LGM climate and vegetation at the southern Alpine foreland: the view from a high-resolution paleoecological record

Long and continuous lake records are renown archives of ecosystems, climate and anthropic history over different time scales. They accurately witness paleoenvironmental and paleoclimate variability, as shown by hundreds of researches worldwide. Northern Italy hosts the first continuous archive of the environmental and climate history of the southern alpine foreland; the core drilled at Lake Fimon (Venetian Plain) contains the whole Late Pleistocene represented by lake sediments, making this record an exceptional evidence of the effects of climate variability and humans on past biodiversity (Pini et al., 2010). We focus here on a specific time interval, corresponding to the coldest extreme of the last glacial cycle, the LGM (Last Glacial Maximum). During the LGM, Earth experienced the development of continental ice sheets complexes over Antarctica, Eurasia, North and South America. Sea-level recorded phases of relative stability followed by abrupt decreases. Terrestrial ecosystems faced major reorganizations. The Lake Fimon paleoecological record describes vegetation changes that occurred regionally during the LGM. Boreal forests thrived for most part of the Middle Würm up to ca. 27.5 ka BP, replaced by open woodlands with xerophytes until the first part of the Lateglacial, and later on followed by broad-leaved forests. Locally, a sedge mire developed between ca. 31-27.5 ka BP, then the mire was buried beneath detrital deposits formed under deep-water conditions (Monegato et al., 2011), an evidence for the abrupt onset of fluvioglacial activity related to the culmination of the Brenta Glacier within the valley (Rossato et al., 2018). Lake level rose suddenly, triggering slope instability and higher sedimentation rates throughout the LGM. Pollen-based quantitative reconstructions of past climate parameters (January and July temperatures, and annual precipitation) were obtained through the application of the Modern Analogue Technique (MAT). For each fossil pollen spectra, the 5 best modern analogues were identified among a calibration set of 239 sample sites of the Euro-Siberian orobiome extracted from the larger EMPD (European Modern Pollen Database: Davis et al., 2013). Climate parameters associated to the 5 best analogues were weighted-mean according to their respective similarity coefficient and attributed to the fossil sample. Climate reconstructions were then compared and fine-tuned with other proxies of climate at alpine and global scale. Warmer Tjan reconstructed at Fimon are related to Greenland Interstadials 3 and 2, with minor shifts between chronologies remaining within the error associated with the radiocarbon determination. Colder reconstructed Tjan seem to be indeed the expression on land of cooling related to massive iceberg discharge in the North Atlantic (Heinrich events 2 and 1, the latter in the early Lateglacial). As soon as the Atlantic Meridional Overturning circulation resumed, pollen-based climate reconstructions point to warmer and moister conditions, favoring the spread of thermophilous forests from the Lateglacial interstadial onwards.