Unexpected enrichment of chlorinated-iodinated carbazoles in deep-sea sediments from the Haima cold-seep area: occurrence, compositional patterns and possible sources.

Polyhalogenated carbazoles (PHCZs) are emerging organohalogen contaminants, yet their molecular diversity, sources, and controlling processes in deep-sea cold seep environments remain poorly understood. In this study, known PHCZs and previously unknown PHCZs (UPHCZs) were investigated in sediment cores from the Haima cold seep area of the South China Sea, including a mature seep site (HMS18), an early-stage seep site (HMW07E), a non-seep site (HMW07N), and coastal sediments. Target analysis showed that Σ12PHCZ concentrations ranged from 0.16 to 3.8 ng g-1 (median, 1.6 ng g-1), with higher levels at HMW07N than at HMS18 and HMW07E. Orbitrap HRMS-based non-target screening further yielded 47 candidate UPHCZs, all tentatively annotated at Level 3, with diverse halogen substitution patterns, including Cl-, I-, Cl/Br-, Cl/I-, Br/I-, and Cl/Br/I-substituted carbazole-like compounds. Σ47UPHCZ concentrations ranged from 3.7 to 170 ng g-1, with a median value of 24 ng g-1, exceeding those of target PHCZs by more than one order of magnitude. These results indicate that conventional target analysis substantially underestimates both the abundance and molecular diversity of halogenated carbazole-like compounds in cold seep sediments. Five-halogenated and Cl/I-substituted congeners dominated the cold seep sediment pool, revealing a distinct halogenation signature. Core-specific correlation analyses showed covariation between PHCZ/UPHCZ compositions and local geochemical gradients, whereas the relationships varied among the three cores. Exploratory comparisons further showed differences in UPHCZ composition between HMS18 and HMW07E, including higher estimated concentrations of more highly halogenated mixed-halogen features at HMS18. The Cl/I-rich and highly halogenated PHCZ/UPHCZ profiles may reflect several unresolved processes, including precursor inputs, halide availability, transformation, transport, and differential preservation. Overall, these findings document diverse halogenated carbazole-like features in sediments from the Haima cold seep area and provide a basis for further investigation of their sources, transformation, and environmental fate.