Vertical profiles of nitrous oxide and dissolved oxygen in marine sediments

Abstract

Abstract The whole core squeezing method was used to simultaneously obtain profiles of nitrous oxide (N2O), nitrogenous nutrients, and dissolved oxygen in sediments of Koaziro Bay, Japan (coastal water), the East China Sea (marginal sea), and the central Pacific Ocean (open ocean). In the spring of Koaziro Bay, subsurface peaks of interstitial N2O (0.5–3.5 cm depth) were observed, at which concentrations were higher than in the overlying water. This was also true for nitrate (NO3−) and nitrite (NO2−) profiles, suggesting that the transport of oxic overlying water to the depth through faunal burrows induced in situ N2O production depending on nitrification. In the summer of Koaziro Bay, sediment concentrations of N2O, NO3− and NO2− were lower than in the overlying water. In most East China Sea sediments, both N2O and NO3− decreased sharply in the top 0.5–2 cm oxic layer (oxygen: 15–130 μM), which may have indicated N2O and NO3− consumption by denitrification at anoxic microsites. N2O peaks at subsurface depth (0.5–6.5 cm) implied in situ production of N2O and/or its supply from the overlying water through faunal burrows. However, the occurrence of the latter process was not confirmed by the profiles of other constituents. In the central Pacific Ocean, the accumulation of N2O and NO3− in the sediments likely resulted from nitrification. Nitrous oxide fluxes from the sediments, calculated using its gradient at the sediment–water interface and the molecular diffusion coefficient, were −45 to 6.9 nmolN m−2 h−1 in Koaziro Bay in the spring, −29 to −21 nmolN m−2 h−1 in the summer, −46 to 37 nmolN m−2 h−1 in the East China Sea, 0.17 to 0.23 nmolN m−2 h−1 in the equatorial Pacific, and