Plant landscape and environmental changes recorded in marginal marine environments: The ancient Roman harbour of Portus (Rome, Italy)

摘要

The interpretation of past environment changes in coastal marshes, river mouths and drainage channels connecting fresh to salt water bodies, is a difficult task since several factors must be considered such as the variable influence of climate, sea level changes, coastline variations and changes in the river discharge. It is even more difficult if this attempt is carried out in basins and channels of an ancient harbour, used for centuries, connected to the sea and to a river. This is the case of the ancient harbour of Rome, located in the Tiber delta area, which had a prominent role during the imperial age in the Mediterranean. A town named Portus arose in the vicinity and, although sacked many times by Barbarians, was still very important during medieval times. The Romans managed the harbour, the city and the surroundings and the impact on the environment must have been very strong. A multi-proxy approach was used to characterize and date environment and landscape changes in the ancient port of Rome area, from the 1st century to the 15th century AD. Plant macrofossil results have been integrated not only with pollen and microcharcoal, but also with the Tiber River bi-millennial record available in literature. This integration, used to improve the chronological framing of the cores, was carried out using ostracod analyses, micro-crustaceans commonly occurring in marginal marine environments and widely used for palaeoenvironmental reconstructions. In particular, the ability of some species to live within specific salinity ranges has been a precious tool to identify the hydrological setting of the depositional environment. The cores recovered allowed recognition of two different sedimentary sequences; the older shows the silting of the harbour during the first centuries, and the younger records the sedimentation that occurred between the 4th and the 15th century AD. This multi-proxy approach applied to the study of plant remains indicates that 1) the human impact was lower during the first centuries AD than during late-Antiquity and Middle Ages or that 2) clues of human presence were easier to detect in a closed marshy area than in an open harbour. The correlations between macroremains, pollen percentage and concentration, freshwater, brackish water and salt water ostracods and the frequencies of Tiber River floods during a 1500 year time span link some environmental changes and the silting of the harbour to the variations in hydrological regime of the Tiber River.