Chemical and structural variations of bivalve shells at the micrometer scale - taking sclerochronology to the next level

双壳类软体动物为古气候研究提供了特殊的潜力。它们几乎栖息于世界上所有的水生环境中，在化石记录中大量出现，可以存活几个月到几百年。最重要的是，它们的外壳周期性地生长，并以精确的时间顺序记录环境变化。然而，从贝壳中提取特定的环境信号仍然是一项非常具有挑战性的任务，因为目前可用的可靠和公认的替代物数量有限。在许多其他生物体中，痕量和微量元素（TME）含量提供了有用的环境代用数据。然而，在双壳类中，它们的解释是出了名的困难，因为TME的摄取、运输和掺入壳中受到强烈的生物控制（生命效应），这叠加了环境信号。通过TME的强大控制，双壳类动物保持了其外壳（最坚固的天然材料之一）的出色机械性能，并确保各个生物矿物单元（碳酸钙和有机大分子的纳米复合物）具有特定的尺寸（nm，µm），形状和方向，并以复杂的顺序排列幸运的是，重要的影响反映在TME的微观结构和协变中。后者表明相互的生物分馏过程在起作用。日本合作伙伴将计算TME回归曲线，并分析/解释协变。我们的目标是识别和数学消除TME的相关性，以隔离环境信号，而不需要确定的分馏机制。TME浓度也强烈耦合到壳的微观结构，这反过来又是由生物矿化过程控制，并集成不同的TME分馏过程。某些微结构类型比其他微结构类型含有更高水平的某些TME。此外，壳的结构在微米尺度上变化很大，导致小尺度的化学异质性。因此，如果同时形成的壳部分的微观结构不同，则它们在化学上彼此偏离。为了隔离环境控制的TME变化，我们（在U美因茨）将进行壳TME和微观结构的综合和系统的映射。然后，我们将参数化不同微观结构中的典型TME水平，以消除TME的非环境变化。最终，我们将共同开发一个结合TME分布、微观结构和环境参数的多维数值模型。如果能够解开TME中的环境信息，双壳类硬年代学将实现重大飞跃，推动古气候研究。提出的新方法和申请人的互补专长沿着以及两个机构的分析基础设施可以将这一愿景变为现实。

Bivalve mollusks offer exceptional potential for paleoclimate research. They inhabit nearly all aquatic environments worldwide, occur abundantly in the fossil record and can live for a few months to several hundreds of years. Most importantly, their shells grow periodically and record environmental changes in precise chronological order. Yet, extracting specific environmental signals from the shells remains a very challenging task owing to limited number of reliable and well-accepted proxies that are currently available. In many other organisms, the trace and minor element (TME) content provides serviceable environmental proxy data. However, in bivalves their interpretation is notoriously difficult, because the uptake, transport and incorporation of TME into the shells is under strong biological control (vital effects) which superimposes the environmental signals. Through the strong control of TME, the bivalve maintains the outstanding mechanical properties of its shell - one of the strongest natural materials - and ensures that the individual biomineral units - nanocomposites of calcium carbonate and organic macromolecules - have a specific size (nm, µm), shape and orientation and are arranged in complex orders (shell microstructures).Fortunately, vital effects are reflected in the microstructure and in covariations of TME. The latter suggests mutual biological fractionation processes were at work. The Japanese collaboration partner will compute TME regression curves, and analyze/interpret the covariations. The goal is to identify and mathematically eliminate TME correlations in order to isolate environmental signals without the need to determine the fractionation mechanisms. TME concentrations are also strongly coupled to the shell microstructure which in turn, is controlled by biomineralization processes and integrates different TME fractionation processes. Some microstructure types contain higher levels of certain TME than others. In addition, the architecture of the shell varies strongly at the µm-scale which results in a small-scale chemical heterogeneity. Thus, contemporaneously formed shell portions deviate chemically from each other if their microstructure differs. To isolate environmentally controlled TME variations, we (at U Mainz) will conduct a combined and systematic mapping of shell TME and microstructure. Then, we will parameterize the typical TME levels in different microstructures in order to eliminate non-environmental variations from TME. Ultimately, we will jointly develop a multidimensional numerical model that combines the TME distribution, microstructure and environmental parameters.If it were possible to unlock the environmental information trapped in TME, bivalve sclerochronology would make a major leap forward and boost paleoclimate research. Proposed novel approaches and the complementary expertise of the applicants along with the analytical infrastructure at both institutions can turn this vision into reality.