Methodology:
Dissolved inorganic carbon concentration ([DIC]) samples were measured by acidification and subsequent release of CO2 with a CO2 cavity-ringdown spectrometer (CRDS; Picarro G1101-i) detector to an accuracy and precision better than ± 3 umol/kg. Precision is based on a set of 15 duplicates. Accuracy is based on certified reference materials prepared at the Scripps Institution of Oceanography of the University of California, San Diego.

Samples for pH were measured spectrophotometrically on board using an Agilent Cary 100 UV-Visible spectrophotometer and m-cresol purple dye (Clayton and Byrne, 1993; Liu et al., 2011). The pH values are reported on the total scale with a precision better than 0.0002 based on duplicate measurements. The estimated accuracy of spectrophotometric pH is better than 0.002.

Natural radiocarbon samples of DIC (∆14C-DIC) were analyzed at the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory at the University of California, Irvine. The accuracy and precision of the ∆14C-DIC measurements is 0.1‰ and 1.7‰, respectively.

Sampling and analytical procedures:
Discrete seawater samples were collected inside Hudson Canyon, US Atlantic Margin aboard the R/V Endeavor from 9 - 13 of July 2014. A total 19 stations and 216 samples were collected along Hudson Canyon and another station was sampled outside the canyon (station E2-HC2). Samples were collected from a bottle Rosette following the procedures in SOP 1 and 6b in Dickson et al., (2007).

[DIC] samples were collected in acid washed and combusted 120 mL serum vials. Vials were allowed to overflow three times their volume, poisoned with 50 uL of a 55 uM HgCl2, capped, and store at 4 degrees C until analysis.

∆14C-DIC samples were collected in combusted 1L borosilicate bottles, poisoned with 200 uL of a 55 uM HgCl2, sealed and stored in the dark at room temperature until isotope analysis.

CH4 concentration analyses were collected by filling 60, 120, or 160 mL glass serum vials. Vials were filled from the bottom with a length of 1/4" Tygon tubing. Vials were flushed with seven vial volumes of seawater to expel any bubbles and ensure collection of a clean sample, and then were sealed with butyl rubber stoppers taking care not to introduce bubbles during sealing. Immediately after sealing, a 10 mL headspace of ultrahigh purity nitrogen was introduced, displacing an equal volume of water, and the samples were preserved by adding 25 uL of a saturated solution of mercuric chloride. The vials were stored inverted to minimize diffusive gas exchange through the butyl rubber stopper.

CH4 concentration measurements were performed using an Agilent 6850 gas chromatograph with a flame ionization detector (GC-FID) following established protocols reported in Weinstein et al. (2016). The uncertainty associated with these measurements was 5.2% based on duplicate measurements.