Dataset: continuous
Deployment: KM1713

Underway/continuous measurements of O2, O2/Ar and optically-based POC

Continuous measurements of surface O2, O2/Ar, POC, and beam attenuation coefficients used seawater supplied from an uncontaminated seawater intake line located at ~7m depth.  All continuous measurements were binned into hourly averages at each station. Continuous measurements of dissolved O2 concentration using an Optode (Aanderaa) and the dissolved O2/Ar  gas ratios using an equilibrator inlet mass spectrometer (EIMS) system [Cassar et al., 2009] were obtained at each station. Dissolved oxygen data (Optode) were corrected for pressure and salinity following Uchida et al. [2008]. O2/Ar measured by the EIMS were calibrated to O2/Ar measurements by mass spectrometer on discrete water samples collected in the mixed layer during the cruise (n=58). Saturated O2 concentrations and O2/Ar values were calculated using the solubility equations of García and Gordon [1992] for O2 and Hamme and Emerson [2004] for Ar. Diurnal changes in O2/Ar data measured by the EIMS instrument were corrected for fluctuations in total gas pressure based on concurrent changes in N2/Ar, yielding O2/Ar*, (i.e., % deviation in O2/Ar* = % deviation O2/Ar – a*% deviation N2/Ar, where ‘a’ represents a calibration factor between N2/Ar and O2/Ar deviations determined at each station). Diurnal changes in O2 were determined by multiplying the relative deviation from the mean O2 measured by the Optode by the mean surface Winkler O2 concentration measured at each station (Eq. 1), where i refers to each datapoint in time, and overbars denotes the mean values while on each station. The diurnal changes in O2/Ar and O2/Ar* were calibrated to Winkler concentrations using the same procedure.

Discrete water samples (collected on combusted GF/F filters with filter blank corrections following the Hawaiian Ocean Time Series (HOT) protocols, http://hahana.soest.hawaii.edu/hot/methods/pcpn.html) were collected from the ship’s uncontaminated seawater system to determine particulate organic carbon (POC) concentrations every ~2.5 hours (n=107 in total) using a semi-automated filtration device [based on the design of Holser et al., 2011]. Underway POC were scaled to mean near-surface (<10 m) samples collected at each station (n=12; r2=0.92) and these corrected values were then used to convert cp measurements to POC using Eq. 2, where  is the mean in situ POC measured at each station.