Chemical and mineralogical changes of calcareous shell material during fossilisation processes – an experimental approach

双壳类是一种逐渐生长的硬组织，它以精确的时间顺序记录了水生环境的生物和环境信息，时间跨度从几年到几个世纪不等。相比之下，蛋壳正在快速矿化（数小时到数天）羊膜动物的硬组织，记录着陆地环境下饮食、体温和气候的信息。我们计划通过体外成岩实验来研究这些生物矿化碳酸盐壳材料的死后蚀变，以及记录的矿物学、化学和微结构代理信息的稳定性。了解双壳类和蛋壳的成岩蚀变对可靠的古气候和古环境重建至关重要。贝壳保存和成岩作用的一个关键因素是钙质壳物质的溶解和再沉淀过程，导致在现有生物矿物单元内形成新形态的CaCO3矿物或结构域，改变了贝壳的化学和同位素组成以及壳的微观结构。仅有为数不多的实验研究系统地研究了双壳类贝壳的化学和矿物学蚀变，但未对蛋壳进行研究，主要是在干燥条件下高温加热或热液蚀变。然而，缺乏典型近地表埋藏和流体条件下的低温（≤90°C）实验。在拟议的研究中，我们将对双壳壳和蛋壳材料在不同同位素标记的水溶液（淡水和海水）中进行受控的体外蚀变实验，模拟典型成岩流体的组成，以评估关键环境、气候和饮食指标对成岩过程的抗性。矿物学、化学和微观结构的变化，钙质硬部分以及晶体内蛋白质相将沿着硬部分的变化梯度进行原位和非原位研究，使用组合的体积和空间高分辨率的原位分析技术。研究材料将在体外低温改造前后进行化学和结构表征，以确定成岩特征，然后将其与化石双壳类和蛋壳样品进行比较。总体目标是更好地了解低温环境下贝壳化石过程的机制，以及这些钙质硬组织的生物矿物和有机阶段的化学、同位素和微结构代用物的保存潜力。

Bivalve shells are incrementally growing hard tissues that record biological and environmental information of aquatic settings in precise chronological order over timespans of years to centuries. In contrast, eggshells are fast mineralising (hours to days) hard tissues from amniotes recording information on diet, body temperature, and climate in terrestrial settings. We plan to investigate the post-mortem alteration of these biomineralised carbonate shell materials and the stability of recorded mineralogical, chemical and microtextural proxy information by in-vitro diagenesis experiments. Understanding the diagenetic alteration of bivalve shells and eggshells is paramount for reliable palaeoclimatic and palaeoenvironmental reconstructions from fossils. A critical factor of shell preservation and diagenesis are dissolution and reprecipitation processes of the calcareous shell material, leading to the formation of neomorphic CaCO3 minerals or domains within existing biomineral units, which changes the chemical and isotopic composition of the shell as well as the shell microstructure. Only a limited number of experimental studies, mostly heating at high temperatures under dry conditions or hydrothermal alteration, systematically investigated chemical and mineralogical alteration of bivalve shells but none of eggshells. Low-temperature (≤90 °C) experiments at typical near-surface burial and fluid conditions, however, are lacking. In the proposed study, we will perform controlled in-vitro alteration experiments of bivalve shell and eggshell material in different isotopically labelled aqueous solutions (freshwater and seawater), mimicking the composition of typical diagenetic fluids, to assess the resistance of key environmental, climatic and dietary proxies against diagenetic processes. Mineralogical, chemical, and microstructural changes of the calcareous hard parts as well as the intra-crystalline protein phase will be investigated along hard part alteration gradients in-situ and ex-situ using a combination of bulk and spatially highly resolved in-situ analytical techniques. The study material will be chemically and structurally characterised before and after the in-vitro low-temperature alteration to identify diagenetic features, which will then be compared to fossil bivalve and eggshell samples. The overarching goal is to gain a better mechanistic understanding of shell fossilisation processes in low temperature settings and the preservation potential of chemical, isotopic and microtextural proxies, both in the biomineral and the organic phase of these calcareous hard tissues.