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.

Radiocarbon dating of closely associated marine mollusk shells and terrestrial material (mammal bones or charred wood) collected from archaeological contexts in northern Atlantic Iberian coastal areas is used to quantify the marine¹⁴C reservoir effect (ΔR) for the coastal waters off the Cantabrian coast of northern Iberia. For the first time, ΔR values were reliably determined for these coastal waters and, also for the first time, a ΔR was calculated for the Late Pleistocene in Atlantic Iberia. Pairs of coeval samples of different carbon reservoirs selected from Upper Paleolithic (Late Pleistocene) and Mesolithic (Early Holocene) contexts yielded ΔR weighted mean values of –117±70¹⁴C yr and –105±21¹⁴C yr, respectively. These values show oceanographic conditions characterized by a reduced offset between atmospheric and surface water¹⁴C contents, suggesting a nonexistent or very weak upwelling and some stratification of the water column. Similar oceanographic conditions have been recorded in other areas of Atlantic Iberia during the Holocene, such as off Andalusian and northwestern Galician coasts. Results not only provide useful information on environmental conditions but also a framework to obtain more precise and reliable absolute chronologies for the Late Pleistocene and Early Holocene in northern Iberia.

In European archaeology, the malacological remains recovered in archaeological contexts have traditionally been considered almost exclusively as food waste. In other cases, this view has been broadened in order to study these remains as an expression of aspects of the social organization of the human groups, based on the use of perforated shells as objects of personal ornamentation. However, the study of these natural resources as raw materials for the manufacture of tools aimed at satisfying the production needs of the human groups has been very limited. This little-developed aspect of research is at variance with the abundant ethnographic information from many different periods and geographical settings showing that malacological resources were used in many complex and varied ways. This paper is an attempt at compiling a small part of this ethnographic information - a contribution which, through its critical application to the archaeological record, is of interest in establishing a methodology for studying this type of evidence. In the specific case of northern Spain, information from ethnographic studies has been used to develop an appropriate methodology with which to approach the analysis of this kind of archaeological evidence, as recently documented for the first time at the classic site of Santimamine (Basque Country). At the same time, the documentation of shell tools could provide an explanation for the scarcity of (i traditional technologies >that characterizes many Mesolithic and early Neolithic sites in northern Spain.Peer Reviewe