Beef and cone-in-cone calcite fibrous cements associated with the end-Permian and end-Triassic mass extinctions: Reassessment of processes of formation

Stephen Kershaw; Li Guo

摘要

Abstract This paper reassesses published interpretation that beef and cone-in-cone (B-CIC) fibrous calcite cements were precipitated contemporaneously just below the sea floor in unconsolidated sediment, in limestones associated with the end-Permian (P/T) and end-Triassic (T/J) mass extinctions. That interpretation introduced the concept of a sub-seafloor carbonate factory associated with ocean acidification by raised carbon dioxide driven by volcanic eruption, coinciding with mass extinction. However, our new fieldwork and petrographic analysis, with literature comparison, reveals several problems with this concept. Two key points based on evidence in the T/J transition of the UK are: (1) that B-CIC calcite deposits form thin scattered layers and lenses at several horizons, not a distinct deposit associated with volcanic activity; and (2) B-CIC calcite is more common in Early Jurassic sediments after the extinction and after the end of the Central Atlantic Magmatic Province volcanism proposed to have supplied the carbon dioxide required.Our samples from Late Triassic, Early Jurassic and Early Cretaceous limestones in southern UK show that B-CIC calcite occurs in both marine and non-marine sediments, therefore ocean processes are not mandatory for its formation. There is no proof that fibrous calcite was formed before lithification, but our Early Jurassic samples do prove fibrous calcite formed after compaction, thus interpretation of crystal growth in unconsolidated sediment is problematic. Furthermore, B-CIC crystals mostly grew both upwards and downwards equally, contradicting the interpretation of the novel carbonate factory that they grew preferentially upwards in soft sediment. Finally, Early Jurassic and Early Cretaceous examples are not associated with mass extinction.Three further key points derived from the literature include: (1) B-CIC calcite is widespread geographically and stratigraphically, not clustered around mass extinctions or the Paleocene–Eocene Thermal Maximum (PETM) event; (2) isotope signatures suggest B-CIC calcite formed under high pressure in burial at 70–120?°C, incompatible with interpretation of formation of B-CIC calcite at the redox boundary below the ocean floor; and (3) B-CIC calcite reported in P/T boundary microbialites in one site in Iran is the only occurrence known despite extensive published studies of similar shallow marine settings, demonstrating its formation is localized to the Iran site.Based on the above evidence, our opinion is that B-CIC calcite is best explained as a later diagenetic feature unrelated to rapid Earth-surface environmental change associated with mass extinctions; thus a novel carbonate factory is highly unlikely. Keywords End-Permian mass extinction ; End-Triassic mass extinction ; Beef ; Cone-in-cone calcite ; Ocean acidification prs.rt("abs_end"); 1. Introduction and aim The occurrence of two types of fibrous diagenetic calcite called “beef” and “cone-in-cone” calcite in limestone has been known in the literature for a long time ( e.g. Lang et?al., 1923 and Richardson, 1923 ). A commonly accepted explanation of these fibrous calcite types (see Cobbold et?al. , 2013 for a review) is that they formed under high hydraulic pressure and raised temperatures in deep burial ( Cobbold and Rodrigues, 2007 ) and this has been used as part of the evidence of stresses in tectonic belts ( e.g. Le Breton et?al. , 2013 ). Evidence that these fibrous calcite growths are additional precipitates on existing limestones ( e.g. Marshall, 1982 ) includes growth on nodules such as the birchii nodules in the Lower Jurassic of southern England reported by Hesselbo and Jenkyns (1995) and well illustrated in photographs of the celebrated website of Ian West ( http://www.southampton.ac.uk/～imw/Lyme-Regis-to-Charmouth.htm ). Interpretations focus on formation in burial as a late diagenetic development. It is therefore of great interest that a completely different interpretation has been proposed by Greene et?al. (2012) in relation to extreme environmental change in the oceans, associated with the Triassic–Jurassic boundary extinction event, employed also by Heindel et?al. (2015 , published online in 2013, but formally published in 2015) for the Permian–Triassic boundary extinction. These two studies presented arguments that such fibrous calcite fabrics were instead formed in the shallow sea floor, contemporaneous with deposition, as a response to enhanced carbon dioxide input into the atmosphere resulting from large-scale volcanic eruptions in the Late Permian and Late Triassic; the carbon dioxide was transferred to the oceans and interpreted to have acidified the seawater. Raised total dissolved inorganic carbon resulting from these changes is proposed by Greene et?al. (2012) to have led to intense precipitation of fibrous calcite below the sea floor, at the redox boundary, for the end-Triassic event. Although much published work exists on “beef” and “cone-in-cone” calc

