Data/hora: 01/06/2024 00:59:09

Provedor de dados: | 5
País: | France

Título:

Composition des isotopes stables du molybdène dans les carbonates du Précambrien : affinement du proxy et applications paléo-environnementales

Molybdenum isotopic compositions of Precambrian carbonates : refinement of the proxy and paleoenvironmental applications

Autores: | Thoby, Mary.
Data: | 2018-12-14
Ano: | 2018
Palavras-chave: | Redox; Précambrien; Carbonates; Molybdène; Isotopes; Attaque séquentielle; Sequential digestion; Precambrian; Molybdenum.
Resumo: | The redox conditions of the oceans evolved considerably during the Precambrian. This is mainly due to the arrival of the oxygen, at first generating oxygen oases, and then accumulating in the atmosphere during the Great Oxidation Event (GOE) ca. 2.45 Ga. However, the earliest traces of free oxygen during these periods, and the mechanisms of their sedimentary expression, remain poorly understood. For over a decade, the isotopic composition of molybdenum (δ98Mo) in sediments has been used as a global marine redox proxy.An elemental and isotopic study of the different phases constituting carbonate rocks was carried out using sequential digestions performed on carbonates of different ages and depositional environments. The results show that Mo is primarily hosted in the organic phase but also within authigenic carbonate phases. These latter influence the isotopic value obtained during the classic pseudo-total digestion by 6N HCl that is most frequently employed for Mo isotope proxy studies in carbonates.Mineralogical and Mo isotopic studies on Mn(II)–rich shales and carbonates in the Boolgeeda and KazputFormations (Pilbara craton, Australia) leveraged the ability of δ98Mo to record local redox conditions in order to understand mechanisms responsible for sedimentary Mn enrichments occurring around the GOE. Coupled with other redox proxies and compared to modern analogues (e.g., the Landsort basin, Baltic Sea), the data argues for a reduction of Mn(IV) oxides within the water column and at the sediment-water interface. These observations bring new perspective on Precambrian Mn enrichments, which have been traditionally considered as the result of Mn oxide reduction occurring at depth in the sedimentary pile.Finally, a study of the maximum values of δ98Mo of analyzed and compiled carbonates compared to compiled isotopic values of black shales and iron formations deposited through geological time make use of the global component of this redox proxy. Data suggest an absence of euxinic conditions required for the recording of seawater values by black shales. This study also highlight the presence of an oxidative cycle of molybdenum since at least the Mesoarchean.
Tipo: | Text
Idioma: | Francês

Identificador:

https://archimer.ifremer.fr/doc/00663/77462/79129.pdf

https://archimer.ifremer.fr/doc/00663/77462/

Editor: | Université de Bretagne Occidentale
Formato: | application/pdf

Fonte:

info:eu-repo/semantics/openAccess

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