The cooling and drying associated with the so-called ‘8.2 ka event’ have long been hypothesized as having sweeping implications for human societies in the Early Holocene, including some of the last Mesolithic hunter-gatherers in Atlantic Europe. Nevertheless, detailed ‘on-site’ records with which the impacts of broader climate changes on human-relevant environments can be explored have been lacking. Here, we reconstruct sea surface temperatures (SST) from δ¹⁸O values measured on subfossil topshells Phorcus lineatus exploited by the Mesolithic human groups that lived at El Mazo cave (N Spain) between 9 and 7.4 ka. Bayesian modelling of 65 radiocarbon dates, in combination with this δ¹⁸O data, provide a high-resolution seasonal record of SST, revealing that colder SST during the 8.2 ka event led to changes in the availability of different shellfish species. Intensification in the exploitation of molluscs by humans indicates demographic growth in these Atlantic coastal settings which acted as refugia during this cold event.

Changes in oxygen isotope ratios from shell carbonates are mainly dependent on sea surface temperature, which enables the estimation of temperatures during periods of shell growth and helps to determine the season of the year when the mollusk died. The marine topshell Phorcus lineatus (Da Costa, 1778) is commonly found in Holocene archaeological deposits of Atlantic Europe and is one of the most abundant subsistence resources utilized during the Mesolithic in northern Spain. Before applying isotopic techniques to ancient samples, calibration of the past isotopic data and its variability must be performed through the study of modern specimens to test their potential as paleoclimate proxy and their suitability for determining the collection season. Although previous studies performed in the region highlighted the existing relationship between sea surface temperatures and isotopic signatures, no systematic works have been done so far. In this paper, calibration of modern P. lineatus shells from northern Spain was carried out using δ¹⁸O analysis. The results showed (1) the existence of a robust inverse correlation between instrumental temperatures ( T_meas) and δ¹⁸O_shell ( R² > 0.9), accompanied by the lack of significant dependence from δ¹⁸O_water variations ( R² = 0.06); (2) the existence of conditions of (or close to) isotopic equilibrium during the formation of the aragonite in the P. lineatus shells; and (3) that using mean annual δ¹⁸O_water values, past temperatures could be calculated with a maximum uncertainty of ±3°C. Moreover, results suggested that P. lineatus generally grew without substantial slow/cessation throughout the year, reflecting the four annual seasons. Therefore, our study not only confirms the potential of oxygen isotope analysis on P. lineatus for paleoclimate reconstruction and archaeological studies highlighted in previous studies but also shows for the first time that the aragonite of those shells grew under conditions of isotopic equilibrium, opening new avenues for future research.

The Mesolithic period in the Cantabrian region, a coastal area located in northern Spain, is characterised by a marked increase in the human use of coastal resources in comparison with previous periods, resulting in the formation of so-called “shell middens”. Archaeological investigations have provided insights into the formation processes of these shell middens, as well as long-term changes in human exploitation of different marine resources and the relationship of foraging strategies to past climate changes. However, efforts to reconstruct the key environmental factor governing coastal subsistence and foraging resilience, the seasonal availability and use of different marine resources, have been limited in the region and, indeed, across coastal Mesolithic Europe more widely. Here, we use stable oxygen isotope analysis of Phorcus lineatus (da Costa, 1778), one of the most widespread molluscs in northern Iberian mesolithic coastal sites, in order to determine the season in which humans collected key coastal resources at the site of EL Mazo (Llanes, Asturias). We demonstrate that P. lineatus was exclusively collected in late autumn, winter and early spring. An experimental programme, in which modern P. lineatus specimens were collected in situ over the course of three years, established that relative meat yield varied within this species throughout the annual cycle, with higher relative meat yield during colder months. We argue that mollusc collection patterns were driven by a cost-benefit principle during the Mesolithic in the Cantabrian region and human populations had intimate knowledge of the seasonal developmental cycles of exploited marine taxa. This also highlights the importance of developing intra-annual records of resource use and climate change if coastal foraging is to be properly understood in prehistory.