A new study refines radiocarbon dating of marine remains and significantly improves the precision with which the human past of the Magdalenian period in the Cantabrian region of Spain can be reconstructed, a key phase of European prehistory dating to around 18,000 years ago.

An international study led by the Institute of Environmental Science and Technology of the Universitat Autònoma de Barcelona (ICTA-UAB) provides new correction values for the radiocarbon dating of marine remains—such as shells—recovered from archaeological sites in the northern Iberian Peninsula. This represents a major advance for more accurately interpreting the chronology of prehistoric human occupations in coastal areas. The study also involved researchers from the universities of Salamanca and Cantabria, the Aranzadi Society of Sciences, and the Max Planck Institute in Germany.

Radiocarbon dating, or carbon-14 dating, is one of the most widely used tools in archaeology for determining the age of archaeological sites. All living organisms incorporate carbon-14 while they are alive, but once they die, this isotope begins to decay progressively. Because its amount is reduced by half every 5,730 years, it is possible to calculate the time elapsed since the death of the organism and place it within a chronological framework.
Radiocarbon dating is most commonly applied to charcoal, human bones, and terrestrial animal remains. However, in many coastal archaeological sites the only available materials are of marine origin—shells, fish, or marine mammals—making it necessary to rely on these remains to establish site chronologies.

This introduces a key challenge: dates obtained from marine organisms may appear older than they actually are when dated using radiocarbon methods. This occurs because marine organisms contain less carbon-14 than their contemporary terrestrial counterparts, as oceanic carbon includes a component of carbon-14 that is already partially depleted. This offset, known as the marine reservoir effect, means that when a marine organism dies, it starts with a lower carbon-14 concentration than a terrestrial organism. If not properly corrected, this effect can make radiocarbon ages appear several hundred years too old.

To correct this offset, a global marine calibration curve is used, to which a local correction factor known as ΔR is added. ΔR varies depending on the region and the time period. “Accurately determining these values is essential for obtaining reliable radiocarbon dates, especially at archaeological and palaeontological sites that contain marine remains, or when dating human remains from populations whose diet included large amounts of marine resources”, explains Asier García-Escárzaga, who conducted this research at ICTA-UAB and the Department of Prehistory of the UAB.

The study, recently published in the journal Radiocarbon, presents new ΔR values that allow radiocarbon dates obtained from marine remains from Magdalenian sites dating to around 18,000 years ago in the northern Iberian Peninsula to be corrected more accurately. To calculate these values, the research team compared radiocarbon dates from marine and terrestrial remains recovered from the Tito Bustillo cave site (Ribadesella), renowned for its rock art and Palaeolithic engravings. “This advance does not mean that archaeological sites are older or younger than previously thought, but rather that we can date them more precisely, fine-tuning the ‘clock’ archaeologists use to reconstruct the history of Palaeolithic human populations”, García-Escárzaga concludes.