Bomb‐produced radiocarbon in the western tropical Pacific Ocean: Guam coral reveals operation‐specific signals from the Pacific Proving Grounds

Abstract

High‐resolution radiocarbon (14C) analyses on a coral core extracted from Guam, a western tropical Pacific island, revealed a series of early bomb‐produced 14C spikes. The typical marine bomb 14C signal—phase lagged and attenuated relative to atmospheric records—is present in the coral and is consistent with other regional coral records. However, 14C levels well above what can be attributed to air‐sea diffusion alone punctuate this pattern. This anomaly was observed in other Indo‐Pacific coral records, but the Guam record is unmatched in magnitude and temporal resolution. The Guam coral Δ14C record provided three spikes in 1954–1955, 1956–1957, and 1958–1959 that are superimposed on a normal 14C record. Relative to mean prebomb levels, the first peak rises an incredible ∼700‰ and remained elevated for ∼1.2 years. A follow up assay with finer resolution increased the peak by ∼300‰. Subsequent spikes were less intense with a rise of ∼35 and ∼70‰. Each can be linked to thermonuclear testing in the Pacific Proving Grounds at Bikini and Enewetak atolls in Operations Castle (1954), Redwing (1956), and Hardtack I (1958). These 14C signals can be explained by vaporization of coral reef material in the nuclear fireball, coupled with neutron activation of atmospheric nitrogen (14C production), and subsequent absorption of 14CO2 to form particulate carbonates of close‐in fallout. The lag time in reaching Guam and other coral records abroad was tied to ocean surface currents and modeling provided validation of 14C arrival observations.

Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

“…Although some portion of Ishigaki coral in 1970s was not able to recover the data, overall structure can be captured clearly. Three early 14 C spikes are observed in the Ishigaki coral for the periods from January 1955 to February 1957, September 1957 to August 1959, and March 1960 to September 1961, which confirms the recent findings for Guam corals [ Andrews et al ., ] (Figure ).…”

Radiocarbon variability recorded in coral skeletons from the northwest of Luzon Island, Philippines

“…3a). The average Δ 14 C value in the 1970s was 146.7 ‰, which lies between the average Δ 14 C values in Guam (Andrews et al 2016) and Ishigaki (Hirabayashi et al 2017a) and is lower than that in Con Dao (Mitsuguchi et al 2007) (Fig. 2).…”

“…We also compare these data with the coral data reported from Ishigaki (Hirabayashi et al 2017a), Guam (Andrews et al 2016), Con Dao (Mitsuguchi et al 2007), and Hon Tre Island (Bolton et al 2016) to understand the relationship between North Pacific western boundary currents and SCS circulation around the Luzon Strait.…”

“…High-resolution 14 C measurements of coral skeletons can be used to reconstruct continuous and seasonal/ interannual variability in ocean conditions (Guilderson et al 2000;Druffel et al 2014;Andrews et al 2016). However, post-bomb 14 C data are scarcely available for the SCS (e.g., Mitsuguchi et al 2007;Shen et al 2003).…”

Upwelling in the Ocean Basins North of the ACC: 1. On the Upwelling Exposed by the Surface Distribution of Δ14C

“…The rest of the Indian Ocean has prebomb deficits between −22 and −28. The smallest deficit in this group is from Cocos atoll at 12°S, which lies along the path of the Indonesian Throughflow from the Pacific (Andrews, Asami, et al, ; Toggweiler & Trumbore, ). While the −22 deficit from Cocos atoll and the −21 deficit in Lombok Strait are similar, the rest of the prebomb deficits in this part of the Indian Ocean are larger.…”