Caesium Isotope Tracer

The caesium isotope tracer uses anthropogenic cesium-137 to trace ocean circulation and water-mass ventilation. Cesium-137 is purely man-made, soluble, and conservative in seawater, with a 30.1-year half-life that suits decadal-scale pathways. Global background came from atmospheric nuclear weapons testing in the 1950s to 1970s, detectable today to about 1000 m depth in the North Pacific. The 2011 Fukushima Daiichi accident added a 15 to 20 PBq pulse to the Northwest Pacific, a dated point source whose surface plume crossed to North America within a few years. Short-lived cesium-134, half-life about 2 years, fingerprints the Fukushima signal against older weapons-test cesium.