Nitrogen cycling during the Cretaceous, Cenomanian‐Turonian Oceanic Anoxic Event II
Abstract
Low δ15 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 δ15 N could suggest a predominance of terrestrially derived organic matter or a diagenetic control on δ15 N variability shifting bulk δ15 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 δ15 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 δ15 N values in this study reflect primary changes in the nitrogen cycle. The δ15 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 δ15 N values, which are significantly lower than bulk δ15 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 δ15 N values is observed. This shift possibly reflects greater utilization of 15 N‐depleted ammonium during a chemocline upward excursion (CUE). Dominance of low δ15 N values from other periods of more widespread marine anoxia is likely the result of similar processes.
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Marine sediments deposited during OAEs may record striking 15N-depletion, with δ15N values commonly <−2‰ interpreted as an expression of strongly reducing water conditions.
“Especially, marine sediments deposited during the OAEs may record striking 15N-depletion, with δ15N values commonly <−2‰ interpreted to be an expression of strongly reducing water conditions28, 29.”
From Heterogenous oceanic redox conditions through the Ediacaran-Cambrian boundary limited the metazoan zonation (2017) · cites this paper as [29]
Sharply negative δ15N deviations (by 2-4‰) from normal marine N cycling can be considered evidence of photic-zone anoxia perhaps induced by a chemocline upward excursion.
“Sharply negative δ15N deviations (by 2–4‰) from normal marine N cycling, then, can be considered as the evidence of photic-zone anoxia perhaps induced by a chemocline upward excursion29.”
From Heterogenous oceanic redox conditions through the Ediacaran-Cambrian boundary limited the metazoan zonation (2017) · cites this paper as [29]
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“…oceanic Cret Campanian 83.60 72.10 77.85 3 55 Meyers et al. (2009) Demerara (ODP 1257-61) unnamed oceanic-med Cret Santonian 86.30 83.60 84.95 4 56 Junium and Arthur (2007) Atlantic (ODP 1261) unnamed oceanic-med Cret Coniac-Santon 90.00 85.95 87.98 23 57 Meyers et al. (2009) Demerara (ODP…”
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“In these anoxic environments, microbial egradation is suggested to retain the initial 15N value or only lightly decrease the 15N value (Altabet et al., 1999; Higgins et al., 010; Junium and Arthur, 2007; Möbius et al., 2010).”
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- Authors
- Junium, C. K., & Arthur, M. A.
- Affiliations
- Pennsylvania State University (US)
- Venue
- Geochemistry, Geophysics, Geosystems
- Publisher
- American Geophysical Union (AGU)
- Published
- 2007-03
- Volume
- 8
- Issue
- 3
- ISSN
- 1525-2027 (ISSN-L)
- OpenAlex
- W1482184946
- Cited by
- 187
- References
- 0
- Citations by year
- 2026: 10 · 2025: 11 · 2024: 20 · 2023: 12 · 2022: 8 · 2021: 11 · 2020: 7 · 2019: 13 · 2018: 12 · 2017: 6 · 2016: 8 · 2015: 15 · 2014: 9 · 2013: 9 · 2012: 5
- FWCI
- 7.05
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Geochemistry Geophysics Geosystems (publishedVersion)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
- Physical Sciences › Earth and Planetary Sciences › Oceanography › Marine and coastal ecosystems
- Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Geology and Paleoclimatology Research
- Keywords
- nitrogen cycling, Cenomanian-Turonian boundary, black shales, nitrogen isotopes, C/N ratio, nitrogen fixation, ammonium utilization, early diagenesis, ocean anoxia, Demerara Rise, upwelling, phosphorus
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