Sources and transport of plutonium in the Indo-Pacific Intersection: Implications for South China Sea freshwater transport into Indonesian Seas

Abstract The Indo-Pacific Intersection (IPI), centered in the Indonesian Seas (IS) and South China Sea (SCS), is a convergence zone of both hydrological and atmospheric circulations and plays an important role in the water cycle and global climatic system. This study presents a unique dataset of both 239+240Pu activities and 240Pu/239Pu atom ratios to trace Plutonium (Pu) source terms and their transport in the IPI, to examine constituent water masses, notably freshwater fluxes. The 240Pu/239Pu atom ratios in surface IPI seawater ranged from 0.194 to 0.258 with an average of 0.223 ± 0.021, values that were consistently higher than global fallout (~0.18). 239+240Pu activities ranged widely from 0.55 to 2.8 mBqm−3 with a generally accumulating trend from the western Pacific Ocean (WP) to the IPI as previously observed. We further reveal, using a two end-member mixing model between global fallout and Pacific Proving Grounds (PPG), that the PPG source contributes 34 ± 8% of the Pu to the IS. In the IS, both 239+240Pu activities and 240Pu/239Pu atom ratios further show values between those in the WP and SCS, indicating a two-endmember mixing scheme between the SCS (characteristically lower-salinity but higher PPG Pu-fraction) and the WP (higher-salinity and higher PPG Pu-fraction). We estimate the fractional contribution of the SCS water mass as 35 ± 16% in the surface mixed layer and 49 ± 14% in the IS 50–235 m depth interval. This water mass transport is equivalent to a freshwater influx of 80 ± 47 mSv from the SCS to the IS (known as the South China Sea Throughflow (SCSTF), a branch of the Indonesian Throughflow (ITF)). It accounts for over half (53 ± 31%) of the total freshwater flux transported by the ITF into the Indian Ocean and supplies ~24% the freshwater net evaporation in the Indian Ocean over the 8°N – 32°S latitudinal range, and it is thus crucial in maintaining the fresh water balance therein. This freshwater influx to the IS from the SCS also leads to a ~ 0.2 kg m−3 decrease in the mean density of the upper 235 m water layer, which serves to inhibit the transport of upper layer warm water from the western tropical Pacific warm pool into the Indian Ocean. Therefore, the SCS, together with the SCSTF, play a critical role in regulating freshwater and heat balance in the IPI and Indian Ocean. These findings have strong implications for the integrative role of the SCS and SCSTF on the Indo-Pacific climate and water cycle.