Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Study Results

Thermodynamic calculations and C1/C2+ ratios indicated that methane in the relatively cooler, shallower layers, was predominantly of biogenic origin. Radiotracer experiments provided direct evidence for the coexistence of multiple methanogenic pathways, hydrogenotrophic, acetoclastic, and methylotrophic, across the sediment column. Methanogenic activity from multiple methanogenic pathways occurred over a wide temperature range (3°C to 80°C), highlighting the unexpectedly high metabolic versatility of methanogens in deep, thermally heated sediments. High methanogenesis rates were detected in near-surface sediments driven predominantly by methylotrophic methanogenesis, followed by hydrogenotrophic pathways. However, these rates declined sharply with depth, particularly within the 40–60°C interval, indicating a transition from mesophilic to thermophilic microbial communities, due to rising temperatures, reductions in gene expression, and decreasing microbial cell densities. Methylotrophic methanogenesis remained detectable down to 320 meters below the seafloor and was the dominant methane-producing pathway at temperatures up to 60°C. In sediments increasingly influenced by sill intrusions, hydrogenotrophic and acetoclastic methanogenesis became the predominant modes of methane production. 

Methanogenic activity rates from multiple substrates at 80°C were comparable to rates in near-surface sediments. This deep, hot activity is attributed to the presence of active microbial biomass and the enhanced reactivity and bioavailability of organic matter in deep, hydrothermally-heated sediments, which provided abundant substrates for methanogenesis. These findings expand the current understanding of methanogenesis in the deep biosphere and reveal the discovery of the contemporaneous activity of multiple methanogenic pathways in deep, hydrothermally-influenced sediments.

See related datasets for all data referenced in these findings.

Subsurface sediment samples were collected from four drilling sites in the Guaymas Basin, Gulf of California, during IODP Expedition 385 “Guaymas Basin Tectonics and Biosphere” using the research vessel R/V JOIDES Resolution. Rates of methanogenesis were determined at four sites. Sites 1545 (27º38.230’N, 111º53.329’W) and 1546 (27º37.884’N, 111º52.781’W) were located roughly 52 km and 51 km, respectively, northwest of the axial graben of the northern spreading segment. Both sites are highly sedimented and a 75-meter thick inactive (~thermally equilibrated) basaltic/doleritic/gabbroic sill was present at site 1546 between ~355 to 431 meters below the seafloor (mbsf). Site U1545 is considered a reference site since it was free of sill intrusions and unaffected by active hydrothermal circulation. Samples to determine methanogenesis rates were collected from the "B" hole at every site (during IODP expeditions, several, closely-spaced holes are drilled at each site so that sufficient sediment is available for all of the proposed work.

The geothermal gradient at hole U1545B was 227ºC/km. The geothermal gradient at site U1546B was similar to that measured in hole U1545B, 221ºC/km, since the sill at 1546B was thermally equilibrated. Sites U1547 (27º30.413’N, 111º40.734’W) was located inside the periphery of an active, sill-associated hydrothermal mound located about 27 km northwest of the axial graben of the northern spreading segment. Temperatures in hole U1547B exceeded 50°C in the upper 50 mbsf. The geothermal gradient at this site was between 511ºC to 960ºC/km. Site U1549 was located near a cold seep sustained by a deeply buried, thermally equilibrated sill intrusion at several hundred meters depth. The geothermal gradient at hole U1549B was 194ºC/km.

Sediment samples were collected using an advanced piston coring system (APC) and a half-length APC (Teske et al. 2021). After retrieval on deck, sediment cores were sectioned in a designated core cutting area. Approximately 0.5 cm of the outer sediment layer was carefully removed to minimize potential contamination. For analysis of hydrocarbon gases, ~ 5 mL sediment was transferred into 21.5 mL clean, pre-combusted glass vials using a cut-off syringe. The vials were sealed immediately with polytetrafluoroethylene septa and aluminum caps and were incubated for 30 min at 70°C to allow dissolved hydrocarbon gases to equilibrate with the headspace before analysis, as described in Teske et al. (2021). For molecular hydrogen analysis, duplicate 3 mL sediment samples were collected immediately after core recovery using a cut-off syringe. The subsample was transferred into 21.5 mL glass vials and incubated and processed according to the procedures of Lin et al. (2012).

Data describing in situ geochemistry, pressure, and temperature were generated shipboard, as outlined in Teske et al. (2021). Porewater for geochemical analyses was extracted from subsurface sediments using a shipboard squeezing apparatus, following established protocols (Manheim and Sayles 1974, Teske et al. 2021). To prevent contamination, ~0.5 cm of the outer sediment was carefully removed prior to transferring 150–300 cm³ of uncontaminated interior material into a titanium squeezing vessel. The vessel was placed in a Carver hydraulic press (Manheim and Sayles 1974), and the expressed porewater was filtered immediately through a 0.2 µm polyethersulfone membrane. Filtrates was collected in HCl-washed syringes and subsampled into acid-cleaned high-density polyethylene vials for onboard determinations of sulfate and ammonium, with samples stored at 4 °C following fixation (acidification with trace metal grade nitric acid for sulfate and phenol solution for ammonium).

