Anthropogenic stones on a remote oceanic island: formation, transport, and burial in a sea turtle nesting beach

This study provides the first five-year baseline assessment of plastic-derived anthropogenic stones on Trindade Island in the South Atlantic Ocean, characterizing their formation, transport, degradation, and burial within a remote green turtle nesting site. Field monitoring and spatial surveys show that in situ plastistone on Parcel das Tartarugas Beach, which initially covered 12 m² in 2019, underwent progressive erosion, resulting in a 45 % decrease in area by 2024. The eroded plasticlasts were found on six beaches, with persistent accumulation at Tartarugas Beach. On this beach, systematic surveys indicate the accumulation of plasticlasts along the strandline, with average abundances of approximately 24 mesoplasticlasts/m² and 22 microplasticlasts/m². Concentrations also occur within green turtle nesting depressions, where plasticlasts reach an abundance of up to 1.3 plasticlasts/m² and are buried to depths of about 10 cm. Morphometric results from 166 plasticlasts indicate trends in coastal transport and deposition. The sub-rounded oblate and sub-rounded prolate shapes dominate the dataset and tend to occur farther from the waterline. In contrast, equant plasticlasts with sub-rounded to sub-angular shapes are abundant near the water and along the strandline. SEM images indicated that plasticlasts feature both mechanically eroded and chemically weathered surface textures, including microfiber generation. Flow structures and vesicle-like cavities are present across the matrix, supporting the melting origins. EDS results suggest that Ca enrichment in the plasticlast matrix reflects early diagenetic interactions with surrounding sediments, supporting their gradual incorporation into geological processes. Raman spectroscopy results show that both in situ plastistone and transported plasticlasts are composed of high-density polyethylene (HDPE) associated with phthalocyanine dyes, linking their origin to partially melted maritime ropes. This study establishes a crucial foundation for tracking plastic pollution in sedimentary systems. Our results indicate that turtle nests act as natural traps for anthropogenic stones, enhancing their accumulation, burial, and preservation in the geological record.