Early Bomb Radiocarbon Detected in Palau Archipelago Corals

In order to evaluate the variability in surface water masses in the western Pacific warm pool, we report high-precision 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 14C from surface thermonuclear weapons testing in the Marshall Islands. Results show a pre-bomb average Δ14C 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 Δ14C cannot be accounted for by air-sea CO2 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