Palynology of the late Quaternary succession of the Arno Plain (northern Italy): new insights on palaeoenvironmental trends and climate dynamics

Palynological analyses, backed by meiofauna (ostracods and benthic foraminifers), sedimentological and radiocarbon data, enabled the identification of late Quaternary turnovers of depositional environments and coeval vegetation dynamics from a ca. 30 m-long core succession (core PA1), retrieved in the Arno delta area (N Italy), 2 km landwards of the innermost outcropping beach ridge. Based on the identification of 213 palynomorph taxa (pollen and spores), eleven ecological groups were elaborated and their relative frequencies stratigraphically plotted, allowing the identification of eight bio-sedimentary units (BSUs) and four vegetation phases dated to the late Pleistocene onwards. The former record palaeoenvironmental changes, mainly in terms of water-table level and salinity. The latter reflect different palaeoclimate conditions at both Milankovitch and sub-Milankovitch timescales, relying on the proportion of montane versus Mediterranean taxa. The vertically stacked pattern of BSUs reveals two transgressive-regressive cycles developed above an alluvial plain succession stratigraphically assigned to the penultimate glacial interval (Marine Isotope Stage-MIS 6). Both depositional cycles show a transgressive portion composed of swampy-lagoonal clays formed under interglacial conditions. Pollen (Mediterranean forest with increasing optimum-like conditions) suggests a MIS 5e age for the lowermost transgressive interval, whereas the palynological assemblage (Mediterranean/sub-Mediterranean forest) and radiocarbon ages consistently indicate an early Holocene age (ca. 9800-7400 cal yrs BP) for the uppermost one. A climate-driven phase of subaerial exposure, likely related to the onset of the last glacial period, affected the MIS 5e lagoon, as testified by the record of an open/sparse pine forest within pedogenised alluvial deposits sandwiched between the two lagoon intervals. By contrast, the early Holocene lagoon experienced a long-term trend of sedimentary filling by river inputs under rather stable interglacial, highstand conditions. Only at the end of climate optimum conditions, marked by a peak of Fagus dated around 5000 cal yrs BP, the lagoon turned into a delta plain. This study documents the successfully use of a multi-proxy, palynological-based approach to investigate the complex interplay between environments, vegetation and climate changes in alluvial-coastal plain contexts and within a sequence-stratigraphic perspective.