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A Closer Look at Guam's Coral Reefs Reveals a Nuclear Timeline

By Sable Liggera, Communications Intern


In the mid to late 1950s, thermonuclear testing was carried out at Bikini and Enewetak atolls in the Marshall Islands - a region that was part of the post-World War II Pacific Proving Grounds. After conducting a radiocarbon (carbon-14) analysis on a coral reef tissue collected off the coast of Guam, Allen H. Andrews, a scientist with NOAA Pacific Islands Fisheries Science Center, found spikes in carbon-14 that correspond with the testing of thermonuclear devices during Operations Castle (1954), Redwing (1956), and Hardtack I (1958).

The Castle Bravo event, the detonation of the most powerful thermonuclear device ever tested by the United States.
Above is a picture of the Castle Bravo event, the detonation of the most powerful thermonuclear device ever tested by the United States.

Thermonuclear devices produce large amounts of carbon-14, which is released into the environment. While most of the carbon-14 from these tests went into the atmosphere as carbon dioxide, some diffused into the marine environment and were captured by carbon-synthesizing marine organisms, such as reef-building corals.

Reef-building corals rely on carbon from the environment to form a calcium carbonate (CaCO3) skeletal structure. This trait makes them valuable as records of changing carbon-14 levels in the environment. By performing radiocarbon analysis on coral cores, Andrews was able to determine the amount of carbon-14 that was present in the marine environment over time. This in turn can be used to validate the age of other marine organisms.

Andrews working with a micromilling machine, used to extract skeletal samples from marine organisms. Photo : NOAA
Above is a picture of Andrews working with a micromilling machine, used to extract skeletal samples from marine organisms. Credit : NOAA

The unique feature of the carbon-14 time series at Guam is that it has provided a direct measure of carbon-14 from not just natural air-sea diffusion, but local fallout at Bikini and Enewetak. These signals were transported in ocean currents to locations throughout the Indo-Pacific and have provided a unique opportunity to describe ocean circulation processes during that time.