Siliceous algae-based seasonal temperature inference and indicator pollen tracking ca. 4,000 years of climate/land use dependency in the southern Austrian Alps

Diatom and chrysophyte cyst-based reconstructions of the dates of spring and autumn lake-mixing enabled us to estimate spring (ST_anom) and autumn (AT_anom) temperature anomalies as well as ice-cover of the last ca. 4,000 years in a lake sediment core (Oberer Landschitzsee, 2,076 m a.s.l.) from the southern slopes of the Austrian Central Alps. The two independently inferred temperature anomalies were significantly correlated. On average, spring and autumn temperatures were lower during the two millennia B.C than during 0–1,300 A.D. Marked spring and autumn temperature minima occurred at about 1,300 and 600 B.C. At about 1,300 A.D, ST_anom declined again. Spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values and paralleled tree-line and glacier fluctuations. The de-coupling of autumn and spring climates, which began during the Medieval period, might indicate changes in major circulation modes. It was assumed that the North-Atlantic influence, triggering winter-rain climate in the Northern Mediterranean, became weaker during Medieval times, resulting in a trend towards warmer autumns and overall more continental climate conditions in the study area. Four pulses of land use, inferred from indicator pollen, occurred during (1) the Early to Late Bronze, (2) the transition from Late Bronze to Early Iron Age (Hallstatt), (3) Late Iron Age (La Tène, Celtic time) to Roman times, and (4) during high to late Medieval times. Climate seemed to be an important, though complex, trigger of Alpine land use.