Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Curation Notes
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Core collection: Cores were collected via a gravity corer (1 meter (m)) in July 2022 during cruise CB22 (R/V Carson, operated by the University of Maryland Center for Environmental Science, https://www.umces.edu/research-fleet/).

Incubation setup: Headspace-free bag incubations were conducted with sediment from each study site. The overall idea was to fill foil bags with a sediment slurry that had been equilibrated with differing concentrations of CH4 and SO4 to mimic in situ geochemical conditions characteristic of each site. Three geochemical zones were targeted for each incubation to reflect the vertical zonation observed in the in situ data: the sulfate zone (SZ; high SO4, low CH4), the transition zone (TZ; low SO4, low CH4), and the methanogenic zone (low SO4, high CH4). First, in situ SO4 data was used to inform the zonation of the collected cores prior to incubation. Next, incubations were set up with experimental and control treatments. For all incubations, a slurry was created in a 2:1 sediment to anoxic MQ ratio. Slurries were sieved with a mesh strainer (steel single weave #20 mesh) to remove shell fragments that could puncture the incubation bags or clog the stopcock fitted to the incubation bag. Prior to starting incubations, storage tests were conducted to ensure the bags were gas-tight and didn’t cause isotopic fractionation. The slurries were then transferred to 500 milliliter (mL) syringes without a headspace. All slurry manipulations were conducted in an anaerobic chamber (Coy). A mixture of 100% CH4, 99% 13C-CH4 (Cambridge Isotope Laboratories), and ultra-high purity (UHP) nitrogen gas (N2) was introduced to the syringe and equilibrated with the slurry by rotating for 90 minutes to achieve CH4 concentrations targeted to in situ concentrations. After equilibration, the headspace was removed and the slurry was injected into a 1 liter (L) N2 -flushed foil sampling bag equipped with a 3-way stopcock (Calibrated Instruments).

Sediment/porewater sampling:
CH4, CO2, δ13C-CH4, δ13C-CO2: 10 mL glass serum vials containing 3 mL sediment slurry were subsampled using the headspace equilibration method following Magen et al. (2014). Briefly, 8 mL of ultra-zero purity air (UZA) was injected into the sediment vial with a plastic syringe equipped with a 22-gauge needle. Vials were shaken and the headspace was mixed for 2 minutes, after which 8 mL of headspace gas sample was removed. Headspace samples were subsequently diluted with UZA to fall within the instrumental range for CH4 and CO2 measurements. Because the sulfide in the incubation subsamples was not bound like in the characterization cores, a copper trap was used at the intake of the CRDS to remove sulfide before entering the machine (Malowany et al., 2015).

SO4 and Cl: 2 mL microcentrifuge tubes containing 40 microliter (uL) porewater sample acidified to pH 2 with 0.1M H3PO4 were diluted (135x) with Milli-Q water prior to analysis. IAPSO certified seawater standards (Ocean Scientific International Ltd.) were used for all samples and precision is +/- 2%.

DOC and δ13C-DOC: 40 mL amber VOA vials containing 1 mL porewater sample acidified to pH 2 with trace metal clean HCl were measured with the wet oxidation method, similarly to the setup detailed in Lalonde et al. (2014). Samples are acidified and sparged to remove inorganic carbon then reacted with Sodium persulfate as wet oxidation. Data were normalized using two different internal organic standards (precision +/- 0.5 ppm for concentrations and 0.2‰ for isotopes).

DIC and δ13C-DIC: 10 mL glass serum vials containing 1 mL porewater sample acidified to pH 2 with H3PO4 were measured at CBL CRDS was used as detailed for the CH4 and CO2 concentrations and stable carbon isotopes.

POC and δ13C-POC: After frozen sediment samples were sampled for CH4 and CO2 concentrations and stable carbon isotope ratios, one of the duplicate samples was uncapped, dried at 60 degrees Celsius (°C) for three days, ground, and acidified with vapor acidification.

Methane, carbon dioxide, and dissolved inorganic carbon concentrations and stable carbon isotope ratios: Software for Picarro was used to obtain the CH4, CO2, and DIC concentrations (ppm) as well as stable carbon isotope ratios (delta notation; per mille). Concentrations were converted to dissolved concentrations (mM) and stable carbon isotope ratios were corrected for the machine offset using Microsoft Excel.

