Stratigraphy, palaeopedology and palinology of Late Pleistocene and Holocene deposits in the landward sector of the Lagoon of Venice (Italy), in relation to the “caranto” level

The present investigation brings new stratigraphic, palaeopedological and palynological data deriving from the study of 4 cores, bored to a maximum depth of 15 m in the central sector of the Lagoon of Venice, along the inner shores, between the mouth of the Dese river and Porto Marghera. In agreement with the general stratigraphy known from previous studies, the lowermost deposits in the cores are fluvial, radiocarbon dated to 21,000-18,000 BP. The correlation to the stratigraphic framework of the central Veneto plain shows that they belong to the Late Pleistocene fluvial sedimentary system of the Brenta river, the Bassano megafan. The pollen record of this alluvium is characterized by typical hydrophilous plants (Typha, Potamogeton, Nymphaea...), which are referred to swampy environments during the Last Glacial Maximum and the Late Glacial. The compact level known as caranto, located at the top of the fluvial sediments at a depth of some metres from the ground surface, has been recognized as consisting of a set of pedogenetic calcic and gley B and C horizons. This palaeosol formed on the alluvial plain of the distal reaches of the Bassano megafan, in the time span comprised between the deactivation of fluvial processes, which took place after 14,500 BP and probably before the beginning of the Holocene, and the lagoon transgression. As the latter apparently interested the study area only in post roman times, pedogenesis could act for 8000-12,000 years. The definition of the pedogenetic nature of the weathering features is based on macro- and micromorphological observations, associated with physico-chemical analyses. The palynological analysis shows that the caranto level is characterized by a very low pollen content. The Late Holocene lagoonal deposits which cover the caranto palaeosol have dominant subtidal / intertidal mud flat and salt marsh facies, within a context of varying water salinity probably related to river inputs. The presence of a slightly pedogenized level developed in a salt marsh environment, indicates a hiatus in the deposition of the lagoonal sediments. Two radiocarbon datings of the organic O horizons of this soil in two cores show that this discontinuity is of medieval age. Furthermore, they provide a reliable chronostratigraphic support to the determination of the recent relative sea level rise. This latter has been estimated 2.3 m since 1055-954 cal BP to present day in Porto Marghera, EniRisorse area, and 1.1 m since 640-592 cal BP to present day in Porto Marghera-Fusina, ABIBES area.