HyFlux - Part II: Subsurface sequestration of methane-derived carbon in gas-hydrate-bearing marine sediments

Authors: Naehr, T.H., Asper, V., Garcia, O., Kastner, M., Leifer, I., MacDonald, I.R., Solomon, E., Yvon-Lewis, S., and Zimmer, B.

Venue: AGU Fall Meeting, San Francisco, CA, December 15-19 2008 -- Session OS25: Methane Flux from Naturally Occurring Marine Gas Hydrates http://www.agu.org

Abstract: The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth measurements of methane flux to the atmosphere" (see MacDonald et al.: HyFlux - Part I) will combine sea surface, water column and shallow subsurface observations to improve our estimates of methane flux from submarine seeps and associated gas hydrate deposits to the water column and atmosphere along the Gulf of Mexico continental margin and other selected areas world-wide. As methane-rich fluids rise towards the sediment-water interface, they will interact with sulfate-rich pore fluids derived from overlying bottom water, which results in the formation of an important biogeochemical redox boundary, the so-called sulfate-methane interface, or SMI. Both methane and sulfate are consumed within the SMI and dissolved inorganic carbon, mostly bicarbonate (HCO3-) and hydrogen sulfide are produced, stimulating authigenic carbonate precipitation at and immediately below the SMI. Accordingly, the formation of authigenic carbonates in methane- and gas-hydrate-rich sediments will sequester a portion of the methane-derived carbon. To date, however, little is known about the quantitative aspects of these reactions. Rates of DIC production are not well constrained, but recent biogeochemical models indicate that CaCO3 precipitation rates may be as high as 120 µmol cm-2a-1. Therefore, AOM-driven carbonate precipitation must be considered when assessing the impact of gas-hydrate-derived methane on the global carbon cycle.

As part of HyFlux, we will conduct pore water analyses (DOC, DIC, CH4, ?13CDIC, ?13CDOC, ?13CCH4, ?18O, and ?D isotope ratios) to evaluate the importance of authigenic carbonate precipitation as a sequestration mechanism for methane-derived carbon. In addition, sediment and seafloor carbonate samples will be analyzed for bulk sedimentary carbonate (?13C and ?18O) and bulk sedimentary organic matter (?13C and ?15N), as well as sulfur, bulk mineralogy, texture and morphological features, and carbonate stable isotopes. We will then combine observational, geochemical, microbiological, and mathematical methods to assess the effectiveness of authigenic carbonate precipitation as a sink for methane-derived carbon under varying environmental conditions. Results of water column flux analysis, air-sea flux modeling, and sediment and pore water analysis will contribute to the development of a working model for quantifying regional fluxes of gas-hydrate-derived methane from the subsurface to the water column and atmosphere.

Related NETL Project
This presentation is related to the NETL project DE-NT0005638, “Remote Sensing and Sea-Truth Measurements of Methane Flux to the Atmosphere". The goal is to obtain accurate measurements of methane flux from the seafloor to the atmosphere with attention to potentially unstable deposits of gas hydrate.

Project Contacts
NETL – Richard (Rick) Baker (richard.baker@netl.doe.gov or 304 285-4714)
Texas A&M University-Corpus Christi – Ian MacDonald (ian.macdonald@tamucc.edu)