Multiproxy reconstruction of late pleistocene-holocene environmental changes in coastal successions: Microfossil and geochemical evidences from the po plain (Northern Italy)

摘要

Despite the overwhelming control exerted on worldwide coastal evolution by the Lateglacial-early Holocene sea-level rise, high-frequency rapid palaeoenvironmental changes are recorded within a thick postglacial succession buried beneath the southern Po Plain (Northern Italy). Amultidisciplinary study involving sedimentology, micropalaeontology (benthic foraminifers and ostracods), geochemistry (major, minor and trace elements) and radiocarbon dating (14C) on a 40m-long core (240-S5) retrieved inland of the Holocene beach ridges, allowed the reconstruction of articulate coastal scenarios and dynamics - including stages of drainage reorganization. An alluvial plain nourished by the Apenninic Savio River developed in response to low sea-level conditions during the last glacial period. During the first stages of deglaciation (Lateglacial period) the study area was flooded and a coastal plain evolved near the coeval lagoon basin. Subtle change in microfossil content documents the occurrence of a short-term progradational episode that led to the temporary establishment of more terrestrial conditions (likely attributed to the Younger Dryas event - ca. 12,500 cal years BP). Complex and unstable Lateglacial palaeoenvironmental scenarios, and a mixed contribution from two Apenninic rivers (Savio River and Fiumi Uniti system), are suggested by the geochemical composition of coastal sediments. A distinctive flooding event, possibly related to the MWP-1B, caused the abrupt replacement of the coastal plain by a barrier-lagoon system. This landward shift of facies was synchronous with a source area change from the Savio River to the Fiumi Uniti system, triggered by an important phase of drainage system reorganization. Short-term changes in microfossil content also highlight the occurrence of two early Holocene episodes of lagoon infilling not accompanied by significant changes in sediment composition. The complete infilling of lagoon, which evolved in a swamp basin, took place only after ca. 7,600 cal years BP. The establishment of hypohaline environmental conditions is supported by subtle changes in geochemical composition.