Ecological risk of Cd and Cr in the black rock series should be noticed: Based on the study of enrichment mechanism, occurrence form in the Lower Cambrian Lujiaping formation.

The heavy metals in soils developed from the black rock series originate from the parent rock, but their sources and enrichment mechanisms in the parent rock remain unclear. This study explores the enrichment mechanisms, occurrence forms, and ecological environmental effects of cadmium (Cd) and chromium (Cr) in the black rock series. Results revealed average concentrations of 1.15 mg/kg for Cd and 193.08 mg/kg for Cr. Cd showed moderate enrichment (Cd=31.03), while Cr had slight enrichment (Cr=4.42). Both metals were mainly in the residual fraction (44.22 % for Cd, 69.02 % for Cr), followed by the Fe-Mn oxide-bound fraction (24.07 % for Cd, 18.51 % for Cr). The Risk Assessment Code (RAC) indicated moderate risk for Cd (10 %≤RAC<30 %) and low risk for Cr (1 %≤RAC<10 %). The Secondary Phase to Primary Phase ratio (RSP) suggested mild Cd pollution (1 <RSP≤2) and no Cr pollution (RSP≤1). Cd poses a significant release risk, necessitating attention to its ecological safety. Hydrothermal and volcanic activities were primary sources of Cd, while TOC and volcanic activities influenced Cr. Anoxic conditions in the Lower Cambrian Ocean facilitated Cd and Cr enrichment. This study enhances understanding of heavy metal enrichment in black rock series and guides pollution management.