Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China

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• 作者：Meng Cheng^a ; ^b ; Chao Li^a ; ^{chaoli@cug.edu.cn" class="auth_mail" title="E-mail the corresponding author} ; Lian Zhou^c ; Thomas J. Algeo^a ; ^c ; ^d ; Feifei Zhang^e ; Stephen Romaniello^e ; Cheng-Sheng Jin^a ; Li-Dan Lei^a ; Lian-Jun Feng^f ; Shao-Yong Jiang^c ; ^g
• 关键词：Mo isotope ; Nanhua Basin ; Fe&ndash ; Mn shuttle ; Niutitang Formation ; Early Cambrian
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 June 2016
• 年：2016
• 卷：183
• 期：Complete
• 页码：79-93
• 全文大小：1146 K

文摘

Molybdenum (Mo) concentrations and Mo isotopes have been widely used as proxies for local and global redox conditions in early oceans (>520 Ma) that were stratified and characterized by dynamically euxinic mid-depth waters. However, the nature of the Mo cycle and accompanying isotopic fractionations in such oceans remain poorly known. To fill this gap, we conducted an integrated study of Mo isotopes and redox-sensitive trace element (RSTE) abundances in the Lower Cambrian Niutitang Formation at Yangjiaping, South China. This section accumulated on the northern shelf margin of the Nanhua Basin, a failed intracontinental rift basin with a good connection to the open ocean during the early Cambrian. The Niutitang Formation contains a ∼18-m-thick lower black shale member, and a ∼56-m-thick upper gray silty shale member. The lower member (LM) is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo isotopic composition (δ⁹⁸Mo = −0.65‰ to +2.14‰), indicative of dominantly euxinic depositional conditions punctuated by ferruginous episodes (as shown by previously reported Fe speciation data). The upper member (UM) shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform Mo-isotopic compositions (δ⁹⁸Mo = +1.16‰ to +1.71‰), indicative of weakly oxic to anoxic-euxinic depositional conditions. Geochemical profiles suggest that the LM-to-UM transition reflects a shift of the O₂/H₂S chemocline from the water column to the sediment. Large δ⁹⁸Mo fluctuations in the LM may record variations of H₂S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ⁹⁸Mo values of the UM are attributed to the effects of a local Fe–Mn particulate shuttle.

In light of the observations at Yangjiaping and other contemporaneous sections in the Nanhua Basin, we propose a new marine Mo biogeochemical model for the early Cambrian ocean. In this model, an Fe–Mn reduction zone may have developed above mid-depth euxinic waters in nearshore areas of the Nanhua Basin due to relatively high availability of fluvial Fe–Mn oxides and their subsequent reduction below the chemocline. Within this redox framework, large sedimentary δ⁹⁸Mo variations observed for the Niutitang shales can be explained through the combined effects of Mo isotopic fractionation associated with adsorption onto Fe–Mn-oxides and spatially variable H₂S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ⁹⁸Mo variations previously observed in the early Cambrian Nanhua Basin and in other Neoproterozoic-Cambrian marine basins can be similarly explained. Our model provides a novel interpretation of the Mo biogeochemistry of early Earth oceans, within the framework of which Mo isotopes may serve to further our understanding of the oxygenation history of the Earth-surface system.