Climate-driven small-scale parasequences from the post-LGM transgressive succession of the Arno valley fill (Tuscany, Italy)

Despite the worldwide report of high-frequency depositiana! cycles from the Lateglacial-Holocene subsurface succession of modern coastal plains, the complex mechanism controlling this sub-Milankovitch cyclicity has not yet been successfully assessed. In particular, the role played by short-term climate changes, and related relative sea level oscillations, is stili matter of debate and no unambiguous evidence of a direct link between post-LGM millennial- scale climatic events and parasequence development has been documented, so far. Detailed sedimentological and micropaleoj1tological (benthic foraminifers and ostracods) analyses performed on a 56 m-long continuous core drilled around the town of Pisa (core S1) allowed to identify a high-frequency depositional cyclicity within the postglacial (~ 13-8 cal. kyr BP) transgressive succession of the Arno valley fili (western Tuscany, ltaly). An aggradational to weakly progradational vertical stacking pattern of three small-scale (8-12 m thick) transgressive- regressive cycles, bounded by latera! equivalents of marine flooding surfaces (parasequences sensu Van Wagoner et al., 1990), is identified in core S1 and traced throughout the valley body up to distallocations. Each parasequence is composed of a thin transgressive succession of estuarine clays overlain by comparatively thicker regressive coastal plain deposits. Micropaleontological analyses allow to refine facies characterization, emphasizing subtle paleonvironmental changes (mainly corresponding to paleosalinity oscillations) occurring across the flooding surfaces and, within each parasequence, at the transition from the transgressive to the regressive portion. Pollen analyses show that these parasequences have a distinct climatic connotation. Expansions of broadleaved forests at parasequence boundaries suggest that rapi d shifts to warmer climate conditions accompanied episodes of rapid sea-level rise. By contrast, stillstand phases correspond to the development of coldtemperate communities (upper parts of parasequences), indicating transition to temporary periods of climate deterioration. On the basis of paleoclimate pollen characterization and radiocarbon data, the three major "regressive" episodes recorded at the top of each parasequence are tentatively correlated with the most important millennial cooling events of the post-LGM period, documented by a variety of indicators in the geologica] record of the North Atlantic region. The coastal sedimentary response to these shortterm phases of climatic cooling is clearly documented by episodes of widespread coastal-plain and bay-head delta progradation, leading to partial estuary infilling and temporary establishment of continental environments in the proximal and centrai sectors of the Arno valley