Late Quaternary glacial-interglacial cycles as revealed by the interfluve architecture of the Arno palaeovalley (Northern Tuscany, Italy)

Palaeovalley systems developed during the last glacial-interglacial cycle and now buried beneath modern delta-coastal plains occupy a central place in the debate about incised valley systems (IVSs) and sequence stratigraphy. However, the stratigraphy of Last Glacial Maximum (LGM) interfluvial areas has attracted little attention compared to the internal fills, because deprived of expanded fine-grained successions and affected by hiatal surfaces, physically represented by palaeosols. Despite remarkable stratigraphic condensation, interfluves are prone to preserve the depositional record of successive transgressive-regressive cycles, as they are not affected by deep river incisions, unless shifts in the valley axis through time. In this study, we analysed the late Quaternary stratigraphic architecture of the southern interfluve of the LGM Arno palaeovalley (Northern Tuscany, Italy) by means of facies-based stratigraphic correlations involving a wealth of sedimentary cores. Facies characterization benefitted from the analysis of the meiofauna (benthic foraminifers and ostracods) found within reference cores. Palynological data from one core supplied important information about vegetation- derived palaeoclimate conditions, supporting the 14C-based chronological framework. Above the laterally extensive LGM palaeosol, ~15 m below sea level (b.s.l.), a progradational stacking pattern of facies testifies the outbuilding of the delta system after the Holocene maximum marine ingression. Below, down to ~40-50 m b.s.l., the interfluve architecture is typified by a few m-thick floodplain clays with closely-spaced palaeosols overlying a weakly pedogenised paludal-lagoon succession, superimposed via a further palaeosol on a tens m-thick alluvial succession, including thick, amalgamated fluvial-channel sands. Pollen (Mediterranean forest with increasing optimum-like conditions) points to a MIS 5e age for the lagoon deposits. On the other hand, alluvial intervals contain pollen assemblages indicative of an open/sparse pine forest typical of glacial periods, specifically the last glacial and the penultimate glacial. Data integration along stratigraphic profiles allowed to: i) relate the MIS 5e lagoon to the inundation of an older interfluve associated laterally to a km-wide channel belt likely formed during the penultimate glacial period (MIS 6) and ii) furnish insights on the evolution of a coastal plain at the onset of glacial conditions, through its emersion and transformation in an interfluvial area subject to pedogenesis. Our data suggest a very slowly subsiding setting for the southern portion of the Arno delta plain and, accordingly, highlight a complex stratigraphic architecture characterised by recurring channel incisions and interfluve development mainly controlled by glacio-eustatic oscillations