Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

The NSF (OCE-2049517) funded ship time, SO4, H2S, DIC, d13C-DIC measurements and partially funded CH4, DOC and d13C-DOC measurements

The DOE NETL interagency agreement with the USGS (#89243320SFE000013) partially funded CH4 measurements

The USGS Mendenhall Research Fellowship Program partially funded DOC and d13C-DOC measurements, and funded TOC and d13C-TOC measurements

Sediment and porewater handling:

PCG02-08- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through 0.45 um polyethersulphone syringe filters.

Hydrates2004- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through 0.2 um arodisc polyethersulphone syringe filters.

AT50-14- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through pre-combusted 0.4 um glass fiber filters.

AT50-29B- Core material was sectioned into acid-washed falcon tubes and refrigerated. Porewater was extracted using Rhizons with a 0.2 um filter size.

Porosity: Fresh sediment was placed in pre-weighed petri-dishes, sealed, and refrigerated until measurement in lab. Petri dishes were weighed upon return to shore, then were played in a drying oven (45℃, 1 week). Petri dishes with dried sediment were weighed again. The difference between the wet and dry sediment is the water weight, and the ratio of the water volume to the total volume is the porosity, assuming a dry bulk density of 2.5.

SO4_mM: 0.5 mL porewater was sub-sampled into 2mL microcentrifuge tubes and dosed with 50 uL 0.1M phosphoric acid. Samples were measured using ion chromatography. IAPSO certified seawater was used for standardization, and precision is ±1.5%.

CH4_uM: Sediment plugs (volume = 3-6 cm3) were sampled and stored in serum bottles sealed with 1-cm-thick butyl rubber septa. 5mL of saturated brine was added to the serum bottles and shaken to equilibrate the headspace. The samples were refrigerated until analysis. For cruises PCG02-08, Hydrates2004, and AT50-14, methane concentrations of serum bottle headspaces were determined by gas chromatography flame ionization detection. These headspace concentrations were used to calculate dissolved concentrations of methane in porewater. For samples measured by GC, dissolved concentrations were calculated following Hoehler et al. (2000).

 For cruise AT50-29B, methane concentrations were determined using cavity ringdown spectroscopy. Check standards for concentration were run approximately every 5 samples and corrected as described by Pohlman and others (2021). For samples measured by cavity ring-down spectroscopy, dissolved concentrations were calculated using the following equation:

((pCH4 * 10^-6 * Vg) /  (R * T_extraction) + (Vw * pCH4 * 10^-6 *  Sol_CH4)) / Vw * 10^6 = [CH4]

Where pCH4 is the headspace concentration of methane in ppm, Vg is the volume of gas sample in mL, R is the gas constant in L-atm/mol-K, T_extraction is the extraction temperature in Kelvin, Vw is the volume of water sample that was extracted in mL, Sol_CH4 is the solubility of methane in mol/L-atm (Wiesenburg & Guinasso, 1979; Yamamoto et al., 1976), and [CH4] is the dissolved concentration of methane in the original natural water sample, in micromoles per liter.

DOC_uM and DOC_d13C_permille: Samples (2-4 mL) were stored in combusted borosilicate glass vials with acid-cleaned teflon-lined septa caps. Vials were pretreated with 20 percent hydrochloric acid (12 microliters per milliliter of sample) to decrease the pH of the sample to 2 or less, and samples were stored at 4 degrees Celsius until analysis. Concentrations were determined with a TOC Analyzer using wet chemical oxidation (cruises PGC02-08 and Hydrates2004) (Heuer et al., 2009; Pohlman et al., 2011) and high temperature catalytic oxidation (cruises AT50-14 and AT50-29B) to convert DOC to CO2. For all cruises, nondispersive infrared detection was used to quantify this CO2. Potassium hydrogen phthalate (KHP) calibration standards were run to quantify DOC concentration. Stable carbon isotopes (d13C-DOC) were measured by isotope ratio mass spectrometry. Stable carbon isotope ratios are reported in standard delta notation relative to Vienna Pee Dee Belemnite (VPDB). Quality assurance included replicate analysis of natural reference materials, field samples and calibration standards. Uncertainty in DOC concentration was 5 percent or less, and uncertainty in d13C was 0.2 permille or less, based on replicate measurements.

DIC_mM and DIC_d13C_permille: Samples for DIC concentration for cruises PCG02-08 and Hydrates2004 were collected as 2 mL aliquots in 5 ml serum vials, sealed with butyl rubber septa, and quantified by coulometry.  Samples for stable carbon isotopes (d13C-DIC) for cruises PGC02-08 and Hydrates2004 were collected as 1 ml aliquots in 2 ml serum vials, sealed with butyl rubber septa and analyzed as CO2 by isotope ratio mass spectrometry following acidification of the sample (Pohlman et al., 2013).  Samples for DIC concentration and stable carbon isotopes for DIC for cruises AT-50-14 and AT50-29B were collected by injecting 1mL porewater into a helium-filled 12 mL exetainer vial pre-filled with 1mL 85% phosphoric acid. The stable carbon isotope ratios and concentrations were determined from CO2 by isotope ratio mass spectrometry. Measurements are standardized with lithium carbonate reference material, and isotope ratios are reported in the standard d-notation relative to VPDB.

