Soil development trajectories, valley alluviation, and the rise and fall of bronze age nuragic monuments: a geoarchaeological case study from the basaltic mesa of siddi, sardinia

Abstract

The classical models of Mediterranean soil-human dynamics focus on long-term pedogenic processes, alongside short-term human-induced erosion. However, geoarchaeological research in other regions shows that prehistoric anthropic interventions too caused rapid changes in soil mineralogy, chemistry, and biology, often occurring as an outcome of population growth and peak cultural activity. Here, we integrate upland soil archives, downstream sediment records, and data from a Bronze Age archaeological excavation to explore the interplay between soil evolution, landscape changes, and prehistoric monument-building societies in Sardinia, western Mediterranean. New geoarchaeological multi-scale investigations mapped soil cover and changes associated with Bronze Age settlement and monument building on the basaltic mesa of Siddi and Pauli Arbarei valley, south-central Sardinia. Over the mesa we identified a soil distribution characterised by a reddish-brown illitic and oxidised Cambisol, and a dark, smectite-rich Vertisol, with contrasting soil microfabrics and multi-elemental compositions. The excavation of a Bronze Age site on the mesa detected the presence of a cambic horizon developed over a former Vertisol. In the valley, the new chronostratigraphy shows alluvial activity initiating in the Middle Holocene, with a major increase in sediment supply around 3,500 BP. From these findings, it is plausible that nutrient- and smectite-rich Vertisols were widespread by the time Bronze Age (Nuragic) monument construction was in full swing. Intensive settlement and land use likely destabilised local soil processes, driving the geochemical and hydrological alteration of Vertisols, including a smectite-to-illite shift coupled with accelerated erosion. This soil trajectory changecould have enhanced the vulnerability of the settled landscape to land degradation, precipitating in monument abandonment and outweighing local climate impacts. The findings challenge traditional soil evolution models in the Mediterranean, offering new insights into rapid human-driven soil changes and contributing to the understanding of past and present soil vulnerability.