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

双壳类贝壳是逐渐生长的硬组织，以精确的时间顺序记录水生环境的生物和环境信息，时间跨度从几年到几个世纪。相反，蛋壳是快速矿化（数小时至数天）的硬组织，记录了陆地环境中的饮食，体温和气候信息。我们计划通过体外成岩实验研究这些生物矿化碳酸盐壳材料的死后变化以及记录的矿物学、化学和微观结构代用信息的稳定性。了解双壳类贝壳和蛋壳的成岩蚀变对于从化石中恢复古气候和古环境至关重要。壳保存和成岩作用的一个关键因素是钙质壳材料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CaCO3矿物或域，从而改变壳的化学和同位素组成以及壳的微观结构。只有有限数量的实验研究（主要是在干燥条件下高温加热或热液蚀变）系统地研究了双壳贝类的化学和矿物学变化，但没有研究蛋壳。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，我们将在不同的同位素标记的水溶液（淡水和海水）中进行双壳类贝壳和蛋壳材料的受控体外改变实验，模拟典型成岩流体的组成，以评估关键环境，气候和饮食代理对成岩过程的抵抗力。矿物学，化学和微观结构的钙质硬的部分，以及晶内蛋白质相的变化将被调查沿着硬的部分蚀变梯度原位和异位使用的组合体和空间高分辨原位分析技术。研究材料将在体外低温蚀变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。总体目标是获得一个更好的机械理解壳结晶过程中的低温设置和保存潜力的化学，同位素和微观代理，无论是在生物矿物和有机相的这些钙质硬组织。

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.