The impact of denitrification on the atmospheric CO2 uptake potential of seawater

In addition to carbonate dissolution, denitrification represents another pivotal geochemical process that produces alkalinity in the marine environment. Previous studies suggested that such alkalinity can increase seawater buffering capacity and thus enhance atmospheric CO2 uptake when the denitrifying water is exposed to the air in the coastal ocean. In this study, we explored the potential responses of seawater pCO2 to denitrification through three approaches: (1) simulating pCO2 variations in response to various denitrification scenarios, (2) verifying in situ pCO2 data in a well-known denitrification “hotbed”—the Arabian Sea—as well as in anammox-dominated oxygen minimum waters in the Eastern South Pacific, and (3) examining published benthic alkalinity and dissolved inorganic carbon (DIC) flux ratios. In the first approach, we showed that the ratios of alkalinity and DIC addition during denitrification of different model compounds were lower than the slopes of alkalinity and DIC (ΔTA/ΔDIC) along a series of CO2 isopleths corresponding to modern-day xCO2 at different temperatures. In the second approach, we showed that water pCO2 level increased with loss of fixed nitrogen. Last, we showed that benthic alkalinity and DIC flux ratios were also lower than the ΔTA/ΔDIC values derived from the above mentioned CO2 isopleths. Overall, these independent approaches support the conclusion that denitrification-generated alkalinity (together with other alkalinity-altering anaerobic respiration pathways) may not be a notable driving force for enhancing atmospheric CO2 uptake, and concurrent DIC production during denitrification has to be taken into account when discussing changes in seawater buffering capacity along with alkalinity production.

► We used three approaches to examine anaerobic processes on seawater pCO2.
► Alkalinity/dissolved inorganic carbon production ratio controls how pCO2 changes.
► Denitrification cannot increase seawater buffering capacity and decrease pCO2.