Table of Contents

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

Sediment pushcores were collected by a remotely-operated vehicle (ROV Doc Ricketts) from the R/V Western Flyer in May 2021 (cruise ID WF05-21) at the Del Mar Seep (32.9043, -117.7824), Santa Monica Seep Mound 800 (33.7993, -118.6465), Santa Monica Seep Mound 863 (33.7891, -118.6683), and Newport Canyon (33.5450, -117.9251). Pushcores were manually sectioned into 1- or 3-centimeter (cm) horizons using plastic rings, and porewaters were extracted using a KC Denmark squeezer. Samples for ion chromatography were frozen shipboard and thawed immediately prior to analysis. Measurements were made using a Thermo Dual Dionex Integrion HPIC ion chromatography system with either a 250-millimeter (mm) Dionex IonPac AS19-4um column (anions) with 50 mm guard column or a 250 mm Dionex IonPac CS16-4um column (cations) with a 50 mm guard column, in the Resnick Water and Environment Laboratory at the California Institute of Technology (Caltech).

100 microliter (uL) samples were diluted 50x in milliQ nanopure water before running. Standard curves were generated using calibration standards diluted similarly to samples, with 100 uL of 500 millimolar (mM) NaCl added to mimic typical seawater sample peak behavior. Analyte peaks were integrated and values were calculated using the standards automatically in the Chromeleon software, with manual QC to ensure regular peak shapes.

Known issues or problems:
Na and Cl are not quantitative, as the standards have an excess of each spiked in, above the upper limit of quantification. Formate values reported as 'ND' indicate they are below the LoQ but not LoD. Ca elutes near the end of the run, and occasionally is not captured in its entirety.

- Imported original file "20230630_WF05-21_Ions_for_NSF.xlsx" into the BCO-DMO system.
- Flagged 'ND' and 'NOT_MEASURED' as missing data identifiers but retained this notation in the dataset.
- Converted the date column YYYY-MM-DD format.
- Created the ISO 8601 date/time field (UTC) using the Date_UTC and Time_UTC columns as input.
- Saved the final file as "908217_v1_porewater_biogeochem.csv".

NSF Award Abstract:
This research examines the role of deep-sea organisms in determining the fate and footprint of methane, a potent greenhouse gas, on Pacific continental margins. The investigators are evaluating the deep ocean methanosphere defined by the microbial communities that consume methane and the animals that directly feed on or form symbioses with methane-consuming microbes. They are also investigating animal communities that gain energy indirectly from methane, as well as those that take advantage of carbonate rocks, the physical manifestation of methane consumption in seafloor sediments. The study of methane seeps in the deep waters of both Alaska (4400-5500 meters) and Southern California (450-1040 meters) is enabling comparisons of the methanosphere under different food-limitation and oxygen regimes. By applying diverse chemical, isotopic, microscopy, and genetic-based analyses to seep microbes and fauna, this study is advancing understanding of the contribution of methane to deep-sea biodiversity and ecosystem function, information that can inform management and conservation actions in US waters. In addition to training for graduate and undergraduate students at their home institutions, the investigators are collaborating with the Alaska Native Science and Engineering Program (ANSEP). They are recruiting Alaskan undergraduates to participate in the research, contributing to ANSEP's online resources that promote interaction between scientists and middle and high school students, and participating in ANSEP's annual residential Career Exploration in Marine Science programs to engage middle school students in learning about deep-sea ecosystems and the variety of career pathways available in marine related fields.

Microbial production and consumption of methane is dynamic and widespread along continental margins, and some animals within deep-sea methane seeps rely on the oxidation and sequestration of methane for nutrition. At the same time, understanding of methane-dependent processes and symbioses in the deep-sea environment is still rudimentary. The goals of this study are to 1) examine the diversity of animals involved in methane-based symbioses and heterotrophic consumption of methane-oxidizing microbes and how these symbioses extend the periphery of seeps, contributing to non-seep, continental slope food webs; and 2) determine whether carbonates on the seep periphery sustain active methanotrophic microbial assemblages, providing a localized food source or chemical fuel for thiotrophic symbioses, via anaerobic oxidation of methane, or free-living, sulfide-oxidizing bacteria consumed by animals. The investigators are addressing these goals by surveying, sampling, and characterizing microbes, water, sediments, carbonates and animals at a deep seep site on the Aleutian Margin and a shallow site off Southern California. Shipboard experiments and laboratory analyses are using molecular, isotopic, geochemical, and radiotracer tools to understand transfer of methane-sourced carbon from aerobic methanotrophs under multiple oxygen levels, pressures, and photosynthetic food inputs. This approach offers a wide lens by which to examine the methane seep footprint, allow reinterpretation of past observations, and identify new scientific areas for future study. Improved characterization of the deep continental margin methanosphere informs climate science, biodiversity conservation, and resource management.