A Fukushima tracer perspective on four years of North Pacific mode water evolution

We present the results of a multi-platform investigation that utilizes tracer information provided by the 2011 release of radioisotopes from the Fukushima Dai-ichi Nuclear Power Plant to better understand the pathways, mixing and transport of mode waters formed in the North Pacific Ocean. The focus is on transition region mode waters and radiocesium (137Cs and 134Cs) observations obtained from the May–June 2015 GO-SHIP occupation of the 152°W line in the Northeast Pacific. Samples include profiles from the surface to 1000 m and surface/subsurface pairs that provide an average 1° of latitude spacing along 152°W. We find a clear Fukushima (134Cs) signal from the surface to 400 m. The core signal (134Cs ∼10 Bq m-3, 137Cs ∼12 Bq m-3) at 41°-43°N lies at 30–220 m where mode waters formed through deep winter mixing in 2011 outcropped in the western North Pacific. The strongest 2015 152°W Fukushima-source radiocesium signal is associated with Dense-Central Mode Waters consistent with the densest variety of these mode waters being formed off the coast of Japan 4 years earlier. The radionuclide signal transited the basin along subsurface mode water isopycnals mainly on the northern side of the subtropical gyre before outcropping at and to the east of the 152°W line. In 2015, the densest 152°W waters with 134Cs lie at ∼435 m in the bottom range of Dense-Central Mode Water at 40°N. There is a weak, but detectable, signal in the boundary current off both Kodiak and Sitka. The deepest detectable 137Cs (weapon's testing background) are found at and to the north of 45°N at 900–1000 m. With the exception of a single subsurface sample near Hawaii, as of spring 2015, the southernmost 134Cs was found above 200 m at 30°N. A total date-corrected 134Cs inventory of 11–16 PBq is estimated. Qualitative comparison to model output suggests good consistency in terms of general location, latitudinal breadth, and predicted depth of penetration, allowing discussion of the bigger picture. However, the model's 2015 152°W radiocesium signal is quantitatively weaker and the core is offset in latitude, potentially due to the lack of consideration of atmospheric deposition. •Northeast Pacific cesium observations highlight subsurface core of mode waters.•At 152°W, Fukushima cesium lies predominantly in Dense Central Model Waters.•Four-year old mode waters reach 400 m and Alaskan coast, but remain north of 30°N.•Northeast Pacific Fukushima cesium suggests total inventory of 11–16 PBq.•The Nort