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

In the low Venetian plain (north-eastern Italy) thick sequences of silt and sand layers alternate with common and 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 a stretch of plain about 100 km large and some 30 km deep. They indicate the presence of wide areas where wet environments developed. Our research aims at understanding the features and the origin of the wet environments by means of sedimentological analysis, pollen, non-pollen palynomorphs and plant macrofossil analyses. A series of medium-deep cores were drilled in the central zone of the low Venetian plain near the coast of the Adriatic Sea and 79 samples have been analysed. The palaeoenvironmental reconstruction based on previous pollen analysis did not underline the presence of the areas where peat layers were formed, suggesting a homogenous steppe environment, typical of a cold and dry climate. They were probably waterlogged for a great part of the year, to allow the formation of peat deposits, and the local plant communities had to consist mainly of aquatic species. The results of macrofossil and pollen analysis suggest that herbaceous plants, such as Cyperaceae and Gramineae (probably Carex fusca and Phragmites australis) and “brownmosses” (mainly Scorpidium scorpioides) were important components of marsh communities. The occurrence of obligate aquatic organisms (Nymphaea, Characeae, Briozoa and Potamogeton remains) and non-pollen palynomorphs as algal resting cells, free cells and colonies (Zygnemataceae, Spirogyra, Mougeotia, Closterium idiosporum, Type 225, Type 229, Type 417B, Botryococcus, Pediastrum cf. boryanum, P. cf. simplex, Ceratium hirundinella, Tetraedron cf. minimum and Type 333), oocytes of aquatic invertebrates (Type 353 A and 353 B) and incompletely known types probably of algal origin (Type 303, Type 74, Type 128 A and 128 B) indicates open water environments. A discontinuous occurrence of fungal spores and remains (Type 200, Type 126, Type 207, Type 351, Type 79, and incompletely known types) suggests frequent fluctuations of the water depth and periodic emersions of the bottom of the ponds or marshes. The nutritive quality of the water preferred by the identified species, or suggested in literature for the recognized fossil types, is typical of mainly eutrophic to mesotrophic waters, rich in cations. Peat formed in wide and low-lying areas between the fluvial ridges, periodically inundated by the alternating outcropping of the groundwater table. Peat accumulation in continuous layers could take place only where the fen organic deposition prevailed on the alluvial minerogenic sedimentation. When the alluvial deposition overwhelmed the organic one, the peat level was buried and incorporated in the stratigraphic record.