A new cryptic Refugium: the Gran Zebrù Nunatak (2900 m asl) during the last glacial maximum in the European Central Alps

In this paper, we present the finding of a new cryptic refugium in the Central European Alps at 2900 m asl, revealed through multiproxy analyses of paleolacustrine sediments at the foot of Gran Zebrù peak. The sediment core revealed the presence of a paleolake from ca 30,000 cal BP until 18,500 cal BP, when a hiatus in sedimentation occurred, followed by an intense weathering phase lasting until 13,700 cal BP. Lake sedimentation then restarted and continued until 9000 cal BP. Throughout this interval, glaciers surrounded the nunatak and flowed into the valleys, as also occurred during at least three subsequent periods (13,700-13,000, 11,500–11,000 and 10,000–9600 cal BP.), when the presence of trees, and therefore of a cryptic refugium, was demonstrated by plant macroremains. Since at least 26,700 cal BP, these climatic phases were also recorded by the second principal component (PC2) of the sediment geochemistry, which explains approximately 20% of the total variance. PC2 is characterized by a strong positive loading of Ca/Ti (0.921) and a weaker negative loading of Si/Ti (−0.364), which closely mirror the temperature reconstructions based on Greenland ice cores. • During Last Glacial Maximum nunatak created oasis of biodiversity in the middle of the Alps. • PC2 values of the sediment geochemistry can be used as a good proxy of the air temperature. • During the LGM the dominance of Staurosira subsalina suggested a very shallow and warm lake. • The recovery of a wood fragment in the core of the LGM supports the cryptic-refugium conditions.