Molecular and isotopic composition of modern soils derived from kerogen-rich bedrock and implications for the global C cycle

Ancient sedimentary organic matter (kerogen) represents the largest terrestrial organic carbon (OC) reservoir on earth and is vulnerable to remineralization upon exposure to earth's atmosphere during the oxidative weathering of sedimentary rocks on the continents. Due to the potential for large carbon-cycle implications, the mechanisms and rates involved in kerogen transformation by oxidative weathering are becoming increasingly well-constrained in contemporary models of the global carbon cycle. Past field studies have focused primarily on areas where high erosion rates deliver large amounts of kerogen to earth's surface, making the relative importance of low-lying landscapes a key unknown in regional or global scale estimates of kerogen recycling. The weathered residuum of organic-rich sedimentary rocks serves as the parent material for many soils. Therefore, some aspects of the chemical structure and biogeochemical cycling of the soil organic matter are likely to be inherited from the bedrock. We used a combination of solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy, and carbon isotope techniques to describe molecular and isotopic changes that occur throughout oxidative weathering of marine kerogens, and the subsequent formation of modern soils, in two outcropping Cretaceous mudstones of the Eagle Ford and Pepper Formations in central, Texas. Increasing production of oxygenated functional groups was coupled with reductions in characteristically abundant aliphatic components of marine kerogens along the weathering profiles. Organic matter structural parameters, derived from C-H dephasing NMR experiments, also provide the basis for a novel weathering index that accounts for the degree of post-sedimentary alteration of kerogen samples along the bedrock-soil continuum. An uncertain future marked by climatic shifts in temperature and/or precipitation and increased continental weathering and denudation rates highlights the potential for enhanced vulnerability of kerogen, and the need for molecular and isotopic tools for quantifying mechanisms and rates involved in kerogen weathering.