High winter precipitation in Southern Europe during the Last Glacial Maximum as inferred from pollen sequences

Abstract

The climate of the Last Glacial Maximum (LGM) in southern Europe has been thoroughly documented. Still, discrepancies exist between the climatic information provided by pollen data, which mainly indicate steppe vegetation and thus arid conditions, and some model simulations that rather suggest large amounts of precipitation, notably in winter. Here, we investigate the regional vegetation and seasonal climate from 28 ka to 10 ka using published pollen sequences from five sites, including core SU81-18 located on the western Iberian Margin, ODP Site 976 in the Alboran Sea, Padul in Southeastern Spain, Lago Grandi di Monticchio (Monticchio) in southern Italy and Xinias in Greece. Pollen-based estimates of seasonal temperatures and precipitation are reconstructed using the Modern Analogue Technique with the latest version of the Eurasian Modern Pollen Database (EMPD2), which comprises numerous samples from cold environments and provides potential analogues for the LGM. The results highlight a coherent regional pattern of vegetation dynamics that appears tightly linked to winter temperature and precipitation influenced by the westerlies. Most importantly, we reconstruct high winter and annual precipitation during the LGM at all study sites, with values comparable to, or even exceeding, those estimated for the Bølling/Allerød. Our results indicate high variability in rainfall, particularly along the Atlantic margin, and lower variability further east, notably in the Alboran Sea, suggesting a climatic front located between the two regions. Two phases of increased precipitation are evidenced in our records, one from 24 to 22.5 ka and the other from 21.5 to 19.5 ka. The timing of these humid intervals is in phase with two main glacier advances in the European Alps, suggesting a consistent regional atmospheric pattern. Increased precipitation is attributed to the southward displacement of the jet stream and the westerlies due to the maximum extension of the Northern Hemisphere ice sheets. Conversely, the dry phases might correspond to a weakening or latitudinal shift of the westerlies. Indications of tenuous arboreal vegetation growth during the LGM are observed at the Iberian sites, but not at Monticchio and Xinias, where a cold climate may have prevented forest development. We hypothesize that the classic Mediterranean gradient with increasing dryness and temperature from west to east may have been amplified by the influence of the Fennoscandian and Alpine ice sheets due to the Rossby wave breaking of the jet stream west of the Alps.