Temporal changes in tritium and radiocarbon concentrations in the western North Pacific Ocean (1993-2012)

The western North Pacific is one of the most studied oceanic basins due to its diverse structure and important role in connection with the adjacent reservoirs. Tritium (³H) and radiocarbon (¹⁴C) have been frequently exploited as oceanographic tracers due to their suitable properties; several extensive observation projects, such as GEOSECS, WOCE and WOMARS, used these two radionuclides to investigate different oceanographic processes, pathways, ocean currents and time scales of deep and bottom water formation. Here we evaluate temporal changes in ³H and ¹⁴C levels in seawater of the western North Pacific Ocean from 1993 to 2012. When compared to the background levels from 1993, the data from 2012 suggests significant impact of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on surface and vertical ³H seawater profiles, increasing its water column inventories in the southern part of the 149°E meridian by a factor of 2-7. On the other hand, ¹⁴C content in surface seawater has been steadily decreasing from 1993, with the accelerated rate from 2005, probably due to downwelling of bomb-produced radiocarbon and its transport along isopycnal layers. The influence of the Oyashio current on ¹⁴C levels in the northern part of the investigated transect and formation of its intrusion was also clearly visible in the collected datasets. Regarding bomb-produced radiocarbon, its water column inventories decreased or remained same from 2005 to 2012 at all stations, except the ones located in the coastal areas of the New Guinea island (3.5°S).