Decadal Variations in Radiocarbon in Dissolved Inorganic Carbon (DIC) Along a Transect in the Western North Pacific Ocean

We report results for the concentrations and radiocarbon (C-14) compositions of dissolved inorganic carbon (DIC) in water samples collected from a north-south transect in the western North Pacific (NP) Ocean in November 2019 and compare our results with values previously reported for similar transects during the WOCE and CLIVAR projects decades ago. We show that the strong bomb C-14 penetration signal in the upper waters has gradually been reduced and diluted by deep water depleted in C-14-DIC over the last 30 years. The rate of decrease in Delta C-14-DIC values in surface waters was greater from 2005 to 2019 than from 1993 to 2005, and the rates of decrease from 2005 to 2019 were fastest (5.4-7.0 parts per thousand yr(-1)) between 25 degrees and 32 degrees N and slowest (2.1-2.8 parts per thousand yr(-1)) between 13 degrees N and 16 degrees N. The downwelling of the water mass could have accelerated the transport of bomb C-14 in the NP subtropical gyre (25 degrees-32 degrees N). The different decadal changes in Delta C-14-DIC in the North Pacific Intermediate Water (NPIW) from north to south prove that bomb C-14 is transported southward to the southwestern NP and that the bomb C-14 peak likely penetrates the ocean interior gradually. Changes in isopycnic circulation and advection diffusion in deep waters might have resulted in shifts in the baseline Delta C-14-DIC signature.