VEGETATION HISTORY AND PALEOCLIMATOLOGY OF THE MIDDLE HOLOCENE - POLLEN ANALYSIS OF ALPINE PEAT BOG SEDIMENTS, COVERED FORMERLY BY THE RUTOR GLACIER, 2510 M (AOSTA VALLEY, ITALY)

In a new profile pollen and spores have been analysed near the actual front of the Rutor Glacier, 2510 m (Aosta Valley, Italy). The profile gives evidence of the following sedimentary phases: glacigenic during Preboreal-Boreal (clay, silt), limnic at the transition Boreal/Older Atlantic (gyttja), telmatic-terrestric during the Older Atlantic (Cyperaceae peat), glacigenic during the Younger Atlantic (moraine), terrestric at the transition of Younger Atlantic/Subboreal (peat), and glacigenic during the Subboreal (moraine). The organic sedimentation started at 8270 BP (Boreal). The peat bog growth began in 6735 BP (Older Atlantic) and ended in 6055 BP. During the Younger Atlantic the peat bog was covered by the glacier (advance during the Piora I-Stage). The peat-forming period between 6735 and 6055 BP represents the optimal phase of warm and dry climate, with a timber-line situated 200-300 m higher than that of today and with the highest pollen influx values. During this time the subalpine larch, the stone pine forest, the montane fire, and the lime formations developed. The amounts of pollen influx are as follows: peat 6795 per cm3 yr-1, gyttja 1457 per cm3 yr-1, and clay only 14 per cm3 yr-1. The rates of growth for peat are 0.5 mm per year and for gyttja only 0.08 mm per year. The Holocene climate optimum lasted between 8300 and 6000 BP. The average July temperature was 1.5-3-degrees-C higher than that of today. There is a certain amount of evidence of late-glacial and mid-Holocene glacier extensions and contractions, of which the oscillations are well documented.