Understanding both the rate and mechanism of weathering is imperative to advancing our knowledge of many dynamic terrestrial processes such as nutrient cycling, soil formation, carbon sequestration, and erosion. In this study, we investigated profiles of chemical weathering and porosity generation in two shales from central Pennsylvania with similar bulk mineral composition but different contents of organic matter (OM) and pyrite. The contrast between the two shales provides an opportunity to study what controls the pyrite and OM oxidation during weathering and how these reactions influence the weathering of other minerals. The gray shale used in this study is the Rose Hill shale at the Susquehanna Shale Hills critical zone observatory (SSHCZO). It contains lower OM (&lt;0.05 wt%) and pyrite (~0.25 wt%) content. In contrast, the Marcellus shale from nearby Frankstown, Pennsylvania is a black shale with higher OM (~10 wt%) and pyrite (~3 wt%) content. Both shales contain ~40% quartz, ~40% illite, ~15% chlorite, and 3-7% plagioclase.<br/><br/>Pyrite oxidation and carbonate dissolution are the two deepest reactions at SSHCZO: both minerals become depleted quickly around the water table (~15-20 m depth along ridgetops). In contrast, weathering of chlorite and plagioclase happened at shallower depths and developed wider front thicknesses. The porosity and water-accessibility of pores measured by neutron scattering on the rocks from boreholes increase gradually above the water table. Despite the similarity in climate and erosion rate in these two sites, the black shale is more weathered.  The black shale formation overlies a layer of carbonate (~11 m below land surface) with low porosity (&lt;3%). All the black shale samples above the carbonate layer are depleted in carbonate, plagioclase, chlorite and pyrite. The porosities in the weathered black shale are twice as high as in the weathered gray shale. The pore size distribution exhibits a broad peak at tens of micrometers, likely due to the loss of OM and dissolution of carbonate during weathering. The formation of micrometer sized pores greatly increases the permeability of shale, and therefore, enhances the infiltration rate. The high rate of infiltration promotes mineral weathering in the black shale.

Role of organic matter and pyrite oxidation on deep shale weathering: A comparison of gray shale and black shale

253rd National Meeting (2017)

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