DE-FE0010120

Goal
The goal of this project is to provide the gas hydrate community with a proven geologically well-preserved proxy for paleo-SMT reconstructions, and thus allow the use of the magnetic susceptibility (χ) paleo-record to assess the natural variability in methane and sulfate fluxes in marine gas hydrate bearing regions. To achieve this goal, this project aims to (1) reconstruct the paleo-positions of the sulfate-methane transition (SMT) using the magnetic susceptibility (χ) and grain size proxy approach in gas hydrate-bearing sediment cores collected on the Cascadia continental margin during ODP Leg 204 and IODP Exp. 311; and (2) utilize the gas hydrate systems specific CrunchFlow reactive transport modules to ultimately model the required methane and sulfate fluxes that best explain the paleo positions of the SMT at sites on both the northern and central Cascadia margin.

Performers
University of New Hampshire, Durham, NH 03824-3585

Background
Methane in marine sediments, often existing ephemerally as gas hydrate, constitutes one of the largest reservoirs of natural gas on Earth, and fluxes of methane in marine sediments may be an important component in the global carbon cycle. Tracking changes in past methane flux, however, remains difficult as there are few available proxies that persist through geologic time. In an effort to better understand the dynamic response of gas hydrate systems and their potential impact on sea-floor stability, ocean ecology, and global climate, researchers intend to reconstruct the paleo-positions of the SMT (sulfate-methane transition) at three sites on the Cascadia margin. This reconstruction will utilize a multi-proxy approach to observe the dynamic behavior of the SMT through glacial-interglacial timescales. These data will be utilized to understand the natural variability in the flux of methane and sulfate implicit from the SMT migration history on the Cascadia margin. These data will also be used to assess whether this approach can be utilized on a future coring expedition to reconstruct the modern and recent past fluxes of methane and sulfate at a site located near the upper hydrate stability boundary, i.e., the region in marine gas hydrate systems that is the most susceptible to environmental change.

Modern methane fluxes, as constrained by porewater geochemistry, provide a snapshot of the present-day SMT. Other proxies of SMT positions such as zones of authigenic barite can provide only a partial record of paleo-SMT positions because barites can easily dissolve in the stratigraphy below the most recent SMT [Von Breymann et al., 1992; Dickens, 2001]. Extraction of biomarkers from methanotrophic bacteria preserved in the sediments [e.g., Hinrichs, 2001; Gontharet et al., 2009] can provide a record of past methane venting, but this proxy needs to be used in an integrated approach, which will be taken in this project to, ultimately, reconstruct methane fluxes. Pyrite, stable both within the sulfate reduction and methanogenic zones, is important to this new proxy approach.

Impact
By identifying intervals where χ has been reduced by the pyritization of magnetite due to anaerobic oxidation of methane at present and past SMT positions, and by constraining sulfate fluxes influenced by sedimentation rate, past changes in methane flux can be tracked. A transport and reaction model like CrunchFlow, involving these fluxes and magnetite dissolution kinetics, can be used to link the migration history of the SMT to the χ record. The approach being developed in this project?using cores from the Indian continental margin and the Cascadia margin?has potential application to several, if not most, methane-bearing marine sequences globally, where significant magnetic iron oxides exist in the primary depositional record. By reconstructing the history of past methane and sulfate fluxes, predictive models describing how modern gas hydrate systems will respond to short- and long-timescale environmental changes can be developed.

Accomplishments
New project awarded October 1, 2012

Current Status (December 2012)
Researchers at the IODP Gulf Coast Repository in College Station are using an XRF core scanner to obtain XRF elemental measurements of the upper ~100 meters of sediment at each Cascadia Margin site (1249, 1252, and 1325) as it allows for high sampling resolution (mm to cm scale) downcore measurements of major chemical elements (e.g., Al, Si, P, S, K, Ca, Ti, Mn, Fe, Sr, Zr, Ba, Rb) in marine sediments cores. From these element distributions in these three records, the Zr/Rb ratio will be examined as a proxy for grain size in these cores and the remaining elements shall be used to track primary and secondary mineral phases throughout the cores.

The project team will also be obtaining sediment samples from cores at the IODP core repository in College Station, Texas for the proposed grain size, radiocarbon, oxygen isotopes, magnetic mineralogy, and carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental analyses, which produces total organic carbon, calcium carbonate, total nitrogen, and total sulfur measurements. These samples will then be shipped to UNH, where the grain size, magnetic mineralogy, and CHNS elemental measurements will be taken, and to WHOI (Woods Hole Oceanographic Institution), where the radiocarbon and oxygen isotope measurements will be made.

Project Start: October 1, 2012
Project End: September 30, 2013

Project Cost Information:
DOE Contribution: $118,786
Performer Contribution: $32,791

Contact Information:
NETL ? John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
University of New Hampshire ? Joel Johnson (joel.johnson@unh.edu or 603-862-4080)