60th Annual Report on Research 2015

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, and authigenic mineralogy 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.  In the first year of our project (Jan. 1, 2013 to Aug. 31, 2014) we completed the first 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 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 signatures (d¹³C and d¹⁸O).  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 magnetic 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.  In the second year of our project (Sep. 1, 2014 to Aug. 31, 2015), we have developed oxygen-isotope stratigraphy and radiocarbon age models.  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 rates, which are constrained with our 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.  In order to establish the dominant pathway for pyritization in these records (AOM vs organoclastic sufate reduction) we have developed an approach to identify excess pyrite in our records, which is consistent with a predominantly AOM pathway for pyritization. The Ph.D. student funded through this ACS PRF award successfully defended his dissertation in August 2015 and we are now in a no-cost extension period of the project as we await some final radiocarbon analyses and prepare our manuscripts for publication.