The impact of denitrification on the atmospheric CO 2 uptake potential of seawater

Abstract

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 CO 2 uptake when the denitrifying water is exposed to the air in the coastal ocean. In this study, we explored the potential responses of seawater pCO 2 to denitrification through three approaches: (1) simulating pCO 2 variations in response to various denitrification scenarios, (2) verifying in situ pCO 2 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 CO 2 isopleths corresponding to modern-day xCO 2 at different temperatures. In the second approach, we showed that water pCO 2 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 CO 2 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 CO 2 uptake, and concurrent DIC production during denitrification has to be taken into account when discussing changes in seawater buffering capacity along with alkalinity production.