Production of bio-refractory fluorescent dissolved organic matter in the ocean interior

Dissolved organic matter in the ocean constitutes one of the largest pools of reduced carbon on the Earth's surface(1,2), but its role in the global carbon cycle is poorly understood. A large part of this pool is thought to be of marine origin(2-4) and to be composed of constituents that are resistant to biological degradation(3-6). It is not clear how this refractory fraction of the carbon pool is produced in the ocean. Here, we analyse observations of fluorescence intensity and apparent oxygen utilization in the interior of the Pacific Ocean, and find that the two parameters are linearly related in the mesopelagic and abyssal layers. We conclude that fluorescent dissolved organic matter is produced in situ in the ocean interior as organic matter is oxidized biologically, and that it is resistant to biological degradation on centennial to millennial timescales. The rate of production of bio-refractory fluorescent dissolved organic matter in the ocean interior is larger than the riverine input of terrestrial humic substances, suggesting that its in situ production is one of the key processes in maintaining the oceanic pool of refractory dissolved organic matter.