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Nitrogen cycling during the Cretaceous, Cenomanian‐Turonian Oceanic Anoxic Event II

2007; Wiley; Volume: 8; Issue: 3 Linguagem: Inglês

10.1029/2006gc001328

1525-2027

Christopher K. Junium, Michael A. Arthur,

Isotope Analysis in Ecology

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