High-resolution sequence-stratigraphy of the post-LGM transgressive succession of the Arno valley fill (Tuscany, Italy)

Despite recent report of millennial-scale cyclicity from Lateglacial-Holocene deposits of severa! coastal plains worldwide, no precise documentation ofthe key fac- ' tors controlling cyclic facies architecture has been made available by previous work. Integrated sedimentologic, micropalaentologic, palynologic and geochemical analyses of a 56 m long core (Sl) from the Pisa area (Tuscany, ltaly). allow reconstruction of a high-resolution sequence-stratigraphic framework for the transgressive systems tract (TST) ofthe post-Last Glacial Maximum (LGM) succession ofthe Arno valley fili. At this location, the post-LGM succession is about 54 m thick and its lower part (early TST) includes a 35 m thick succession of alternating inner- and central- estuarine deposits (sensu Dalrymple et al. 1992). Three high-frequency depositional cycles, about 8-12 m thick and spanning intervals of ti me of about 1500 years each, are identified in core Sl. These cycles can be physically traced throughout the valley body, grading seaward to outer estuarine deposits (Amorosi et al. 2008). Radiocarbon dates constrain the age ofthe valley fili between 13 and 8 cal. kyr BP. Around 8 cal. kyr BP, transgressive sedimentation spread onto the interfluves and a coastal environment established ali throughout the study area (late TST). The three small-scale cycles are bounded by latera! equivalents of marine flooding surfaces (parasequences) and display a characteristic internai architecture, with a thin transgressive central-estuarine succession overlain by comparatively thicker, regressive inner-estuarine deposits. Differentiation of eight microfaunal (benthic foraminifer and ostracod) associations, allows to refi ne the stratigraphic framework, emphasizing subtle changes in palaeosalinity across parasequence boundaries. Diagnostic changes in vegetation patterns, driven by opposite climate conditions, enable precise documentation of parasequence development as a function of climate change around the Pleistocene-Holocene boundary. Pollen spectra invariably show expansions of broad-leaved forests at parasequence boundaries, suggesting that rapid shifts to warmer climate conditions accompanied episodes of rapid sea-level rise. In contrast, sti llstand phases saw the development of cold-temperate communities (upper parts of parasequences), suggesting transition to temporary colder climate conditions. Geochemical characterization of the three parasequences allows detailed recon struction of palaoenvironmental changes within the valley, as a function of small-scale sea-level fluctuations. Specifically, peaks in [Cr/Ba]/[Mg/Al] at the onset ofbrackish conditions (lower parts of parasequences) are interpreted to reflect a major sediment contribution from the Arno River to the estuary during transgressive pulsations. In contrast, sediment provenance from Serchio River is recorded by the upper parts ofparasequences. This suggests increasing sediment contribution from atributary incised valley at times of sea-level stillstand, accompanying coastal progradation.