The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling

Abstract

To investigate the weathering of sedimentary organic matter and its role in regulating atmospheric oxygen, a theoretical modeling study is presented that addresses the fundamental controls on atmospheric oxygen uptake: erosion rate, organic matter content, and reaction rate. We compare model results with the previous part of this study that analyzed a drill core of black shale from the New Albany formation (Upper Devonian, Clay City, KY) for total and organic carbon, pyrite sulfur, porosity, permeability and specific surface area. As was observed in the field study, the model predicts that the 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 pyrite loss. The model is based on kinetic control of reaction of organic matter and pyrite with O2 dissolved in soil water. The downward diffusion of gaseous O2 partitions with dissolved O2 in water films on sediment grains via Henry's law. Once a weathering profile is developed, the downward migrating O2 reacts with shale organic matter and pyrite. Pyrite reacts faster with O2 than does organic matter (for a given local concentration of oxygen) making the pyrite front generally deeper than the organic matter front. We explore the influence of differing erosion rates, atmospheric O2 concentrations, organic matter contents, porosities, tortuosities, and rates of reaction (that could include possible acceleration due to microbes) on the oxygen consumption. We conclude, based on our modeling, that the erosion rate and the concentration of buried reduced matter, as opposed to the level of atmospheric O2, normally limits the rate of drawdown of atmospheric oxygen. For the vast majority of erosion rates and Phanerozoic oxygen levels, essentially all ancient reduced material is oxidized before reaching the surface. Only in regions of unusually rapid erosion or during very low atmospheric oxygen levels can rates of diffusion of O2 in soils and rates of reaction control O2 drawdown, leading to weathering that is O2-dependent. In this case erosion and rapid reburial of unoxidized organic matter would occur.