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[MIS22-05] 129I and halogen distributions at gas hydrate field offshore Hikurangi subduction zone
Keywords:Pore water, Radioactive iodine isotope, 127I, Halogens, IODP Exp.372/375, Hikurangi Subduction Margin
Due to their biophilic characteristics on living organisms, iodine and bromine have a strong affinity with organic matter in sediments and methane dissolved in pore water. Iodine in pore water is almost 100% stable isotope (127I), and a minimal amount of radioactive isotope (129I) has a half-life of 15.7 million years. Since 129I is an isotope with a long half-life, lower isotopic ratios indicate an older origin of iodine, which has been used as a tracer for methane and organic matter in plate subduction zones and gas hydrate fields. Chloride is a conservative ion; therefore, the concentration is altered by water molecules input or consumption, such as gas hydrate formation/dissociation.
This study investigates the distribution of 129I and halogens dissolved in interstitial water in the gas hydrate field, Hikurangi subduction zone. Coring at Site U1517 and U1518 was conducted by JOIDES Resolution during IODP Exp. 372 and 375.
Bromine and iodine concentrations at Site U1517 generally increase with depth from seawater concentration to 1500 µM and 580 µM between the seafloor and 180 mbsf, respectively. However, bromine and iodine concentrations rapidly decrease, similar to the chloride concentration by freshened water derived from gas hydrate decomposition at 150 mbsf. Iodine concentration decreases to 300 µM and strongly reflects the effect of the hydrate decomposition water compared to bromine. 129I/I decrease rapidly from 361×10-15 to 144×10-15 between 0 mbsf and 70 mbsf, indicating well mixing with young iodine derived from seawater with high 129I/I. Below 70 mbsf, 129I/I are constant at 200×10-15 to 250×10-15, indicating that old iodine derived from deep sediment predominates regardless of the depth of the gas hydrate. At Site U1518, bromine and iodine concentrations increase to 1600 µM and 600 µM between 0 and 60 mbsf, respectively, and remain constant with no significant concentration fluctuations from 60 mbsf to 500 mbsf. At five depths (33, 52, 71, 155, and 391 mbsf), local chloride depletion due to freshened water from hydrate decomposition was observed, suggesting the presence of multiple thin gas hydrate layers. Bromine and iodine concentrations also decreased at the same depths, indicating the influence of freshened water. 129I/I decrease from 361×10-15 to 144×10-15 rapidly between 0 mbsf and 70 mbsf, suggesting that the iodine isotope ratios are well mixed with young iodine derived from seawater with high isotope ratios, as is the case with U1517. However, from 70 to 160 mbsf, the isotope ratio increases from 164×10-15 to 180×10-15, and young iodine predominates with depth. This indicates that younger iodine is supplied from deeper sediments due to the inversion layer caused by the thrust fault. The concentration of halogens at Site U1517 and U1518 well reflects the depth of the gas hydrates. On the other hand, 129I/I strongly reflects the dominance of deep-derived old fluid, regardless of the presence of gas hydrates.
This study investigates the distribution of 129I and halogens dissolved in interstitial water in the gas hydrate field, Hikurangi subduction zone. Coring at Site U1517 and U1518 was conducted by JOIDES Resolution during IODP Exp. 372 and 375.
Bromine and iodine concentrations at Site U1517 generally increase with depth from seawater concentration to 1500 µM and 580 µM between the seafloor and 180 mbsf, respectively. However, bromine and iodine concentrations rapidly decrease, similar to the chloride concentration by freshened water derived from gas hydrate decomposition at 150 mbsf. Iodine concentration decreases to 300 µM and strongly reflects the effect of the hydrate decomposition water compared to bromine. 129I/I decrease rapidly from 361×10-15 to 144×10-15 between 0 mbsf and 70 mbsf, indicating well mixing with young iodine derived from seawater with high 129I/I. Below 70 mbsf, 129I/I are constant at 200×10-15 to 250×10-15, indicating that old iodine derived from deep sediment predominates regardless of the depth of the gas hydrate. At Site U1518, bromine and iodine concentrations increase to 1600 µM and 600 µM between 0 and 60 mbsf, respectively, and remain constant with no significant concentration fluctuations from 60 mbsf to 500 mbsf. At five depths (33, 52, 71, 155, and 391 mbsf), local chloride depletion due to freshened water from hydrate decomposition was observed, suggesting the presence of multiple thin gas hydrate layers. Bromine and iodine concentrations also decreased at the same depths, indicating the influence of freshened water. 129I/I decrease from 361×10-15 to 144×10-15 rapidly between 0 mbsf and 70 mbsf, suggesting that the iodine isotope ratios are well mixed with young iodine derived from seawater with high isotope ratios, as is the case with U1517. However, from 70 to 160 mbsf, the isotope ratio increases from 164×10-15 to 180×10-15, and young iodine predominates with depth. This indicates that younger iodine is supplied from deeper sediments due to the inversion layer caused by the thrust fault. The concentration of halogens at Site U1517 and U1518 well reflects the depth of the gas hydrates. On the other hand, 129I/I strongly reflects the dominance of deep-derived old fluid, regardless of the presence of gas hydrates.