First evidence of plutonium and uranium fallout on a Southern Ocean Island (Macquarie Island, 54°S, 158°E)
Dr Krystyna Saunders, Mr David Child, AD Griffiths, JJ Harrison, Dr Michael Hotchkis, G Motta, SJ Roberts, A Zawadzki
Plutonium and uranium concentrations and isotopic signatures may be used to trace the changing patterns and locations of nuclear testing activities and the resulting spatial distribution of fallout. Virtually nothing is known about the fallout distribution of plutonium and uranium in the Southern Ocean region. Although only 10% of the atmospheric nuclear weapons detonations between 1945 and 1980 were conducted in the Southern Hemisphere, atmospheric transport meant that it received a much larger proportion of fallout – about one third of the total inventory. Lake sediment and peat records from the few remote sub-Antarctic islands in the Southern Ocean provide unique and undisturbed archives for investigating fallout histories, and an opportunity to bridge the spatial gap between the Southern Hemisphere mid-latitudes, where records show mixed regional and global signatures, and Antarctica, where global signatures dominate.
Here, we present the first nuclear weapons testing fallout history for a sub-Antarctic island using a lake sediment core from Macquarie Island. Macquarie Island is nearly equidistant from Tasmania, New Zealand and Antarctica at 54°S, 158°E. It has an area of 120 km2 and lies just north of the Polar Frontal Zone in the core belt of the Southern Hemisphere westerly winds, which dominate the climate of the Southern Hemisphere mid-high latitudes.
A 54.0 cm sediment core was collected from a small lake on the western edge of the Macquarie Island plateau. It was sampled at 0.5 cm increments, dated using 210Pb (ages calculated using the constant flux, constant sedimentation model) and 14C, and scanned with micro x-ray fluorescence and hyperspectral (400-1000 nm) core scanners to identify stratigraphic and geochemical variations through time.
The core spanned ca. 2000 years, with the upper 18.0 cm dating back to ca. 1900 based on 210Pb dating. Plutonium and uranium fallout isotopes 239Pu, 240Pu, 241Pu and 236U were measured on this part of the core. The profile demonstrated the onset of nuclear weapons testing at 12.0-12.5 cm (late 1940s based on 210Pb dating) with a 240/239Pu atom ratio of 0.24. This coincides with ratios typical of US testing in the Pacific Proving Grounds, which were the dominant source of global fallout at the time. Concentrations of all isotopes sharply increased between 12.5-9.5 cm and peaked at 9.5-10.0 cm, which, based on 210Pb dating corresponds to ca. 1963. In the late 1950s and early 1960s, there was a change to a lower 240/239Pu atom ratio of 0.17 coinciding with the shift from US testing dominating global fallout to that of the former Soviet Union. A replicate sediment core from another lake on Macquarie Island is currently being analysed to verify these findings, as well as records from other sub-Antarctic islands, with the aim of tracing the changing patterns and locations of nuclear testing activities and resulting spatial distribution of fallout in the mid-late 20th century.
Krystyna Saunders is a senior research scientist at the Australian Nuclear Science and Technology Organisation. She primarily works in the fields of palaeoclimate and palaeoecology, with a focus on lake sediment and peat records combined with present-day surveys from sub-Antarctic islands.