Calcareous nannofossils and Mesozoic oceanic anoxic events
Calcareous nannofossils and Mesozoic oceanic anoxic events
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DOI：
10.1016/j.marmicro.2004.04.007
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发表时间：
2004-08-01
影响因子：
1.9
通讯作者：
Erba, E
中科院分区：
文献类型：
--
作者：
Erba, E
25年前，中生代海洋缺氧事件（OAE）被记录和正式的有机物的广泛到全球沉积的间隔。Toarcian，早期Aptian（OAE 1a）和最新的Cenomanian（OAE 2）OAE在性质上是真正全球性的，通常碳酸盐贫乏，通常以富含有机碳的黑色页岩为代表。在某些地区，这些OAE还具有丰富的放射虫砂和粉砂的特征。它们与Sr-87/Sr-86记录中的负向和正向偏移有关，此外，全球碳酸盐和/或有机物中的碳同位素异常也很大，这是由全球碳收支的重大扰动造成的。翁通爪哇（和马尼希基）和加勒比板块形成期间火山活动的增加，以及卡鲁-费拉尔圈闭，被认为分别导致了与OAE 1a，OAE 2和Toarcian OAE相关的地质反应。大气中过量的火山成因二氧化碳很可能将气候转变为温室模式，加速大陆风化，并通过河流径流增加海洋表层水的养分含量。全球海洋更高的生育力也可能是由海底火成事件直接引发的，这些事件在热液羽流中引入了大量的生物限制金属。由于中生代OAE通常以贫碳酸盐沉积物为代表，因此对钙质超微化石的定量研究已被应用于探索（a）相对于微型植物的增加和危机，火成/构造事件和气候变化的原因和影响，以及（B）溶解事件和（c）成岩改造。OAE期间钙质超微植物的特征可以提高我们对海洋生态系统和光合作用等生物过程的理解（生物泵）和生物矿化（碳酸盐泵），影响有机和无机碳循环，以及在海洋中的大气CO2的吸附。类型和速率的nannoflancoma的生产和演变的解释跟踪的主要古海洋和古气候事件的影响。在选定的部分，它已被证明，钙质nannofloras迅速反应，以新的条件下的生育力和更高的pCO（2），大大减少钙化。在现代海洋中，在OAE的营养物质和大气中的CO2的增加诱导较高丰度的nannoplankton生产小板石和抑制深透光区nannoconids和nanosphaerellids.Similarly的“nannoconid危机”沉积前的早期Aptian OE 1a黑色页岩，一个“nanosphaerellid危机”之前检测到的Toarcian OAE。这两个OAE的进一步特点是一个快速nannoflora物种形成，开始约1.5万年前的OAE，但没有precentions。在最新的森诺曼OAE 2的全球变化产生了不同的影响，经历了营业额，由于最极端的环境条件下的钙质nannoflancoma。事实上，这一事件是一个由钙质超微化石内的起源所引起的进化时期。OAE 1a、OAE 2和Toarcian OAE之前、期间和之后事件的精确时间表明，它们是海洋生产力提高的间隔，缺氧/缺氧发生在生物变化之后。(C)2004 Elsevier B. V.保留所有权利。
Twenty-five years ago, Mesozoic oceanic anoxic events (OAEs) were documented and formalised as intervals of widespread to global deposition of organic matter. The Toarcian, Early Aptian (OAE1a) and latest Cenomanian (OAE2) OAEs are truly global in nature, commonly carbonate-poor, and typically represented by organic carbon-rich black shales. In some areas, these OAEs are also characterised by abundant radiolarian-sands and silts. They are associated with negative and positive excursions in the Sr-87/Sr-86 record, in addition to large global carbon-isotope anomalies in carbonate and/or organic matter, caused by a major perturbation of the global carbon budget. Increased rates of volcanism during the formation of the Ontong Java (and Manihiki) and Caribbean Plates, and the Karoo-Ferrar Traps, are believed to have caused the geological responses associated with OAE1a, OAE2, and the Toarcian OAE, respectively. Excess volcanogenic CO2 in the atmosphere most probably turned the climate into a greenhouse mode, accelerating continental weathering and increasing nutrient content in oceanic surface-waters via river run-off. Higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes.Because Mesozoic OAEs are often represented by carbonate-poor sediments, quantitative studies of calcareous nannofossils have been applied to explore (a) the causes and effects of igneous/tectonic events and climate changes, relative to nannofloral increases and crises, as well as (b) dissolution events, and (c) diagenetic modifications. Characterization of calcareous nannofloras in OAE intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO2 in the oceans.Types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events. In selected sections, it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pCO(2) by drastically reducing calcification. As in the modem oceans, during OAEs the increase of nutrients and atmospheric CO2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep-photic zone nannoconids and schizosphaerellids.Similarly to the 'nannoconid crisis' preceding deposition of the Early Aptian OAE1a black shales, a 'schizosphaerellid crisis' is detected prior to the Toarcian OAE. Both OAEs are further characterised by a rapid nannofloral speciation, beginning approximately 1.5 myr before the OAE, but without extinctions. Global changes during the latest Cenomanian OAE 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions. This event, in fact, was a time of extinctions followed by originations within calcareous nannofossils. Precise timing of the events before, during and after OAE1a, OAE2 and the Toarcian OAE indicates that they were intervals of enhanced oceanic productivity and that anoxia/dysoxia post-dated biotic changes. (C) 2004 Elsevier B.V. All rights reserved.