Dataset: Dark DIC Fixation Rates collected from CliOMZ AT50-10 in the Eastern Pacific Ocean from May to June 2023 (CliOMZ project)

Water samples were collected during cruise AT50-10 in the eastern tropical and subtropical Pacific Ocean. Discrete water samples were obtained using a CTD rosette sampler equipped with 24 x 10 L Niskin bottles. Different depths were sampled ranging from 55 meters to 1500 meters. The sampling strategy focussed on targeting low oxygen zones within the water column.  For depths where O2 concentrations were above 20 µM, water was dispensed into 40 mL glass vials with teflon septa (TOC-certified, Fisher Scientific). For depths where in situ O2 concentrations were ≤20 µM, water was sampled directly from the Niskin bottle into 60 mL glass serum bottles using tygon tubing, allowing approximately three volumes of sample water to overflow the bottle prior to collection. Serum bottles were closed bubble-free with deoxygenated butyl rubber stoppers (De Brabandere et al. 2012) and sealed with aluminum crimps. A 20 mL helium (He) headspace was introduced to each serum bottle and oxygen was then added back to reach in situ concentrations by injecting air using gas-tight syringes (Hamilton) with volumes calculated using the solubility equations of Garcia and Gordon (1992).

For each depth, seven replicate bottles were filled, sealed and spiked with 50 µCi [14C]-bicarbonate (specific activity 56 mCi mmol−1/2.072 × 109 Bq mmol−1; PerkinElmer). To three bottles, 10 µM phenylacetylene dissolved in DMSO (0.01% final concentration) was added to inhibit ammonia oxidation activities as described in Bayer et al. 2024. One bottle served as killed control to which formaldehyde (3% vol/vol) was added at the start of incubation. Bottles were incubated in the dark at in situ temperature and live incubations terminated after 24-72 hours by adding formaldehyde (3% vol/vol). After 30–60 minutes, samples were filtered onto 0.2 μm polycarbonate filters (GTTP, 25mm, Millipore) and rinsed with 10 mL of artificial seawater at a vacuum pump pressure of ~100 mbar. Incubation times were chosen according to the productivity of the sampling region (Bayer et al. 2024). The filters were transferred to scintillation vials, 10 mL of scintillation cocktail (Ultima Gold; PerkinElmer) was added, samples were shaken for ca. 30 seconds and incubated in the dark for at least 24 hours prior to counting the disintegrations per minute (DPM) in a scintillation counter (Perkin-Elmer Tri-Carb 2910 TR) for 15 minutes. Total radioactivity measurements were performed to verify added [14C]-bicarbonate concentrations and DIC fixation rates were calculated as described previously (Bayer et al. 2022).