Reconstructing surface ocean circulation with I time series records from corals.

The long-lived radionuclide I (half-life: 15.7 × 10 yr) is well-known as a useful environmental tracer. At present, the global I in surface water is about 1-2 orders of magnitude higher than pre-1960 levels. Since the 1990s, anthropogenic I produced from industrial nuclear fuels reprocessing plants has been the primary source of I in marine surface waters of the Atlantic and around the globe. Here we present four coral I time series records from: 1) Con Dao and 2) Xisha Islands, the South China Sea, 3) Rabaul, Papua New Guinea and 4) Guam. The Con Dao coral I record features a sudden increase in I in 1959. The Xisha coral shows similar peak values for I as the Con Dao coral, punctuated by distinct low values, likely due to the upwelling in the central South China Sea. The Rabaul coral features much more gradual I increases in the 1970s, similar to a published record from the Solomon Islands. The Guam coral I record contains the largest measured values for any site, with two large peaks, in 1955 and 1959. Nuclear weapons testing was the primary I source in the Western Pacific in the latter part of the 20th Century, notably from testing in the Marshall Islands. The Guam 1955 peak and Con Dao 1959 increases are likely from the 1954 Castle Bravo test, and the Operation Hardtack I test is the most likely source of the 1959 peak observed at Guam. Radiogenic iodine found in coral was carried primarily through surface ocean currents. The coral I time series data provide a broad picture of the surface distribution and depth penetration of I in the Pacific Ocean over the past 60 years.