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

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.

文石（aragonite）与方解石（calcite）同为构成生物骨骼结构的结晶碳酸钙最常见的同质多象变体，但其保存潜力相对较低。在环境条件且有水存在的情况下，文石会转变为稳定的同质多象变体方解石。由于文石的溶解度更高，因此在波兰与乌克兰上白垩统地层典型的透水硅质灰岩（opokas）中，原始文石质骨骼要么完全溶解并以铸模和铸型的形式留存，要么转变为次生方解石，而原始方解石质壳体则得以完好保存。与这类多孔碳酸盐沉积物中缺乏文石的普遍认知相悖，本研究证实，文石残余物可作为头足类铸模上的珍珠质内衬，或是包裹在新生方解石中的薄透镜状结构得以保存。基于观测到的文石蚀变中间阶段，我们提出了扩展的珍珠质成岩作用模型。在原本以文石为主要组分的生物群中，仅鹦鹉螺类与菊石类保留了原始骨骼的残余物。这种仅对部分文石结构（珍珠质层而非文石棱柱层）的选择性保存，表明头足类壳体层之间的微观结构与生物化学差异，可能为原始生物矿化结构的溶解、溶解-沉淀过程或保存状态设定了阈值。