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Formation of microbial organic carbonates during the late Jurassic from the Northern Tethys (Amu Darya Basin, Uzbekistan): Implications for Jurassic anoxic events
Global and Planetary Change ( IF 3.9 ) Pub Date : 2020-03-01 , DOI: 10.1016/j.gloplacha.2020.103127
Mehdi Carmeille , Raphaël Bourillot , Pierre Pellenard , Victor Dupias , Johann Schnyder , Laurent Riquier , Olivier Mathieu , Marie-Françoise Brunet , Raymond Enay , Vincent Grossi , Cécile Gaborieau , Philippe Razin , Pieter T. Visscher

Abstract The Late Jurassic was a period of major global carbon cycle perturbations with episodes of anoxia leading to regional accumulation of organic matter in sediments worldwide. The Tubiegatan section (SW Gissar Mountains, Uzbekistan) located in the Northern Tethys, shows atypical organic-rich limestone and marl deposits (up to 6% of total organic carbon) marked by pronounced negative excursions of δ13Ccarb (amplitude of ca. 12‰) and δ13Corg (amplitude of ca. 4‰) recorded during the Middle Oxfordian (Transversarium Zone). A transdisciplinary approach including sedimentology, palynofacies characterization, mineralogy, organic and inorganic geochemistry was carried out to elucidate the origin of these organic-rich deposits. Highest TOC are measured in nodular limestones, and lowest δ13Ccarb values in thinly laminated facies consisting in alternances of infra-millimeter-thick organic and carbonate laminae. In the latter, the presence of organic-carbonate peloids and of possible remnants of exopolymeric substances associated with clay indicate that these structures are probably mineralized laminated benthic microbial mats (i.e., stromatolites). Rock-Eval pyrolysis coupled to palynofacies analyses point to a dominant altered marine organic matter of probable algal/microbial origin, with subordinate continental phytoclasts inputs in the upper part of the organic-rich interval. Trace elements (U/Th, V/Cr and Mo/Al ratios) indicate two anoxic episodes coinciding with the highest TOC, punctuated by dysoxic periods. Such O2-depleted conditions have allowed the preservation and probably the development of anaerobic microbial communities in the microbial mats. In these latter, sulfate reduction probably had a significant contribution to the production of carbonates, which would explain the precipitation of pyrite and the relatively low δ13Ccarb values. The progressive decrease then disappearance of kaolinite from the base of the organic-rich interval, is interpreted as a progressive aridification of the Amu Darya Basin during the Transversarium Zone, culminating with the progradation of a large-scale gypsum sabkha overlying the organic deposits. Overall, the organic-rich deposits could record the onset of the disconnection of the Amu Darya Basin from the open sea to the south, induced by compression and subsequent uplifts in the Afghan and Central Iranian blocks. The elevated evaporation, coupled with the presence of hydrological barriers (such as coral reefs) could have led to the formation of local to regional anoxic conditions in the Amu Darya Basin. Similar microbial organic accumulations are recently known throughout the Tethys (e.g., Arabian Plate, Western Europe) and from other oceans (e.g., Central Atlantic, Pacific) during the Late Jurassic, suggesting common controlling factors. The increase of organic matter storage worldwide coupled with potential methane release could have in turn induced major perturbations of the carbon cycle during the Oxfordian-Kimmeridgian interval. The relatively shallow anoxia model proposed in this study contrasts with the well-known organic carbon-rich pelagic models proposed for the Jurassic anoxia (e.g., Toarcian, Kimmeridgian) and Cretaceous OAEs.

中文翻译:

北特提斯(乌兹别克斯坦阿姆河盆地)晚侏罗世微生物有机碳酸盐的形成:对侏罗纪缺氧事件的影响

摘要 晚侏罗世是全球主要碳循环扰动时期,缺氧事件导致全球沉积物中有机质的区域性积累。Tubiegatan 剖面(乌兹别克斯坦西南吉萨尔山脉)显示了非典型的富含有机物的石灰岩和泥灰岩沉积物(占总有机碳的 6%),其特征是 δ13Ccarb 的显着负偏移(幅度约 12‰)和 δ13Corg(幅度约 4‰)记录在牛津中期(横穿带)。包括沉积学、孢粉相特征、矿物学、有机和无机地球化学在内的跨学科方法被用来阐明这些富含有机物的矿床的起源。最高的 TOC 在球状石灰岩中测量,在由亚毫米厚的有机层和碳酸盐层交替组成的薄层状相中,δ13Ccarb 值最低。在后者中,有机碳酸盐球体的存在以及与粘土相关的外聚物质的可能残余物的存在表明这些结构可能是矿化的层压底栖微生物垫(即叠层石)。Rock-Eval 热解与孢粉相分析相结合,表明可能是藻类/微生物来源的主要改变的海洋有机物质,在富含有机物层段的上部具有次要的大陆植物碎屑输入。微量元素(U/Th、V/Cr 和 Mo/Al 比率)表明两次缺氧事件与最高 TOC 一致,被缺氧期打断。这种氧气耗尽的条件允许微生物垫中厌氧微生物群落的保存和可能的发展。在后者中,硫酸盐还原可能对碳酸盐的产生有显着贡献,这可以解释黄铁矿的沉淀和相对较低的 δ13Ccarb 值。高岭石从富含有机质层段的底部逐渐减少然后消失,被解释为在横向带期间阿姆河盆地的逐渐干旱化,最终以覆盖有机沉积物的大规模石膏 sabkha 的进积而告终。总体而言,富含有机物的沉积物可以记录阿姆河盆地从公海向南断开的开始,由阿富汗和伊朗中部区块的压缩和随后的抬升引起。蒸发增加,加上水文障碍(如珊瑚礁)的存在,可能导致阿姆河盆地形成局部到区域缺氧条件。最近在晚侏罗世期间在整个特提斯(例如阿拉伯板块,西欧)和其他海洋(例如,中大西洋,太平洋)中发现了类似的微生物有机积累,这表明存在共同的控制因素。世界范围内有机物质储存量的增加以及潜在的甲烷释放可能反过来导致牛津阶 - 金默里吉阶期间碳循环的重大扰动。
更新日期:2020-03-01
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