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Perturbation des environnements marins à la limite Frasnien-Famennien (Dévonien terminal) : apport de la géochimie inorganique et du magnétisme des roches.

Abstract : The Late Devonian times are a transitional period characterized by important climatic varia-tions and by several environmental perturbations recorded both in the continental domain (mountain uplift, development of vascular plants) and marine domain (sea-level fluctuations, biological crisis, burial of organic matter). For this study, we focus our attention to the deposition of worldwide organic matter-rich sedimentary levels: the Late Frasnian Kellwasser horizons, and their impact on carbon cycle. The pluridisciplinary study of sedimentary sections, located on the both sides of the Eovariscan belt, allows to reconstruct the variations affecting marine environments at the Frasnian-Famennian boundary, in the areas of the Rhenisch Massif, Harz Mountains (Germany), Western Meseta and Anti-Atlas (Morocco). Our works are based mainly on geochemical analyse and rock magnetism studies. From our results, a model is proposed to account for the formation of the Late-Frasnian Kellwasser horizons.

The analyses of magnetic signal show the detrital inputs gradually decreased during the Fras-nian. Detrital inputs were the lowest during the two Late Frasnian Kellwasser horizons. Conversely, the Early Famennian recorded an increase of detrital inputs. These variations were associated with global sea-level fluctuations, recorded during the Late Devonian, and profound changes of erosional rates on land. Our study suggests the evolution of detritism and carbonate productivity on the conti-nental margins were controlled by major climatic variations. This boundary would correspond to tran-sitional period from hot and humid conditions of the Devonian greenhouse climate to cooler and drier conditions, resulting in the establishment of an icehouse climate during the Carboniferous.
The geochemical analyses of the Kellwasser horizons highlight the increase of primary pro-ductivity and confirm the oxygen-depleted conditions in bottom-water in most of the studied marine environments. Our study evidences contrasting degrees and duration of oxygen depletion for the two Kellwasser horizons. The lower Kellwasser horizon is characterized by the onset of dysoxic conditions in shallower environments (platforms or submarine rises), whereas the upper Kellwasser horizon re-cords the onset of anoxic to euxinic conditions in the deepest environments (basins) and oxic to dy-soxic in the shallower environments.

During the lower Kellwasser horizon deposition, the oxygen depletion would have resulted from the eutrophication of shallow marine environments, induced by an enhanced supply of nutrients derived from continents that stimulated primary productivity. This mechanism would be the result of the intensification of weathering, caused by the development of vascular plants and the uplift of Eovariscan belt, and favoured by warm and humid climate. During the upper Kellwasser horizon deposition, the oxygen depletion would have resulted from water stratification confining the deepest environments, in relation with oceanic water circulations. The water stratification was accentuated during the highstand period of the late Kellwasser horizon. The anoxic water may have impinged on the platform environments because of sea-level rise. The onset of anoxic to euxinic conditions fa-voured the diffusion of nutrients released by the anaerobic decay of organic matter. The transfer of dissolved nutrients from anoxic bottom water to ocean surface could happen during episodic disrup-tions of water stratification and could enhance the primary productivity.

It is proposed that this period of organic carbon burial in the sediments had highly perturbed the long-term carbon cycle, leading to a noticeable drop of the atmospheric CO2 pressure and to a cooling phase during the early Famennian. Thus, the formation of the Kellwasser horizons resulted from a conjunction of diversified phenomena, including tectonics, evolution of plants, physiography of oceans, marine productivity, all of these being in direct or indirect interactions with the climate.
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https://tel.archives-ouvertes.fr/tel-00206195
Contributor : Laurent Riquier <>
Submitted on : Wednesday, January 16, 2008 - 4:52:04 PM
Last modification on : Friday, April 5, 2019 - 8:11:09 PM
Long-term archiving on: : Tuesday, April 13, 2010 - 6:06:28 PM

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Laurent Riquier. Perturbation des environnements marins à la limite Frasnien-Famennien (Dévonien terminal) : apport de la géochimie inorganique et du magnétisme des roches.. Géochimie. Université des Sciences et Technologie de Lille - Lille I, 2005. Français. ⟨tel-00206195⟩

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