A historical record of the impact of nuclear activities based on 129I in coral cores in Baler, Philippines: An update.

In a previous study in 2016, we presented how 129I in coral cores from the east (Baler) and west (Parola) sides of the Philippines recorded the impacts of human nuclear activities, including nuclear weapons testing, nuclear fuel reprocessing, and nuclear accidents. However, the 2016 Baler dataset only had a two-year time resolution and a crude age model based on growth band counting. Here we present a new 2020 Baler 129I/127I atomic ratio dataset that features at least annual time resolution and a more accurate age model constructed using 3D X-ray Computed Tomography. Results show that the bomb peaks in Baler primarily came from the Pacific Proving Grounds or PPG with a time lag of about 1.8 years (or more specifically, between 1.3 and 2.4 years). Moreover, a review of the Parola dataset shows that PPG signals may have been transported to Parola in the West Philippine Sea via two pathways: the northward and southward bifurcations of the North Equatorial Current, reaching Parola about 4.5 and 8.5 years after detonation, respectively. Moreover, a prominent peak in the year 2014.7 in Baler possibly came from the 2011 Fukushima Accident, transported by the Kuroshio Recirculation Gyre and the North Pacific Mode Waters with a 3.5-year time lag. This study contributes to the understanding of the impact and transport of human-made radionuclides to the Philippines and the relevant oceanographic processes in the Western Equatorial Pacific region.

High-resolution 129I bomb peak profile in an ice core from SE-Dome site, Greenland.

129I in natural archives, such as ice cores, can be used as a proxy for human nuclear activities, age marker, and environmental tracer. Currently, there is only one published record of 129I in ice core (i.e., from Fiescherhorn Glacier, Swiss Alps) and its limited time resolution (1-2 years) prevents the full use of 129I for the mentioned applications. Here we show 129I concentrations in an ice core from SE-Dome, Greenland, covering years 1956-1976 at a time resolution of ∼6 months, the most detailed record to date. Results revealed 129I bomb peaks in years 1959, 1962, and 1963, associated to tests performed by the former Soviet Union, one year prior, in its Novaya Zemlya test site. All 129I bomb peaks were observed in winter (1958.9, 1962.1, and 1963.0), while tritium bomb peaks, another prominent radionuclide associated with nuclear bomb testing, were observed in spring or summer (1959.3, and 1963.6; Iizuka et al., 2017). These results indicate that 129I bomb peaks can be used as annual and seasonal age markers for these years. Furthermore, we found that 129I recorded nuclear fuel reprocessing signals and that these can be potentially used to correct timing of estimated 129I releases during years 1964-1976. Comparisons with other published records of 129I in natural archives showed that 129I can be used as common age marker and tracer for different types of records. Most notably, the 1963 129I bomb peak can be used as common age marker for ice and coral cores, providing the means to reconcile age models and associated trends from the polar and tropical regions, respectively.

Historical record of nuclear activities from 129I in corals from the northern hemisphere (Philippines).

Iodine-129 is a long-lived fission product that is majorly released in human nuclear activities (HNA) such as nuclear bomb testing, nuclear fuel reprocessing, and nuclear accidents. It is a good environmental tracer and former measurements of 129I in corals from the southern hemisphere show the increasing trend of 129I concentrations in the marine environment caused by HNA. Here we show time series of 129I/127(stable)I isotopic ratios in two coral cores from the northern hemisphere (Philippines) and how these record 129I released from HNA in even greater, unprecedented detail. Corals were taken from the Pacific Ocean (Baler) and South China Sea (Parola) sides of the Philippines. We observed nearly identical peaks (129I/127I âˆ¼ 31.5 × 10-12) in both the Baler and Parola records, each attributed to the year 1962 - the year with the highest recorded amount of 129I release from nuclear bomb testing. This 1962 129I bomb signal offers a new time marker that can be used to establish or confirm age models of corals, comparable to or possibly better than the well-known coral 14C bomb peak. We also observed nuclear fuel reprocessing and Chernobyl accident 129I signals in years 1977, 1980, and 1986, concurrently in Parola and with 9 to 11-year lags in Baler. This discrepancy in timing suggests that 129I was transported to the South China Sea and Pacific Ocean sides of the Philippines directly from the atmosphere and through prevailing ocean currents, respectively. Lastly, we observed surprisingly high 129I/127I isotopic ratios (i.e., 22.8 to 38.9 × 10-12) in the Parola record after the year 1996, which is in contrast to the decreasing trend observed in the Baler record and in published 129I releases of different HNA. These results possibly indicate the presence of unknown sources of 129I in the South China Sea region.