Elemental composition of cold-seep bivalve shells under acid conditions

Ocean acidification (OA) significantly interferes with the recorded geochemical information of bivalve shells by affecting their structural stability, thereby affecting their reliability of reconstructing marine paleoenvironments. A series of pH-controlled dissolution experiments (pH 2, 4, 6, 7, 8, and 10) on two dominant cold-seep bivalves from the Haima cold seep in the South China Sea—Archivesica marissinica (clam) and Gigantidas haimaensis (mussel)—was conducted to evaluate shell mass loss and trace element preservation in different anatomical regions (ventral, dorsal and posterior). Results show a strong inverse relationship between shell mass retention and acidification intensity, with mass loss reaching up to 70% at pH 2, whereas under neutral to alkaline conditions (pH ≥7), the mass loss was 0-2%. The elemental loss patterns (Ca, Mg, Sr, and Ba) were highly pH-dependent and positively correlated with Ca²⁺ concentrations (Mg: R²=0.76; Sr: R²=0.97; Ba: R²=0.78). The inter- and intra-species variability was evident: A. marissinica shells exhibited higher resistance to acidic dissolution than G. haimaensis, and ventral regions were consistently more resilient than dorsal and posterior areas in both species. These findings clarify the pH thresholds for reliable shell-based trace element proxies and reveal anatomical controls on geochemical signal preservation. This study provided critical insights for refining paleoenvironmental reconstructions and interpreting diagenetic alteration in bivalve shells in cold seeps.