Insights Into Microscale Biomineralization in Deep‐Sea Callogorgia Coral via Halogen Banding

Abstract Halogen incorporation into coral carbonate skeletons serves as a valuable proxy for reconstructing paleoenvironments. However, the underlying mechanisms remain poorly understood. This study employed femtosecond laser ablation‐inductively coupled plasma time‐of‐flight mass spectrometry (fs‐LA‐ICP‐TOFMS) to perform multi‐elemental (I, Br, P, Ca, Mg, Sr, S, Ba, and Fe) imaging of cross‐sections from carbonate skeleton of deep‐sea Callogorgia sp. coral (octocoral) collected from the Mariana Arc at a depth of 1,386 m. Previously unrecognized iodine‐rich ring patterns and bromine‐rich fan‐like distributions were identified, likely influenced by the biological rhythms of polyps, such as polyp apoptosis‐replacement or migration, which refresh the polyp‐skeleton interface and facilitate halogen accumulation. Additionally, higher iodine and bromine concentrations were consistently observed in the calcitic layers compared to the aragonitic layer, suggesting that biological factors, particularly halogenated organics, dominate halogen incorporation, outweighing mineralogical controls. A comparison of halogen distributions between Callogorgia and bamboo corals demonstrated species‐specific halogen incorporation patterns. These findings provide new insights into coral biomineralization, emphasizing the role of biological rhythms in skeletal formation and trace element incorporation.