| The commonly overlooked process of oxidative weathering of sedimentary organic matter [OM] forms a balance with OM burial in sediments that, on geologic time scales, maintains a roughly constant atmospheric O 2 concentration. This study presents the first detailed description of changes in abundance and composition of OM within black shale weathering profiles. A wide array of analytical techniques (elemental analysis, infrared spectroscopy, analytical pyrolysis/gas chromatography and 13C nuclear magnetic spectroscopy) were employed. Measured OM loss from shales during weathering is between 60% to nearly 100%. This variability is not related to OM type or composition, but instead is tentatively linked to local erosion rate and thus cumulative O2 exposure time, a factor that has been invoked to explain preservation of OM in modern sediments.; Results from this study indicate that OM type influences the character, if not the rate, of OM weathering. Highly aliphatic OM is simply lost during weathering, with long n-alkyl fragments selectively retained and little if any accumulation of oxidation products. This is analogous with selective preservation of aliphatic OM during burial. Alternatively, shale OM that is more aromatic and contain more branched alkyl carbons is more reactive during weathering. This material accumulates carbonyl oxidation products and preferentially loses aliphatic relative to aromatic carbon.; If erosion rate, and not OM type, is the dominant control on OM weathering, then tectonic forcing may play a strong control on the evolution of atmospheric O2. Greater erosion rates result in less weathering within the outcrop and a greater degree of truncation of the weathering profile. A mathematical model of shale weathering profiles was developed for this study. Results from this model indicate that both erosion rate and atmospheric O2 concentrations are the dominant controls on the efficiency of OM weathering; under higher O2 concentrations, more OM is oxidized within the outcrop at a given erosion rate. Thus the possibility exists for a feedback between atmospheric O2 concentration and OM weathering rates. |
