Methane‐Driven Arsenic Biogeochemical Cycling in Deep‐Sea Cold Seeps: Implications for Oceanic Arsenic Geochemistry

Arsenic accumulation in seafloor cold seeps is poorly understood. Here, we investigate the interplay between arsenic and methane biogeochemical cycles at the Haima cold seep, South China Sea. Geochemical analyses showed elevated arsenic levels in seep sediments, primarily as sulfide‐bound forms, with strong correlations between dissolved arsenic and methane oxidation proxies (DIC, R 2 = 0.64, p < 0.05; δ 13 C DIC , R 2 = 0.86, p < 0.05). Metagenomic sequencing revealed diverse functional genes related to arsenic‐, methane‐ and sulfur cycling, including co‐occurrence of arrA and dsrA in sulfate‐reducing bacteria Desulfobacterota , which are symbionts of Anaerobic Methane Oxidizing Archaea. We propose that anaerobic oxidation of methane (AOM) and the enhanced “metal particle shuttle effect,” both associated with methane release, drive arsenic sequestration. Cold seeps may sequester 0.04–1.81 × 10 3 kg arsenic annually. These finding highlights cold seeps are hotspots of arsenic cycling with implications for oceanic arsenic chemistry.