Death assemblages from contemporary marginal marine settings carved into ancient shell deposits are composed of fossil shells exhumed by currents or tides and shells derived from living populations. A better understanding of the bias produced by such a mixing process is of interest for studies that use modern death assemblages as analogues of similar past habitats. In order to evaluate the magnitude of reworking and redeposition of fossil shells in modern environments, a taxonomic (composition, abundance, and richness) and taphonomic (taphofacies) study was carried out in the Mar Chiquita coastal lagoon, Argentina (37°40′S, 57°20′W). The nature and extent of reworking was explored along a gradient in tidal energy from the outer to the inner reaches of the coastal lagoon. Results indicate that modern death assemblages in the lagoon are composed mostly of fossil (late Holocene) reworked shells and that reworking varies along a gradient in tidal energy, being higher in the outer reaches of the coastal lagoon, where tidal action is more significant. Temporal mixing in the coastal lagoon appears to be associated with condensation (remanié) rather than with a subtle mixing of shells, as occurs in time-averaged deposits. This reworking process leads to an abundance of old shells in modern death assemblages, which has negative consequences for their utilization as modern analogues of past lagoons. Multidisciplinary studies involving various biological indicators need to take this type of bias into consideration in order to avoid erroneous inferences on the Quaternary evolution of coastal lagoons.