A solid state 13C-NMR study of kerogen degradation during black shale weathering

Abstract

Abstract-Solid state 13 C nuclear magnetic resonance (NMR) spectroscopy is used to examine kerogen composition in weathering profiles of the Monterey, Green River, Woodford, and New Albany formations. Techniques include cross polarization (CP) and Bloch decay (BD) spectral acquisition, dipolar dephasing (DD), spin counting, experiments to provide estimates of relaxation times (T 1 H and T 1 H), and proton spin relaxation editing (PSRE). It is demonstrated that CP/MAS (cross polarization/magic angle spinning) spectra obtained on isolated kerogens provide reliable characterization of kerogen composition (compared with BD spectra and whole-rock samples). Highly aliphatic (polymethylenic) kerogens are not appreciably altered during weathering. Aromatic and/or branched aliphatic kerogens accumulate oxidation products and preferentially lose aliphatic relative to aromatic carbon during weathering. No relation is observed between T 1 H times and either kerogen composition or degree of weathering; T 1 H times correlate with aromaticity. Two distinct components within kerogens are discerned by PSRE: one highly aliphatic (largely polymethylenic) component and one mixed aliphatic/aromatic component. During weathering, the highly aliphatic component remains largely unaltered, while the mixed component loses aliphatic carbon and accumulates carbonyl oxidation products. Thus it appears that kerogen weathering is dominated by two separate processes: Linear alkyl fragments are cleaved without oxidation, and aromatic/branched alkyl fragments are oxidized while attached to the kerogen macromolecule and then cleaved.

Characterization of macromolecular organic matter in atmospheric dust from Guangzhou, China

“…A little carbohydrate carbon was also present in the spectra. These characteristics are similar to results of the elemental analysis discussed above but differ slightly from modern BC derived from the combustion of fossil fuels, which completely lacks carbohydrate carbon (Petsch et al, 2001). The presence of this type of carbon in the fraction indicates that some parts of the BC are from biomass burning.…”

Studies of the Compositions of Humic Acids from Amazonian Dark Earth Soils

“…They depict typical signals from alkyl C groups (0-46 ppm), probably of plant origin, such as cutin, suberin, wax, and dehydroxylated lignin side chains with high resistance to degradation (Baldock et al, 1992;Preston, 1996;Knicker et al, 2005b), or from microbial neosynthesis (Baldock et al, 1990). The persistence of signals at 21 and 30 ppm in the DD spectra (Figure 3) indicate the presence of terminal CH 3 and long-chain mobile CH 2 , respectively (Hu et al, 2000;Petsch et al, 2001;Lorenz and Preston, 2002;Knicker et al, 2005a). The signal for methoxyl C is expected at 56 ppm.…”

The preservation state of organic matter in meteorites from Antarctica

“…Furthermore, common effects of weathering on terrestrial organic matter are well-constrained and provide clues as to what may be expected for extraterrestrial materials. A common response of terrestrial organic matter to weathering is a reduction in the number of aliphatic structures relative to aromatic structures (Fredricks et al 1983;Martinez and Escobar 1995;Tekely et al 1987;Petsch et al 2001). If present, these types of changes in organic constitution due to parent body processing and Antarctic weathering should be readily detectable by pyrolysis-GC-MS. Figure 1 shows the total ion chromatogram (TIC) and summed ion chromatograms (SIC) of the pyrolysis products from the Mighei meteorite.…”