Assessing seawater metal pollution using oyster shells as archives: Trace element variations along an intertidal reef...

This study reconstructs the historical baseline of trace metal pollution along the South Yellow Sea coast by establishing a chronological framework for the Liyashan oyster reef, analyzing major and trace elements preserved in oyster shells, and comparing the enrichment of Cu, Zn, Pb, Cd, and Hg among shells, soft tissues, and sediments. Radiocarbon dating revealed a significant positive linear correlation between reef depth and age (p = 0.002, R² = 0.924). Distinct enrichment patterns were observed among the three media: soft tissues accumulated higher levels of Cu, Zn, and Hg; sediments exhibited the highest Pb concentrations; and shells were markedly enriched in Cd. Most elements remained stable along the reef profile; however, shell Cu/Ca, Zn/Ca, Pb/Ca, and Cd/Ca ratios decreased significantly with depth (p < 0.05), indicating a clear increase in trace metals toward younger reef layers. Surface shells (0.1 m depth layer) showed enrichment factors of 1.71 for Cu, 4.84 for Zn, 2.65 for Pb, and 5.04 for Cd, relative to pre-industrial baseline values (1 m depth layer; estimated age 476 calibrated years BP). This study provides direct evidence of intensifying trace metal pollution along the South Yellow Sea coast since pre-industrial times and validates the use of bivalve shells as effective natural archives for tracking long-term coastal environmental change.