Wetlands in the Venetian Po Plain (northeastern Italy) during the Last Glacial Maximum: Interplay between vegetation, hydrology and sedimentary environment

Abstract

In the low Venetian plain (northeastern Italy) thick sequences of silt and sand layers alternate with common, thin layers of peat and organic silt; the organic layers in the topmost 30 m of the Late Pleistocene alluvial series span between 23,000 and 14,000 yr BP (radiocarbon dating), in an area measuring 100 km by 30 km. They indicate broad areas where wetlands developed. We aim to understand the features and the origin of the wetlands by undertaking sedimentological, pollen, non-pollen palynomorph and plant macrofossil analyses. Thirteen cores were drilled in the central zone of the low Venetian plain near the coast of the Adriatic Sea and 79 samples were analysed. The palaeoenvironmental reconstruction based on previous pollen analysis did not emphasize the areas where peat layers were formed, suggesting a homogenous steppe environment, typical of a cold and dry climate. They were probably waterlogged for most of the year allowing the formation of peat and the development of local plant communities of mainly aquatic species. Macrofossil and pollen analyses suggest that herbaceous plants, such as Cyperaceae and Poaceae (probably Carex fusca and Phragmites australis), and brown mosses (mainly Scorpidium scorpioides) were the most important components of wetland communities. Fossils of obligate aquatic organisms indicate open water environments, these include macrofossils of Nymphaea, Characeae, Bryozoa and Potamogeton, and non-pollen palynomorphs such as algal resting cells, free cells and colonies (Zygnemataceae, Spirogyra, Mougeotia, Closterium idiosporum, Type 225, Type 229, Botryococcus, Pediastrum cf. boryanum, P. cf. simplex, Ceratium hirundinella, Tetraedron cf. minimum and Type 333), oocytes of aquatic invertebrates (Type 353A and 353B) and incompletely known types probably of algal origin (Type 303, Type 74, Type 128A and 128B). A discontinuous occurrence of fungal spores and other microfossils (Type 200, Gaeumannomyces (Type 126), Glomus (Type 207), Type 351, Type 79, and incompletely known types) suggests frequent fluctuations of the water depth with periodic emersions of the bottom of the ponds or fens. The water quality preferred by the identified species, or suggested in literature for the fossil types, is mainly eutrophic to mesotrophic and rich in cations. Peatland formed in wide, low-lying areas between the fluvial ridges which were periodically inundated by the fluctuating groundwater. Peat accumulated in continuous layers only where the fen organic deposition prevailed the alluvial minerogenic sediment. When alluvial deposition buried the organic deposit, the peat level was incorporated into the stratigraphic record.