Late Holocene pollen record from Fiume Morto (Dead River), a palaeomeander of Tiber River near Ancient Ostia (central Italy)

Pollen and non pollen palynomorph analyses were carried out in a sediment core (MO2) drilled in the southern lobe of a palaeomeander, Fiume Morto (Dead River), in the Tiber delta area, near Ancient Ostia (central Italy). Since the Roman period, the Tiber River flowed close to Ancient Ostia and its saltworks, Salinae Ostiensis. The Tiber meander was cut off during the Tiber River flood of 1557 AD and transformed into an oxbow lake. During the nineteenth century the Fiume Morto pond was reclaimed and at present the area is about 3 km distant from the present shoreline and intensely transformed by human settlements. According to radiocarbon dates, the pollen record, not continuous, spans from the fourth century BC to the nineteenth century AD. It shows first a riverine phase before the meander cut off of sixteenth century AD and probably only the last centuries BC were preserved from erosion. The river deposits record riparian vegetation (mainly tamarisk and alder) with mesophilous (mainly deciduous oaks) and Mediterranean (mainly evergreen oaks, heather and olive tree) elements. The human presence is clear, probably related to the development of Ancient Ostia and evidenced by synanthropic taxa. The second phase corresponds to the oxbow lake formed after the meander cut off. Several peaks of pine pollen are tentatively ascribed to Tiber flood events: the first peak is found just in correspondence with the meander cut off at 1557 AD. The numerous floods we interpret in the following part of the diagram could be linked to the increase in extreme events and precipitation that occurred during the Little Ice Age. The last phase, in which freshwater plants are present and chenopods decrease, shows the saltworks abandonment that occurred in nineteenth century. This study turned out to be of key relevance to reconstruct the palaeoenvironmental evolution of the ancient Holocene Tiber meander during the last two and a half millennia.