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Climate of the Past An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 Oct 2019

Submitted as: research article | 08 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

Cretaceous Oceanic Anoxic Events prolonged by phosphorus cycle feedbacks

Sebastian Beil1, Wolfgang Kuhnt1, Ann Holbourn1, Florian Scholz2, Julian Oxmann2, Klaus Wallmann2, Janne Lorenzen3, Mohamed Aquit4, and El Hassane Chellai5 Sebastian Beil et al.
  • 1Institute of Geosciences, Christian-Albrechts-University, Ludewig-Meyn-Str. 10-14, D-24118 Kiel, Germany
  • 2Geomar Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1–3, D-24148 Kiel, Germany
  • 3Janne Lorenzen, Christian-Albrechts-University, Ludewig-Meyn-Str. 10–14, D-24118 Kiel, Germany
  • 4OCP S.A., Direction de Recherche et Développement, Recherche Géologique, 46300 Youssoufia, Morocco
  • 5Department of Geology, Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco

Abstract. Oceanic Anoxic Events (OAEs) document major perturbations of the global carbon cycle with repercussions on the Earth’s climate and ocean circulation that are relevant to understand future climate trends. Here, we compare sedimentation patterns, nutrient cycling, organic carbon accumulation and carbon isotope variability across Cretaceous Oceanic Anoxic Events OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their δ13C excursion with long-lasting negative carbon isotope excursions preceding the onset of both anoxic events, supporting the view that OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. Based on analysis of cyclic sediment variations, we estimated the duration of the individual phases within the carbon isotope excursions. For both events, we identify: (1) a precursor phase lasting ~ 430 kyr and ~ 130 kyr, (2) an onset phase of ~ 390 and ~ 70 kyr, (3) a peak phase of ~ 600 and ~ 90 kyr, (4) a plateau phase of ~ 1400 and ~ 200 kyr and (5) a recovery phase of ~ 630 and ~ 440 kyr, respectively. The total duration of the positive carbon isotope excursion is estimated as 3400 kyr and 790 kyr and that of the main carbon accumulation phase as 980 kyr and 180 kyr, for OAE1a and OAE 2 respectively. The extended duration of the peak, plateau and recovery phases requires fundamental changes in global nutrient cycles either (1) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge or (2) through nutrient-recycling from the marine sediment reservoir. We investigated the role of phosphorus on the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg/Preact and Ntotal/Preact) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in Corg/Preact and Ntotal/Preact above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs.

Sebastian Beil et al.
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Sebastian Beil et al.
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