Table of Contents

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

This dataset contains porewater geochemistry and rate data obtained from Gulf of Mexico sediments collected using both push core and multiple core technology. Sediment push cores were collected across multiple dives of HOV Alvin, during cruises AT18-02 (2010) and AT26-13 (2014) aboard the R/V Atlantis. Multiple cores were collected during cruises EN527, EN528, EN529, and EN586 aboard the R/V Endeavor. Procedures for sample processing were identical for both types of cores.

Sediment cores were collected immediately upon retrieval from the seafloor and stored at 4 degrees C until further processing. Sediment samples were collected from select cores in 3 cm intervals (reported as average depth below seafloor, in centimeters) for geochemical and rate analysis, using aseptic techniques at in situ temperature (4 degrees C). Methane samples were collected first: A 3 ml subsample was collected in a cut-end syringe, placed in a 20 ml serum vial containing 4 ml 2M NaOH to stop microbial activity, sealed with a butyl rubber stopper, crimp sealed, vortexed to homogenize, and stored at room temperature until analysis. Porewater samples were extracted from whole sediment under pressure into acid washed syringes using a custom argon-purged squeezer as described by Joye et al., 2004. A subsample was collected for sulfide determination and analyzed on board using Cline (1969) colorometric methods. DIC samples were collected in 20 ml Hungate tubes and sealed with butyl stoppers. Samples were preserved with 1 ml saturated CuSO4 and 1 ml of 56 mM NaMoO4 in 10% v/v H3PO4. The headspace DIC was methanized and analyzed by GC-FID. Subsamples were collected for determination of porewater pH and salinity: pH was measured on board using a ROSS pH electrode, calibrated with salinity corrected buffers (pH 4, 7, and 10) chilled to in situ temperature (Bowles et al., 2011), and salinity was measured visually on a 500µl subsample using a handheld refractometer (Cole-Parmer RSA-BR60). The remaining sample was filtered-sterilized through a washed 0.2 µm Target filter and subsampled further for ammonium, major ions, and nutrients.

A 2 ml ammonium subsample (amm1_micro_m) was immediately analyzed on board using the method of Soloranzo (1969). A 5 ml subsample for major ions (sodium, magnesium, potassium, calcium, sulfate, and chloride) was preserved with concentrated nitric acid (0.1 micromole per L final concentration) and stored at room temperature until analysis by ion chromatography (Joye et al., 2004). The remaining nutrient subsample was frozen at -20 degrees C until shore-based laboratory analysis. Total dissolved nitrogen (TDN) was analyzed via the oxidative combustion-chemiluminescence technique of Salgado and Miller (1998) using a Shimadzu TOC 5000 coupled to an Antek model 7020 NO analyzer (Joye et al., 2004). NOx (nitrate + nitrite) was analyzed on a Lachat FIA 8000 Autoanalyzer using method 31-107-04-1-A, phosphate via the molybdate blue colorometric method, total dissolved phosphate (TDP) by high-temperature combustion and hydrolysis (Monaghan and Ruttenberg 1999), dissolved inorganic carbon (DIC) by gas chromatography (flame ionization detector; GC-FID), dissolved organic carbon (DOC) by oxidative combustion-infrared analysis, methane by GC-FID, and stable 13C isotopes of DIC by headspace analysis using a Picarro G2201-i isotope analyzer (Joye et al., 2004, Bowles et al., 2016). An additional ammonium subsample was analyzed from the frozen nutrient split to check sample integrity (amm2_micro_m), again using the method outlined by Soloranzo (1969). Whole sediment subsamples for radiotracer sulfate reduction rates and anaerobic oxidation of methane rates were collected in triplicate from a parallel core and were handled, injected, incubated, and analyzed using the methods described in Joye et al., 2004.

Data Processing: Raw instrument data were converted to concentration units by applying linear regression of standard curves, using Excel V 16.38 for Macintosh. Sulfate reduction rates were calculated using the relationships outlined in Fossing and Jorgensen, 1989. Anaerobic oxidation of methane rates were similarly calculated using the procedure outlined in Iversen and Blackburn, 1981.

Problem Report: Missing data (represented by 'nd') are due either to deliberate choices, or from accidental loss of sample.

BCO-DMO Processing:
- renamed fields;
- converted date format o YYYY-MM-DD;
- converted date-time field to ISO8601 format.

