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

Hermann W. Bange, Annette Kock, Nicole Pelz, Mark Schmidt, Florian Schütte, Sylvia Walter, Anton F. Post, Burton Jones, Benjamin Kürten

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4 Scopus citations

Abstract

Nitrous oxide (N2O) 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 N2O. However, its oceanic distribution is still largely unknown. Here we present the first measurements of the water column distribution of N2O in the Gulf of Aqaba and the Red Sea. Samples for N2O 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 N2O concentrations in brine pool samples collected in the northern and central Red Sea (January 2005 and August 2016). In the Gulf of Aqaba, N2O 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 N2O water column distribution was observed. We suggest that this variability is a reflection of the interplay between N2O production by nitrification and its consumption by N2 fixation in the layers below 150 m during summer. N2O 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, N2O in the water column seemed to result from production via nitrification. Low N2O water column concentrations in January and August 2016 indicated a significant removal of N2O. We speculate that either in-situ consumption or remote loss processes of N2O such as denitrification in coastal regions were responsible for this difference. Strong meso- and submesoscale processes might have transported the coastal signals across the Red Sea. In addition, enhanced N2O concentrations of up to 39 nmol L−1 were found at the seawater-brine pool interfaces which point to an N2O production via nitrification and/or denitrification at low O2 concentrations. Our results indicate that the Red Sea and the Gulf of Aqaba are unique natural laboratories for the study of N2O production and consumption pathways under extreme conditions in one of the warmest and most saline region of the global oceans.
Original languageEnglish (US)
Pages (from-to)90-103
Number of pages14
JournalDeep Sea Research Part II: Topical Studies in Oceanography
Volume166
DOIs
StatePublished - Jul 2019

Bibliographical note

KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We thank the chief scientists of the Station A sampling trips, the RS05 cruise, and I. Schulz (NC4 chief scientist), as well as the crews and scientific parties of R/V Queen of Sheba, R/V Thuwal, and R/V Urania for their support at sea and assistance with sample collections. CTD and nutrient data from Station A were provided by the ‘Israel National Monitoring Program of the Gulf of Eilat’ and are available from https://iui-eilat.huji.ac.il/research/nmpabout.aspx. The N2O data presented here have been archived in MEMENTO (The MarinE MethanE and NiTrous Oxide database) and are available from https://memento.geomar.de. Furthermore, we thank the Ocean Biology Processing Group (OBPG) for data service regarding the satellite data, which is freely available at https://oceancolor.gsfc.nasa.gov. We thank Damian Arévalo-Martínez and Carolin Löscher for helpful discussions of the results and comments on an early version of the manuscript. We thank two anonymous reviewers for their helpful comments. The EU's EUROMARGINS project 01-LEC-EMA21F (STO110/39-1) funded the RS05 cruise. SW's work in the Gulf of Aqaba was supported by the Deutsche Forschungsgemeinschaft through grant WA1434/1. FS was supported by the DFG funded Collaborative Research Centre 754 (SFB 754) ‘Climate-Biogeochemistry Interactions in the Tropical Ocean’. The research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST) through the baseline funds to BJ and the Competitive Centre Funding program. This is a contribution to the 2nd International Indian Ocean Expedition (IIOE-2) programme (www.iioe-2.incois.gov.in/).

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