Rapid dating of recent sediments in Loch Ness: inductively coupled plasma mass spectrometric measurements of global fallout plutonium

The 239+240Pu activity profile is determined for a sediment core collected from 170-m depth at Loch Ness, Scotland. These measurements use magnetic sector inductively coupled plasma mass spectrometry for rapid determination of Pu activities and 240Pu/239Pu atom ratios. A 239+240Pu detection limit of 0.1 Bq/kg is obtained for 2 g of acid-leached sediment; 242Pu is used as a spike isotope. The Pu activity profile exhibits a maximum of 42.7±0.3 Bq/kg 239+240Pu in the 9–10-cm depth interval. The position of this maximum coincides with peaks in the 241Am and 137Cs activity profiles. These peak activities are ascribed to the 1963/1964 peak fallout from atmospheric testing of nuclear weapons. The 240Pu/239Pu atom ratios are in the range 0.15–0.20, in agreement with the expected range of 0.166–0.194 for Northern Hemisphere fallout, and do not suggest the presence of other contributing sources. This study demonstrates that ICPMS has considerable potential for rapid determination of the chronology of post-1950 sediments, and also for validating 210Pb dates where chronologies over longer time-scales are needed.