Transport of Aged Dissolved Organic Carbon via the Surface Current Revealed by Radiocarbon

The East Sea (also known as the Japan Sea) is connected to the Northwest Pacific via shallow straits and has independent deep water circulation, as a model miniature ocean. The radiocarbon age of dissolved organic carbon (DOC) in the East Sea ranged from 2,000 to 3,700 years, exceeding the water turnover time (similar to 100 years). The oldest DOC was found in the subsurface layer characterized by the Tsushima Warm Water. Comparison of the radiocarbon content and concentration of DOC in the East Sea to those in the ocean suggests that aged DOC was transported conservatively from the Northwest Pacific to the East Sea via the shallow Tsushima Warm Current. The fractions of DOC released by serial-oxidation of the oldest DOC sample had identical radiocarbon ages, implying that refractory DOC was produced in situ and added to the DOC pool in the East Sea. The East Sea (also known as the Japan Sea) is connected to the Northwest Pacific exchanging surface water only through shallow passages. The radiocarbon age of DOC in the East Sea is old, ranging from 2,000 to 3,700 years. This cannot be explained by DOC production and aging in the East Sea. Instead, it appears that old DOC is transported into the East Sea from the Northwest Pacific. The oldest DOC was found in the subsurface layer instead of the deepest layer. We show that this oldest DOC is a mixture of the old DOC from the North Pacific and modern DOC produced in the East Sea however their characteristics in terms of resistance to oxidation were modified. These findings provide clues for understanding the global DOC cycling especially in the surface ocean. Radiocarbon ages of dissolved organic carbon (DOC) in the East Sea (Japan Sea) ranged from 2,000 to 3,700 years, exceeding the water turnover timeThe oldest DOC at subsurface depths in the East Sea was transported from the North Pacific via the Tsushima Warm CurrentThe aged DOC from the North Pacific is mixed with younger DOC produced in situ and spreads to the deep waters in the East Sea

Automated summary

The East Sea, connected to the Northwest Pacific, is a miniature ocean with independent deep water circulation. The radiocarbon age of dissolved organic carbon (DOC) in the East Sea is older than its water turnover time, indicating the transportation of aged DOC from the Northwest Pacific. The oldest DOC is found in the subsurface layer and is a mixture of old DOC from the North Pacific and modern DOC produced in the East Sea. These findings provide insights into the global cycling of DOC in the surface ocean.