The weathering of sedimentary organic matter as a control on athmospheric O2: I. Analysis of a black shale

Abstract

To investigate the weathering of sedimentary organic matter and its role in regulating atmospheric oxygen, a drill core of black shale from the New Albany formation (Upper Devonian, Clay City, KY) has been analyzed for total and organic carbon, nitrogen, phosphorus, and sulfur and for porosity, permeability and specific surface area. The distribution of organic matter and pyrite in the shale has also been examined by means of electron photomicrography and element mapping. Element mapping indicates that shale organic matter is present mainly as 0.5 to 50 μm discoidal clots and discontinuous laminae (" stringers") and not as monomolecular coatings on clay grains. Loss of organic matter by oxidative weathering takes place across a reaction "front" where organic carbon content decreases sharply toward the land surface along with organic nitrogen and sulfur, but not organic phosphorus which remains relatively constant over the same depth range. Accompanying the decrease in organic matter is an increase in porosity. These results agree with earlier work on a single stratigraphic layer that found also that the oxygen content of the organic matter increases sharply toward the land surface across a similar carbon oxidation front. Pyrite in both the core and the layer was found to decrease toward the surface more continuously than organic matter and at intermediate depths is essentially absent in the presence of high levels of organic matter. These results suggest that organic matter weathering in shales can be treated in terms of the reaction of organic matter with gaseous O2 and O2 dissolved in groundwater. Once a weathering profile is developed the inward or downward migrating O2 reacts first with modern soil organic matter and subsequently with low concentrations of remaining pyrite and ancient organic matter before reaching the "front". Pyrite apparently reacts faster with O2 than does organic matter (for a given local concentration of oxygen) as evidenced by reduced pyrite concentrations accompanying high organic matter concentrations at the front. Upon further inward migration lower levels of O2 react with higher concentrations of pyrite and organic matter. These data provide an important basis for the theoretical modeling of organic matter oxidation during weathering.