Following the Trail of Fukushima Tracers: Introducing Part II – Seven Years of North Pacific Mode Water Evolution

Here we present an extension our previous investigation by making use of 7-years of observations of North Pacific radio-cesium (¹³⁷Cs and ¹³⁴Cs with ~30 and 2 year half-lives, respectively) discharged from the Fukushima Dai-ichi nuclear power plants in northern Japan in 2011. The new dataset includes samples measured along the 2018 GEOTRACES 152°W transect (GP15) repeating some of the sampling performed in 2015 along the same line. The GP15 cesium observations will be used in combination with findings from the earlier cesium survey along with the hydrography from both cruises and Argo observations to investigate the changes that have occurred in the intervening three years. The cesium distributions will provide a temporal extension of current knowledge of the pathways and timescales associated with Fukushima-derived waters, including their journey around the subarctic and subtropical gyres. The signal will be linked to the various North Pacific mode waters in which they reside to provide insight into the evolution and fate of their characteristic properties. It is likely that detection of ¹³⁴Cs over most or all the region can be achieved, the knowledge of ¹³⁴Cs/¹³⁷Cs distributions on the 152°W line gained from the 2015 GO-SHIP data set will allow us to tag Fukushima-derived waters with ¹³⁷Cs alone. We also intend to compare findings with the published results coming from the Line-P time series.