In the 1950s, U.S. nuclear tests were conducted at Bikini and Enewetak; decades later, Guam corals preserved carbon-14 spikes linked to tests in 1954, 1956, and 1958

Beneath Guam's coastal waters, centuries-old coral reefs hold a chemical record of the Cold War's most powerful nuclear tests. NOAA Fisheries scientists examined the reefs and found chemical spikes from 1954, 1956, and 1958, the years the U.S. detonated its largest thermonuclear devices at Bikini and Enewetak atolls in the Pacific Proving Grounds, according to a 2016 study by NOAA researcher Allen H. Andrews published in the Journal of Geophysical Research: Oceans.

Uncovering history buried in Pacific coral reefs

The mid-20th century marked an unprecedented era of atmospheric nuclear testing in the Marshall Islands, located roughly 1,200 miles east of Guam. As these massive detonations vaporized coral reefs and superheated the surrounding air, they released intense neutrons that created high concentrations of radiocarbon. While scientists knew that atmospheric testing dispersed radioactive isotopes across the globe, understanding how those particles traveled through oceanic pathways remained uncertain. Analyzing the coral core gave researchers an unusually detailed, high-resolution record, one NOAA describes as unmatched in magnitude and resolution among Indo-Pacific coral cores, of how nuclear fallout moved through the tropical Pacific.


​How corals store radiocarbon

This natural tracking method is based on coral features. Coral species called Porites build skeletons that are made of calcium carbonate. These skeletons get their dissolved carbon from the seawater. When the fireballs exploded at Bikini and Enewetak atolls, the extreme heat broke down the reef structure into tiny pieces of carbonates. These pieces absorbed carbon-14 before sinking into the ocean.

The three distinct atomic test spikes

Three separate radiocarbon spikes have been noted in the coral core extracted from Guam. These correspond to specific nuclear operations that increased marine radiocarbon values above background levels. Operation Castle in 1954 caused the largest increase in marine radiocarbon values, a spike of about 700% (parts per thousand) above pre-bomb levels. A follow-up assay with finer resolution later revised this peak upward to roughly 300% above the surrounding trend. Operation Redwing in 1956 caused the smallest of the three increases, with radiocarbon values rising by about 35% above the trend. Operation Hardtack I in 1958 caused the third spike, occurring during 1958–59. Radiocarbon values rose by about 70% above the background trend, roughly double Redwing's spike, but far smaller than Castle's.

The record's full sixty-year span

The finding mattered not only because of the three individual spikes, but because one coral sample contained three chemically distinct markers linked to three separate nuclear operations. A closer look at the same data later revealed additional structure in the first (1954) peak that wasn't captured in the original measurement, suggesting the core holds more detail than initially reported.

The period covered in this dataset extends well beyond the bomb-testing period. According to the NOAA Fisheries InPort record, the usable time frame of the radiocarbon concentration in this coral covers 1939 to 2000, providing a roughly six-decade record of the ocean's chemical state before, during, and after those tests. The full dataset, available from the NOAA National Centers for Environmental Information, allows researchers to compare radiocarbon concentrations before and after the tests. That long-term view is what gives the record its lasting value: scientists still use it today to determine the true age of marine species, including reef fish and giant clams. It enables comparison of radiocarbon concentrations before and after the events. Such long-term data is crucial to the record's significance, as it is used today to determine the true age of marine species, including reef fish and giant clams.

Ocean transport dynamics and modern research value

These sudden, intense peaks were unusual because typical marine bomb-radiocarbon signals enter surface waters slowly through air-sea gas diffusion, producing a more gradual curve. However, the rapid entry of these Guam peaks confirmed that they were fallout from pulverized coral material moving through fast oceanographic currents. The dataset's NOAA record shows that ocean surface currents carried these radiotracers to Guam.

What the coral records show

Marine biologists have used these radiocarbon markers mainly to validate the ages of long-lived reef species. The pronounced spike in the 1950s can serve as a marker for calculating the age and growth rate of marine life, ocean currents, and climatic variations.