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Climate of the Past An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/cp-2020-114
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-114
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Oct 2020

02 Oct 2020

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This preprint is currently under review for the journal CP.

Deoxygenation dynamics above the western Nile deep-sea fan during sapropel S1 at seasonal to millennial time-scales

Cécile L. Blanchet1, Rik Tjallingii1, Anja M. Schleicher2, Stefan Schouten3, Martin Frank4, and Achim Brauer1 Cécile L. Blanchet et al.
  • 1GFZ Potsdam, Department of Climate and Landscape Dynamics, Telegrafenberg, 14743 Potsdam, Germany
  • 2GFZ Potsdam, Department of Inorganic and Isotope Geochemistry, Telegrafenberg, 14743 Potsdam, Germany
  • 3Royal NIOZ, Department of Marine Microbiology and Biogeochemistry, Landsdiep 4, 1797SZ 't Horntje, Texel, The Netherlands
  • 4GEOMAR, Research Unit Paleoceanography, Wischhofstrasse 1–3, D-24148 Kiel, Germany

Abstract. Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled the changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 thousand years (10 kyr BP) and located on a bathymetric transect offshore the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial time-scales. The regular alternations of detrital, biogenic and chemogenic sublayers in the laminated sequences are interpreted in terms of seasonal changes. Our microfacies analyses reveal distinct summer floods and subsequent plankton blooms preceding the deposition of inorganic carbonates formed in the water-column during spring-early summer. The isotopic signature of these carbonates suggests year-round anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralisation of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics on millennial time-scales obtained by our multi-proxy study show that runoff-driven eutrophication played a central role in driving rapid changes in oxygenation state of the entire Levantine Basin. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP while deeper cores recorded more stable anoxic conditions. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.

Cécile L. Blanchet et al.

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Short summary
The Mediterranean Sea turned repeatedly into an oxygen-deprived basin during the geological past, as evidenced by distinct sediment layers called sapropels. We use here records of the last sapropel S1 retrieved in front of the Nile River to explore the relationships between riverine input and seawater oxygenation. We decipher the seasonal cycle of fluvial input and seawater chemistry as well as the decisive influence of primary productivity on deoxygenation at millennial time scales.
The Mediterranean Sea turned repeatedly into an oxygen-deprived basin during the geological...
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