Discovery of new pathways of Pacific sea water currents through radiotracer studies

currents
Pacific Ocean Waves © jondavatzphoto - Fotolia.com

JRC scientists, in close collaboration with Japanese research organisations, investigated the dissolution of radioactivity in the Pacific Ocean following the Fukushima incident.

The accident in the Fukushima Dai-ichi nuclear power plant in March 2011 led to the release of vast amounts of radioactivity into the Pacific Ocean. The silver-lining of this story is that the radioactivity is relatively quickly diluted into benign levels and transported with the ocean currents around the globe. The radionuclide ceasiuem-134 (Cs-134) is only produced in nuclear reactions and not present naturally on earth. If Cs-134 is detected in sea water collected anywhere, it becomes obvious to determine that it came from Fukushima. Therefore, it is possible to trace ocean currents. Only very little is known about ocean currents and generally about the dynamics in the oceans. This poses a problem as ocean currents are the main driving force of climate change because they contain and transport enormous amounts of heat (think e.g. of the Gulf-stream in the Atlantic). A Japanese-lead research team has been collecting sea water samples all over the Pacific (using e.g. commercial cargo ships). As the level of radioactivity is very low ( ~1 mBq/L), it is difficult to measure. The JRC-Institute for Reference Materials and Measurements (IRMM) contributed with underground gamma-spectrometry measurements of Cs-134 (and other radionuclides) to this study.

The first results are now available and reveal new facts. Two important oceanographic conclusions are that most of the surface radioactivity transported to the east (towards the United States of America)  will submerge to a depth of 400 m near the international date line and then move towards the south-west.

Also,  the impact of the so-called mesoscale Eddie currents has been noted in this study.  The Eddie currents, ranging from a few hundred meters to a few kilometres can appear suddenly and last from a few days to some weeks. These currents mean a lot for transporting nutrients and oxygen to near-shore regions.

Read more in: M. Aoyama et al.: “134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part one: surface pathway and vertical distributions“, J. Oceanogr.  72 (2015) 53-65, doi:10.1007/s10872-015-0335-z

JRC publication: 134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part one: surface pathway and vertical distributions