The goal of this project is to provide the gas hydrate community with a proven, geologically well-preserved proxy for paleo- sulfate-methane transition (SMT) reconstructions using magnetic susceptibility (χ) and grain size measurements to track diagenetic changes that are associated with the anaerobic oxidation of methane. To achieve this goal, this project aims to (1) reconstruct the paleo-positions of the (SMT) using a 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 Integrated Ocean Drilling Program (IODP) Exp. 311, and (2) utilize 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. 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.
University of New Hampshire (UNH), Durham, NH 03824-3585
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.
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 also provide a record of past methane venting, but these compounds are often not well preserved in the sediment record. 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 at three sites on the Cascadia margin. This reconstruction will utilize a multi-proxy approach based on magnetic susceptibility to observe the dynamic behavior of the SMT through glacial-interglacial timescales.
Recent work by UNH on the Indian continental margin [Phillips et al., 2012] shows that magnetic susceptibility (χ), constrained by magnetic properties, and integrated with core sedimentology (including grain size), authigenic mineralogy, and porewater geochemistry, can be used to track the paleo-positions of the SMT. As others have documented [e.g. Kasten et al., 1998 and Riedinger et al., 2005], anaerobic oxidation of methane (AOM) at the SMT can result in dissolution of existing ferrimagnetic minerals (e.g., magnetite) and precipitation of authigenic carbonate, pyrite, and magnetic iron sulfide minerals (e.g., pyrrhotite and greigite), altering the original χ of the bulk sediment. The dissolution of magnetic iron oxides and the re-precipitation of magnetic and non-magnetic iron sulfides [Canfield and Berner, 1987] occurs during hydrogen sulfide production at the SMT during either AOM [Kasten et al., 1998] or sulfate reduction via organic matter oxidation [Passier et al., 1996]. In anoxic marine sediments under reducing conditions, magnetite is more resistant to dissolution than other iron oxides and oxyhydroxides, but more prone to dissolution than iron (Fe) bound in silicate minerals [Canfield et al. 1992]. Thus, if sediments with magnetic mineralogy dominated by magnetite have depleted zones of χ indicating dissolution of magnetite and re-precipitation of Fe-sulfides, the drawdown in χ likely indicates a long-term paleo-position of the SMT.
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 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.
The project research has been completed and a final report is available below under "Additional Information".