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

Type Article
Acceptance Date 2020 IN PRESS
Language English
Author(s) Blanchet Cécile L1, Tjallingii Rik1, Schleicher Anja M2, Schouten Stefan3, Frank Martin4, Brauer Achim1
Affiliation(s) 1 : GFZ Potsdam, Department of Climate and Landscape Dynamics, Telegrafenberg, 14743 Potsdam, Germany
2 : GFZ Potsdam, Department of Inorganic and Isotope Geochemistry, Telegrafenberg, 14743 Potsdam, Germany
3 : Royal NIOZ, Department of Marine Microbiology and Biogeochemistry, Landsdiep 4, 1797SZ 't Horntje, Texel, The Netherlands
4 : GEOMAR, Research Unit Paleoceanography, Wischhofstrasse 1–3, D-24148 Kiel, Germany
Source Climate of the Past (1814-9332) (Copernicus GmbH) In Press
DOI 10.5194/cp-2020-114
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.

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Blanchet Cécile L, Tjallingii Rik, Schleicher Anja M, Schouten Stefan, Frank Martin, Brauer Achim Deoxygenation dynamics above the western Nile deep-sea fan during sapropel S1 at seasonal to millennial time-scales. Climate of the Past IN PRESS. Publisher's official version : https://doi.org/10.5194/cp-2020-114 , Open Access version : https://archimer.ifremer.fr/doc/00653/76539/