April Luncheon - Todd Longbottom, Pioneer Natural Resources

Petrogenic carbon (OCpetro) represents one of the largest terrestrial organic carbon reservoirs 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 OCpetro 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 OCpetro to Earth’s surface, making the relative importance of low-lying landscapes a key unknown in regional or global scale estimates of OCpetro recycling. Additionally, 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 13C nuclear magnetic resonance (NMR) spectroscopy, and carbon isotope techniques to describe molecular and isotopic changes that occur throughout oxidative weathering of OCpetro, and the subsequent formation of modern soils, in two outcropping Cretaceous mudrocksof the Eagle Ford and Pepper Formations in central, Texas. 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 OCpetro, and the need for molecular and isotopic tools for quantifying mechanisms and rates involved in OCpetro weathering.