Scientists from the 'Plutonium bio-signature as tracer of climate changes in ocean transport' (Plutotrace) initiative have tested the hypothesis that concentrations of plutonium (Pu) in the surface water are related to the depth of the mixed layer. This layer is where turbulence is generated as a result of wind, cooling and other processes. Project partners have used a technique based on mass spectrometry (MS) to determine isotope ratios in corals and shells. Samples of coral were obtained with the aid of Japanese and Korean partners from three sites in the north-west Pacific. Sampling sites were situated along the path of the North Equatorial Current (NEC) and the Kuroshio Current (KC). Seasonal growth layers of coral were analysed using MS to reveal changes in the Pu isotope ratio over space and time. Coral analysis has enabled scientists to reconstruct past surface mixed layers in areas subject to the NEC and KC and compare the results with past sea surface temperatures. The findings have been used to build a model showing changes in water mass transport from the tropical Pacific to the north-west Pacific. The work of the Plutotrace project has enabled scientists to study ocean current changes resulting from climate change. The techniques developed can be used by researchers studying the oceans in other parts of the world to help obtain a clearer picture of the marine effects of the changing climate.