Concentration and isotopic composition of marine nitrous oxide, in particular the oxygen-17 isotope excess

The oxygen isotope excess Δ(17O) is a potential tracer of biological nitrous oxide (N2O) cycling. This study presents the first measurements of Δ(17O) in marine N2O together with details about the design and development of a custom-built Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS) analytical system for δ(15N), δ(18O) and Δ(17O) of N2O. Δ(17O) values between 0.1 and 4.6 ‰ (relative to VSMOW) were observed during three field campaigns in the temperate, subtropical and tropical Atlantic Ocean, the Scotia Sea and the Weddell Sea. This indicates a biological source for oxygen isotope excess in N2O, as oceanic N2O was not in equilibrium with the atmosphere (0.9 ‰). δ(15N) values ranged from 2.3 to 25.1 ‰ (relative to Air-N2). δ(18O) measured in the Weddell Sea ranged from 44.9 to 48.8 ‰ (relative to VSMOW). The dataset from the Atlantic Ocean and the Atlantic sector of the Southern Ocean suggest nitrification and nitrifier-denitrification as the main N2O production pathway in the oxic, deep ocean. High δ(15N) values in the south Atlantic Gyre are presumably associated with a denitrification source. A novel off-the-shelf N2O analyser was tested in combination with an equilibrator for semiautonomous concentration measurements in the surface ocean. The subtropical gyres in the Atlantic Ocean were confirmed to be weak sinks ((-0.14±0.31) μmol m-2 d-1 N2O flux to the ocean in the northern, and (-0.16±0.33) μmol m-2 d-1 in the southern gyre) and the equatorial region was a source of N2O to the atmosphere (flux of 0.53 μmol m-2 d-1). New data from the Scotia Sea identified a strong source region ((2.9±2.7) μmol m-2 d-1), while the Weddell Sea was closer to equilibrium with the atmosphere ((0.9±1.0) μmol m-2 d-1).

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of East Anglia

Year dc:date.issued

2013

Author dc:creator

• Grefe, Imke

Language dc:language

en