Photochemistry and the dissolved organic carbon cycle in the global ocean

The oceanic DOC pool is comparable to the carbon inventory in both atmospheric CO2 and in the total biomass on earth. It’s reactivity, defined by its timescale of oxidation, reflects the complexity of this massive carbon pool and forms a continuum from labile, semi-labile, to refractory carbon constituents. Our studies using HOOH and O2- proxies, as well as novel direct measurements of CO2 photoproduction demonstrate that the capacity for DOM photooxidation to CO2 is not linear with photon dose, showing a strong loss of CO2 production efficiency within minutes to hours as irradiation proceeds. This argues that previous estimates of CO2 photoproduction based on extended irradiations do not capture the initial rates critical for photochemical models and consequently suggests that the photochemical sink for DOM in the surface ocean may have been significantly underestimated. The most refractory DOC compounds, thought to be ~4000-6000 yrs old on average, could represent over 90% of the inventory of oceanic DOC and are only rarely exposed to sunlight. Shipboard and laboratory irradiations of samples collected from 0 to 5000m as part of a Gulf of Alaska survey allowed photochemical comparison of the proposed DOC reactivity continuum using CO, superoxide, and HOOH production. Unlike surface irradiations, results suggest that photochemistry has been overestimated as a sink for this deep refractory DOC pool and plays only a minor role (<10%) in its direct removal.  Here we bring these elements together to provide a reevaluation of the role of photochemical oxidation in oceanic DOC budgets, providing new estimates based on new insight regarding reactivity and initial rates.