『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 50μm 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.』
Introduction
Methods
Sampling
Chemical analyses
SEM, porosity, permeability and specific surface area
Results and discussion
Organic carbon and pyrite sulfur
Scanning electron photomicrographs
Organic phosphorus and nitrogen
Porosity, permeability and specific surface area
Summary
Acknowledgments
References