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

Abstract

AbstractWe report results for the concentrations and radiocarbon (14C) 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 14C penetration signal in the upper waters has gradually been reduced and diluted by deep water depleted in 14C‐DIC over the last 30 years. The rate of decrease in Δ14C‐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‰ yr−1) between 25° and 32°N and slowest (2.1–2.8‰ yr−1) between 13°N and 16°N. The downwelling of the water mass could have accelerated the transport of bomb 14C in the NP subtropical gyre (25°–32°N). The different decadal changes in Δ14C‐DIC in the North Pacific Intermediate Water (NPIW) from north to south prove that bomb 14C is transported southward to the southwestern NP and that the bomb 14C 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 Δ14C‐DIC signature.