In general, procedures followed those of Reuer et al. 2007. Briefly, 1 and 2-L bottles were pre-poisoned with 200 μL and 400 μL saturated HgCl2 solution, dried, and then evacuated to a pressure of < 10-3 mbar before the gas-tight valve was closed. The Louwer-Hapert 9mm valve sidearm was then filled with distilled water and capped, making sure to remove all visible bubbles, to retard any atmospheric leaks. During sampling, water from the bottom of the Niskin flask was allowed to flow into the sidearm for several seconds (to flush out the distilled water) before the vacuum valve was opened to allow water to be siphoned into the sampling bottle. After sampling—each bottle typically being half full—the sidearm was filled again with seawater and capped until analysis. All gas samples were analyzed within 4 months of sampling.

Before gas analysis, bottles were gently agitated on a shaker table for 48h to equilibrate dissolved gases with the 
headspace at room temperature (25C). Next, bottles were inverted and the seawater pumped out. Finally, the headspace gases were extracted and purified according to Li et al. (2019), Yeung et al. (2016), and Yeung et al. (2018), in which O2 is separated from Ar and N2 by gas chromatography. O2/Ar ratios are standardized relative to atmospheric O2 and air dissolved in distilled water. Bulk δ18O and 17Δ values are standardized against atmospheric O2 and O2 derived from laser-fluorination of San Carlos Olivine (Yeung et al. 2018). Clumped-isotope Δ35 and Δ36 values are standardized against laboratory-generated photolytic and heated-gas standards (Yeung et al. 2014).

O2/Ar ratios were determined by integrating the peak areas for each species on an Aglient 7890B gas chromatograph utilizing thermal conductivity detection. Dissolved O2 saturation was determined from these O2/Ar measurements and checked independently through manometric determination of O2 yield and weighing the seawater from which it was extracted. Isotopologue analyses were performed on a custom Nu Instruments Perspective IS high-resolution isotope ratio mass spectrometer at Rice University.
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Data was processed as was done in Yeung et al. (2018) for δ18O and 17Δ values and Yeung et al. (2016) for clumped-isotope values. Dissolved oxygen saturation is calculated using potential temperature and salinity and the O2 and Ar saturation calculated using MATLAB programs by Roberta Hamme (O2sol and Arsol from https://web.uvic.ca/~rhamme/download.html). These calculated values were compared to measured values to determine O2/O2sat.