Bomb Radiocarbon and the Hawaiian Archipelago: Coral, Otoliths, and Seawater

Abstract

Corals of the Hawaiian Archipelago are well situated in the North Pacific Gyre (NPG) to record how bomb-produced radiocarbon has been sequestered and transported by the sea. While this signal can be traced accurately through time in reef-building corals and used to infer oceanographic processes and determine the ages of marine organisms, a comprehensive and validated record has been lacking for the Hawaiian Archipelago. In this study, a coral core from Kure Atoll in the northwestern Hawaiian Islands was used to create a high-resolution bomb 14C record for the years 1939–2002, and was then used with other 14C measurements in fish otoliths and seawater to explore differences and similarities in the bomb 14C signal throughout the Hawaiian Archipelago. The Kure Atoll sample series produced a well-defined bomb 14C curve that, with some exceptions, was similar to other coral 14C records from the Hawaiian Archipelago. Subtle differences in the coral 14C records across the region may be explained by the large-scale ocean circulation patterns and decadal cycles of the NPG. The most rapid increase of 14C, in the 1950s and 1960s, showed similar timing across the Hawaiian Archipelago and provides a robust basis for use of bomb 14C dating to obtain high-precision age determinations of marine organisms. Reference otoliths of juvenile fish demonstrated the use of the post-peak 14C decline period as a viable reference in the age validation of younger and more recently collected fishes, and effectively extended the utility of bomb 14C dating to the latest 30 yr.

Bomb‐14C Peak in the North Pacific Recorded in Long‐Lived Bivalve Shells (Mercenaria stimpsoni)

“…The D 14 C estimates in Hachinohe and Hakodate, north of Otsuchi, and in Shiogama, south of Otsuchi , both of which are under the influence of the Tsugaru Current, are consistent with our estimate based on M. stimpsoni shells ( Figure 6). The D 14 C values in the Tsugaru Current (Otsuchi, this study) were 20& lower during 1930-1950 than those in the Kuroshio Current, as represented by Hawaiian corals (Andrews et al, 2016b;Druffel et al, 2001) ( Figure 6) (R 5 372 6 22 yr, N 5 7). Similarly, the D 14 C values in the Tsugaru Current (Hachinohe, Hakodate, and Shiogama; Yoneda et al, 2007;Yoshida et al, 2007) were 5& lower during 1890-1910 than those in the Kuroshio Current (R 5 380 6 16 yr, N 5 3).…”

“…The oldest seawater D 14 C value in the western subtropical gyre in the GLODAP dataset was obtained at GEOSECS Station 222 (water depth of 10 m). As representatives of the Kuroshio Current, the D 14 C records of reef-building corals at Kure Atoll (288N, 1788W, yellow; Andrews et al, 2016b) and Hawaii (218N, 1608W, black; Druffel et al, 2001), which are outside of Figure 6a, are also shown. In both pre-bomb and post-bomb eras, the D 14 C values are higher in the Kuroshio Current (Northwestern Pacific Subtropical Gyre) and lower in the Oyashio Current (Western Subarctic Gyre).…”

“…(a) The bomb-14 C record reconstructed from M. stimpsoni shells (red) and seawater D 14 C values in Funakoshi Bay (blue diamonds) and Otsuchi Bay (light blue triangles). For comparison, the bomb-14 C records reconstructed from reef-building coral skeletons in the tropical to subtropical Pacific are shown: Galapagos (green, 18S, 908W;Druffel, 1987), Nauru (blue, 18S, 1678E;Guilderson et al, 1998Glynn et al, 2013), and Kure Atoll(yellow, 288N, 1788W;Andrews et al, 2016b). Atmospheric D 14 C records (black lines) for the years 1930-1950 are from compiled data from INTCAL13…”

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

“…To provide a regional context to the Guam Δ 14 C reference, other coral records from previous studies were considered (Figure ). The closest complete records were from Okinawa and Hawaii (FFS and Kure Atoll) to the north and Nauru and Palau to the south [ Konishi et al ., ; Druffel , ; Guilderson et al ., ; Glynn et al ., ; Andrews et al ., ]. Two fragment records were also available from Pohnpei (1971–1979) [ Konishi et al ., ] and the southwestern coast of Guam at Taelayag Reef (1978–1981) [ Moore et al ., ], each of which provided some initial information on how the full bomb 14 C curve may behave for Guam.…”

“…Based on general latitudinal trends in seawater Δ 14 C of the Pacific Ocean—attributed to CO 2 saturation states of tropical versus temperate waters [ Broecker and Peng , ]—it was hypothesized that the bomb 14 C pattern for Guam would lie between existing coral Δ 14 C records from locations north and south of Guam. The coral records from Okinawa [ Konishi et al ., ] and Hawaii (French Frigate Shoals (FFS) and Kure Atoll) [ Druffel , ; Andrews et al ., ] were expected to be the greatest and timeliest due to their more northerly position (between 25°N and 30°N). The Δ 14 C coral record for Nauru (0.5°S) exemplifies a southern hemisphere signal [ Guilderson et al ., ]—defined as more deeply phase lagged and attenuated—by residing near the equator and separated oceanographically from North Pacific waters by the North Equatorial Current (NEC) and North Equatorial Counter Current (NECC).…”