Dataset: Stable carbon isotopic signatures of solid-phase extracted DOC from seawater collected on three GO-SHIP Repeat Hydrography Cruises in 2015-2018 along transects P16N, P18, and I07N.

Seawater samples were obtained from three GO-SHIP Repeat Hydrography transects to measure stable carbon isotopic composition of solid-phase extracted dissolved organic matter (SPE-DOC δ¹³C). Transect P16N (151°W) sampled in the year 2015, P18 (103-110°W) in 2016 and 2017, and transect I07N (50-70°E) in 2018 aboard the NOAA Ship Ronald H. Brown (see Figure 1), at or within 1 station of an associated total DOC sample (Druffel et al., 2019, 2021, 2023). “Surface” and “deep” samples were collected at 0-200 meters and 2000-4000 meters, respectively. Due to the integration of seawater among different stations and depth, we report the weighted average depth which is the weighted mean that incorporates the relative amount of each water depth into the final value. (See Supplemental Files section for the equation). 

 Due to excess water availability, two samples (called "seawater duplicates or SD) were collected from P18 stations 77-79 (in deep water), and 168 (in surface water). For seawater samples from P18 and IO7N, solid phase extraction (SPE) was performed at sea using PPL cartridges (Bond Elut-PPL, 1 gm 6 ml, Part No. 12255002) according to Dittmar et al. (2008) and stored in the dark at 4°C until elution at UC Irvine. Seawater samples from the P16N cruise were processed after four years of freezing. Samples were then acidified to pH 2 with sulfuric acid (ACS grade, Lot #160,128) and extracted onto PPL cartridges. Before elution, PPL cartridges were dried under a steady stream of ultra-high purity N₂ gas for 30 min to remove residual MilliQ water (3 ppb DOC). SPE-DOC was eluted with LC/MS grade methanol following established methods (Dittmar et al., 2008). However, the elution volume of methanol was increased to 32 mL to increase yield of SPE-DOC (Lewis et al., 2020). SPE-DOC was eluted into pre-weighed and pre-combusted (540°C/2h) 50 mL centrifuge vials (VWR product #89091-460), and weighed after elution to determine the exact volume. 

 Splits were dried in a centrifugal evaporator with a −60°C chiller trap (Savant SpeedVac SC200) for a minimum of 12 hours. Pre-baked cupric oxide and silver wire were added, and the outside of these quartz tubes were cleaned with acetone, dried, and placed inside larger 9 mm diameter, 20 cm long quartz vials. These “double tube” samples were sealed under vacuum and combusted at 850°C for two hours. Sample CO₂ was extracted cryogenically on a vacuum line and quantified manometrically.

Stable carbon isotopic measurements (δ¹³C) were made using a Gas Bench II and Thermo Electron Delta Plus mass spectrometer, and corrected using calibrated isotopic standards (NIST NBS-19). Measurements are reported as standard permille (‰) relative to V-PDB (Vienna Pee-Dee Belemnite) scale. Duplicate and triplicates taken from certain SPE-DOC extracts from all three cruises show our SPE-DOC δ¹³C measurement precision averages better than 0.2‰ providing confidence that the variability seen throughout different ocean regions is real.

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This work was supported by the National Science Foundation Chemical Oceanography Program (OCE-1458941 and OCE-1951073 to E.R.M.D.), the U.S. American Chemical Society Petroleum Research Fund New Directions Grant (55430-ND2 to E.R.M.D. and B.D.W.), the Fred Kavli Foundation, and the Keck Carbon Cycle AMS Laboratory. A portion of this work was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada through a Discovery Grant, Accelerator and Launch Supplements (RGPIN-2020-06501, RGPAS-2020-00071, DGECR-202000256; to B.D.W.), the New Frontiers in Research Fund (NFRFE-2019-00794 to B.D.W.) and the Canada Research Chairs program (to B.D.W.). A portion of this work was also funded by the New Zealand Ministry of Business, Innovation and Employment (MBIE) through the Global Change through Time programme (Strategic Science Investment Fund, contract C05X1702)