Water samples were collected using a 12 x 10 L Niskin bottle rosette sampler equipped with a conductivity, temperature, and pressure instrument package (SBE9, Sea-Bird Electronics, Bellevue, Washington, U.S.A.) and a sensor for dissolved oxygen (SBE43, Sea-Bird).
N2O samples were collected in duplicate in 160 mL serum vials. The tubing was placed at the bottom of each container and water overflowed by approximately five volumes. The samples were preserved using 100 μL of a saturated mercuric chloride solution. The serum vials were sealed using butyl septa and aluminum crimp tops, and stored at room temperature, which was cooler than sampling temperature, until analysis.
A 30 mL ultra-high purity N2 headspace was added to each sample using a syringe with a vent needle inserted in the septa to drain sample water. Each headspace was over-pressurized with an additional 2.5 mL of N2 to avoid atmospheric contamination upon headspace sample removal. The headspace was equilibrated with the underlying seawater by gentle shaking at room temperature for at least 2 hours.
N2O concentrations were measured using a headspace equilibration method and analyzed on a Shimadzu GC-14B Gas Chromatograph (GC) equipped with a Porapak-Q packed column and an electron capture detector (ECD) (Elkins 1980). N2O concentrations were calculated according to Walter et al. 2006.
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