Seawater 129I/127I ratios in the various seas of the Philippines

Dr Angel Bautista VII1, Ms. Sophia Jobien Limlingan1, Ms. Mary Margareth Bauyon1, Mr. Remjohn Aron Magtaas1, Mrs. Eliza Enriquez1, Mr. Jeffrey Munar2, Mr. Edwin Dumalagan, Jr.2, Dr. Haruka Kusuno3, Dr. Caroline Marie Jaraula2, Prof. Fernando Siringan2, Prof. Hiroyuki Matsuzaki3

1Philippine Nuclear Research Institute (DOST-PNRI), Quezon City, Philippines, 2University of the Philippines – Marine Science Institute (UP-MSI), Quezon City, Philippines, 3Micro Analysis Laboratory Tandem Accelerator (MALT), The University of Tokyo, Tokyo, Japan

Iodine-129 is a long-lived radioactive isotope produced as a fission product in human nuclear activities (HNAs) such as nuclear accidents, nuclear fuel reprocessing, concentration of weapons-grade nuclear material, and nuclear weapons testing. Previous measurements of 129I in coral cores from the West Philippine Sea (WPS) have shown elevated levels compared to those observed from the Pacific Ocean side of the Philippines (Bautista et al., 2016, 2021). For this reason, we wanted to look at the current 129I levels in seawater in the WPS compared to the Pacific Ocean and the rest of the country to understand better the possible sources, implications, and applications of the elevated 129I in the WPS.

Materials and Methods
A total of 79 surface seawater samples were collected over the course of three years from the West Philippine Sea, Philippine Rise, the Tubbataha Area in the Sulu Sea, and several other areas across the country. Iodine was extracted from the samples via multi-stage solvent extraction procedures and then analyzed for 129I and 127I via Accelerator Mass Spectrometry (AMS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in MALT, The University of Tokyo.

Results and Discussion
The average 129I/127I ratios in the WPS have been found to occur at around 2.4 to 3.7 x 10-11, which is at an average of 9.1%, 29.9%, and 13.3% higher than those observed in the Philippine Rise, Sulu Sea, and in the other areas studied, respectively. The seawater samples have also shown location-based variation, suggesting that the elevated 129I in the WPS comes from sources located along the prevailing currents.

Using data from all over the Philippines, we demonstrate that the elevated 129I level in the WPS initially detected in corals has been sustained and isolated in the WPS. Results suggest a regional source for the 129I in the WPS. Moreover, this study is proof that, since 129I is elevated in the WPS, it can be used as a quantitative oceanographic tracer for the complicated circulation and water mass mixing in the WPS, South China Sea, and other neighboring bodies of water.


Angel Bautista VII is a chemist and the current Section Head of the Nuclear Materials Research Section of the Philippine Nuclear Research Institute (DOST-PNRI). His research interests include applications of AMS in studies on paleoenvironment, hydrology, climate change, disasters, archaeology, and the nuclear industry. Some of his ongoing research projects include I-129 in coral cores and seawater in the Philippines and in an ice-core from SE-Dome Greenland.

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Nov 18 2021


10:00 am - 10:55 am