The geochemical carbon cycle exerts a major influence on the evolution of Earth's atmosphere (Po2, Pco2) over geologic time scales. As part of this cycle, oxidation of sedimentary organic matter [SOM] comprises more than 80% of the total oxygen consumption required to maintain an equable Po2. However, little prior effort has been made to examine the pathways, efficiency or rates of SOM oxidation, and thus an entire link in the geochemical carbon cycle remains poorly defined. Oxidation of organic matter to carbon dioxide is assumed to be the overall, long-term weathering process, with the common expression CH2O +O2>CO2+H2O implying a single-step reaction acting on an homogeneous material. The reality is that SOM is complex and heterogeneous, so that oxidation likely proceeds through an as yet undefined series of intermediate products and sequential oxidation reactions prior to conversion to inorganic carbon. Thus, to better understand the process of SOM oxidation, detailed characterization of these structures and compositions obtained at various stages of SOM oxidation is required.
This study proposes to investigate the pathways, rates and efficiency of the chemical weathering of organic matter contained in black shales by characterizing uptake of oxygen and other oxidation/weathering reactions, which alter the molecular structure of SOM. Samples have already been collected along weathering profiles developed on organic carbon-rich rocks. The selected formations (Green River, Woodford, New Albany, and Monterey) represent specific end-members of mineralogy and organic matter type. Thus, the results of this study will both reveal the sequence of progressively weathered SOM, and indicate the role that lithology and organic matter type may play in defining this sequence. The rate of SOM weathering will be constrained by comparing pyrite loss between profiles and by examining profiles of known age. Preliminary results from six shale profiles indicate between 70-90% loss of total organic carbon across a distance of 3-5 m during weathering. A small but significant mass of relict organic matter remains at the surface of each profile, implying incomplete SOM remineralization efficiency. Further analyses of the weathering profiles will determine elemental, isotopic and petrographic composition of the organic matter, along with shale mineralogy and physical data such as grain size and permeability. Organic geochemical analytical tools, specifically nuclear magnetic resonance, infrared spectroscopy, and flash pyrolysis in tandem with gas chromatography-mass spectrometry, will be used to decipher molecular level structural characteristics such as carbon skeleton types and oxygen-containing functionality. 14C age dating will be used to distinguish samples contaminated by modern soil carbon.
By addressing the rate, efficiency and pathways of SOM weathering and supply of relict (radiocarbon dead) organic matter to contemporary organic carbon reservoirs (soils, lacustrine and/or marine sediments), this study will have a valuable impact on several disciplines of the Earth sciences including chemical oceanography, surficial geochemistry, geochemical cycle modeling, environmental geochemistry, and Quaternary geology. The collaborative nature of the project and the range of analytical tools, chemical techniques and numerical methods employed will generate a solid foundation of experience and knowledge for graduate students and undergraduates involved in this project. This study of SOM weathering (including development of the project and preparation of this proposal) comprises a crucial part of the Ph.D. thesis research for a graduate student, Steven Petsch, and the inclusion of undergraduates in the research will enhance both their education and the research.
