Applying OSL for dating mortar, plaster and soil samples at the archaeological site underneath the Dubrovnik cathedral

The 20th century has brought unexpected archaeological discoveries under the Baroque Cathedral in Dubrovnik (the UNESCO World Heritage Site). In the 1980s, a church complex older than the historically recorded Romanesque basilica was excavated. The dating of these finds is crucial, not only for understanding the historical changes of the Cathedral site but also for the earliest development of the historical centre of the city of Dubrovnik. After the archaeological research of the 1980s, in 2018, the first, more extensive processing of all findings was initiated through the cooperation of five Croatian scientific institutions (the project Discovering the Old Dubrovnik Cathedrals – DiODuCat). Within the framework of the IPERION HS platform, the DiODuCat 2022 collaboration was established with the CENIEH institute in Burgos (Spain) and OSL dating was applied to mortar and plaster collected from the internal layers of key structures in the sanctuary of the “first church”, as well as to the soil collected within its nave. The resulting data supplemented radiocarbon dating (AMS ¹⁴ C) of plaster samples applied in 2021 and 2022. The new data combined to indicate a somewhat different chronological development of the "first church" than was suggested by previous scholarly works, implying that it had been constructed by adapting an earlier, Late antique edifice while giving a more precise chronology to the church's refurbishments during the early Middle Ages. These data complement the already formulated scientific assumption of the DioDuCat research team based on the marble sculpture analysis and, primarily, the observations of the in situ layers of plaster in the church's sanctuary.