Arid climate 2.5 Ma in the Plio-Pleistocene Valdarno Basin (Northern Apennines, Italy)

Distinctive, eolian-dominated sandy deposits 35 m thick exposed in the SW margin of the Plio-Pleistocene Valdarno Basin in the Northern Apennines, Italy, reflect an unusually arid period in the region. Paleomagnetic investigation and 40Ar/39Ar dating of a tephra layer, 10 m above the eolian sand, suggest that the sand was deposited between about 2.7 to 2.4 Ma. This age is comparable to the global climatic deterioration that occurred at the Middle-Upper Pliocene transition. Eolian sediments are represented by cross-bedded sand, rippled sand and horizontal-bedded sand. The alluvial deposits, associated with the eolian sediment, consist of sheet-like beds of coarse sand and erosively based pebbly sand bodies. Eolian sand, together with fluvial sandy deposits, were formed in the medial-distal portion of an alluvial system, and represent the Rena Bianca Sand Unit. The eolian and alluvial deposits are arranged into sedimentary cycles (2-6 m thick), displaying wetting-drying-wetting upward trend recording second-order climatic oscillations of ca 40 ka. Each cycle is included between major bounding surfaces, defined by iron mineral encrustations. These bounding surfaces represent periods of reduced eolian sand accumulation caused by a rising water table. During the 2.5 Ma global cooling the Valdarno Basin experienced a period of arid conditions owing to the influence of cold and dry eastern and northeaster trade wind. In contrast, during the Pleistocene, the uplift of the Pratomagno Ridge protected the Valdarno Basin from eastern and north-eastern trade winds, giving rise to relatively warm and wet conditions, and mitigating the regional and global climatic changes.