The Unaccounted Oceanic Sink of Anthropogenic Nitrous Oxide and Its Relationship With Anthropogenic Carbon Dioxide

Abstract

Since 1800, the concentration of greenhouse gases like nitrous oxide (N₂O) and carbon dioxide (CO₂) has significantly increased due to anthropogenic activities. Oceanic anthropogenic CO₂ (C_ant) uptake from the atmosphere has been quantified and periodically re-evaluated given the implications for climate change. However, the potential oceanic uptake of N₂O has been largely overlooked. This study quantifies the uptake of N₂O of anthropogenic origin (N₂O_ant) taken up by the global ocean and how it relates with the anthropogenic CO₂. The oceanic inventory of N₂O_ant has been quantified using two approaches which consider the anthropogenic perturbation of N₂O and CO₂ as conservative tracers: first, a direct approach using the Transient Time Distribution (TTD) method; and second, indirectly through a novel method, founded on the direct proportionality between the excess of both N₂O and CO₂ in the atmosphere since 1800. Our results show that the North Atlantic Ocean is a key region of maximum accumulation of N₂O_ant due to the confluence of cold and ventilated waters. The global oceanic uptake of N₂O_ant from the pre-industrial times to 2010 was estimated to be 11.5 ± 2.3 Tg-N, with an annual uptake rate of 0.23 ± 0.05 Tg-N yr⁻¹. The study shows that oceanic sequestration contributes to a small portion of global N₂O inventories, but it is comparable to other N₂O oceanic budget numbers derived from atmospheric nitrogen deposition. Furthermore, connecting the N₂O_ant and C_ant oceanic distributions is a valuable tool for linking the perturbation of the Anthropocene on the N and C cycles in the ocean.