Low δ ¹⁵N values, ranging from +1.2‰ to −3.9‰, and atomic carbon/nitrogen (C/N) ratios of 25–50 are characteristic of “black shales” deposited during the Cenomanian-Turonian boundary interval and Oceanic Anoxic Event II (OAE II). An observed antithetic relationship between C/N ratios and δ ¹⁵N could suggest a predominance of terrestrially derived organic matter or a diagenetic control on δ ¹⁵N variability shifting bulk δ ¹⁵N values lower. However, Hydrogen Indices (HI) generally >450, and a positive correlation of HI with C/N mitigates against a significant terrestrial organic matter fraction. High C/N values are likely the result preferential degradation of labile, N-rich compounds during early diagenesis and loss of N as ammonium from sediments through time. A hypothetical model that considers the degradation of a ¹⁵N-enriched labile protein fraction yields only small, 1–2‰ negative shifts in δ ¹⁵N. However, ¹⁵N depletion during diagenesis is contrary to normal isotope kinetics which should result in ¹⁵N enrichment of bulk organic matter. Therefore we conclude that the bulk δ ¹⁵N values in this study reflect primary changes in the nitrogen cycle. The δ ¹⁵N data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient-nitrogen poor, phosphorus replete waters during OAE II and from the mid-Cenomanian to Santonian at Demerara Rise. The low δ ¹⁵N values, which are significantly lower than bulk δ ¹⁵N values in modern regions where nitrogen fixation is known to be important, probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation. During the peak of OAE II a marked shift to lower δ ¹⁵N values is observed. This shift possibly reflects greater utilization of ¹⁵N-depleted ammonium during a chemocline upward excursion (CUE). Dominance of low δ ¹⁵N values from other periods of more widespread marine anoxia is likely the result of similar processes.