Sampling:
During the ship-based repeated hydrographic observations, discrete seawater samples for δ13C-DIC were collected according to procedures outlined in the PICES Special Publication, Guide to Best Practices for Ocean CO2 Measurements from a profiling Conductivity, Temperature, and Depth (CTD) instrument paired with Niskin bottles. One or two duplicate samples were taken at each station. Pre-combusted (550 degrees Celsius for 4 hours) 250-milliliter (mL) borosilicate glass bottles were rinsed three times with the sample seawater before being filled from the bottom, allowing it to overflow for approximately twice the time needed to fill the bottle to the top. Bottles were capped and left in the room for about 30 minutes (to bring cold deep-water samples to near room temperature). Then, 1 mL of water was extracted from each bottle to allow thermal expansion, and 50 microliters (μL) of saturated HgCl2 solution was added to poison biological activities. Sample bottles were sealed with Apiezon-L grease, and stoppers were fixed with rubber bands and clips. The samples were stored at room temperature for at least 24 hours before onboard analysis or in coolers for transporting back to the home laboratory.

Analysis:
A G2131-i Isotope and Gas Concentration CRDS Analyzer (Picarro, USA) was employed along with an AS-D1 δ13C-DIC Analyzer (Apollo SciTech, USA) for sample injection, CO2 extraction, instrument control, and data acquisition in δ13C-DIC measurements. The analytical procedure began with drawing 0.7 mL of phosphoric acid brine (2% vol./vol. H3PO4 with 7% wt./vol. NaCl) into a 10 mL syringe by a digital syringe pump (Precision ≤ 0.05%, Tecan, USA) coupled to a 20-port valve and followed up by injection of the acid brine into the reactor. This step also cleaned residues from the previous cycle. While this pre-acid was bubbled in the reactor with a CO2-free air stream, an additional 0.9 mL of the acid brine was drawn into the syringe, followed by a 6.6 mL sample (or standard). The excess of acid brine ensured that all DIC in the sample could completely convert to CO2. Once a stable baseline of near zero CO2 was reached in the reactor and the CO2 detector, the sample and acid brine in the syringe were injected into the reactor at a controlled low speed to allow the acid brine to clear the sample DIC attached to the syringe wall into the reactor, where all carbonate species were converted to CO2. The CO2 was extracted and carried to the CRDS analyzer at a rate of 60 mL-1 by CO2-free compressed air from a 40-liter (L) cylinder. The CRDS concurrently reported CO2 concentration (12CO2 + 13CO2) and δ13C-CO2 values at 1 Hz for about 500 seconds, with data similarly captured by the AS-D1's data processing and control module. The analytical cycle would complete when CO2 levels drop below a set threshold (i.e., the deviation between 15 successive data points of CO2 reading was less than 5 parts per million (ppm) above the initial baseline), followed by a 120 second purge with carrier gas before the next cycle. Measurements occur under room temperature (20 ± 1 degree Celsius), each lasting about 13 minutes.

During the cruise, a total of 3825 samples were collected from 150 CTD casts, including 320 duplicates. Of these, 2875 samples were analyzed onboard, while the rest were analyzed ashore within 3 months.