Calcium peroxide capping stabilizes organic carbon in anoxic sediments via mineral protection

The application of calcium peroxide (CaO₂) to remediate anaerobic sediments presents a fundamental paradox: its direct oxidative potential is expected to mineralize organic carbon (OC) into CO₂, while its indirect geochemical effects (elevated pH and Ca²⁺) promote mineral precipitation, which can protect OC. This study aimed to resolve this conflict by disentangling these competing mechanisms in a simulated capping experiment. Our findings reveal that the mineral protection mechanism overwhelmingly dominates. Instead of promoting net degradation, CaO₂ treatment led to a significant stabilization of the sediment OC pool, evidenced by the inhibited shift from particulate to labile OC. We demonstrate that direct oxidation was confined to a superficial sediment layer (∼1.5 mm), whereas Ca²⁺ penetrated deep into the anoxic zone (>20 mm), triggering widespread mineral precipitation that physically encapsulated the OC. This geochemical sequestration was corroborated by a significant reduction in microbial chemoheterotrophic functions, confirming that the OC was rendered biologically inaccessible. These results provide critical insights into carbon turnover mechanisms during sediment remediation and the potential of CaO₂ to stabilize OC in treated sediments.