LAUSR

ABSTRACT Surface radiocarbon (Δ14C) in the North Pacific has been monitored using a commercial volunteer observation ship since the early 2000s. Here we report the temporal and spatial variations in Δ14C in the summer surface water when the surface ocean is vertically stratified over a 13-yr period, 2004–2016. The long-term Δ14C decreasing trend after the late 1970s in the subtropical region has continued to the present and the rate of decrease of the Kuroshio and Kuroshio Extension, North Pacific and California current areas is calculated to be –3.3, –5.2 and –3.3 ‰/yr, respectively. After 2012 the Δ14C of the Kuroshio and Kuroshio Extension area, however, has remained at an approximately constant value of around 50‰. The result may indicate that subtropical surface Δ14C in the western North Pacific has reached an equilibrium with atmospheric Δ14CO2. The Δ14C in the subarctic region is markedly lower than values in the subtropical region and it seems that the decreasing tendency of surface Δ14C has changed to an increasing tendency after 2010. The results may indicate that bomb-produced 14C, which has accumulated below the mixed layer in the past few decades, has been entrained into the surface layer by deep convection.