Mid-late Holocene fluvial evolution of the Arno coastal plain (Tuscany, Italy)

Mid-late Holocene successions buried beneath modern deltaic and alluvial plains represent excellent archives for developing high-resolution stratigraphic studies and reconstructing palaeogeographic changes occurred in the recent past. A large number of core data and piezocone tests were used for the first time to reconstruct the midlate Holocene palaeohydrography of the Arno plain (western Tuscany, Italy), considerable for its relations with protohistoric and historic human activities. Special attention was focused around the city of Pisa, characterized by a very high density of subsurface data and by the presence of several archeological sites spanning from the Iron Age to Late Medieval times. The data set consists of 35 continuously-cored boreholes (10-m100 deep) and hundreds of well logs. Sediment mean grain size, sand composition, texture, color, sedimentary structures and accessory materials combined with stratigraphic correlations along cross-sections, transverse to the Arno River course, allowed to identify the main depositional facies and architectural elements recorded within the upper 15- 20 meters of the Late Holocene sequence. The overall late highstand fluvio-deltaic succession, underlain by laterally extensive lagoonal deposits, is composed of isolated to amalgamated, 2-8 m-thick channel sand bodies within predominant flood-basin fine-grained deposits. Four channel-belt units (I-IV), clustered at distinct stratigraphic levels ranging in age from late Neolithic time to the Middle Ages, were recognized defining a crono- stratigraphic succession of fluvial events. Although an overall aggradational architecture is documented, as expected from strongly subsiding areas under highstand sea-level conditions, Units I and II erode the underlying lagoonal deposits up to 4 m. Proximity to the coast suggests a possible control driven by small-amplitude sealevel fall. Integration with absolute ages derived from radiocarbon dating and archeological artifacts enabled the elaboration of palaeogeographical maps, reporting the main modifications that affected the drainage network during the mid-late Holocene due to autogenic and allogenic processes and anthropogenic activity. Between pre- Roman age and the XII century AD, the Arno plain showed a complex palaeohydrographic system (Units I-III). According to historical sources and geomorphological maps based on photograph and satellite image analysis, our stratigraphic data evidence that Pisa emerged at the confluence of two large rivers, Arno and Auser, an old branch of Serchio River. Only one active fluvial course, almost coincident with the present Arno River, is recorded after the XII century AD (Unit IV), when the Auser was embanked and forced to flow northward to reduce flooding risk. Our results confirm that late highstand successions constitute excellent archives to establish the fluvial responses to climate and anthropogenic changes in areas with abundant archeological sites. They also corroborate the validity of a stratigraphic-based approach to reconstruct the palaeohydrography of modern deltaic and alluvial plains. 786