Climate warming and vegetation response after Heinrich event 1 (16 700-16 000 cal yr BP) in Europe south of the Alps

Chironomids preserved in a sediment core from Lago di Origlio (416ma.s.l.), a lake in the foreland of the Southern Swiss Alps, allowed quantitative reconstruction of Late Glacial and Early Holocene summer temperatures using a combined Swiss-Norwegian temperature inference model based on chironomid assemblages from 274 lakes. We reconstruct July air temperatures of ca. 10 degrees C between 17 300 and 16 000 cal yr BP, a rather abrupt warming to ca. 12.0 degrees C at ca. 16 500-16 000 cal yr BP, and a strong temperature increase at the transition to the Bolling/Allerod interstadial with average temperatures of about 14 degrees C. During the Younger Dryas and earliest Holocene similar temperatures are reconstructed as for the interstadial. The rather abrupt warming at 16 500-16 000 cal yr BP is consistent with sea-surface temperature as well as speleothem records, which indicate a warming after the end of Heinrich event 1 (sensu stricto) and before the Bolling/Allerod interstadial in southern Europe and the Mediterranean Sea. Pollen records from Origlio and other sites in southern Switzerland and northern Italy indicate an early reforestation of the lowlands 2000-1500 yr prior to the large-scale afforestation of Central Europe at the onset of the Bolling/Allerod period at ca. 14 700-14 600 cal yr BP. Our results suggest that these early afforestation processes in the formerly glaciated areas of northern Italy and southern Switzerland have been promoted by increasing temperatures.