机译：摘要本文重新评估了已发表的解释，即牛肉和圆锥形（B-CIC）纤维方解石水泥在未固结的沉积物中，同时在与二叠纪末期（P / T）和末尾相关的石灰岩中同时沉淀在海床下方。三叠纪（T / J）灭绝。这种解释引入了一个海底碳酸盐工厂的概念，该工厂与火山喷发驱动的二氧化碳升高引起的海洋酸化有关，这与大规模灭绝相吻合。然而，我们的新的实地考察和岩相学分析以及文献比较揭示了该概念的一些问题。基于英国T / J转换的证据，有两个要点：（1）B-CIC方解石沉积物在几个水平面上形成了薄的分散层和晶状体，而不是与火山活动有关的独特沉积物; （2）B-CIC方解石在灭绝后的早期侏罗纪沉积物中更常见，并且在中大西洋岩浆省的火山活动结束后，提议提供所需的二氧化碳。我们的样品来自三叠纪晚期，侏罗纪早期和白垩纪早期石灰岩英国南部的研究表明，B-CIC方解石既存在于海洋沉积物也存在于非海洋沉积物中，因此，形成海洋过程不是强制性的。没有证据表明在方解石化之前就形成了方解石纤维，但是我们的早侏罗纪样品确实证实了在压实之后就形成了方解石纤维，因此对未固结沉积物中晶体生长的解释是有问题的。此外，B-CIC晶体大部分向上和向下均等生长，这与新颖的碳酸盐工厂的解释相反，即它们优先在软质沉积物中向上生长。最后，早侏罗世和早白垩纪的例子与大灭绝无关。从文献中得出的另外三个要点包括：（1）B-CIC方解石在地理和地层上广泛分布，不聚集在大灭绝或古新世-始新世热最大值附近（PETM）事件; （2）同位素特征表明，B-CIC方解石是在70-120°C的高压下在埋藏中形成的，这与海床以下氧化还原边界处B-CIC方解石的形成解释不符。（3）尽管对类似浅海环境进行了广泛的公开研究，但在伊朗一个地点的P / T边界微辉石中报告的B-CIC方解石是唯一已知的事件，表明其形成局限于伊朗地点。，我们的观点是，B-CIC方解石最好被解释为后期成岩特征，与大规模灭绝相关的快速地球表面环境变化无关;因此，建立一个新的碳酸盐工厂的可能性很小。关键词二叠纪末次大灭绝;三叠纪末期大灭绝;牛肉 ;锥形方解石;海洋酸化prs.rt（“ abs_end”）; 1.引言和目的石灰岩中两种类型的成岩纤维方解石的形成过程分别是“牛肉”和“锥内”方解石（例如Lang等人，1923年和理查森），1923年）。对这些方解石纤维类型的普遍接受的解释（参见Cobbold等人，2013年综述）是它们是在高水压和深埋温度升高的情况下形成的（Cobbold和Rodrigues，2007年）。构造带应力证据的证明（例如，Le Breton等，2013）。这些纤维状方解石生长是现有石灰石上的额外沉淀物的证据（例如，Marshall，1982年）包括在结节上的生长，例如Hesselbo和Jenkyns（1995年）报道的英格兰南部下侏罗统的birchii结节，并在著名照片中很好地说明Ian West的网站（http://www.southampton.ac.uk/～imw/Lyme-Regis-to-Charmouth.htm）。解释着眼于成岩后期的成岩过程。因此，引起极大兴趣的是格林等人提出了完全不同的解释。（2012）关于海洋的极端环境变化，与三叠纪-侏罗纪边界的灭绝事件有关，Heindel等人也采用了这种方法。（2015年，于2013年在线发布，但于2015年正式发布）针对二叠纪-三叠纪边界的灭绝。这两项研究提出了这样的观点，即方解石纤维而是在浅海底形成，与沉积同时发生，是对二叠纪晚期和三叠纪晚期大规模火山喷发导致二氧化碳向大气中输入量增加的反应。二氧化碳被转移到海洋，并被解释为酸化了海水。由这些变化引起的总溶解无机碳的增加是由格林等人提出的。（2012年）导致三叠纪末海底氧化还原边界处的方解石纤维大量沉淀。尽管在“牛肉”和“圆锥内”计算上存在许多已发表的著作

Beef and cone-in-cone calcite fibrous cements associated with the end-Permian and end-Triassic mass extinctions: Reassessment of processes of formation

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