The study, published in the peer-reviewed Chinese language journal Environmental Chemistry, traced plutonium particles found in sediments on the seabed of the South China Sea to nuclear weapons tests in the US Pacific Proving Ground, a series of atolls and islands in the South Pacific nearly 3,000 miles to the east, where the US detonated dozens of nuclear weapons between 1946 and 1962.
Their conclusion was based on a precise identification of the ratio of two plutonium isotopes, 240Pu and 239Pu, which is a unique part of the process of manufacturing plutonium. The ratio in the South China Sea sediments is identical to that left by US nuclear weapons tests at the PPG, with a range of 0.306-0.36.
According to the scientists, the radioactive sediment was carried westward by the North Pacific Gyre, an ovular ocean current that rotates clockwise. The current comes west through the Marshall Islands, Micronesia, and Caroline Islands before hitting the Philippines and Taiwan, swinging north, and then turning east along the southern coast of Japan. On the California coast, it turns south until it is forced westward again by other currents along the Mexican coast.
The sediments would have passed along this current, entering the South China Sea via the Luzon Strait that separates Luzon from Taiwan.
The research team in particular used sediments collected from Nansha, or the Spratly Islands, but also looked at prior research from other parts of the South China Sea, and found that US nuclear weapons created 7.15% and 15.89% of the plutonium in Nansha and 87% of the overall plutonium pollution in the northern parts of the South China Sea that are close to the Luzon Strait. However, the amount of pollution across the sea varies widely.
The US began using several remote Pacific islands to test nuclear weapons in the period immediately after World War II, which was forced to an end by Washington’s use of nuclear bombs to destroy the Japanese cities of Hiroshima and Nagasaki in August 1945.
Between 1946 and 1958, the US detonated 67 nuclear devices in the Marshall Islands, amounting to a total explosive yield equal to roughly 210 megatons of TNT, or an average of two Hiroshima-sized bombs every two days. The tests included underwater, surface, and upper atmospheric detonations, testing the viability of the devices as anti-naval, anti-surface, and even anti-satellite weapons.
Castle Bravo, the US’ first thermonuclear bomb, detonated in 1954 on Bikini Atoll, was much larger than scientists anticipated and threw up vast amounts of radioactive dust into the atmosphere, which fell across a wide region of sea that included several inhabited islands and a Japanese fishing boat, Daigo Fukuryū Maru, afflicting the crew of 23 with acute radiation poisoning and killing one of them.