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Biogeochemical implication of massive episodic flood deposition: Model-Data integration
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  • Stanley Ifeanyi Nmor,
  • Eric Viollier,
  • Lucie Pastor,
  • Bruno Lansard,
  • Christophe Rabouille
Stanley Ifeanyi Nmor
Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL,CEA-CNRS-UVSQ-Université Paris Saclay

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Eric Viollier
Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL,CEA-CNRS-UVSQ-Université Paris Saclay
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Lucie Pastor
ABF Décisions
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Bruno Lansard
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Christophe Rabouille
Laboratoire des Sciences du Climat et de l'Environnement, CNRS, Gif sur Yvette
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Coastal deltas are depocenter for materials transported from riverine channels. Under regime of extreme flood events, this zone can experience large sediment deposition within a short period. However, the biogeochemical consequences of such disturbances on the carbon and other element cycles are not fully understood. Using a coupled data-model approach, we explore the early diagenesis responses of coastal sediment influenced by two intense flood discharges (in spring and fall) by the Rhône river in 2008. The data set shows that biogeochemical fluxes and rates responded abruptly to this almost instantaneous change in sediment deposition. These flood-related depositions increased organic carbon mineralization by a factor of 2 to 6 compared to pre-flood levels, previously dominated by sulfate reduction (72%), and methanogenesis (8%). The two floods represented (organic-poor in spring and organic-rich in fall) cause different responses of the diagenetic system in terms of dissolved inorganic carbon (DIC) fluxes - the organic-poor flood deposition induced a large storage of DIC in porewaters, whereas the organic-rich induced a large efflux of DIC along the entire relaxation. The model reveals that intense redox cycling and mineral precipitation were responsible for the non-euxinic (sulfide-free) sediment after flood deposition. The sequential flood depositions reveal a temporary memory effect (i.e. an interaction between two successive floods), with stronger effect for methane (44%), whose relatively long relaxation timescale limits complete recovery before the next event 6 months after the first one. Increasing frequency and intensity of these events could lead to memory accumulation of flood biogeochemical signature.
11 Jan 2024Submitted to ESS Open Archive
29 Jan 2024Published in ESS Open Archive