Isotopic Composition and Distribution of Plutonium in Northern South China Sea Sediments Revealed Continuous Release and Transport of Pu from the Marshall Islands

Abstract

The (239+240)Pu activities and (240)Pu/(239)Pu atom ratios in sediments of the northern South China Sea and its adjacent Pearl River Estuary were determined to examine the spatial and temporal variations of Pu inputs. We clarified that Pu in the study area is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds in the Marshall Islands where above-ground nuclear weapons testing was carried out during the period of 1952-1958. The latter source dominated the Pu input in the 1950s, as evidenced by elevated (240)Pu/(239)Pu atom ratios (>0.30) in a dated sediment core. Even after the 1950s, the Pacific Proving Grounds was still a dominant Pu source due to continuous transport of remobilized Pu from the Marshall Islands, about 4500 km away, along the North Equatorial Current followed by the transport of the Kuroshio current and its extension into the South China Sea through the Luzon Strait. Using a simple two end-member mixing model, we have quantified the contributions of Pu from the Pacific Proving Grounds to the northern South China Sea shelf and the Pearl River Estuary are 68% ± 1% and 30% ± 5%, respectively. This study also confirmed that there were no clear signals of Pu from the Fukushima Daiichi Nuclear Power Plant accident impacting the South China Sea.

129I and its species in the East China Sea: level, distribution, sources and tracing water masses exchange and movement

“…Besides those injected to the stratosphere and contributed to the global fallout, large amount of radioactive substances from these NWTs were deposited in the southern Pacific Ocean surrounding the Marshall Islands (close-in fallout). A number of investigations have reported that the close-in fallout of plutonium in the PPG has being carried by the north equatorial current, which is followed by the Kuroshio Current (KC) and continuously transported to the South China Sea and ECS2829. The constantly significant PPG derived Pu signal observed in the ECS was attributed to the continuous re-suspension of the highly deposited plutonium in the sediment in the PPG region2829.…”

“…A number of investigations have reported that the close-in fallout of plutonium in the PPG has being carried by the north equatorial current, which is followed by the Kuroshio Current (KC) and continuously transported to the South China Sea and ECS2829. The constantly significant PPG derived Pu signal observed in the ECS was attributed to the continuous re-suspension of the highly deposited plutonium in the sediment in the PPG region2829. Compared to plutonium, iodine is water soluble and conservative in ocean, the close-in fallout of 129 I at the PPG can be also transported to the ECS in early time of the NWTs at PPG.…”

Fukushima Daiichi–Derived Radionuclides in the Ocean: Transport, Fate, and Impacts

“…In addition, Pu from the FDNPPs was not detected in the marginal seas of the Pacific, such as the South China Sea (Wu et al 2014). The challenge in these studies, as on land, is in separating the weapons-testing component of Pu that was already present in the environment from an even smaller additional component of FDNPP Pu.…”

“…Although Pu and Am are more particle reactive in their geochemistry, measurements of their activities and isotopic ratios in sediments reflect atmospheric weapons testing rather than a smaller FDNPP source (Zheng et al 2012, Bu et al 2013, Wu et al 2014, Oikawa et al 2015.…”

Continuous transport of Pacific-derived anthropogenic radionuclides towards the Indian Ocean

“…It has been known for several years that plutonium from the PPG can be found in the Northwest Pacific marginal seas, particularly in the East China Sea91011 and along the Japanese west coast12. Detectable PPG signals are also reported in sediments from the Sulu and South China Seas1314.…”