RationaleStrontium isotope ratios (Sr-87/Sr-86) in modern-day marine environments are considered to be homogeneous (0.7092). However, in the Baltic Sea, the Sr ratios are controlled by mixing seawater and continental drainage from major rivers discharging into the Baltic. This pilot study explores if variations in Sr can be detected in marine mammals from archaeological sites in the Baltic Sea. Methods(87)Sr/Sr-86 ratios were measured in tooth enamel from three seal species by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The method enables micro-sampling of solid materials. This is the first time that the method has been applied to marine samples from archaeological collections. ResultsThe analyses showed inter-tooth Sr-87/Sr-86 variation suggesting that different ratios can be detected in different regions of the Baltic Sea. Furthermore, the intra-tooth variation suggests possible different geographic origin or seasonal movement of seals within different regions in the Baltic Sea through their lifetime. ConclusionsThe method was successfully applied to archaeological marine samples showing that: (1) the Sr-87/Sr-86 ratio in marine environments is not uniform, (2) Sr-87/Sr-86 differences might reflect differences in ecology and life history of different seal species, and (3) archaeological mobility studies based on Sr-87/Sr-86 ratios in humans should therefore be evaluated together with diet reconstruction.