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).
N2:Ar samples were collected in triplicate in 12 mL exetainers directly from the Niskin bottles using a small piece of tubing. The tubing was placed at the bottom of each container and water overflowed by approximately five volumes. The samples were preserved using 20 μL of a saturated mercuric chloride. The serum vials were sealed using butyl septa and aluminum crimp tops, and both sample types were stored at room temperature, which was cooler than sampling temperature, until analysis.
N2:Ar was measured on a membrane inlet mass spectrometer (MIMS) equipped with a flow-through capillary membrane inlet (Bay Instruments, Easton, Maryland) following Kana et al. (1994).