The last glacial coldest extreme: the LGM climate at the southern alpine fringe reconstructed from a high-resolution palaeoecological record.

The Last Glacial Maximum (LGM) represents the coldest extreme of the last glacial cycle. During the LGM glaciers extended over large areas of the Northern and the Southern Hemispheres, building up continental ice sheets complexes (Eurasian, North-American, Patagonian, Antarctic and others). Sandy deserts expanded. Globally-averaged changes describe a period of relatively low sea level decrease between 35-31 ka cal BP, a rapid fall between 31-29 ka cal BP, constant or slowly increasing ice volumes between 29-21 ka and the onset of deglaciation at ca. 21-20 ka cal BP. Biological proxies such as micro- and macroscopic plant remains can be used to reconstruct LGM terrestrial biomes9. Progresses in the definition of modern pollen-climate calibration models and their application to fossil data10 provide the chance to look at palaeoecological records not only for their power to reconstruct vegetations, but also as descriptors of past climates. We present here quantitative reconstructions of the LGM climate at the southern alpine fringe based on a high-resolution record obtained on a sediment core from Lake Fimon (23 m asl, venetian Plain, northern Italy). The core was analyzed for its palynological content; 14C AMS dates obtained on terrestrial plant remains (seeds, charcoal fragments, periderms, twigs, needles, etc.) provide a robust age-depth model which sets the analyzed sequence between ca. 30 - 12,5 ka cal BP. Mean time resolution of the palaecological record is 1 sample/180 yrs over the whole period, increasing to 1 sample/160 yrs for the LGM. Up to ca. 27,5 ka cal BP the pollen record from Lake Fimon speaks for boreal forests dominated by Pinus sylvestris/mugo (Scots/Dwarf mountain pine) covering the valley floor, the adjacent hills and possibly largely extending over the nearby venetian plain. Plant communities with Artemisia (sagebrush), Juniperus (juniper) and Larix (larch) characterized the landscapes up to ca. 17,5 ka cal BP, followed by the renewed expansion of mixed pine - birch (Betula) forests, with broadleaved temperate trees (oaks, linden, hazel and elm) thriving from the onset of the Bølling-Allerød interstadial. In order to produce pollen-based quantitative reconstructions of January and July temperatures (Tjan and Tjul), and annual precipitation (Pann) from the Lake Fimon pollen data, the Modern Analogue Technique (MAT) was adopted. Starting from a modern calibration training set of 3019 sample sites from the larger European Modern Pollen Database (EMPD11), a DCA (Detrended Correspondence Analysis) gradient analysis revealed similarities between the Lake Fimon fossil spectra with modern pollen assemblages from the Euro-Siberian orobiome (Ural Mountains, southern and western Siberia). In this area, the humid semi-continental siberian climate meets with drier continental air masses, producing a mosaic of altitude and climate-depending vegetations such as taiga, hemiboreal forests, steppes, forest-steppes and tundra, recalling full-glacial plant communities of Europe. For each fossil pollen spectra, the 5 best modern analogues were identified among the 239 sample sites of the Euro-Siberian orobiome. Climate parameters associated to the 5 best analogues were weighted-mean according to their respective similarity coefficient and attributed to the fossil sample. The high-resolution of the palaeoecological record and of the resulting climate reconstructions allow comparison and fine-tuning with other proxy records of LGM climates at alpine and global scale. Relatively warmer Tjan reconstructed from the Fimon pollen data at 27,5 - 27,7 ka cal BP chronologically corresponds to lighter ?18O isotopic values and lower dust concentrations in the NGRIP ice core, related to Greenland Interstadial (GI) 3. A 2,5 millennia-long phas