Project abstract:
Light hydrocarbon gas mixtures are commonly found in organic-rich marine sediments. Methane (C1) is typically the dominant constituent in these mixtures, but ethane (C2) and propane (C3) are nearly always present in trace amounts. C1 dynamics are typically associated with either thermal cracking of deeply buried organic matter or the metabolic end-product of organic matter degradation. Ethane and propane production had typically been associated with thermocatalytic processes in deeply buried sediments, but limited studies suggested C2/C3 production in biogenic C1 gas mixtures was likely attributable to the activity of methanogenic archaea. However, very few of these studies looked at C1/C2 production in deep-sea sediments, and quantification of rates had either not been attempted, or were absent from the literature. We attempted to use organic-rich, cold seep sediments from the Green Canyon area of the Gulf of Mexico (GC600) to determine C2/C3 dynamics in the first ten meters of sediment (i.e. 0 – 10 m). We found C2/C3 production in near surface cold-seep sediments to be indistinguishable from the background degassing signatures of clay minerals. Surface sediments (i.e. < 4 m) are hypothesized to be dominated by communities of organisms that oxidize C2/C3 compounds, rather than communities that produce them. Experiments determining the controls and magnitude of C2/C3 oxidation in surface sediments in cold-seep environments are ongoing. We hypothesized that C2/C3 production likely occurs deeper in the sediment column (i.e. > 4 m), based primarily on ethane and propane profiles of similar environments. Such material proved difficult to acquire; efforts are ongoing to obtain deep piston cores (i.e. >10 m) for environmental profiling and experimental manipulation in the deeper sediment layers where C2/C3 production likely occurs.

C-DEBI project page: https://www.darkenergybiosphere.org/award/quantifying-biological-production-of-ethane-and-propane-in-deep-subsurface-sediments/

The mission of the Center for Dark Energy Biosphere Investigations (C-DEBI) is to explore life beneath the seafloor and make transformative discoveries that advance science, benefit society, and inspire people of all ages and origins.

C-DEBI provides a framework for a large, multi-disciplinary group of scientists to pursue fundamental questions about life deep in the sub-surface environment of Earth. The fundamental science questions of C-DEBI involve exploration and discovery, uncovering the processes that constrain the sub-surface biosphere below the oceans, and implications to the Earth system. What type of life exists in this deep biosphere, how much, and how is it distributed and dispersed? What are the physical-chemical conditions that promote or limit life? What are the important oxidation-reduction processes and are they unique or important to humankind? How does this biosphere influence global energy and material cycles, particularly the carbon cycle? Finally, can we discern how such life evolved in geological settings beneath the ocean floor, and how this might relate to ideas about the origin of life on our planet?

C-DEBI's scientific goals are pursued with a combination of approaches:
(1) coordinate, integrate, support, and extend the research associated with four major programs—Juan de Fuca Ridge flank (JdF), South Pacific Gyre (SPG), North Pond (NP), and Dorado Outcrop (DO)—and other field sites;
(2) make substantial investments of resources to support field, laboratory, analytical, and modeling studies of the deep subseafloor ecosystems;
(3) facilitate and encourage synthesis and thematic understanding of submarine microbiological processes, through funding of scientific and technical activities, coordination and hosting of meetings and workshops, and support of (mostly junior) researchers and graduate students; and
(4) entrain, educate, inspire, and mentor an interdisciplinary community of researchers and educators, with an emphasis on undergraduate and graduate students and early-career scientists.

Note: Katrina Edwards was a former PI of C-DEBI; James Cowen is a former co-PI.

Data Management:
C-DEBI is committed to ensuring all the data generated are publically available and deposited in a data repository for long-term storage as stated in their Data Management Plan (PDF) and in compliance with the NSF Ocean Sciences Sample and Data Policy. The data types and products resulting from C-DEBI-supported research include a wide variety of geophysical, geological, geochemical, and biological information, in addition to education and outreach materials, technical documents, and samples. All data and information generated by C-DEBI-supported research projects are required to be made publically available either following publication of research results or within two (2) years of data generation.

To ensure preservation and dissemination of the diverse data-types generated, C-DEBI researchers are working with BCO-DMO Data Managers make data publicly available online. The partnership with BCO-DMO helps ensure that the C-DEBI data are discoverable and available for reuse. Some C-DEBI data is better served by specialized repositories (NCBI's GenBank for sequence data, for example) and, in those cases, BCO-DMO provides dataset documentation (metadata) that includes links to those external repositories.