Significant anaerobic production of fluorescent dissolved organic matter in the deep East Sea (Sea of Japan)

The distribution of fluorescent dissolved organic matter (FDOM) in the East Sea (Sea of Japan) was examined by excitation‐emission matrix spectroscopy with parallel factor analysis (PARAFAC). Humic‐like FDOM (FDOMH) increased with depth and was significantly correlated with Apparent Oxygen Utilization (AOU), indicating that FDOMH in the deep water is mainly produced by oxidation of organic matter. In addition, a surprisingly large excess of FDOMH relative to that expected from the observed AOU was found from 1000 m to the bottom (up to 3500 m). Based on the high‐resolution geographical distribution and characteristics of FDOM in the East Sea, we conclude that this excess likely originates from anaerobic FDOMH production in subsurface bottom sediments. This FDOMH flux accounts for 8–15% of the total FDOM production in the water column. Our results suggest that anaerobic activities in subsurface sediments are an important hidden source of FDOM in the ocean. Key Points Excess FDOM was observed in the deep East Sea (Sea of Japan) FDOM is produced by an anaerobic process in subsurface bottom sediments Excess FDOM fluxes account for about 10% of the deep‐sea CDOM production