Geochemical and palaeoecological record of the Cenomanian-Turonian Anoxic Event in the carbonate platform of the Preafrican Trough, Morocco
Geochemical and palaeoecological record of the Cenomanian-Turonian Anoxic Event in the carbonate platform of the Preafrican Trough, Morocco
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DOI：
10.1016/j.palaeo.2012.10.005
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
V. Lebedel;C. Lézin;B. Andreu;M. Wallez;E. M. Ettachfini;L. Riquier
中科院分区：
文献类型：
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作者：
V. Lebedel;C. Lézin;B. Andreu;M. Wallez;E. M. Ettachfini;L. Riquier
在摩洛哥南部前非洲海槽碳酸盐岩台地的Cenomania-下图伦纪的3个露头剖面(Goulmima，Ziz和Tazzouguert)采集的岩石样品和薄片的基础上，进行了常量元素和微量元素的地球化学分析和古生物研究，以确定缺氧条件是否影响台地，并强调氧变化对碳酸盐生产的可能影响，特别是在Cenomarian晚期发生的大洋缺氧事件2(OAE2)期间。在此期间，碳酸盐台地由两个截然不同的环境组成。东部(Ziz和Tazzouguert段)发育了浅水环境(潮间至中坡)，以底栖生物的高度多样性和丰度为特征，在某些情况下，以浮游生物的高度多样性为特征，并伴随着低浓度的自生标志物(U、Mo、V和Cr：氧化还原代用品)，表明有氧条件。在西部(Goulmima段)，环境较深(中坡道/外坡道)，氧化还原代用品记录到高度浓缩，这是减少条件的指标，与生物多样性和丰度低以及存在Buliminidae有关。底层水体缺氧，海面古生产力低。因此，在晚更新世，前非洲海槽地区的氧合条件相对较好，即使在OAE2时期也是如此，但在向西更深的环境中除外，那里的底层水域氧气匮乏。在塞诺马尼亚海侵期间，更准确地说，在OAE2期间，这种温度失调与从大西洋和/或特提斯洋更深的缺氧盆地流入含氧量较低的水有关。在地台外斜坡环境中，下都罗统到处都记录到了地球化学、古生物和沉积学的扰动。古生物学指标显示，种群多样性大幅下降，底栖有孔虫消失，只有浅水浮游有孔虫幸存下来。氧化还原条件和古生产力的地球化学指标显示，三个露头剖面都有明显的富集区，西部的丰度较高。这种温差是对气候变化的响应，从温差季节到炎热潮湿，导致地台西南部反地图集的化学风化作用增加。这种营养物质的大量输入增加了古生产力，最终由于有机质的腐烂而增加了O2的消耗。此外，由于土壤的淋失，新生代晚期的海侵导致向平台提供了额外的营养物质。
Geochemical analyses (major and trace elements) and palaeontological studies were carried out based on rock samples and thin sections collected from three outcrop sections (Goulmima, Ziz and Tazzouguert) in the upper Cenomanian–lower Turonian in the carbonate platform of the Preafrican Trough (South Morocco), in order to determine if oxygen-depleted conditions affected the platform, and to highlight the possible impact of oxygen variation on carbonate production, particularly during the Oceanic Anoxic Event 2 (OAE2), which took place in the late Cenomanian. During this interval, the carbonate platform was composed of two contrasting environments. A shallow-water environment (peritidal to mid-ramp) was developed in the East (Ziz and Tazzouguert sections), characterized by a high diversity and abundance of benthic organisms, and in some cases, by a high diversity of planktonic organisms, associated with low concentrations of authigenic markers (U, Mo, V and Cr: redox proxies), indicating oxic conditions. In the West (Goulmima section), the environment was deeper (mid ramp/outer ramp) and redox proxies record a high enrichment, an indicator of reducing conditions, associated with a low diversity and abundance of organisms and the presence of Buliminidae. Bottom waters were dysoxic and the palaeoproductivity was low at the sea surface. Thus, during the late Cenomanian, oxygenation conditions were relatively good over the area of the Preafrican Trough, even during the OAE2, except in the deeper environments toward the West, where bottom waters were depleted in oxygen. During the Cenomanian transgression, and more precisely during the OAE2, this dysoxia was linked to the influx of poorly-oxygenated waters from the deeper anoxic basins of the Atlantic and/or Tethys oceans. Geochemical, palaeontological and sedimentological disturbances are recorded in the lower Turonian everywhere in the outer-ramp environments of the platform. Palaeontological proxies show a huge decrease in group diversity, with the disappearance of benthic foraminifera – except for the Buliminidae which proliferated – and only shallow-water planktonic foraminifera survived. Geochemical proxies of redox conditions and palaeoproductivity show a marked enrichment in all three outcrop sections, with higher values in the West. This dysoxia occurred in response to a climatic change, from warm with contrasting seasons to hot and wet, which induced an increase of chemical weathering in the Anti-Atlas to the southwest of the platform. This substantial input of nutrients increased the palaeoproductivity, which finally enhanced the consumption of O2owing to the decay of organic matter. Moreover, the late Cenomanian transgression led to the supply of additional nutrients to the platform, due to the leaching of soils.