Holocene vegetation history and quantitative climate reconstructions in a high-elevation oceanic district of the Italian Alps. Evidence for a middle to late Holocene precipitation increase

摘要

We reconstructed the vegetation and climate history during the last 10 ka in a high-elevation sedimentary record (Armentarga peat bog, 2345 m asl) on the southern flank of the European Alps through the study of paleoecological and sedimentary proxies. We included a specific elevational transect of modern Pollen Accumulation Rates for timberline-forming trees and shrubs (Alnus viridis, Pinus sylvestris/mugo, Pinus cembra). Quantitative reconstructions of July temperature and annual precipitation were obtained by applying numerical transfer functions built on an extensive pollen-climate calibration set from the European Alps. Changes in elevational vegetation arrangement were primarily driven by phases of precipitation increase, and to a lesser extent by millennial-scale temperature changes already known from glacier, timberline, chironomids and speleothem records at Alpine scale. Changes in pollen-inferred annual precipitation occurred in three main steps. An early Holocene moderately humid phase is mirrored by the early spread of Alnus viridis dwarf forests. Precipitation started to increase at 6.2 ka cal BP. A further, prominent step forward at the Middle to Late Holocene transition led to the high values of snowfall and runoff characterizing today's oceanic elevational climates of the outer Italian Alps. This change led to timberline depression and grassland expansion. Locally, human impact was weak at the Late Neolithic/Bronze Age transition. This event correlates with lake level oscillations in the northern Mediterranean borderlands, suggesting intensification of southern air masses conveyed by Tyrrhenian cyclones towards windward districts. (C) 2018 Elsevier Ltd. All rights reserved.