Nitrous oxide in the northern Gulf of Aqaba and the central Red Sea

Abstract Nitrous oxide (N 2 O) is a climate-relevant atmospheric trace gas. It is produced as an intermediate of the nitrogen cycle. The open and coastal oceans are major sources of atmospheric N 2 O. However, its oceanic distribution is still largely unknown. Here we present the first measurements of the water columndistribution of N 2 O in the Gulf of Aqaba and the Red Sea. Samples for N 2 O depth profiles were collected at the time-series site Station A in the northern Gulf of Aqaba (June and September 2003, and February 2004) and at several stations in the central Red Sea (October 2014, January and August 2016). Additionally, we measured N 2 O concentrations in brine poolsamples collected in the northern and central Red Sea (January 2005 and August 2016). In the Gulf of Aqaba, N 2 O surface concentrations ranged from 6 to 8 nmol L −1 (97–111% saturation) and were close to the equilibrium with the overlying atmosphere. A pronounced temporal variability of the N 2 O water columndistribution was observed. We suggest that this variability is a reflection of the interplay between N 2 O production by nitrificationand its consumption by N 2 fixation in the layers below 150 m during summer. N 2 O surface concentrations and saturations in the central Red Sea basin ranged from 2 to 9 nmol L −1 (43–155% saturation). A pronounced temporal variability with significant supersaturation in October 2014 and undersaturation in January and August 2016 was observed in the surface layer. In October 2014, N 2 O in the water columnseemed to result from production via nitrification. Low N 2 O water columnconcentrations in January and August 2016 indicated a significant removal of N 2 O. We speculate that either in-situ consumption or remote loss processes of N 2 O such as denitrification in coastal regions were responsible for this difference. Strong meso- and submesoscale processes might have transported the coastal signals into the central Red Sea. In addition, enhanced N 2 O concentrations of up to 39 nmol L −1 were found at the seawater- brine poolinterfaces which point to an N 2 O production via nitrificationand/or denitrification at low O 2 concentrations. Our results indicate that the Red Sea and the Gulf of Aqaba are unique natural laboratories for the study of N 2 O production and consumption pathways under extreme conditions in one of the warmest and most saline regions of the global ocean.