Paleo-coastline reconstruction based on the hydrogeochemistry of coastal aquifers

Abstract Coastlines have undergone significant changes from the last glacial period to the present. Peak sea levels occurred approximately 6000 years ago. Since then, the coastline has progressively retreated as a consequence of the progradation of sedimentary systems, especially in deltaic environments. Seawater infiltrated into the coastal aquifers during the transgressive peak can become trapped in sedimentary deposits, allowing for the reconstruction of the paleo-coastline. Two coastal aquifers, the Cabo de Gata and Andarax Delta, located along a 20-km strip of coastline in southeastern Spain, have been investigated to identify the possible presence of trapped fossil waters. Based on chemical and isotopic data ( 14 C, δ 13 C, δ 18 O, and δ 2 H) in saline waters, it has been possible to reconstruct the paleo-coastline of this region and its distinct sedimentary environments. Samples taken along vertical depth profiles show that the shallower and fresher waters are associated with recent recharge, while the saline waters are composed of both recent and ancient recharge (up to 11 ka). In many cases, the chemical data do not allow for the differentiation between the two types of saline waters, requiring the use of radiogenic isotopes such as 14 C. The proposed paleogeographic reconstruction is in accordance with the Neolithic archaeological sites present in the area. The use of hydrogeochemical data from saline waters in coastal aquifers would be a valuable tool for making paleogeographic reconstructions. The existence of fossil saline waters trapped in coastal aquifers worldwide may be a more common phenomena than previously assumed.