Organic geochemistry data of bulk sediments from the Escanaba Trough, off the coast of Northern California, USA, from May-June 2022.

SEDIMENT COLLECTION. Samples were collected aboard the R/V Thomas Thompson via push coring using the ROV Jason. Once recovered shipboard, pore waters were extracted via rhizon samplers, cores were subsectioned into one to five cm slices, the exterior sediment which was in contact with the core liner was removed, and remaining sediment was stored frozen in pre-combusted glass jars.

ORGANIC CARBON GROUP ISOLATION: Bulk sediments from two depth horizons in each core (surface and near bottom of core) were subsampled for extraction of lipids, acid-soluble (AS), and acid-insoluble (AI) organic matter (Hwang and others , 2005; Wang and others, 1996). Between 500 and 1500 mg of dried sediment was weighed into pre-combusted glass vials. The total lipid extra was extracted using a modified version of the method presented in Bligh and Dyer (1959). Two bed volumes of hexanes were added to each sample and samples were mixed, sonicated for 15 minutes (min), then centrifuged for five min at 2000 revolutions per min. The supernatant was piped to a combusted Turbovap flask. This process was repeated two more times with hexanes and three times with 9:1 DCM:MeOH, combining the supernatant from each sample. Supernatant was dried under N2 gas at 27 degrees Celsius using a TurboVap Evaporation Concentrator (Zymark Corp, NC, USA) until nearly dry, then were transferred to 4 mL vials. The fully dry total lipid extract (TLE) was weighed for each sample. Remaining sediments were subject to acid hydrolysis by adding 6N HCl to each sample, evacuating the headspace with N2 gas, and heating at 110 degrees Celsius for 20 hours. Samples were then mixed, centrifuged at 2200 rpm for five minutes, and supernatant pipetted off to new vial. This process (mixing, centrifuge, and removal of supernatant) was repeated with 1 mL 6N HCl and four 1 mL rinses of MilliQ H2O. The supernatant from all HCl and MilliQ H2O rinses were combined and stored in the freezer (AS fraction). Additional MilliQ H2O was then added to the residual sediments (AI fraction), mixed, centrifuged, and H2O discarded, until pH of pore fluids was about 4. Sediments were then freeze dried.

RATIOS OF STABLE ISOTOPES OF CARBON 13/12 (d13C), NITROGEN (d15N), TOTAL ORGANIC CARBON, and TOTAL NITROGEN: Acid insoluble material was analyzed for percent TOC, TN, d13C, and d15N using a CE Instruments NC2500 elemental analyzer (EA) coupled to a Thermo Scientific DELTAplus XP isotope ratio mass spectrometer (IRMS) via a Thermo Scientific Conflo III at the University of California at Santa Cruz Stable Isotope Lab. The total lipid extract was measured for TN and d15N at Texas A&M using a Thermo Scientific GC-Isolink EA coupled to a Thermo Scientific Delta V Advantage IRMS. At both labs, measurements were corrected to VPDB (Vienna PeeDee Belemnite) for d13C and AIR for d15N against in-house standard reference material (PUGel at UCSC, USGS 40 and 41a at Texas A&M). The total lipid extract was measured for d13C at NOSAMS via accelerator mass spectrometry (AMS) using a custom built continuous flow AMS.

RADIOCARBON ANALYSES: Total lipid extract and acid insoluble material were analyzed for D14C values via accelerator mass spectrometry (AMS) using a custom built continuous flow AMS system at NOSAMS. D14C values were corrected for any addition of carbon during the process step ORGANIC CARBON GROUP ISOLATION by processing 14C modern and depleted grass samples in the same manner as the samples according to Hwang and Druffel (2005) and Hanke and others (2017).