Sulfate and chloride: Chromeleon 7 software was used to obtain areas for SO4 and Cl. Areas were converted to concentrations (mM) using Microsoft Excel.

Particulate organic carbon concentrations and stable carbon isotope ratios: EAS Clarity 3.0.0.154 and Isodat Acquisition Version 3.0 were used to obtain POC concentrations (%) and stable carbon isotope ratios (delta notation; per mille). Concentrations and stable carbon isotope ratios were corrected for machine drift using Microsoft Excel.

- Imported sheet 1 of the original file "BCO_DMO_CB_v2.xlsx" into the BCO-DMO data processing system.
- Converted Sampling_Date column from string (format %m-%d-%y) to a date type, output as %Y-%m-%d.
- Saved the final file as "1002493_v1_inc_exp_cb.csv".

NSF Award Abstract:
Dissolved organic carbon (DOC) is a key component of the ocean’s food web and carbon cycle, and carbon exchanged between oceanic DOC and the atmosphere has influenced atmospheric CO2 levels on timescales ranging from recent decades to the geologic past. Production by marine algae in the surface ocean is the largest source of DOC and its effects on the ocean carbon cycle are widely appreciated. However, the contribution of DOC from additional sources such as rivers, hydrothermal vents, and methane seeps and their impact on ocean ecology and chemistry are not well understood. Each source differs in terms of its biological utilization and age, which affects the storage and distribution of DOC among the ocean basins. Methane seeps located along continental margins are particularly significant because they may transfer globally significant quantities of carbon stored below the seafloor as natural gas and gas hydrate to the oceans. This project will investigate the production, flux, composition and potential for biological utilization of DOC at Hydrate Ridge, located offshore Oregon. Hydrate Ridge is a prominent methane seep with massive accumulations of gas hydrate and a node of the Ocean Observatories Initiative telecommunications cabled array on the Juan De Fuca tectonic plate, which provides a continuous stream of real-time regional oceanographic data. We will sample and chemically characterize methane, DOC, and other materials to provide information about where the materials originated (deep vs shallow), how they have been chemically altered, to what extent they may feed deep ocean organisms, or contribute to the long term storage of DOC in the ocean. Experiments and analysis will be conducted using sediment cores and bottom water samples collected using either the remotely operated vehicle Jason or the human occupied vehicle Alvin during a 7-day ocean expedition. Additionally, this project will place osmotically-driven pumps on the seafloor to continuously sample fluids for approximately one year, thereby allowing us to monitor the movement of methane and DOC expelled from the seafloor to the ocean and constrain processes that regulate the release of carbon to the oceans at methane seeps. This project will support one graduate student and several undergraduates from a community college in Maryland and a college located in a lower-income urban center in southeastern Massachusetts. We will disseminate project findings to the public with a series of videos for public TV.

This study will investigate the production, flux and reactivity of methane-derived dissolved organic carbon (DOC) from methane (CH4) seeps at Hydrate Ridge, Offshore Oregon. The study will address four fundamental questions to determine the significance of CH4-derived DOC within the ocean carbon cycle: (1) How much CH4-derived fossil DOC do seeps contribute to the oceans? (2) To what extent is CH4-derived C incorporated into DOC during anaerobic oxidation of CH4? (3) Is seep DOC bioavailable or recalcitrant when released into the deep ocean? (4) How does the flux of DOC to the water column vary over time? We will employ an interdisciplinary strategy that includes in situ sampling, laboratory incubations, and a comprehensive analytical geochemistry program. Data from the Ocean Observatories Initiative Regional Cabled Array at Southern Hydrate Ridge will be used to provide context for field and experimental data. The composition and abundance of organic and inorganic chemical species along with the stable and radiocarbon isotope composition of pore water, bulk sediment, and water column C pools will be used to identify DOC sources and quantify fluxes from cold seeps characterized by a range of advection rates. The centerpiece of the investigation will be a 7-day research cruise to Hydrate Ridge to collect sediments, pore fluids, and water column samples, and deploy OsmoSamplers for continuous time series fluid sampling. The results will form the foundation for estimating the contribution of CH4-derived DOC to the oceanic DOC pool.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.