A field study of the chemical weathering of ancient sedimentary organic matter

Weathering profiles developed on organic carbon-rich black shales were studied to examine the loss and degradation of organic matter (OM) during weathering and its role in the geochemical carbon cycle. Analysis of weathered shales reveals between 60 and nearly 100% total organic carbon (TOC) loss in highly weathered samples relative to initial, unweathered TOC content. Pyrite loss coincides with or precedes organic carbon loss. Elemental analysis and flash pyrolysis–gas chromatography (Py–GC) of kerogen concentrates indicate that there is little or no selective enrichment or depletion of Norg-containing, Sorg-containing, alkylaromatic, branched alkyl or long-chain n-alkyl moieties in most profiles during weathering. Kerogen O/C ratios consistently increase with TOC and pyrite loss. Infrared spectroscopy (IR) reveals an increase in the relative abundance of C=C and C=O bonds relative to alkyl C–H bonds in progressively weathered samples. These results suggest a two component model for kerogen weathering: largely non-selective oxidation and hydration, followed by cleavage/dissolution of oxidized kerogen fragments. The extent of weathering in a given outcrop is likely limited by a combination of the rate of physical erosion and exposure of the rock to oxidizing surface waters, with OM type/composition playing a lesser role.