Climate change signature of small-scale parasequences from Lateglacial–Holocene transgressive deposits of the Arno valley fill

Abstract

Despite recent report of short-term cyclicity from Lateglacial–Holocene deposits of several coastal plains worldwide, no precise documentation of the key factors controlling cyclic facies architecture has been made available by previous work. Detailed sedimentological analysis of a continuously-cored borehole, around the town of Pisa, in western Tuscany, provides evidence for the occurrence of three high-frequency, transgressive–regressive cycles within the post-Last Glacial Maximum (LGM) transgressive succession (13–8 cal. kyr BP) of the Arno incised-valley fill. These cycles, which are bounded by lateral equivalents of marine flooding surfaces, are 8–12 m thick and correspond to small-scale parasequences. Micropalaeontological (foraminifers and ostracods) investigations based upon differentiation of eight microfossil associations, allow to refine 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. 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, stillstand phases saw the development of cold-temperate communities (upper parts of parasequences), suggesting transition to temporary colder climate conditions. Reconstruction of parasequence architecture on the basis of adjacent stratigraphic data, combined with palaeoclimate characterization and radiometric dating enable identification, within the transgressive Arno valley body, of three major “regressive” pulsations that are tentatively correlated with the most important cooling events of the post-LGM period. The sedimentary response to these short-term 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 central sectors of the valley.