TOC_wt.% and TOC_d13C_permille: Sediment samples from all cruises were freeze-dried and ground with a mortar and pestle. Homogenized sediment was weighed into silver capsules for acid fumigation. Samples were acid fumigated to remove carbonate and dried at 45℃ for 24 hours, then wrapped into tin capsules. Samples were run on an elemental analyzer interfaced with an isotope ratio mass spectrometer. Samples from PGC02-08 were normalized to an acetanilide standard and samples from AT50-14 were run with a peptone reference material, as well as standards USGS 40 226 and USGS 41. Stable carbon isotope values are reported in d notation relative to VPDB.

H2S_mM: Porewater sulfide (ΣH2S) was determined by sparging the sample aliquot after acidification with 25 % phosphoric acid, and trapping evolved H2S in sulfide antioxidant buffer (SAOB) solution for measurement with a sulfide specific electrode. Standards were prepared from a sodium bisulfide stock solution that was titrated with lead nitrate to determine its concentration, daily. Also on a daily basis, the stock solution was serially diluted with SAOB solution to produce a five-level calibration.

Sampling for this dataset includes push-coring, video-guided multi-coring, and piston coring. Samples from cruise PGC02-08 were collected by piston coring, and horizontal positioning information for cores was achieved with an acoustic bottom transponder constellation, which provided an accuracy of 1-2 meters. Samples from cruise Hydrates2004 were collected by push-coring using the ROV ROPOS. Samples from cruise AT50-14 were collected by push-coring using the HOV Alvin, and by video-guided multi-coring (sample IDs containing ‘MUC’). Samples from cruise AT50-29B were collected by push-coring using the ROV Jason.

SO4_mM: For cruises PGC02-08 and Hydrates2004 sulfate was measured using a Dionex DX-120 ion chromatograph equipped with a 4 mm AS-9HC column.  For cruises AT50-14 and AT50-29B sulfate was measured with a Thermo Scientific Aquion ion chromatograph (IonPac AG22 4x50 mm guard column, IonPac AS22 4x250 mm analytical column, and AERS 300 4 mm suppressor) with an AS40 Autosampler.

CH4_uM: For cruises PGC02-08 and Hydrates2004 methane was measured by gas chromatography using a Shimadzu GC 14-A and flame-ionization detector.  For cruise AT50-14 samples were measured using an Agilent 6890 Gas Chromatograph with a Flame Ionization Detector and fitted with a HaySep Q packed column, 1/8”, 80/100 Mesh, 6ft long, operated at a constant flow of 20 ml/min. Samples were injected with a modified sample loop. For samples from AT50-29B, a Picarro G2201-i CRDS was used. 

DOC_uM and DOC_d13C_permille: For cruises PGC02-08 and Hydrates2004 samples were measured on an OI Analytical 1010 TOC analyzer with a nondispersive infrared detector interfaced to a ThermoFinnigan DELTAplus XP mass spectrometer. For cruises AT50-14 and AT50-29B DOC samples were analyzed on an O.I. Analytical Aurora 1030C auto-analyzer by high temperature catalytic oxidation and quantified with a nondispersive infrared detector. d13C-DOC values were quantified using a ThermoFinnigan DELTAplus XL Isotope Ratio Mass Spectrometer interfaced to the Aurora 1030C following the method of Lalonde and others (2014).

DIC_mM and DIC_d13C_permille: For cruises PGC02-08 and Hydrates2004 DIC concentrations were determined using a model 5011 UIC carbon dioxide coulometer, and stable carbon isotope ratios were determined using a ThermoFinnigan Delta S isotope ratio mass spectrometer.  For cruises AT50-14 and AT50-29B, DIC concentrations and carbon isotope values measured using a GasBench II system interfaced with a Delta V Plus isotope ratio mass spectrometer at the University of California Davis Stable Isotope Facility.

TOC_wt.% and TOC_d13C_permille: For cruise PGC02-08, TOC content and carbon isotope values were quantified using a Fisons EA-1100 Elemental Analyzer interfaces to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer. For cruise AT50-14, TOC content and carbon isotope values were quantified using a Thermo Flash Elemental Analyzer interfaced with a Delta V Plus isotope ratio mass spectrometer at the Alaska Stable Isotope Facility.

H2S_mM: Sulfide was measured with a Cole-Parmer Silver/Sulfide Ion-Selective Electrode (27502-41)

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.