[SCG56-P25] Origin and migration of fluid in the Sumatra subduction input: application of a geochronological method using a long-lived radioisotope of dissolved iodine
Keywords:129I, Iodine geochronology, Interstitial water, Sumatra subduction input, IODP Exp.362
Concentrations of iodine and methane increase and TOC decreases toward 200 meters below seafloor (mbsf) and 1250 mbsf, which indicate degradation of organic matter around that depth and subsequent release of iodine and methane into the interstitial water. Iodine gradient is controlled by the degradation of iodine-rich organic matter microbially in shallow sediment (at 200 mbsf) and geothermally in deep sediment (at 1250 mbsf), respectively. The 129I/127I ratios between 0 and 400 mbsf vary between 300 ×10-15 and 800 ×10-15, and the lowest ratio of 300×10-15 is found at 175 mbsf. There are some thick sand layers > 60 meters between 0 and 400 mbsf. These porous layers transport allochthonous dissolved iodine and interstitial water, which are relatively old. However 129I/127I of 1300×10-15 at 67 mbsf corresponding to the sediment age of 3 Ma at 200 mbsf which is the zone of degradation of organic matter. 129I/127I between 400 to 1480 mbsf decrease from 700 ×10-15 to 400×10-15, resulting from mixing with deep (old) iodine and shallow (young) iodine. In the input sediment, iodine gradient is strongly controlled by degradation of organic matter and the migration is controlled by vertical diffusion which indicates poor fluid migration or slow migration rate. However, the horizontal advection/migration of interstitial water is significant within the thick-porous sandy layers.