Time-resolved record of ²³⁶U and ^239,240Pu isotopes from a coral growing during the nuclear testing program at Enewetak Atoll (Marshall Islands)

A comprehensive series of nuclear tests were carried out by the United States at Enewetak Atoll in the Marshall Islands, especially between 1952 and 1958. A Porites Lutea coral that was growing in the Enewetak lagoon within a few km of all of the high-yield tests contains a continuous record of isotopes, which are of interest (e.g. ¹⁴C, ²³⁶U, ^239,240Pu) through the testing period. Prior to the present work, ¹⁴C measurements at ∼2-month resolution had shown pronounced peaks in the Δ¹⁴C data that coincided with the times at which tests were conducted. Here we report measurements of ²³⁶U and ^239,240Pu on the same coral using accelerator mass spectrometry, and again find prominent peaks in the concentrations of these isotopes that closely follow those in ¹⁴C. Consistent with the ¹⁴C data, the magnitudes of these peaks do not, however, correlate well with the explosive yields of the corresponding tests, indicating that smaller tests probably contributed disproportionately to the debris that fell in the lagoon. Additional information about the different tests can also be obtained from the ²³⁶U/²³⁹Pu and ²⁴⁰Pu/²³⁹Pu ratios, which are found to vary dramatically over the testing period. In particular, the first thermonuclear test, Ivy-Mike, has characteristic ²³⁶U/²³⁹Pu and ²⁴⁰Pu/²³⁹Pu signatures which are diagnostic of the first arrival of nuclear test material in various archives.