Black shale deposition during Toarcian super-greenhouse driven by sea level
Black shale deposition during Toarcian super-greenhouse driven by sea level
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DOI：
10.5194/cp-9-2703-2013
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发表时间：
2013-01-01
影响因子：
4.3
通讯作者：
Pellenard, P.
中科院分区：
文献类型：
--
作者：
Hermoso, M.;Minoletti, F.;Pellenard, P.
Toarcian海洋缺氧事件（T-OAE）最难以捉摸的方面之一是碳同位素之间的矛盾，表明强烈的全球初级生产力和有机碳埋藏在全球范围内，在欧洲缺氧的延迟表达。在最早的Toarcian，没有黑色页岩沉积在欧洲陆表海航道，和大多数有机碳富集的沉积物的结束后，总体积极的趋势，在碳同位素的特点T-OAE。在目前的研究中，我们试图建立一个层序地层格架的早托阿西沉积从一个核心钻在巴黎盆地使用相结合的矿物学（石英和粘土的相对丰度）和地球化学（Si，Zr，Ti和Al）测量。结合氧化还原敏感元素（铁，钒和钼）的演变，数据表明，缺氧的表达受到阻碍，在欧洲陆表海在大多数的T-OAE（定义的正碳同位素趋势），由于水深不足，防止分层的水柱。只有欧洲第一次出现的黑色页岩才与“全球性”事件相对应。该层段的特征在于>10%的总有机碳（TOC）含量，与随后的黑色页岩层相比，该层段含有相对低浓度的钼。此外，这第一个黑色页岩的出现与主要的负碳同位素漂移（CIE）的记录是同时代的，可能对应于一个短暂的温室效应加剧的时期，可能是由于大量的碳注入到大气-海洋系统。作为对风化作用增强和河流径流的一种反应，向盆地供应的淡水增加可能通过水柱的盐分层促进了完全缺氧条件的发展。相比之下，T-OAE后的黑色页岩在serpentinum和bifrons区仅限于陆表海（较高的钼TOC比）在相对高的海平面期间，和碳同位素返回到前T-OAE值。比较古氧化还原代理与推断的层序地层学桑塞尔表明，短期有机碳富集的情节主要是由三级海平面变化。这些黑色页岩在水柱中的氧可用性方面表现出非常好的高频周期性，其性质仍有待通过旋回地层分析来确定。
One of the most elusive aspects of the Toarcian oceanic anoxic event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the end of the overarching positive trend in the carbon isotopes that characterises the T-OAE. In the present study, we have attempted to establish a sequence stratigraphic framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE (defined by the positive carbon isotope trend) due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by >10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the atmosphere-ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales during the serpentinum and bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the oxygen availability in the water column whose nature has still to be determined through cyclostratigraphic analysis.