This article presents a set of Late Pleistocene marine mollusk radiocarbon (AMS) age estimates of 30–50 14C kyr BP, whereas a MIS5 age (>75 ka) is indicated by quartz and feldspar OSL dating, biostratigraphy, U-Th dating, and age-depth relationships with sea level. These results indicate that the 14C dates represent minimum ages. The age discrepancy suggests that the shells are contaminated by younger carbon following shell death. The enigmatic 14C dates cannot be “solved” by removing part of the shell by stepwise dissolution. SEM analysis of the Late Pleistocene shells within a context of geologically younger (recent/modern, Holocene) and older (Pliocene) shells shows the presence of considerable amounts of an intracrystalline secondary carbonate precipitate. The presence of this precipitate is not visible using XRD since it is of the same (aragonitic) polymorph as the original shell carbonate. The combination of nanospherulitic-shaped carbonate crystals, typical cavities, and the presence of fatty acids leads to the conclusion that the secondary carbonate, and hence the addition of younger carbon, has a bacterial origin. As shell material was studied, this study recommends an assessment of possible bacterial imprints in other materials like bone collagen as well.