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Reports: ND853006-ND8: Developing a New Proxy Approach for Reconstructing Paleo-Methane and Sulfate Fluxes in Gas Hydrate Bearing Stratigraphy

Joel E. Johnson, Ph.D., University of New Hampshire

Methane in marine sediments constitutes one of the largest reservoirs of natural gas on Earth, and fluxes of methane in marine sediments are an important component in the global carbon cycle.  Tracking changes in past methane flux, however, remain difficult and there are few available proxies that persist through geologic time.  We are working to develop a new proxy approach using magnetic susceptibility, constrained by magnetic properties, and integrated with core sedimentology, authigenic mineralogy, and geochemical modeling to reconstruct paleo-methane and sulfate fluxes by tracking the past positions of the sulfate-methane transition (SMT).  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 sulfide minerals (e.g. pyrrhotite and greigite), altering the original magnetic susceptibility of the bulk sediment.  To develop our proxy approach for paleo-methane and sulfate flux reconstruction, we utilize existing and proposed data from two sediment records in the Krishna-Godavari Basin, offshore eastern peninsular India.  Both of these records contain zones of reduced magnetic susceptibility above and below the modern SMT, including evidence of past methane venting to the seafloor. We hypothesize that the variations in magnetic susceptibility reflect the migration history of the SMT in these records.  In this project we couple the magnetic susceptibility signal with multiple depositional and diagenetic data sets and an age model to effectively test this proxy approach.  Paleo-SMT depths will then be used with reactive transport models to reconstruct past changes in methane and sulfate fluxes at these sites.

In the first year of our project (Jan. 1, 2013 to Aug. 31, 2014) we have completed the first major steps in developing a new proxy approach for reconstructing paleo-methane and -sulfate fluxes in gas hydrate-bearing sediments. To confirm the locations and origins of diagenetic drawdowns in magnetic susceptibility in the Krishna-Godavari Basin cores, we have measured total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), d13C and d15N of organic matter, and particle size distribution in bulk sediments at these two sites.  In addition, we have characterized the mineralogy of authigenic carbonates in these records using X-ray diffraction (XRD) and stable isotopic signature (d13C and d18O).  We have also begun preparing benthic foraminifer samples for stable isotope analysis to provide an age model for these records.

We have integrated these data with existing records of X-ray fluorescence (XRF) elemental profiles, magnetic susceptibility, and magnetic mineral assemblage data from isothermal remanent magnetization (IRM) to identify intervals of diagenetic loss of magnetic susceptibility via dissolution of magnetite (Fe3O4) with H2S to form pyrite (FeS2).  By predicting original detrital magnetic susceptibility using heavy mineral proxies from core scanning XRF data, we can quantitatively estimate the amount of magnetic susceptibility and magnetite loss, and the corresponding pyrite and sulfur gain. Our approach works best when magnetic iron sulfides are absent, but even in the presence of iron sulfides, it can still provide useful indications of diagenetic alteration of magnetic susceptibility.  By comparing our predicted sulfur precipitation to measured TS in the record, we successfully demonstrate the diagenetic loss of magnetic oxide minerals via precipitation of non-magnetic iron sulfides.  We are currently preparing a manuscript documenting these results and establishing this integrated approach for identifying intervals of diagenetic alteration of magnetic properties in methane bearing marine sediments.

Entering the second year of our project (Sep. 1, 2014 to Aug. 31, 2014), we will develop oxygen-isotope stratigraphy and radiocarbon age models, and run CrunchFlow reactive-transport models for these sites to link our observed results to methane and sulfate cycling.  A key factor in diagenetic loss of magnetic susceptibility is exposure time to H2S which is controlled by the balance of upward methane and downward sulfate fluxes.  Sulfate availability is largely influenced by sedimentation rate, and to calculate sedimentation rates at our sites we are working to establish an age model comprised from benthic d18O stratigraphy and radiocarbon analysis. By identifying the sediment accumulation influence on sulfate fluxes, we can constrain possible changes in methane flux required to create the observed magnetite loss and sulfur gain.  In addition, we observe large variation in TOC content from less than 1 wt. % to over 2.5 wt. % corresponding to one of the observed intervals of decreased magnetic susceptibility. Organoclastic sulfate reduction is another pathway for H2S production in sediments. We will also utilize the model to account for the role of TOC in sulfidization of the sediments.  Our modeling efforts will be summarized and presented in a publication to be submitted near the end of the grant period.