Role of alternating redox conditions in the formation of organic-rich interval in the Middle Devonian Marcellus Shale, Appalachian Basin, USA

Geochemical, isotopic and petrographic analyses were performed on a 30-m core of the Marcellus Shale obtained from Greene County, Pennsylvania, to understand the dominant controls on organic matter deposition in the black shale study units. Our study suggests that the regeneration of nutrients, such as nitrogen (N) and phosphorous (P), may have played a key role in the formation of organic carbon (OC)-rich intervals in the Marcellus Shale. The decomposition of organic matter is likely to have released N and P into the water column, and the episodic upward mixing of these nutrient-enriched waters enhanced primary production. The ratios of Corg/Nbulk and Corg/Ptot are significantly higher in the OC-rich zone of the core (i.e., defined as TOC > 4% and located between 2393 and 2406.5 m depth). The high Corg/Ptot and Corg/Nbulk ratios of the preserved organic matter may reflect the release of N and P by microbial processes, indicating the recycling of nutrients during deposition of the OC-rich interval. In addition, our data show a positive relationships between the Corg/Ptot and Corg/Nbulk ratios and the organic carbon content, which indicate that the recycled nutrients may have promoted primary productivity, resulting in higher OC accumulation in some intervals. The geochemical and stable isotopic results also suggest that the alternating redox conditions in the water mass were favorable for nutrient regeneration. Highly variable trace metal concentrations (e.g., U, Mo, and V) in the OC-rich zone and a wide range of δ15N and δ34Spyr values towards the top of the OC-rich zone suggest fluctuations between anoxic and suboxic water conditions. Finally, sedimentary features and agglutinated benthic foraminifera in the OC-rich zone support the existence of short-term fluctuations between suboxic and anoxic conditions near the sediment-water interface. Multiple lines of evidence suggest that nutrient regeneration due to alternating redox conditions may have played an important role in the formation of OC-rich intervals in the Marcellus Shale.