Data were collected on the Oregon-Washington continental shelf (between 44.5 N,124.2 W and 46.5 N, 124.6 W) and at water depths between 30-80 meters during the following cruises and dates:

R/V Oceanus OC1802B (dates: 2018-02-26 to 2018-03-01)
R/V Oceanus OC1805B (dates: 2018-05-07 to 2018-05-11)
R/V Oceanus OC1807A (dates: 2018-07-03 to 2018-07-05)
R/V Oceanus OC1810A (dates: 2018-10-04 to 2018-10-08)

R/V Oceanus OC1901A (dates: 2019-01-11 to 2019-01-15)
R/V Oceanus OC1904A (dates: 2019-04-22 to 2019-04-26)
R/V Oceanus OC1907A (dates: 2019-07-07 to 2019-07-11)

R/V Robert Gordon Sproul SP2215 (dates: 2022-07-22 to 2022-07-25)
R/V Robert Gordon Sproul SP2219 (dates: 2022-09-14 to 2022-09-19)

Sediment cores were collected from sites along the Newport Hydrographic Line on the Oregon shelf during the the February, May, July, and October 2018, January, April, and July 2019 cruises, and from 8 different sites, along the Oregon-Washington mid-shelf during the July and September 2022 cruises. Sediment cores were incubated using a unique method designed around the core tube itself. The method is described in Hughes et al. (2024). Briefly, sediment cores were collected in acrylic tubes with a radius of 5.3 centimeters (cm) and length of 94 cm using a hydraulically dampened Gravity Corer (Reimers et al., 2012). Once onboard, cores were immediately transported to a cold van, pre-set to the approximate in situ bottom water temperature. The core's overlying water was siphoned down to ~10 cm above the sediment surface and the core was sealed with a custom-made black Delrin puck. The pucks included two side grooves, two through-wells threaded at each end, two eye screws, and top and bottom axially aligned cavities. The two grooves each held an X-ring seal to ensure a seal with the core tube sides. One of the wells was fitted with a barbed connector joined to Tygon tubing (inner diameter of 3 millimeters (mm) and ranging in length from 0.66 meters (m) to 0.88 m) to enable overlying water sampling. The other well was plugged into the waterside. The top cavity was fitted with a 4-inch long threaded, clear PVC pipe (McMaster-Carr, part no. 4677T33) which contained an externally powered 6-volt DC mini electric motor. The output shaft of the motor ended in a small Delrin disk, fitted with four rare earth magnets (McMaster-Carr, part no. 5862K108). The bottom cavity held a second Delrin disk, with its top side fitted with four rare-earth magnets as well. The stir bar was set to rotate at 30 rpm at the top of the overlying water column. The resealed core was placed in a temperature-equilibrated, water-filled, incubation chamber with a lid to ensure the core remained at bottom water temperature and under low light conditions inside the cold van.

Incubations ran between 24 and 36 hours, with three to six time-point measurements conducted per core (about every 4 to 12 hours) before oxygen saturation dropped below 10%. For each measurement, the stirring motor of the core was stopped, and a custom device was used to slowly depress the black puck caps against the overlying water, expelling small, carefully measured volumes through the open sampling tube into a 10-milliliter (mL) gas-tight glass syringe with a luer-lock fitting. Following a flush of the sample tube, the sample water was directed to the flow-through cell of a PreSens Microx T3 micro-fiber optic oxygen sensor via a three-way valve at the end of the tubing, and its dissolved oxygen (DO) was recorded as percent air-saturation. The luer-lock fitting of the gas-tight glass syringe was attached to the exit side of the flow-through cell to collect timepoint water samples. These samples included 7 mL for shore-based dissolved inorganic carbon (DIC) analysis that were transferred to 8 mL borosilicate scintillation vials with polyethylene screw caps with poly-seal cone liners, pre-poisoned with 50 microliters (µL) of mercuric chloride to halt any biological activity. Another 10 mL was stored in a Nalgene™ HDPE bottle, and frozen (-20 degrees Celsius) for shore-based nutrient analysis. The second sample enabled a repeat oxygen measurement through the flow cell; however, the initial oxygen measurement was used for the flux calculation. After samples were collected, the stirring motor for the core was turned back on for the core to continue to incubate.

Nutrient samples from sediment core incubation experiments were analyzed onshore using standard auto-analyzer colorimetric methods at Oregon State University (Gordon et al., 1993). For DIC samples collected during the 2018 and 2019 cruises, samples were analyzed using a UIC Coulometer with a 1 mL sample loop, however, the results were not found to be reliable and therefore these data are not reported. For the DIC samples collected during the 2022 core incubations, samples were processed for DIC with a Finnigan GasBench-II headspace sampler online with a Finnigan DELTAplusXL gas-source isotope-ratio mass spectrometer, with reported precision between 0.5 and 1%, using a procedure outlined in Torres et al. (2005). Additional details on core collection, incubations, and sample analyses can be found in Hughes et al. (2024).