| 黔北早奥陶世疑难钙藻化石Nuia形态分类与钙化机制 |
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| 引用本文： | 王冬梅,杨宇宁,刘伟,崔琳浩.黔北早奥陶世疑难钙藻化石Nuia形态分类与钙化机制[J].地质学报,2023,97(7):2093-2110. |
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| 作者姓名： | 王冬梅 杨宇宁 刘伟 崔琳浩 |
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| 作者单位： | 1) 贵州大学资源与环境工程学院，喀斯特地质资源与环境教育部重点实验室，贵州贵阳，550025;2) 陕西省早期生命与环境重点实验室, 西北大学地质学系，陕西西安，710069 |
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| 基金项目： | 本文为国家自然科学基金项目(编号 42162003，41890845，41902007，41702022，41772021)、贵州省科技计划项目(黔科合基础2018- 1053，黔科合平台人才2017- 5788)、中国科学院南京地质古研究所现代古生物学和地层学国家重点实验室开放课题(编号 173130)、贵州省人才基地项目(编号 RCJD2018- 21)和贵州大学大学生创新创业训练计划项目(贵大国创字2019- 012)联合资助的成果 |
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| 摘 要： | Nuia是早奥陶世备受争议的一种疑难钙化微体化石，其分类位置长期以来因内部构造常被成岩作用破坏而未得到解决。本文在薄片镜下观察的基础上，结合拉曼光谱和扫描电镜分析技术，对黔北桐梓地区下奥陶统红花园组生物礁中的Nuia化石进行形态解剖分析并探讨其钙化机制。研究结果显示，Nuia由发射状丝状体(filaments)聚集而成的胶群体(colony)以及外围的钙化外鞘(calcified encrustation)组成，丝状体基部往往有异形胞和(或)厚壁孢子聚集成暗色的中央区。据此，Nuia 可能是蓝藻门念珠藻目胶须藻科(Rivulariaceae, Nostocales, Cyanophyta)的早期代表。通过类比现代蓝藻的钙化机制模型，Nuia 可能具有比较复杂的钙化过程：原植体通过二氧化碳聚集机制(carbon dioxide concentrating mechanisms, CCMs)进行光合作用的过程中，碳酸盐颗粒首先在原始丝状体的基部胶鞘内沉淀形成黑色的中央区；随着藻丝的生长和假分枝的出现，丝状体的相应位置可形成反映季节变化的钙化带，显示为特殊的分层现象，藻丝生长停止后，藻体表面形成一层公共胶鞘，并经浸染作用和(或)结壳作用形成钙化外鞘；丝状体在早期成岩阶段受到重结晶作用而被交代为方解石纤维。此外，Nuia的中央区和公共胶鞘可能有细胞降解残留的有机大分子，限制了矿物晶体的快速生长，钙化过程中只形成细小的方解石矿物，因而在薄片中呈现出比围岩和钙化丝状体更深的颜色。
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| 关 键 词： | Nuia 形态分类 钙化机制 早奥陶世 红花园组 黔北红花园剖面 |
| 收稿时间： | 2022/2/17 0:00:00 |
| 修稿时间： | 2022/3/27 0:00:00 |
Morphological identification and calcification mechanism of the problemetic Nuia (calcified rivulariacean) from northern Guizhou during the Early Ordovician |
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| WANG Dongmei,YANG Yuning,LIU Wei,CUI Linhao.Morphological identification and calcification mechanism of the problemetic Nuia (calcified rivulariacean) from northern Guizhou during the Early Ordovician[J].Acta Geologica Sinica,2023,97(7):2093-2110. |
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| Authors: | WANG Dongmei YANG Yuning LIU Wei CUI Linhao |
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| Abstract: | Calcified Nuia is a problematic microfossil with a distribution range limited to the Early Ordovician. Its affinity has long been controversial due to the absence of morphological details as for an eodiagenetic taphotaxon. Here, we investigate a collection of Nuia is identified in the patch reefs at the top of the Honghuayuan Formation (Floian Stage, Early Ordovician) in northern Guizhou. Thin section investigations along with Raman spectroscopy (RAM) and scanning electron microscopy (SEM) reveal an unexplored pathway for its morphological deconstruction and calcification mechanism. Nuia is featured here as an irregular thallus colony surrounded by an external calcified encrustation and with densely filaments radially arranged from the dark central area that corresponds to the emplacements of heterocysts and/or aktinetes on the base of the filaments. We, therefore, temporarily assign Nuia to an early taxon of Rivulariaceae (Nostocales, Cyanophyta). Integrating the model of vivo cyanobacterial calcification by carbon dioxide concentrating mechanisms (CCMs)- enhanced photosynthesis, we propose a calcification process of Nuia to be as follows: at the beginning, calcium carbonates induced by CCMs deposit in the base sheaths of initial filaments, forming a black central area of Nuia; as the trichomes grow and produce false branching, seasonal pattern of growth and calcification is formed in zones where the filaments develop false branching, appearing as unique multilayers. When the trichomes stop growing, an outer common sheath formation and CCMs- induced precipitate of calcites is formed on the surface of colony by impregnation and/or encrustation, and the filaments are rapidly metasomatized by calcite fibers during early diagenesis. In addition, residual organic macromolecules probably gathered in the central area and the common sheath, limit the rapid growth of crystals and eventually form tiny calcite minerals during the calcification process and, furthermore, make them darker than the calcified filaments and the surrounding rocks. |
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| Keywords: | Nuia morphology calcification mechanism Early Ordovician Honghuayuan Formation Honghuayuan section in northern Guizhou |
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