Geochemical investigation of a sediment core from the Trajan basin at Portus, the harbor of ancient Rome

From the 1st century AD and for the duration of the Roman Empire, the Portus complex was the main harbor of Rome. Its location on the Tiber delta next to the Tyrrhenian Sea produced rapid environmental changes that, together with historical vicissitudes, largely determined the fate of the harbor. We have assembled data on the mineralogy, sedimentology, geochemistry, and ostracod populations of a sediment core drilled in the access channel of the hexagonal basin of Trajan, with the expectation that such a combined data set will shed new light on how the connections of the inland Trajan basin with the Tiber river, the earlier Claudius harbor on the nearby shoreline, and the sea evolved through the centuries. The data define four distinct periods which geochemistry characterizes by different conditions of salinity and oxygenation. These in turn can be related to historical periods and events by means of 14C data. The early Imperial Period was dominated by input of well-oxygenated freshwater from the Tiber. During the Late Empire, harbor water became relatively more influenced by seawater and increasingly oxygen deficient, which attests to a decommissioning of the Canale Trasverso connecting the harbor to the Tiber. The strong anthropogenic signal, which is visible very clearly in geochemical parameters, attests to the human occupation of the harbor area up to the Early Middle Ages, when human activity was brought to an abrupt end. The simultaneous use in this study of multiple complementary tracers has allowed for the sedimentary sources of the different classes of particles in the harbor basin to be identified and assigned to either the freshwater supply from the Canale Trasverso or the seawater of the Claudius harbor.