Impact of mid-to late Holocene hydroclimatic variability and sediment dynamics on coastal flooding and human settlement in the Lez delta plain, southern France

The acceleration of sea level rise caused by global warming increases the risk of coastal flooding for people living on river deltas, as well as the erosion of archaeological sites along the Mediterranean coasts. The inhabitants of deltaic areas from the northwestern Mediterranean were exposed to coastal changes during the Holocene, and especially to geomorphic evolution driven by regression and progradation dynamics. Coastal flooding related to transgression can lead to a reduction in terrestrial areas available for human activities, whereas the emergence of new lands during delta progradation can provide opportunities for the development of cities and agriculture, although it can also increase the vulnerability of coastal infrastructures and settlements to sediment accretion.

The archaeological site of Lattara is one of the oldest coastal cities of the northwestern Mediterranean and is particularly interesting to study the impact of flooding on human settlements. This ancient city was built on a delta lobe of the Lez River during the Iron Age in the late 6th century BCE. Already, Middle Neolithic settlements were present in the northern part of the city. However, the absence of human occupations between ca. 3000 and 800 BCE suggests an abandonment of the site over two millennia. Geoarchaeological and environmental studies showed that this period was characterized by high groundwater levels in the Lez delta plain and relatively deeper water in the lagoon south of Lattara. Coastal flooding could thus explain the absence of human settlements at Lattara in the Late Neolithic and most of the Bronze Age, but this hypothesis needs to be investigated further.

Here we present the relation between hydroclimatic changes, sedimentation, coastal flooding and human settlements in the Lez delta plain during the mid-to late Holocene using bioindicators (ostracods, molluscs), geomorphological features (accommodation space, sediment accumulation rates), hydrological parameters (sea level change, water depth, discharge rates), age models based on radiocarbon dating, and archaeological data. Our data points toward the evidence of low sediment accumulation rates in a humid climate from 6 to 3 kyr cal BCE. These low sedimentation rates in a context of continuous sea level rise led to increasing of an accommodation space in the lagoon.

Our new results are compared with multi-millennial environmental records in the northwestern Mediterranean to evaluate the role of hydroclimate changes on coastal flooding. Besides, hydroclimatic parameters from instrumental data were investigated to determine if the relationships between climate change and hydrological processes over the past millennia were similar to those of the last decades.