ice-sheets, namely Marine Isotope Stages (MIS) 9e and 5e. We provide new pollen and sea surface temperatures records for MIS 9e from IODP Site U1385. This pollen record shows a forest expansion in southern Iberia over a 14 ky interval, bracketed by the millennial-scale cooling events of Termination IV and MIS 9d. Between 334.5 and 332.5 ka, forest expansion reached a maximum, suggesting increased winter moisture during early MIS 9e. Model-data comparison for MIS 9e and 5e shows that insolation is the main driver of the orbital-scale vegetation and precipitation changes in Iberia, atmospheric CO2 forcing playing a secondary role. The high-frequency component of the MIS 9e and 5e forest timeseries highlights the early interglacial forest and precipitation maxima as prominent suborbital events lasting similar to 2 ky. We propose that the primarily insolation-driven forest and precipitation optima were fostered by the non-equilibrium conditions generated by the millennial-scale deglacial variability during the early interglacials. Additionally, the early end of these optima may have been favored by a cooling and drying event that is part of the persistent intra-interglacial variability.