Dataset: Continuous underway data

Measurements
Measurements for O2/Ar dissolved gas ratios, temperature, salinity, and fluorescence were continuously measured from an underway seawater system (10 m depth) on basin-wide transects of the North Pacific between Hong Kong and Long Beach, California onboard the M/V OOCL Tianjin and the M/V OOCL Tokyo (each individual transect has a unique Cruise ID). Sea surface temperature and salinity at the time of sample collection were determined using a Sea-Bird Electronics SBE45 thermosalinograph (TSG) installed in the ship’s seawater intake. Fluorescence was measured using a Seapoint Chlorophyll Fluorometer.  This dataset presents uncalibrated data; the discrete calibration data are in https://www.bco-dmo.org/dataset/626855.

Underway measurements of O2/Ar dissolved gas ratios were made using continuous flow equilibrator inlet mass spectrometry (EIMS), following the method of Cassar et al. (2009). Water from the underway seawater system was pumped into an equilibrator cartridge (Membrana MicroModule G569, 0.75” x 1”), the headspace of which was delivered to a quadrupole mass spectrometer (Pfeiffer Prisma QMS) that measured individual ion currents at one-second intervals. To prevent biofouling that could cause respiration in the ship’s seawater lines (Juranek et al., 2010), intake lines between the anticorrosive sea chest and the sampling port were purged with bleach and freshwater between every cruise.

NOTE: Since the samples were collected underway on a vessel moving ~24 knots and samples for all parameters were collected by a single shiprider, ship transit from the time that the location coordinates were recorded to the time of actual sampling could reflect a transit distance offset from the recorded location of up to 40 kilometers.

Methods and Calculations
Methods are described in detail in Clayton et al. (in preparation for JGR: Oceans). 

To aid in interpretation of the O2/Ar data provided here, we include calculations of the air-sea flux of biological oxygen driven by the biological saturation anomaly (O2Arbiosat), termed “bioflux” by Jonsson et al. [2013]:
                             Air_sea_flux = k*[O2]eq*(O2Arbiosat/100)

Where k represents the wind speed-dependent air-sea gas transfer velocity and [O2]eq is the oxygen concentration that would be expected in the mixed layer were it in equilibrium with the atmosphere. We calculate k from daily wind speed data from the NOAA National Climatic Data Center’s multiple-satellite Blended Sea Winds product (https://www.ncdc.noaa.gov/data-access/marineocean-data/blended-global/blended-sea-winds) following the Nightingale et al. (2000) equation and the 60-day (Reuer et al., 2007) time-dependent weighting scheme. We report k here as “Wkn” representing the weighted k value using the Nightingale algorithm. In conditions with limited influence of physical advection, entrainment, and transient changes due to non-steady state conditions over the dissolved gas residence time in the mixed layer, bioflux is equivalent to net community production. However, there are potential biases in quantifying net community production based on observed O2/Ar in dynamic western boundary current regions where steady state assumptions are likely invalid and strong vertical turbulent dissipation rates have been observed.

The following are additional details for relevant fields in this dataset:

• TrueO2Ar:  ratio of dissolved O2/dissolved Ar in surface seawater as measured by continuous underway equilibrator inlet mass spec, and calibrated with discrete samples from existing dataset (https://www.bco-dmo.org/dataset/626855).
• O2Arsat: O2/Ar ratio expected if both gases were in equilibrium with the atmosphere, which is a function of Salinity and Temperature (Garcia and Gordon, 1992; Hamme and Emerson, 2007)
• O2Arbiosat:  percent supersaturation of O2 in surface seawater, calculated from the TrueO2Ar and O2Arsat [O2Arbiosat = (TrueO2Ar/O2Arsat – 1)*100]
• MLD: climatological mixed layer depth from monthly World Ocean Atlas 2013 (with 1 degree gridded data)
• Wkn: air-sea gas transfer velocity in m/day, calculated using a relationship between wind speed and gas transfer from Nightingale et al. (2000) and the NOAA/NCDC Blended Sea Winds satellite wind speed product (Zhang et al. 2006), and using the Reuer et al. (2007) time-dependent weighting scheme and the MLD values given here.