Distribution of Stable and Radioactive Iodine Dissolved in Interstitial Waters Within the Subduction Input Sediment Offshore Sumatra Subduction Zone

Subduction input (sediment before subduction)-located seaward of the trench is one of the largest iodine budgets on the earth's surface. It is responsible for the deep iodine source in the landward of the trench where the iodine flux is significantly high. However, the distribution in the subduction input is poorly understood, contrary to the subducted sediment (sediment after subduction) landward of the trench. We determined iodine concentration and ¹²⁹I/¹²⁷I ratio of the interstitial water from the seafloor to the basement continuously at a subduction input site ~250 km southwest of the Sunda Trench for the first time to understand the iodine distribution. In the study site, the iodine concentration increased with depth linearly to ~100 μM at 1400 mbsf. Iodine isotope ratios (¹²⁹I/¹²⁷I) remained constant as low as ~400 × 10⁻¹⁵ from 400 to 1400 mbsf, suggesting that the iodine distribution was mainly controlled by old iodine-rich fluid (low ¹²⁹I/¹²⁷I ratio and high iodine concentration) supplied along the basement and by mixing with seawater (high ¹²⁹I/¹²⁷I ratio and low iodine concentration). The linear iodine gradient was changed at ~200 and ~1200 mbsf, where the methane concentration rapidly increased and total organic carbon decreased. This indicates that young iodine (low ¹²⁹I/¹²⁷I ratio) was released from the organic materials in the sediment into the interstitial water at these depths. This is the first observation of in situ iodine/methane addition to the interstitial water associated with the organic decomposition. The iodine concentration and ¹²⁹I/¹²⁷I ratio indicated that iodine in the subduction input was either derived from the in situ sediment or allochthonous fluid transported from subducted sediment due to differences in physical properties and permeability. This allochthonous iodine transportation to the subduction input may broaden the concept of the iodine cycling in the subduction system, including the sediments after and before subduction.