10:45 〜 12:15
[MIS18-P09] 129I and halogen distributions at gas hydrate field offshore Hikurangi subduction zone
キーワード：ガスハイドレート、ハロゲン、129I、間隙水、国際深海科学掘削計画 第372・375次航海
Iodine and bromine have a strong affinity with organic matter in sediments and methane dissolved in pore water due to their biophilic characteristics on living organisms. 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 and mineral dehydration.
In this study, we investigate 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.
At site U1517, bromine and iodine concentrations increase with depth from 0 to 180 mbsf to 1500 µM and 580 µM, respectively. However, bromine and iodine concentrations decrease similarly to the chloride concentration by freshened water derived from gas hydrate decomposition at 150 mbsf. Iodine concentration decreases to 300 µM and more strongly reflects the effect of the GH decomposition water compared to bromine. 129I/I decrease rapidly from 361×10-15 to 144×10-15 between 0 to 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 to 500 mbsf. At Site U1518, bromine and iodine concentrations increase to 1600 µM and 600 µM between 0 and 60 mbsf, respectively, and are constant with no significant concentration fluctuations from 60 to 500 mbsf. At five depths (33, 52, 71, 155, and 391 mbsf), localized chloride depletion due to freshened water from gas hydrate 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 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 than shallower sediments due to the presence of younger sediments in the lower part of the formation caused by the inversion layer caused by the thrust fault. The halogen distribution at the two sites indicates that the halogen concentration well reflects the location of the gas hydrates. On the other hand, the 129I/I does not sufficiently reflect the presence or absence of gas hydrate, indicating that the detailed differences among sites strongly reflect the relationship between the age of host sediments and the deep-derived fluids.
In this study, we investigate 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.
At site U1517, bromine and iodine concentrations increase with depth from 0 to 180 mbsf to 1500 µM and 580 µM, respectively. However, bromine and iodine concentrations decrease similarly to the chloride concentration by freshened water derived from gas hydrate decomposition at 150 mbsf. Iodine concentration decreases to 300 µM and more strongly reflects the effect of the GH decomposition water compared to bromine. 129I/I decrease rapidly from 361×10-15 to 144×10-15 between 0 to 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 to 500 mbsf. At Site U1518, bromine and iodine concentrations increase to 1600 µM and 600 µM between 0 and 60 mbsf, respectively, and are constant with no significant concentration fluctuations from 60 to 500 mbsf. At five depths (33, 52, 71, 155, and 391 mbsf), localized chloride depletion due to freshened water from gas hydrate 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 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 than shallower sediments due to the presence of younger sediments in the lower part of the formation caused by the inversion layer caused by the thrust fault. The halogen distribution at the two sites indicates that the halogen concentration well reflects the location of the gas hydrates. On the other hand, the 129I/I does not sufficiently reflect the presence or absence of gas hydrate, indicating that the detailed differences among sites strongly reflect the relationship between the age of host sediments and the deep-derived fluids.