Bomb-Produced Radiocarbon Across the South Pacific Gyre—a New Record From American Samoa With Utility for Fisheries Science

Abstract

Coral skeletal structures can provide a robust record of nuclear bomb produced 14C with valuable insight into air-sea exchange processes and water movement with applications to fisheries science. To expand these records in the South Pacific, a coral core from Tutuila Island, American Samoa was dated with density band counting covering a 59-yr period (1953–2012). Seasonal signals in elemental ratios (Sr/Ca and Ba/Ca) and stable carbon (δ13C) values across the coral core corroborated the well-defined annual band structure and highlighted an ocean climate shift from the 1997–1998 El Niño. The American Samoa coral 14C measurements were consistent with other regional records but included some notable differences across the South Pacific Gyre (SPG) at Fiji, Rarotonga, and Easter Island that can be attributed to decadal ocean climate cycles, surface residence times and proximity to the South Equatorial Current. An analysis of the post-peak 14C decline associated with each coral record indicated 14C levels are beginning to merge for the SPG. This observation, coupled with otolith measurements from American Samoa, reinforces the perspective that bomb 14C dating can be performed on fishes and other marine organisms of the region using the post-peak 14C decline to properly inform fisheries management in the South Pacific.

Atmospheric Radiocarbon for the Period 1950–2019

“…After reaching its peak levels, atmospheric Δ 14 C in the NH and SH has decreased since 1963-1964 and 1964-1965, respectively. Decreases in atmospheric Δ 14 C from the mid-1960s to mid-1980s are mainly due to rapid exchange between the atmosphere and the biosphere and oceans (Oeschger et al 1975;Druffel and Suess 1983;Levin and Hesshaimer 2000), while combustion of fossil fuels free of 14 C is the main causal factor for the Δ 14 C decline since the late 1980s and early 1990s (Levin et al 2010;Graven et al 2012a). Since the early and late 2000s, the atmospheric Δ 14 C values have been lower than those of the surface waters in the North and South Pacific Gyres, respectively, indicating the oceans might become a net 14 C source (instead of a net 14 C sink) of the atmosphere (Andrews et al 2016(Andrews et al , 2021Wu et al 2021). The last data points in our compiled monthly data at 2019.375 have respective F 14 C values of 1.0084 and 1.0195 for the NH and SH (see Supplementary Tables 2a-e), which are very close to the pre-bomb F 14 C value of slightly lower than 1.…”

“…Decreases in atmospheric Δ 14 C from the mid-1960s to mid-1980s are mainly due to rapid exchange between the atmosphere and the biosphere and oceans (Oeschger et al 1975; Druffel and Suess 1983; Levin and Hesshaimer 2000), while combustion of fossil fuels free of 14 C is the main causal factor for the Δ 14 C decline since the late 1980s and early 1990s (Levin et al 2010; Graven et al 2012a). Since the early and late 2000s, the atmospheric Δ 14 C values have been lower than those of the surface waters in the North and South Pacific Gyres, respectively, indicating the oceans might become a net 14 C source (instead of a net 14 C sink) of the atmosphere (Andrews et al 2016, 2021; Wu et al 2021).…”