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From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones

机译：从原始文石到块状方解石：白垩纪多孔石灰石中头足纲珍珠层的异常保存和成岩作用

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摘要

Aragonite (along with calcite) is one of the most common polymorphs of the crystalline calcium carbonate that forms the skeletal structures of organisms, but it has relatively low preservation potential. Under ambient conditions and in the presence of water, aragonite transforms into calcite, the stable polymorph. Aragonite is also more soluble therefore, in water-permeable siliceous limestones (opokas) that are typical of Upper Cretaceous deposits of Poland and Ukraine, the primary aragonitic skeletons are either entirely dissolved and found as moulds and casts or transformed into secondary calcite, whereas the primary calcitic shells remain well preserved. Contrary to the common notion of the lack of aragonite in such porous carbonate deposits, we show that relics of aragonite can be preserved as a nacreous lining on cephalopod moulds or as thin, lenticular structures entrapped in neomorphic calcite. Based on the observed intermediate steps of aragonite alteration, we propose an extended model of nacre diagenesis. Among the originally aragonitic biota, only nautilids and ammonites have retained relics of pristine skeletons. Such selective preservation of only some aragonitic structures (nacre but not the prismatic aragonitic layers) points to the role of microstructural and biochemical differences between cephalopod shell layers that may set a threshold for the dissolution, dissolution/precipitation or preservation of original biomineral structures.
机译：文石（与方解石一起）是形成生物体骨骼结构的结晶碳酸钙中最常见的多晶型物之一，但其保存潜力相对较低。在环境条件下和水的存在下，文石转变为方解石，即稳定的多晶型物。因此，文石还更易溶于波兰和乌克兰上白垩统沉积层中典型的透水硅质石灰岩（opokas）中，主要的文石骨架要么完全溶解并以铸模和铸模形式发现，要么转变为次方解石。初级钙质壳保存完好。与这种多孔碳酸盐矿床中缺少文石的普遍观念相反，我们表明文石的遗迹可以保留为头足类霉菌中的珍珠质衬里，也可以保留为新变方解石中包裹的薄而透镜状的结构。基于观察到的文石蚀变的中间步骤，我们提出了珍珠质成岩作用的扩展模型。在最初的石蜡生物区系中，只有鹦鹉螺和am虫保留了原始骨架的遗迹。仅选择性地保存某些文石结构（珍珠层而不是棱柱状文石层）表明了头足类动物壳层之间的微观结构和生化差异的作用，这可能为原始生物矿物结构的溶解，溶解/沉淀或保存设定了阈值。

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