Observation of the deep Indonesian throughflow using helium isotopes

Helium isotopes in the ocean are very useful tracers of hydrothermal input and deep-sea circulation. Unlike labile trace elements, noble gases are inert and do not react with other substances in the water column. Previous studies in the Indian Ocean showed that helium isotopes in the region were discharged mainly from the central Indian midocean ridge, convergent boundaries, and the Pacific Ocean via currents that flow through the Indonesian archipelago. These Indonesian waters then exit out as the Indonesian Throughflow (ITF) into the Indian Ocean through shallow sills. The ITF plays a significant role in global climate, because it serves as the main pathway of thermocline waters between the Pacific Ocean and Indian Ocean. We present new helium isotope data from International Indian Ocean Expedition (IIOE-2) at the east Indian Ocean in December 2018. Helium isotopes were measured using a Noble Gas Mass Spectrometer at the Atmosphere and Ocean Research Institute, The University of Tokyo. We compare our data with those from previous expeditions and discuss the source and distribution of helium isotopes near Java. Waters with high excess(3)He from the ITF were observed at 1000 m. Application of two-components mixing model enabled us to estimate ITF fraction in the observed area and to clarify the ITF flow in the deep layer. Moreover, flow rate of the deep ITF from Savu sill to Indian Ocean was estimated to be similar to 0.8-1.6 Sv. Based on stations near the Java coast, there could be a deep current or hydrothermal source at 3000 m that advects water enriched in(3)He southwards along the Java Island.