Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China
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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动态弱生中深水背景下的海洋钼生物地球化学：以华南下寒武统牛蹄塘页岩为例
DOI：
10.1016/j.gca.2016.03.035
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发表时间：
2016-06
期刊：
影响因子：
--
通讯作者：
Shao-Yong Jiang
中科院分区：
文献类型：
--
作者：
Meng Cheng;Chao Li;Lian Zhou;Thomas Algeo;Feifei Zhang;Stephen Romaniello;Cheng-Sheng Jin;Li-Dan Lei;Lian-Jun Feng;Shao-Yong Jiang
钼（Mo）浓度和Mo同位素已被广泛用作早期海洋（>520 Ma）局部和全球氧化还原条件的代用指标，这些海洋具有分层性，并以动态富氧的中层沃茨为特征。然而，钼循环的性质和伴随的同位素分馏在这样的海洋仍然知之甚少。为了填补这一空白，我们对杨家坪下寒武统牛蹄塘组的钼同位素和氧化还原敏感微量元素（RSTE）丰度进行了综合研究。该剖面堆积于南华盆地北方陆架边缘，南华盆地是一个早寒武世与大洋有良好联系的失败的陆内裂谷盆地。牛蹄塘组下部为黑色页岩段，厚度约18 m，上部为灰色粉砂质页岩段，厚度约56 m。下段（LM）中等至强烈富集Mo、U和V，并且Mo同位素组成不均匀（δ 98 Mo =-0.65 ‰至+2.14‰），表明以富氧为主的沉积条件，并伴有铁质事件（如之前报道的Fe形态数据所示）。上段（UM）Mo、U、V的富集程度较低，Mo/U比值较高，Mo同位素组成中等偏均匀（δ 98 Mo = +1.16‰ ~+1.71‰），沉积环境为弱氧-缺氧-富氧。地球化学剖面表明，LM到UM的过渡反映了O2/H2S化学跃层从水柱到沉积物的转变。LM中δ 98 Mo的大幅度波动可能记录了中深度缺氧沃茨中H2S浓度的变化。根据南华盆地杨家坪等剖面的观测结果，提出了早寒武世海洋钼地球化学新模式。在这个模型中，一个铁锰还原带可能已开发以上的中深度的缺氧沃茨在南华盆地近岸地区，由于相对较高的可用性河流铁锰氧化物和它们的后续还原低于化学跃层。在此氧化还原框架内，大的沉积δ 98 Mo的变化，观察牛蹄塘页岩可以解释通过与吸附到铁锰氧化物和空间变化的H2S浓度在富氧水体中的Mo同位素分馏的综合影响。通过对已有资料的整理，我们发现南华盆地早寒武世和新元古代-寒武纪海相盆地的δ 98 Mo变化也可以得到类似的解释。我们的模型提供了一个新的解释早期地球海洋的钼地球化学，在此框架内，钼同位素可能有助于我们进一步了解地球表面系统的氧化历史。
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.
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