Dissolved inorganic carbon (DIC) was collected into 2 mL glass vials and preserved with 10 µL of saturated HgCl₂ for shore-based analysis. For acetate analysis, 2 mL porewater subsamples were preserved immediately after collection and stored frozen at −20 °C in pre-combusted glass vials with Teflon-coated screw caps (Heuer et al. 2009). Samples for methanol and trimethylamine (TMA) measurements were transferred immediately into pre-combusted and N₂-purged 20 mL glass vails that were sealed with gas-tight butyl rubber stoppers, stored at −80°C until analysis.

Hydrocarbon gas analysis was performed by extracting about 5 mL of headspace gas from the 21.5 mL vials using a gas-tight syringe and directly injecting it into an Agilent 7890A gas chromatograph with a flame ionization detector (FID) (Teske et al. 2021b). Sulfate concentrations were measured by ion chromatography using a Metrohm 850 Professional IC system (Murray et al. 2000, Teske et al. 2021b). Ammonium concentrations were determined by a colorimetric method involving phenol diazotization followed by oxidation with bleach (Clorox) to produce a blue color, measured at 640 nm using a Cary Series 100 UV-Vis spectrophotometer (Agilent Technologies) (Solorzano 1969, Teske et al. 2021b). Samples were diluted beforehand to keep ammonium below 1000 μM. Total alkalinity was measured by Gran titration with a Metrohm 794 Basic Titrino autotitrator (Gieskes et al. 1991, Teske et al. 2021b). Methanol and TMA (~5 mL) in porewater were simultaneously quantified using a purge and trap pre-treatment system coupled with a gas chromatograph (Jiang et al. 2024). An Agilent Technologies Inc. (USA) 8890 gas chromatograph equipped with a purge and trap system (Acrichi PTC, Beiging Juxin Zhuifeng Technology, Ltd, China), an Agilent splitter based on proprietary capillary flow technology installed between the capillary column (HP-PLOT Q; 30m x 0.32mm, Agilent Technologies Inc., USA) and detectors, and dual nitrogen phosphorus (methylamine) and flame ionization (methanol) detectors was used for this analysis. The method followed the protocol outlined in Jiang et al. The molar fraction of H₂ in headspace gas was quantified using a Reduction Gas Analyzer (Peak Performer 1, Peak Laboratories LLC, USA). All geochemical data were retrieved from the IODP database (https://web.iodp.tamu.edu/OVERVIEW/) (Teske et al. 2021b), while DIC concentrations in interstitial waters were reported by Torres and Kim (2022).

- Imported "OCE-2023575_Methanogenesis_geochemistry_csv.csv" into BCO-DMO system
- Created new field "methane_flag" and moved all "b.d.l." values to this new flag field and removed them from the "methane" field
- Created new field "methane_higher_alkane_ratio_bdl_flag" and moved all "b.d.l." values to this new flag field and removed them from the "methane/ higher alkanes" field
- Renamed fields to comply with BCO-DMO naming conventions, removing units, special characters, and spaces
- Exported file as "991374_v1_methanogenesis_geochem_guaymas_basin"

NSF Award Abstract:
This research will explore carbon cycling in one of the largest carbon reservoirs on Earth, marine sediments, located at bottom of the ocean. This carbon is recycled gradually over time through interacting geological, chemical, and biological processes. This project will document how each of these processes transforms carbon in marine sediments from the Guaymas Basin (Gulf of California). This setting offers the chance to study carbon cycling across a broad range of chemical and temperature gradients, providing an opportunity to tease apart the factors regulating carbon cycling in marine sediments. This project will investigate the role of ocean sediments in the global carbon cycle. These research objectives represent key science priorities in a time of global environmental change. For outreach activities, the scientist, in collaboration with Jim Toomey Education, would continue the "Adventures of Zack and Molly" educational video series. In this instance, the video would document results from this study and its broader significance. The scientist also would create a learning guide for teachers. Both the video and the learning guide would be disseminated to educators. One graduate and one undergraduate student would be supported and trained as part of this project.

Subsurface sediments in the Guaymas Basin (Gulf of California) offer an accessible window for investigating carbon cycling in a dynamic, yet tractable, marine environment. This work will study how heating of subsurface sediments affects the production, consumption, and fate of low molecular weight dissolved organic carbon. The research will track the fate of key carbon species – including formate, acetate, and methanol – as they are processed through a gauntlet of microbial-mediated processes. Samples were collected during Expedition 385 of the International Ocean Discovery Program in September-October 2019. Some experiments were conducted on the research vessel and additional experiments will be conducted in the laboratory. The study will constrain the magnitudes of transformation and the fate of low molecular weight carbon substrates using a combination of direct rate, pool size, and stable isotopic measurements coupled to thermodynamic modeling and probative laboratory experiments. Key topics for investigation include: (1) What is the dominant production mode for organic compounds in subsurface sediments? (2) What are the dominant pathways of methanogenesis along geochemical and temperature gradients? (3) What are the temperature limits of microbially-driven carbon cycling processes? (4) How does the fate of organic compounds change along geochemical and/or temperature gradients?

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.