Damage triggered by bioerosion and surface residence time in Holocene shell beds from Brazil

Holocene shell beds along the Brazilian coast provide critical records of sea level variations influenced by climatic changes. This study examines the taphonomic implications of bioerosional traces in bioclastic accumulations from coastal sites in Rio de Janeiro State, Brazil. By integrating sedimentological, taphonomic, and taxonomic data, the relationship between bioerosion and shell damage signatures was explored, focusing on three Quaternary localities rich in mollusk shell remains. Detailed analyses using thin sections, X-ray microtomography, and compaction tests identified four distinct bioerosion incipient- traces: Entobia, Caulostrepsis, Meandropolydora, and Oichnus. The results show that the one active depositional site has a higher proportion of bioclasts with bioerosion but fewer shells with taphonomic damage compared to two Holocene sites. The higher fragmentation rates at Holocene sites are linked to longer exposure times of bioeroded shells before final burial. These differences suggest that shorter exposure times lead to lower fragmentation rates despite higher bioerosion levels. Mechanical tests confirmed that bioeroded shells are more fragile, breaking into angular fragments, thereby contributing significantly to shell damage. Thus, bioerosion coupled with moderate surface residence times, plays a crucial role in shell fragmentation, with hydrodynamic conditions being a secondary agent of shell damage in high-energy environments