The Nitrogen Biogeochemistry of Devonian Black Shale-Forming Ecosystems

Explicating the role of nitrogen in Devonian black shale-forming ecosystems is particularly challenging because the multiple microbially-mediated reduction/oxidation reactions that characterize the modern marine N cycle leave only indirect evidence in the geological record. In this dissertation, I examine the geochemical record of Late Devonian black shale-forming ecosystems in an effort to reconstruct the biogeochemistry of N. In order to do so, I have employed elemental and stable isotope analyses of C, N, P, Fe, and S to investigate the fate of organic matter and the redox state of the water column and underlying sediment. Core and outcrop samples were collected from a variety of representative sections. Results confirm that N biogeochemistry was a determinative factor in the development of black shale facies. For instance, terrestrially-derived reactive N was an important macronutrient source for primary production during deposition of the Upper Kellwasser interval at the Frasnian/Famennian boundary in western New York. Based upon data from the Indiana Basin, I have proposed that the accumulation of lipids in green algal biomass elicited by N-limitation contributed to the competetive advantage of prasinophyte green algae over cyanobacterial diazotrophs and the stability of water column stratification. Although the vertical structure of microbial communities in stratified water columns largely precluded loss of reactive N through coupled nitrification/denitrification during black shale deposition, the increasing ventilation of the water column through the latest Devonian permitted increasing nitrification/denitrification that is recorded in progressively more enriched δ15N values in a core from southwest Virginia. Finally, based upon a compiliation of data, I refute the methodological assumption of a constant inorganic N fraction in a stratigraphically arrayed set of samples as a poor one that leads to the erroneous conclusion that the ratio of organic C to organic N is constant within that set regardless of differences in organic matter source or diagenetic history. The role of nitrogen biogeochemistry in the genesis of black shales has been largely subordinated to that of phosphorus. This dissertation contributes to righting that imbalance.