Nitrogen cycling during the cretaceous, Cenomanian-Turonian oceanic anoxic event II

[1] Low delta N-15 values, ranging from + 1.2 parts per thousand to -3.9 parts per thousand, 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 delta N-15 could suggest a predominance of terrestrially derived organic matter or a diagenetic control on delta N-15 variability shifting bulk delta N-15 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 N-15-enriched labile protein fraction yields only small, 1 - 2 parts per thousand negative shifts in delta N-15. However, N-15 depletion during diagenesis is contrary to normal isotope kinetics which should result in N-15 enrichment of bulk organic matter. Therefore we conclude that the bulk delta N-15 values in this study reflect primary changes in the nitrogen cycle. The delta N-15 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 delta N-15 values, which are significantly lower than bulk delta N-15 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 delta N-15 values is observed. This shift possibly reflects greater utilization of N-15-depleted ammonium during a chemocline upward excursion ( CUE). Dominance of low delta N-15 values from other periods of more widespread marine anoxia is likely the result of similar processes.