A journey through the Roman coastal landscape of Campi Flegrei and its surroundings: new insights from geomorphological, stratigraphic, geophysics, and geoarchaeological surveys

The Campi Flegrei is one of the widest and dangerous active volcanic complexes in the Mediterranean basin, worldwide known for the sudden vertical ground movements (bradyseismic crisis) characterizing its post-calderic volcanic activity since the Late Pleistocene. Despite its intense volcano-tectonic activity, the area has been densely inhabited since the Greek-Roman time and still preserves in its submerged sector many archaeological remains whose particular features can be interpreted as high-precision sea-level markers.

This research presents the results from a multidisciplinary study aimed at reconstructing the coastal landscape of Campi Flegrei and its surroundings during Roman Time by using a complex multi-technique approach.

The study area, ranging from the western margin of Torregaveta Promontory to the eastern area of Castel dell’Ovo in the City of Naples, was surveyed by a team of specialized divers (archaeologists and geomorphologists) and by using different indirect methods. High-precision data related to the underwater morphologies were obtained by coupling extensive morpho-acoustic surveys (Multibeam and Side Scan Sonar) with integrated detailed surveys using an Unmanned Surface Vessel (USV) equipped with acoustic and optical sensors.  3D reconstructions of seabed morphologies and submerged archaeological remains were integrated with sedimentological analysis of several boreholes, in order to obtain the geological, geomorphological, and archaeological interpretation related to the Roman coastal seascape.

The landscape evolution was evaluated both in terms of coastline variations, with related prograding/retreating trends, and quantification of the effects of the local volcano-tectonic activity recorded by the high-precision sea-level markers, leading to the identification of a differential behavior in terms of vertical ground movements along the different sectors of the study area.  Indeed, the RSL variation in the area is non-monotonic being controlled by the alternation of phases of positive and negative VGMs associated with multiple bradyseismic crises.

In particular, by comparing the collected RSL data with different GIA models, it was possible to observe that 2.1 ka BP the central part of the caldera was affected by a general subsiding trend with peak of acceleration up to -3.4 mm a^-1 between 2.1 and 1.9 ka BP. On the other hand, along the Posillipo and Chiaia coastal sectors, outside the eastern margin of the Campi Flegrei caldera, the RSL datapoints testified an overall subsiding trend during the Holocene which indicate general subsidence with rates between -1.2 and -2.5 mm ^a-1 at least in the last 2.0 ka.

Moreover, the combination of this multi-technique approach allowed the realization of a geoarchaeological map of the coastal landscape of Campi Flegrei and its surroundings between the II century BC and the I century AD, highlighting how the Romans gave rise to a new approach to the coastal outline and utilization becoming the precursors of an ever-increasing anthropization.