Early Bomb Radiocarbon Detected in Palau Archipelago Corals

Abstract

In order to evaluate the variability in surface water masses in the Western Pacific Warm Pool, we report highprecision radiocarbon measurements in annual and seasonal bands from Porites lutea corals collected from the Palau Archipelago (7N, 134E). Annual coral bands from 1945 to 2008 and seasonal samples from 1953 to 1957 were analyzed to capture the initial early input of bomb 14 C from surface thermonuclear weapons testing in the Marshall Islands. Results show a pre-bomb average  14 C value of-54.9‰ between 1945 and early 1953. Beginning early in 1954, there is a rapid increase to a maximum of-23.1‰ at the start of 1955. Values continued to rise after 1957 to a post-bomb peak of 141‰ by 1976. The large initial rise in  14 C cannot be accounted for by air-sea CO 2 exchange. Results therefore suggest that the primary cause of this increase is the lateral advection of fallout-contaminated water from the Marshall Islands to Palau via the North Equatorial Current and then to the North Equatorial Countercurrent.

Interannual Coral Δ14C Records of Surface Water Exchange Across the Luzon Strait

“…Compared with the WPO and SCS annual records (Figure ), Houbihu prebomb Δ 14 C values are lower than most other records in the region (i.e., −64.6 ± 2.5‰, n = 8, 1 σ , Figure b). In contrast, Palaui prebomb Δ 14 C (−58.4 ± 1.6‰, n = 12, 1 σ ) is regionally comparable to Hon Tre, Vietnam (−57.1 ± 3.7‰, n = 10, 1 σ , Bolton et al, ), Ishigaki, Japan (−57.9 ± 7.6‰, n = 18, 1 σ , Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ), Guam (−54.0 ± 5.4‰, n = 18, 1 σ , Andrews et al, ), and Palau (−54.9 ± 2.4‰, n = 9, 1 σ , Glynn et al, ; Figure b). Con Dao, Vietnam (−47.9 ± 3.3‰, n = 6, 1 σ , Mitsuguchi et al, ) on the SCS shelf has the highest Δ 14 C values during this period (Figure b).…”

“…Guam also shows distinct bomb‐related peaks in 1957 and 1959, similarly captured by the high‐resolution Δ 14 C record in Ishigaki, but with a year lagged (Andrews et al, ; Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ). Palau, nearest to Guam, records an early Δ 14 C rise in 1954 but sustained a lower bomb peak that lasted until late 1956 (Glynn et al, ). Houbihu Δ 14 C shows an earlier rise and broader bomb peak than that of Palaui, but both records reached initial peak values in mid‐1955, a year after the early bomb detonations.…”

“…The occurrence of early bomb peaks in both Houbihu and Palaui Δ 14 C records indicates influence of the western Pacific circulation across the LS. Relative to air‐sea CO 2 exchange, which typically takes a decade (Nydal, ), the quick Δ 14 C rise in Houbihu and Palaui recorded 1 year after the initial bomb detonations is likely due to a nuclear fallout signal distributed by the advection of tropical Pacific currents (i.e., NEC and KC) originating at the testing sites at Bikini and Enewetak atolls (Andrews et al, ; Glynn et al, ; Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ). This current system delivers bomb‐contaminated NEC waters through Guam, with a current speed of ~0.1 to 0.2 m/s, until it bifurcates to the northward flowing KC and the southward flowing MC whose speeds are roughly 5 to 10 times faster (Andrews et al, ; Lien et al, ; Qu et al, ; Figure ).…”

“…Wang & Li, ), and the ITF (Gordon et al, ). Colored symbols indicate the location of coral Δ 14 C records used in this study: blue circle = Houbihu, Taiwan and red circle = Palaui, Philippines (this study); gray diamond = Guam, USA (Andrews et al, ); light green diamond = Palau (Glynn et al, ); yellow diamond = Ishigaki Island, Japan (Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ); orange triangle = Langkai, Indonesia (Fallon & Guilderson, ); light blue triangle = Lombok Strait, Indonesia (Guilderson et al, ); green square = Currimao, Philippines (Hirabayashi, Yokoyama, Suzuki, Miyairi, Aze, Siringan, & Maeda, ); pink square = Hon Tre, Vietnam (Bolton et al, ); and brown square = Con Dao, Vietnam (Mitsuguchi et al, ). The red line across the strait at 120.8°E running from 18°N to 22°N define the transect where surface zonal currents and sea surface heights from the Simple Ocean Data Assimilation data set (SODA v.2.2.4, http://apdrc.soest.hawaii.edu/dods/public_data/SODA/soda_pop2.2.4) are derived (shown in Figures and c, respectively).…”

Seasonal radiocarbon and oxygen isotopes in a Galapagos coral: Calibration with climate indices

“…The average of all ∆ 14 C results in the Galapagos coral (−68.4 ± 3.8‰ SD n = 129) from 1939 to 1954 is the lowest of any tropical or subtropical location studied to date in the nonpolar oceans. For comparison, the prebomb ∆ 14 C value at Palau (7°17′N, 134°15′E) was −56.7 ± 1.6‰ ( n = 18) [ Glynn et al ., ], Hawaii (19°31′N, 155°58′W) was −50.8 ± 2.0‰ ( n = 12) [ Druffel et al ., ], Palmyra (5°52′N, 162°07′W) was −57.9 ± 1.8‰ ( n = 15) [ Druffel‐Rodriguez et al ., ], Fanning (3°55′N, 159°19′W) was −47.1 ± 11.8‰ ( n = 129) [ Grottoli et al . ], Langkai in the Indonesian throughflow (5°02′S, 119°04′E) was −56.4 ± 3.0‰ ( n = 144) [ Fallon and Guilderson , ], Rarotonga in the southwest Pacific (21°14′S, 159°49′W) was −50.8 ± 4.3‰ ( n = 123) [ Guilderson et al ., ], and Nauru (0°32′S, 166°30′E) was −57.9 ± 6.1‰ ( n = 77) [ Guilderson et al ., ].…”