Variation of ^239,240Pu in Coral and Its Response to the Climate System in South China Sea

With the acceleration of climate change, understanding the behavior of the anthropogenic radioactive substances─particularly their responses to the climate system─has become critical for assessing their transport, transfer, and impact on the ecosystems. However, this remains underexplored, particularly in the South China Sea (SCS), where radioactivity is derived from both the close-in fallout of the Pacific Proving Ground (PPG) and the global fallout. Additionally, this region is quite sensitive to climate change. A coral core collected from Xisha Island, SCS, was initially analyzed for high-radiotoxicity ^239,240Pu. Approximately 72-84% of plutonium in coral originated from the close-in fallout of PPG through ocean current compared to the direct global fallout. However, the ^239,240Pu concentration still remains in background levels and does not show a significant radiation risk. After 1980, a distinct pattern emerged characterized by a "higher" concentration but a "lower" ²⁴⁰Pu/²³⁹Pu atom ratio compared to the levels in the open west Pacific. This is primarily attributed to the seasonal upwelling of subsurface seawater on the continental shelf of SCS, driven by the prevailing southwest monsoon. Significantly elevated ^239,240Pu concentrations were observed during typical ENSO years 1983, 1988, and 1997. This is due to the elevated temperature, coral bleaching, and expulsion of symbiotic zooxanthellae. After expulsion, zooxanthellae containing higher ^239,240Pu compared to skeleton rapidly die, and their debris directly deposit onto the coral skeleton, in contrast to the metabolic way of ^239,240Pu during normal years. This finding offers critical insights into ecosystem protection in SCS amid global changes and the potential threat of nuclear contamination.