Abstract
Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and
mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption
that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest
that soil production rates have on average been faster for this black shale compared to the gray shale in similar
climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz,
illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25 wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite.
During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular
pores and fractures. Some of these pores are eitherfilled with organic matter or air-filled but remain
unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated
with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows
water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase
and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the
soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface,
mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale
chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity,
ii) greater average length of connected pores, and iii) a more branched pore network compared to the
unweathered sample.
This work highlights the impact of shale–water–O2interactions in near-surface environments: (1) black shale
weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and
(2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and
mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in
Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the
environment if reactions between water and black shale are accelerated by gas development activities in the
subsurface just as they are by low-temperature processes in ourfield study.
