Dataset: OC1703A and AT36 sediment N isotope ratios
Deployment: AT36

Sediment cores were collected in the northwest Pacific Ocean off the coast of San Francisco, CA on the R/V Oceanus during cruise OC1703A in March 2017 and in the northeast Atlantic off the coast of Woods Hole, MA on the R/V Atlantis during cruise AT36 in July 2016. Sediment cores in the northwest Pacific were 15–30 cm long and collected at five sites (100–4475 water depths) using an MC-800 multicore (Ocean Instruments). Sediment cores in the northeast Atlantic were 15 cm long and collected at two sites (1252–1496 water depth) using an MC-400 multicore (Ocean Instruments). Both multicores were modified with Go-Pro cameras in custom pressure housing to provide real-time video feeds and guide sampling (MISO Facility, Woods Hole Oceanographic Institute). Cores were stored at 4ºC for ≤24 hours until they were sectioned on board in 2.5, 3, or 5 cm depth increments.

Sediment from two cores at each of the five Pacific sampling sites was sectioned in 5 cm intervals (3–5 sections per core) and mixed with 0.2 µm-filtered, argon-sparged bottom water (1:1–1:2 ratio of sediment to seawater). About 60 mL of sediment slurry was aliquoted into each 120 mL glass serum bottle, sealed with NaOH pre-treated black butyl rubber stoppers (Geo-microbial technologies, Ochelata, OK, USA) and aluminum crimp caps, and the headspace exchanged using argon. Incubations from each sediment depth were amended with ¹⁵N-ammonium (final concentration: 500 µM; Cambridge Isotopes, NLM-467-5, Lot I-19633L), ¹³C-glucose (50 µM; Cambridge Isotopes, CLM-1396-1, Lot PR-27921), ¹³C-bicarbonate (final concentration 1.15 mM; Cambridge Isotopes, CLM-441-5, Lot PR-27797), or 5 mL of ¹⁵N₂ (Cambridge Isotopes, NLM-363-PK, Lot I-19197). ¹⁵N₂ gas was passed through an acid trap before addition to incubation bottles to scavenge any undetected contaminants. Select ¹⁵N-ammonium and ¹³C-glucose incubations were additionally amended with non-isotope labeled glucose (50 µM) and ammonium (500 µM), respectively. Incubations were over-pressured to 30 psi using argon. All incubations were conducted in duplicate, with the exception of the ¹⁵N₂ incubations, which were conducted in triplicate. All Pacific incubations were subsampled at 2, 4, 8, and 12 weeks for bulk C or N stable isotope analysis.

For the Atlantic samples, cores were sectioned into 3 cm sediment horizons and stored under anaerobic conditions at 4ºC for 3 months. Incubation set up was the same as for the Pacific samples, except only 20 mL of slurry was aliquoted into each 60 mL serum bottle and they were sealed with blue butyl rubber stoppers (Bellco Glass, Vineland, NJ, USA). All incubations were amended with ¹⁵N-ammonium (final concentration: 1 mM; Cambridge Isotopes, NLM-467-5, Lot I-19633L) or 5 mL ¹⁵N₂ (Cambridge Isotopes, NLM-363-PK, Lot I-19197) and incubations were subsampled at 0 and 30 days. A total of nearly 300 incubations of Pacific and Atlantic sediment were prepared.

Sediment slurries were subsampled with a needle and syringe and sampled slurry was immediately centrifuged at 4,000 x g for 30 seconds. The supernatant was removed and stored at -20ºC, and the sediment pellet frozen at -20ºC. For incubations amended with ¹⁵N-ammonium , sediment pellets were washed three times with PBS and KCl after defrosting and before drying as previously described (Dekas et al., 2014). Briefly, 1 x PBS was added to sediment pellets, vortexed, centrifuged at 15,100 x g for 15 min, the supernatant removed, and repeated with 2M KCl (incubated at 1 h at room temperature) and then 1 x PBS. For incubations amended with ¹³C-glucose and ¹³C-bicarbonate, sediment pellets were also washed, but we replaced the KCl incubation with another 1 x PBS wash step. Sediment pellets were dried overnight at 60ºC and left to equilibrate to ambient humidity for at least three days. For N isotope analysis, samples were weighed into tin capsules. For C isotope analysis, samples were weighed into silver capsules and acid fumigated to remove carbonates as previously described (Harris et al., 2001). Acid fumigated samples were then encapsulated into tin capsules. All samples were analyzed using Elementar Vario EL Cube or Micro Cube elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany) interfaced to either an Isoprime VisION IRMS (Elementar UK Ltd, Cheadle, UK) or a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) at UC Davis Stable Isotope Facility. They report a long term deviation of 0.2‰ for ¹³C and 0.3‰ for ¹⁵N.