Temporal trends and tracing capabilities of plutonium in the western North Pacific Ocean

Plutonium is a valuable temporal and spatial tracer in biogeochemical research due to its strong chemical reactivity, serving as a significant resource for tracking water mass movement. Because two major sources of plutonium exist in the North Pacific Ocean - global fallout (GF-Pu) from nuclear weapons tests and close-in fallout from the US Pacific Proving Ground (PPG-Pu), investigating its distribution and cycling is highly viable. Here, we examine temporal and spatial changes in ^239,240Pu activity concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in surface and deep waters by analyzing Pu data from 1960 to 2020. Surface ^239,240Pu in the subarctic North Pacific has showed a declining trend after 2000, with a rate of decrease comparable to that in the subtropical region (apparent half-life of 6.4 y). Deep ^239,240Pu levels in the North Pacific also declined on a decadal timescale, likely reflecting the northward flow of deep water with low ^239,240Pu concentrations near 20°N. The ²⁴⁰Pu/²³⁹Pu atom ratio in surface waters of the subtropical North Pacific indicates that GF-derived Pu (²⁴⁰Pu/²³⁹Pu atom ratio of 0.18) dominated surface Pu levels until 1980, after which PPG-derived Pu (²⁴⁰Pu/²³⁹Pu atom ratio of 0.33) became the dominant component. In the deep waters of the North Pacific, PPG-Pu signals were detected in the subarctic region during 1981 and 1988. Consequently, the ²⁴⁰Pu/²³⁹Pu atom ratio serves as a powerful tool for unraveling the complexities of Pu cycling. These observations are important for better understanding of surface and deep-water flows, vertical motion, and biogeochemical processes in the western North Pacific Ocean.