Radiocarbon in the water column of the southwestern North Pacific Ocean - 24 Years after GEOSECS

In the framework of the Worldwide Marine Radioactivity Studies (WOMARS) project, water profile samples for radiocarbon measurements were taken during the IAEA'97 cruise at 10 stations in the southwestern North Pacific Ocean. While ¹⁴C concentrations were rapidly decreasing from the surface (Δ¹⁴C about 100‰) down to about 800 m at all visited stations (Δ¹⁴C about-200‰), the concentrations below 1000 m were almost constant. Some stations were in proximity to the GEOSECS stations sampled in 1973; thus, ¹⁴C profiles could be compared after a 24-yr interval. Generally, ¹⁴C concentrations had decreased in surface waters (by 50-80°‰) and increased (by about the same amount) in intermediate waters when compared with GEOSECS data. In deep waters (below 1000 m), the observed ¹⁴C concentrations were similar to GEOSECS values. The bomb-produced ¹⁴C inventory had increased by more than 20% over the 24 yr from 1973 to 1997 and was estimated to be about (32 ± 5) 10¹² atom m^-2, with an annual ¹⁴C flux of (1.3 ± 0.3) 10¹² atom m^-2 yr^-1. The results suggest that bomb-produced ¹⁴C has been advected northwards by the Kuroshio Current and the Kuroshio Extension and stored in the intermediate layer as North Pacific Intermediate Water.