Long-term distribution of radiocarbon in the Pacific Ocean within prevailing climate change

Many studies have been already published documenting that prevailing global warming and climate change are affecting the world ocean, with some of them suggesting the Pacific Ocean being one of the most impacted basins. Among others, the physical state of the ocean can be derived from the distribution of its constituents, e.g., radionuclides with suitable characteristics. Radiocarbon (¹⁴C) was used in the past as a tracer for studying various oceanic processes such as air-sea gas exchange and thermocline ventilation. A complex image about the distribution of radiocarbon in the form of dissolved inorganic carbon (DIC) in the Pacific Ocean between 1990s and 2010s is described in the current study. Except for the northeastern part, observed accelerating decrease of a radiocarbon signal (Δ¹⁴C) in the surface seawater was quantified between 3.5 ± 0.3 and 3.9 ± 0.3‰ y^-1 on decadal timescale. Several factors contributing to the alteration of distribution, including fossil fuel combustion leading to Suess effect, inverted air-sea exchange of ¹⁴CO₂, and shifted ocean circulation and ventilation, were discussed as possible influences. The change was also visible for the upper mixed layer whose ¹⁴C concentration vertical profile showed fewer local maxima and was extended into greater depths, in parallel with generally higher mean penetration depth (MPD) of bomb fraction, which may be related to increasing stratification in the Pacific Ocean. The impact of environmental conditions within climate change was mainly noticeable in upper layers which is supported by a shift in the bomb radiocarbon inventories distribution in the eastern Pacific Ocean after 2000s, dominantly accumulated at the depth above 1 km.